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THE IMPACT OF LOSS OF FOREST COVER ON
RIVER SYSTEM HYDROLOGY AND HUMAN SETTLEMENTS:
A Case study from the Se San River Basin
in Vietnam and Cambodia
By Alastair Fraser and Nick Jewell
CONTENTS
Acronyms
Preface
Summary
1.
Introduction
2.
Methods
3.
Data and information
3.1.
Data sources
3.2.
Access to data
3.3.
Data quality
3.4
Data processing
4.
Description of the Study Area within the Greater Mekong River Basin
4.1.
Geomorphology
Slope
Catchment gradient
4.2.
Forests and ecology
Evergreen forest
Deciduous forest
Medium and low density forest, scrub and secondary forest
4.2.
Climate and hydrology
Precipitation
River discharge
River water levels
4.4.
Population and settlements
Major settlements
Villages
Population density
Population changes
4.5.
Socio-economic conditions
5.
Results
5.1.
Changes in forest cover and land-use
5.2.
Changes in rainfall and run-off
5.3.
Changes in population and economic activity
5.4.
Rainfall and flooding events
5.5.
Economic impact
6.
Conclusions
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7.
Recommendations for policy and follow-up investigations
Appendices & maps
SELECTED ACRONYMS
ADB
Asian Development Bank
AIT
Asian Institute for Technology
ESRI
Environmental Systems Research Institute
GIS
Geographic Information Systems
GTZ
German Technical Cooperation Agency
LMB
Lower Mekong Basin
MRC
Mekong River Commission
RETA
Regional Environmental Technical Assistance
RS
Remote Sensing
RSAP
Regional Centre for Asia and the Pacific
SEF
Strategic Environment Framework
SEMIS
Subregional Environmental Monitoring and Information Systems
EWIS
Early Warning Information System
UNEP
United Nations Environment Programme
UNEP-GRID
United Nations Environmental Program – Global Resources Information Data Base
PREFACE
This report has been produced by Alastair Fraser, (Forest Policy and Economics Consultant) and Nick Jewell (GIS
Consultant), and is based solely on data available from various archives held by the MRC, UNEP and ADB. The study is
intended as a preliminary investigation of the impact of deforestation in the Mekong River Basin on flooding and other
environmental degradation using readily available data. One purpose of the study is to identify any important gaps or
deficiencies in the data that would prevent a thorough economic analysis of deforestation and environmental degradation.
Much of the work was undertaken at the UNEP offices at AIT in Bangkok, and the authors would like to thank staff from
UNEP, AIT the MRC and the ADB for their help in tracking down sources of data, and in particular help with scanning
documents to obtain records from the time before digital data became the norm.
SUMMARY
1. The Sub-regional Environmental Training and Institutional Strengthening (SETIS) Regional Technical Assistance
(RETA) is concerned with, identification and analysis of critical issues of mutual concern to the Greater Mekong Subregion in institutional strengthening, policy reforms, legislative initiatives, organisational structuring and capacity building
for environment and natural resource management and policy priorities. This study gives suggestions and proposals for
further action required to strengthen institutional capacity for environment management in the GMS in relation to
environment and natural resource management. In particular the response to the impact of deforestation on the hydrology
of the river basin, and changes in the nature, extent and frequency of flooding and the severity of damage to property and
livelihoods brought about by floods.
2. The study has been carried out at two levels. First at the sub-regional level; trends in deforestation within the Lower
Mekong Basin covering those parts of Laos, Thailand, Cambodia and Vietnam that lie within the Mekong catchment; and
second, the catchments of the Sesan and Srepok rivers that rise in the Central Highlands of Vietnam and join the main
Mekong river in Cambodia at Stung Treng. In the second part of the study the relationship between deforestation trends
and the land-use that has replaced forest has been examined, together with changes in the hydrology of the two rivers,
patterns of rainfall and the extent of flooding. It was not possible to collect the necessary socio-economic data from
secondary sources to allow an assessment of the balance of costs and benefits arising from the changes that have taken
place.
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3. Maps of forest cover at four occasions between 1975 and 1997 were obtained, but because these were prepared by
different agencies, the classification used for the forest differed on each occasion so that direct comparisons were difficult.
Some surveys class according to the condition of the forest such as "closed", "medium density" and "mosaic", while others
use the ecological class such as "evergreen", "deciduous" and "secondary forest" and these classes overlap. The
deciduous forest covers a substantial proportion of the area, and as its appearance on satellite imagery varies with the
season, it is clear that this has led to serious problems of interpretation. Only by assuming that areas classed as
"deciduous forest" in the later surveys would have been forest in the earlier surveys, was it possible to reconcile the areas
of forest at each of the surveys.
4. The rainfall and river flow data was mostly obtained from the Mekong Hydrological Journal by scanning old records, as
the only data available in digital form is very recent. The records from this source go back a long way, but they are very
patchy and a considerable effort would be required to digitise all the available data. The socio-economic data was
obtained from the MRC, but the available data only gave population statistics at the District level for Cambodia and some
basic "ranks" for a number of possible indicators related to poverty for both Cambodia and Vietnam, though the ranks
referred to different indicators in each country.
The study shows that since 1975, there has been severe loss of forest area within the Greater Mekong Basin and serious
degradation of the forest areas that remain. These losses of forest cover have been most severe on the steeper slopes,
but this is almost certainly due to the fact that little forest remained on the flatter land in 1975. The basins of the Sesan
and Srepok have suffered a similar fate to the GMB as a whole. Very little closed evergreen or deciduous forest remains
throughout the GMB, and most of the remaining forest is now classed as "open or "mosaic of forest and cropland". The
loss of forest cover in the Sesan and Srepok basins is associated with changes in the pattern of run-off in the rivers. Over
the decade from 1985 to 1994, the rainfall/run-off ratio decreased, so that a given amount of rainfall was associated with a
greater run-off. The additional run-off from heavy rain, especially when it occurs late in the season contributes to an
increased risk of flooding in the main Mekong channel.
In the lower Mekong basin in Cambodia a very small rise in the water level above a critical point, results in very extensive
floods. Once Tonle Sap lake is full, a small additional flow produces a flood that covers about 40% of the area of the
country. During the period for which river flow and rainfall data was obtained, the water level only rose above the critical
level in one year, 1994; the most recent year for which data was obtained. It is difficult to say how much the general loss
of forest cover both in the Mekong as a whole and in the Sesan and Srepok basins in particular, contributed to this high
level of flow in the river. The evidence from the Sesan and Srepok basins suggests very strongly that loss of forest cover
would have increased the flow for a given amount of rainfall, and so turned a rainfall event that might have been within the
capacity of the lower Mekong basin to absorb, into a flood.
The loss of forest cover in the Sesan and Srepok basins is related to a combination of population growth and indirectly to
the cultivation of coffee. The latter has generally been expanded on the best soils in the Central Highlands and has
resulted in a demand for more land for cultivation of food crops. The population growth has been around 3.8% in the upper
Sesan and Srepok basins, partly due to immigration, and newcomers and new households have been forced onto more
marginal land as a result of the coffee boom that led to the occupation of most of the best land. The loss of forest cover
was most severe in the more densely populated Districts, where 20 to 60 ha of forest were cleared for each additional
household. In the less densely populated Districts the area cleared per additional household was generally less than 10ha.
However despite these relatively large areas that have been cleared in relation to additional households, the increase in
cropland per additional household appears to be only around 2 ha. Part of the difference between forest area cleared and
additional cropland, may be due to the use of some of the land for coffee and other plantation crops, that do not yet show
up clearly in satellite imagery, but it seems that much of the land, having been cleared is then abandoned.
It is not possible to assess the economic impact of the deforestation from the available data. However, with more than 9
million people being affected whenever the water level in the Mekong rises past the critical level, even quite small costs for
each person or household will be a very large sum in aggregate terms. The results of the study suggest that the value of
forest for watershed protection depends very much on the local conditions, and that some specific areas will have very
high value, while other areas will provide relatively little benefits. It is therefore important to better understand the specific
attributes of forests that have a positive impact on water conservation and river flow so that such areas can be protected
and managed properly. This will depend on the topography and geology of the basin, the pattern of rainfall, the soil
conditions and the state of the human communities and settlements in the lower reaches of the catchment. This study has
shown that in a monsoonal climate with very seasonal rainfall, the rainfall is mainly absorbed by the soil in the early part of
the rainy season, and that run-off from a given amount of rainfall increases as the season progresses. The removal of
forest appears to have an impact on the soil’s ability to absorb and store water, and so gives rise to increased run-off.
In the Mekong basin, a serious flood event affects about 80% of the population, but this effect falls relatively more on the
better off. Only about 500,000 of those affected by flooding have an aggregate poverty rank of >3 out of a total in the
country of about 960,000. However, it is not clear how the ranking relates to other definitions of poverty, since only 8% of
the population have an aggregate rank of more than 3. Proportionally flooding affects the poorest less than the better-off,
mainly because the poorest live in the remoter uplands that are not affected by flooding. However, the data on poverty
rank is only available at the District level, and it is not possible to say whether flooding affects those that are poor within
the flood prone areas, more severely, as might be expected, if they tend to live along the river banks.
The season of 2001 has also proved to be one of serious flooding, though it was not possible to collect the rainfall or river
flow data so soon after the event. Data could not be readily obtained on historical flood events, but the data is likely to be
present in the Mekong Hydrological Journal and other sources. However, the former needs to be digitised to facilitate the
type of analysis necessary, and the latter is difficult to access according to the Starbuck report. (see para 13).
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The apparent severity of the deforestation that has taken place within the Mekong basin as a whole is cause for serious
concern. It is recommended that ADB and other concerned agencies take steps to collate ground inventory data for the
region in order to clarify the uncertainties over definition of forest types, establish some standard definitions and verify the
extent to which the findings of this study reflect the reality on the ground, and the degree to which they are affected by
inconsistencies in the interpretation of remotely sensed data. This would involve collaboration between national
governments and other international agencies, including MRC, ADB and UNEP.
The results of the study that show an apparent link between deforestation and increased run-off is sufficiently strong to
justify further more detailed analysis within the watersheds in order to elucidate more precisely the conditions under which
forest removal is having an impact. This would enable critical areas within watersheds to be identified, and for a value to
be attributed to them for water conservation, so that they can be given priority for conservation and protection. It could
lead to instruments to internalise the benefits of water conservation for forests. These conclusions should be discussed
with the MRC and consideration given to using the Katoomba Group as a means of promoting private sector involvement
in financing conservation measures where the benefits can be internalised.
In view of the development goals of poverty reduction, and the indication that the poorest Districts are generally those in
the remoter uplands where most of the forest remains, it is important to examine the relationship between poverty and
deforestation. The results suggest that the poorer communities with little access to social services may also lack access to
other resources, and so have little alternative to clearing forest to survive. This would imply that investment in the Central
Highlands of Vietnam and other upland areas in Laos and Cambodia associated with forest management and
conservation could have environmental benefits beyond the immediate locality of the investment especially if conservation
efforts were focussed in areas that bring the greatest benefits or have the greatest impact on reducing costs.
With at least 40% of Cambodia being at risk of flooding, further studies are needed of forest cover and management in the
upland areas throughout the Mekong basin to ensure that critical areas are either not deforested or those that have been
deforested are restored. Any social and economic development within Cambodia must take a holistic approach and
include measures to reduce the risk of damage from exceptional floods in the future. This may include a combination of
local measures and measures in the upper parts of river catchments. Climate change may bring about changes in rainfall
patterns. If the result is greater variation in rainfall, with heavier downpours, the risk of flooding in Cambodia could become
greater. Some assessment should be made of the possible changes in the risks of flooding in order to assess the need for
any investment in preventative measures.
1. INTRODUCTION
1. The scope of the Sub-regional Environmental Training and Institutional Strengthening (SETIS) Regional Technical
Assistance (RETA) covers inter alia, identification and analysis of critical issues of mutual concern to the Greater Mekong
Sub-region in institutional strengthening, policy reforms, legislative initiatives, organisational structuring and capacity
building for environment and natural resource management and policy priorities. The RETA has provision for the
preparation of a study encompassing the issues identified in order to give suggestions and proposals for action plans for
strengthening institutional capacities for environment management in the GMS. One issue that has been identified as
having mutual concern to the countries of the GMS in relation to environment and natural resource management is the
potential impact of deforestation on the hydrology of the river basin, and the possibility that part of that impact could be
changes in the nature, extent and frequency of flooding and the severity of damage to property and livelihoods brought
about by floods.
2. There is scientific evidence that forest cover has an impact on the hydrology of river systems through its effect on
rainfall interception and transpiration. In general, total run-off into rivers is lower from forest-covered land over a long
period compared to vegetation such as grassland due to greater interception and evapotranspiration losses. However, this
relatively small reduction in total run-off is offset by changes in the pattern of run-off after heavy downpours, with the
beneficial effect of forests in reducing the peak run-off, and extending the time to return to the base flow after heavy
downpours being reduced or lost. The higher peak flows after heavy downpours can in many circumstances lead to
increased flooding, though the effect depends on the characteristics of the river basin. The effect of removing forest cover
on the propensity to flooding is partly dependent on the proportion of the catchment that has lost its forest cover, and the
impact is generally limited to smaller river basins, because of the generally localised nature of heavy storms. From time to
time unusually large storms do occur and although these are likely to result in flooding regardless of the vegetation cover,
it is often claimed that such floods will be more severe in the absence of forest cover in the more steeply sloping parts of
the catchment, where run-off will be almost instantaneous in the absence of forests. In such circumstances the risk of
erosion and landslides is also increased and this will affect the silt load in the river as well as the flow.
3. Most of the research undertaken to date on the impact of removing forest cover, has been on the hydrological impact as
discussed above. Work has been undertaken, most notably in Indonesia, Malaysia, Sri Lanka and in Brazil, which has
confirmed the general validity of the impact referred to above at a micro catchment level. Very little work has been
reported on the economic impact of such changes, especially related to downstream impacts in populated catchments.
Some work has been done in North America on the impact of changes in forest cover on fish stocks, which have shown
that some species of fish can be adversely affected by removal of forest due to changes in water temperature and the silt
load. Very little work has been done in tropical countries on the economic impact of loss of forest cover, despite the many
claims of increased flooding and erosion. Floods have severe economic costs that can be particularly serious for poor
people who have few reserves to survive damage or loss of their homes or crops. There may be additional losses of social
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capital in the form of public buildings and infrastructure and these can impose an additional burden on the poor through
creating local shortages and raising prices for essential goods and services.
2. METHODS
4. The objective of this study is to assess the relationship between land use change, precipitation, and runoff, in two sub
catchments of the Lower Mekong Basin (LMB), namely the Se San and Sre Pok river basins, and to test whether any
measurable causal relationship exists between these factors. An additional aim is to put the changes occurring in these
two areas into perspective by looking also at the changes that have taken place in forest cover in all the other major 132
sub basins of the LMB since 1973. Two sets of analysis were made, one looking in general terms at the types of land use
change over the whole LMB and a second one looking in detail at the Se San and Sre Pok catchments. Subsequent
analysis seeks to relate rainfall and river discharge figures with the land use data, and to the socio-economic data that
exists for the administrative districts in both the Vietnamese and Cambodian sides of the catchments.
5. Standard spatial modelling techniques using ESRI Arcview software and data processing using grid (raster) format have
been the main GIS approach used in this study in order to produce a variety of maps and statistics. The grid format has
been used as it is a particularly appropriate and efficient method of summarizing the area of one variable (ie forest cover)
within the area of another given theme and according to additional sets of given criteria, such as slope angle and
ecological zone. The units of analysis for review of the LMB as a whole are those 132 major catchments or watersheds
identified by the MRC for management purposes (Figure xx). For the detailed analysis of the SeSan and Srepok basins,
administrative units (districts) of Cambodia and Vietnam have been used, within which socio economic data exists.
3. DATA AND INFORMATION
3.1. Data sources
6. Core datasets required for this analysis include historical data on land cover, precipitation, runoff, and topography. A
certain amount of this thematic data is directly available at UNEP-GRID at AIT in Bangkok, as part of UNEPs’ statutory
role in the GRID programme as a Regional Site for the Asia Pacific region (RSAP). However much of this GIS data tends
to be at a reconnaissance scale of around 1:1,000,000 which is not appropriate for a spatially detailed study.
7. Ideally, historical data on land use change would be available at an appropriate scale for a sub-regional study – say
1:250,000, and together with the hydrological data, to cover a time span equal to that of the major changes in land use
and human populations that have taken place. In practice, reliable digital data covering the LMB is scarce and often
appears to be anomalous. Other data, for example that on historical flooding extent is almost non existent prior to the late
1990’s, although this would more probably be due to the difficulty of acquiring the information in the absence of data from
modern remote sensing instruments.
8. Descriptions of other more detailed data sets held by national agencies are available at UNEP, although these data are
not directly accessible in Bangkok, and require a direct application to be made to the agency concerned. In principle,
highly relevant national datasets - for example that held by Vietnam’s National Environment Agency (NEA) - are also
available, though there appear to be significant practical difficulties in accessing and using the data which effectively
precludes their use in short term studies such as this.
9. Much of the baseline data used (country boundaries, towns, hydrological network) was originally collected as part of the
Early Warning Information System (EWIS) for the Strategic Environment Framework of the GMS (SEF project TA No.
5783). This data set was supplemented by elements supplied from the Mekong River Commission (MRC), and
hydrological data was digitized by scanning historical records (Mekong Hydrological Journal, held at AIT library, Bangkok),
and other data downloaded from the internet.
3.2. Access to data
10. The catalogue of spatial data held at the MRC in Phnom Penh indicated that the agency held much of the data needed
for this present study. A visit to Phnom Penh was arranged to physically obtain the data for analysis. Access to the 50
meter digital elevation model of the LMB was a particular priority, in addition to reliable historical data on land cover,
precipitation and surface hydrology in digital form. Staff at MRC cooperated fully in making some of this data available,
although acquiring data on precipitation and hydrology in digital form proved to be difficult.
11. Given the short time available requests to governmental agencies for data were made asking for digital data to be sent
as email attachments and data from NEA in Vietnam and MRC in Phnom Penh was sent to the consultant in Bangkok in
this form. In the case of certain core data sets including the 1:50,000 provincial map database of Vietnam, consent to
make these data available was declined by the NEA, citing an official policy that precludes external access to this detailed
information. Clearly the issue of open data access within the GMS, while agreed to in principle by many organisations may
not necessarily function in practice, and further effort by national agencies towards reaching a consensus on data
standards, public access and sharing would greatly improve efficiency in this regard. Many of the issues in sharing and
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access to geographic information are complex, including questions that concern data ownership, national standards and
agreed formats. Poor public access to detailed data of a non-commercial nature is a real constraint that agencies should
consider investing further effort in resolving, perhaps through the aegis of UNEP. Starbucks (2001)
12. Problems of data sharing and cooperation between countries within the GMS may be improved via a third party
agency such as the MRC, which is independent and being involved with all four countries of the LMB is in an ideal position
to act as a neutral agency in which all current data in the GMS region can be stored and accessed by the public according
to set rules and criteria. Most of the thematic data that were made available were prepared for SEMIS / EWIS (see
Starbuck Report) including the following files;
Table 1 – EWIS Datasets used this study provided by UNEP
File Name
Data Description
Scale
Source
Asia_bnd.shp
Regional Boundaries for countries in Asia
1:5M
ESRI
Gms-riv.shp
Greater Mekong River System
1:1M
UNEP
Pov_cam.shp
Poverty data for Cambodia
-
UNEP
Pov_viet.shp
Poverty data for Vietnam
Mrc_forest.shp
MRC forest cover for the lower Mekong Basin
1:250K
MRC
Gms_city.shp
Locations of cities
1:1M
UNEP
Gms_prov.shp
Provincial boundaries in the GMS
1:1M
UNEP
Gms_dist.shp
District and county boundaries in the GMS
1:250K
MRC
Mrc_ws.shp
Watershed Boundaries in the GMS
1:250K
MRC
Gms_soil.shp
Soil map of the GMS
1:250K
MRC
MRC_hyd_sta
Hydrological stations in the GMS
1:250K
MRC
13. Other potentially useful data held as part of the RSAP archive at UNEP ( such historical data on flooding extent, soils,
precipitation) on inspection was often poor in quality, (lacking topology or important attributes), and much of the data
anticipated could not be used for the study. This general observation is also mentioned in the report by Starbuck (2001)
who emphasizes the lack of a clear assignment of responsibility for maintenance of the individual pan GMS datasets, the
absence of which renders some of them useless after a few years.
Land cover information - data needed and data available.
14. There is a need for consistent data with which to assess the relationship between land cover change, precipitation,
and runoff over an adequate time period. Most of the earliest data on land cover available for the GMS region dates from
mapping work carried out in the early 1970s. Prior to this, information is generally sparse and uses inconsistent
classification schemes across national borders.
15. Considerable effort has been made in the review and preparation of forest cover data sets prior to any analysis taking
place, in order to ensure spatial and temporal consistency and therefore greater validity in the conclusions that are drawn.
Some of the data held at the UNEP-RSSAP data that were used in this study required some additional processing to
ensure that any results are as far as possible related to actual changes and not to differences in methodology employed in
the original mapping.
3.3. Data quality
16. Some obvious anomalies that were observed in the data immediately raise questions as to its accuracy and therefore
preclude its use. Land use maps of Cambodia prepared in 1971 and 1988 (UNEP/EAP-AP ref CAM0041 and CAM0044)
showed an apparent change from semi-evergreen to dry forest with a significant increase in area (figure 1). Differences
such as this are almost certainly an anomaly due to differences in classification method rather than an actual increase in
forest cover on the ground. Although much of the LMB has been mapped by UNEP using 1 kilometer NOAA data it is
generally accepted that NOAA provides a general indication of vegetation cover and is not suitable or reliable for detailed
study at scales less than 1:500,000.
17. In view of such anomalies and in order to acquire consistent data across national boundaries, data from Michigan
State University’s Tropical Rain Forest Information Centre (TRFIC) were used for the general overview of the LMB. This
data was downloaded from the internet assembled into a data files covering the entire LMB. Data processing prior to
analysis included geometric correction and edge matching. The 57m data resolution (derived from the original Landsat
MSS data) were re-sampled to 100m to reduce processing time and improve data storage. For the overview of the entire
LMB, the TRFIC data were the most consistent data source available, overcoming national differences in classification
schemes that tend to show an abrupt change in forest cover when moving from one country to another.
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18. From the possible land cover data sources at UNEP (Appendix table 1) only those generated by the MRC / GTZ
project in Vientiane and held by the MRC in Phnom Penh were found to be unambiguous and at a sufficiently detailed
scale (1:250,000) to use for the detailed study of the Sesan and SrePok basins. Land use data from the MRC, prepared
from Landsat TM data in 1993 and 1997-8 as part of a GTZ funded project were identified as the best available for several
reasons.
Fig 1: Land Use of NE Cambodia in 1971 (left) and 1985-86 (right). Arrows indicate local anomalies in land use
mapping within both Sesan and Srepok catchments where forest has apparently ‘increased’ and changed type
Consistent methodology had been used in classification, allowing for reasonably accurate change estimates between
years.
19. The GTZ project had also generated a land use classification for 1975, that covered both the Sesan and Sre Pok
catchments although this did not distinguish deciduous from evergreen forest types. A GIS procedure based on ecological
zoning was used to differentiate the 1975 forest / non forest land cover map into evergreen and deciduous forest.
Seasonality of vegetation cover
20. The difficulties in acquiring a consistent forest cover map of large parts of the LMB appear to be caused by the marked
seasonal pattern of the deciduous forest. Comparison of the Landsat data acquired on April 03, 2000 (at the end of the dry
season) with the MRC/GTZ vegetation map of 1997 shows the forest is virtually indistinguishable from areas that are
mapped as agriculture or non-forest (Figure 2).
Figure 2: Top Landsat TM dated 03 April 2000. Below GTZ/MRC vegetation map of part of upper Srepok
catchment
3.4. Data processing
All GIS data processing was performed using ArcView using the grid (spatial analyst) extension. Data for the detailed
study was processed in a similar manner to the data covering the whole LMB, though using the administrative regions of
Cambodia and Vietnam (and hence the socio economic data associated with these) rather than the watersheds to
summarise the data. Processing followed similar paths using the elevation data to extract slope information, quantify the
type of forest or land use within those areas and then extract these statistics according to watershed or district boundary
(Figure 3).
Figure 3: Steps followed in the processing of data sets from different sources and types
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4. DESCRIPTION OF THE STUDY AREA WITHIN THE GREATER MEKONG RIVER BASIN
4.1. Geomorphology
24. Figures 4 and 5 below show the geographic extent of the Greater Mekong Basin and the distribution of land according
to elevation. Figure 5 shows that most of the basin has an altitude of less than 280m and higher elevations are limited to
the northern and eastern margins of the basin.
Figure 4: Location of Se San (red) and Se San (green) watersheds within Lower Mekong Basin, showing main
towns
Figure 5: Boundaries of main 130 sub basins in LMB digital elevation model
Topography and Soils
21. The MRC GIS department in Phnom Penh kindly made available a 50m digital elevation model (DEM) that was
generated through a co-operation with the University of Berne as part of the watershed classification project. This DEM
was used to generate slope data for both the entire LMB and for the Sesan and Srepok that was then used to examine
patterns of deforestation and land use change according to topography and slope.
22. Some pre-processing of the data was required to remove negative values prior to generating the slope maps. A
preliminary classification of slope types within the Sesan and Srepok into four categories: less than 10 degrees, between
10 and 21 degrees, over 21degrees and over 30 degrees. See table 2 and Figure 6 below for the distribution of the slope
classes within the catchments. The results indicate that the topography is predominantly flat in both catchments with the
Sesan having a slightly greater area of steep slopes.
Table 2. Classification of land in the Sesan and Srepok sub-basins by slope class
Slope class
Below 10 deg
Sesan (ha)
as percent
Srepok (ha)
as percent
1,129,510
60.8
2,595,238
83.4
10 – 21deg
517,235
27.7
387,730
12.4
Over 21
189,345
10.1
112,508
3.6
Over 30
34,826
1.8
15,168
0.4
1,870,916
100
3,110,645
100
Total
Figure 6. Distribution of slope classes within the Sesan and Srepok sub-basins
23. Soil characteristics were derived from the EWIS soils map. Soil types in the two study areas are most commonly clay
(Acrisols and Ferrasols) to loamy silt (other soils); These soils are acid and very acid (pH KCl varied from 3.5 to 5.0 with a
central point between 4.2-4.3); The decline of soil organic matter (on average, organic carbon in the cultivated soils,
represents only 40-60% of that found in the soils under forests); They have low cation exchangeable capacity (CEC) and
base saturation (BS) and are poor in nutrients. Figure 7 below shows the distribution of soils within the Sesan and Srepok
basins, from which it can be seen that the Acrisols predominate in the upper parts and the margins of the two sub-basins,
with Gleysols more generally being found in the lower areas near the rivers. The Vertisols are mainly in the intermediate
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areas and the Ferrasols are on the lower ground in the upper reaches of the two basins.
Figure 7: Distribution of soil types within the Sesan and Srepok sub basins from FAO-UNESCO Soil Map
Catchment gradient
24. The topographic profiles sampled from the digital elevation model along several of the river channels allows a
comparison to be made of the overall gradient, and the variation in gradient along the length of the river. Figure 8 below
compares the vertical drop over a fixed distance in the upper part of the Mekong basin with the Nam Ngum, a tributary in
the upper reaches of the Mekong basin, and the Sesan and Srepok sub-basins in the middle reaches of the basin. The
results show that the Srepok and Sesan basins are generally flatter than similar sized catchments in the northern part of
the LMB but have much steeper sections in their headwaters. These areas are very steep (>30 degrees) and hence
maintenance of forest cover is crucial for erosion protection.
Figure 8: Comparison of the gradient in the upper Mekong, Nam Ngum, Sesan and Srepok sub-basins
On the basis of some evidence from studies in Vietnam that removal of forest cover has the greatest impact on slopes
steeper than 21º, the two steepest slope classes were combined to give only three slope classes recognised for this study.
The three slope classes that have been distinguished are:
steep slopes > 21º
intermediate slopes > 10º and < 21º
gentle slope/flat <10º
The Lower Mekong Basin contains 129 sub basins, and the proportion of the basin area in each of the slope classes has
been determined. This analysis showed that the sub-basins could be divided into three broad classes, based on the
proportion of the basin area having steep slopes. Approximately 20% of the Lower Mekong Basin lies in sub-basins with
more than 20% of their area on steep slopes. A similar proportion lies in basins with no steep slopes and the remaining
60% has less than 20% of steep slopes. These three classes are referred to hereafter as coarse, smooth and intermediate
terrain basins. Table 3 below gives a summary of the distribution of terrain within the Lower Mekong Basin and includes
the Sesan and Srepok sub-basins for comparative purposes.
Table 3: Distribution of area by terrain classes within the Lower Mekong Basin and within the Sesan and Srepok
sub-basins.
Terrain
Description
Number
of Area (‘000ha.)
sub-basins
Percent
of Percent of area of terrain type according
total area of to slope class
LMB
Gentle
Intermed.
Steep
Coarse
>21% steep
28
12,292
20.5
28
35
37
Intermed.
1-21% steep
54
37,422
59.9
81
12
6
Smooth
no steep
47
12,789
19.7
99
1
0
Page 10 of 22
Total LMB
128
62,505
100
75
14
11
Sesan
Intermed.
1
1,871
3.0
60
28
12
Srepok
Intermed.
1
3,111
5.0
83
12
4
25. The data presented in Table 3 shows that the Sesan and Srepok sub basins are both of the intermediate terrain class,
with the Sesan having slightly coarser terrain than the Srepok; (higher proportion of land on steep and intermediate
slopes)
Figure 9: Slope categories generated from 50m DEM, courtesy MRC, Phnom Penh
26. Figure 9 shows that the steeper slopes are generally found in the northern part and to a lesser extent along the
eastern border of the Lower Mekong Basin. Figure 11 shows all the sub-basins, shaded according to the proportion of
their area made up of steeply sloping land. The Sesan and Srepok sub basins stand out as being among the larger subbasins, and as having a significant proportion of steep slopes.
Figure 10: Sub-basins of the Lower Mekong Basin classified according to the proportion of their area in the
steepest slope class
27. Figure 11 below shows the distribution of slope classes within the Sesan and Srepok sub-basins at an enlarged scale.
This shows clearly the greater preponderance of sloping land within the Sesan sub-basin compared with the Srepok. The
steeper slopes in the Srepok basin are mainly in the upper reaches of the sub-basin in the south-east part of the basin,
and therefore it is to be expected that the impact of deforestation would be less in the Srepok basin than in the Sesan.
Figure 12 shows the altitude of the land within the two sub-basins and the generally greater preponderance of high altitude
land within the Sesan sub-basin.
Figure 11: Distribution of sloping land within the Sesan and Srepok sub-basins; (Pink < 10 degrees; Green 10-21; Red
> 21).
Figure 12. Land elevation within the Sesan and Srepok sub-basins
4.2. Forests and ecology
27. At the opening of the 20th century forests covered about three quarters of the land surface of Thailand (McKinnon
1997) and it is reasonable to assume that they covered a similar proportion of the Lower Mekong Basin, in Laos,
Cambodia and parts of the central highlands of Vietnam. The vast majority of the population lived in the Mekong, Chao
Phrya and Red River deltas and coastal plains and practised irrigated agriculture, while a minority of "hill tribes" lived in
small scattered communities in the hill areas and practised mainly shifting cultivation.
28. The climax forest type over most of the Mekong basin is semi-deciduous or deciduous monsoon forest as a result of
the seasonal characteristic of the rainfall and the pronounced dry season. In the southern part of the basin to the south of
about 10°, which is considered to be more or less the boundary between the evergreen and monsoon forest types, the
climax forest is evergreen rainforest. To the north of this boundary a mosaic of evergreen and deciduous forest may be
found where local climate, drainage and soil conditions result in moister conditions and favour the development of
evergreen species. The species composition of these two major forest types is different, and further local differences may
occur at higher latitudes, higher altitudes and in riparian, wetland and coastal forests.
29. The first comprehensive survey of forest cover for the basin in 1973 suggests that forest cover had been reduced to
about 55% over the basin as a whole, with an estimated forested area of about 34.5 million ha within the basin area of
62.5 million ha. Figures 13 to 16 show the distribution of forest cover over the Lower Mekong Basin in 1973, 1985, 1993
and 1997. Over the 24 years from 1973 to 1997, almost 16 million ha of forest were lost reducing forest cover to around
30%.
Figure 13 Closed forest (1973) from Landsat MSS data . Data from TRFIC, Michigan State University
Figure 14. Closed forest (1985) from Landsat MSS. Data from TRFIC, Michigan State University
30. The 1973 and 1985 surveys were undertaken by Michigan State University, and distinguished only closed forest.
Since much of the forest in the Lower Mekong Basin is deciduous, it must be assumed that the Landsat imagery used was
taken during the rainy season when the deciduous forest is in leaf and would appear as closed forest. In the two most
recent surveys, large tracts of forest are shown as "open forest", but where they coincide with closed forest in the earlier
survey, they are assumed to be deciduous forest in a leafless condition, so that they would appear as open forest in
satellite imagery. This illustrates the difficulty in comparing time series of forest cover data where different agencies use
different definitions of forest, and seasonal affects can be very important.
Figure 15. 1993; Closed, (green) and Open Forest (brown) and Forest Mosaic. Data from MRC, Phnom Penh
Page 11 of 22
Figure 16. 1997; Closed, (green) and Open Forest (brown) and Forest Mosaic. Data from MRC, Phnom Penh
31. Examination of the distribution of forest cover according to slope classes in 1973 and the subsequent deforestation
reveals that deforestation was relatively much higher on the steeper slopes than on the flatter land. Of the 16 million ha of
forest that were lost during the period 1975-1997 more than a half appears to have been on slopes steeper than 21%
although this slope class only accounts for about 11% of the total area of the basin. (see Table 1). Table 4 gives a
summary of the apparent loss of forest cover during the intervals between the surveys on each of the slope classes. Table
5 gives the apparent annual loss of forest cover by slope class during the periods between the surveys, expressed as a
percentage of the forest area at the beginning of the period.
Table 4: Change in forest area 1973-1997 by slope class in the Lower Mekong Basin
Slope class
Steep
Forest area (‘000ha.)
1973
1985
1993
1997
Loss 73-97
6,459.9
11,085.1
3,560.5
2,679.5
8,868.6
17,047.9
14,901.2
12,520.9
6,442.2
Intermediate
Gentle
4,030.4
3,600.0
3,990.8
3,494.9
535.4
Total
34,541.6
31,733.0
22,452.6
18,695.4
15,846.2
Table 5: Average annual rate of change of forest cover 1973-1997 by slope class in the Lower Mekong Basin
Slope class
Rate of change in Forest area (%)
1973-85
1985-93
1993-97
1973-93
1973-97
Steep
0.28
6.83
5.32
2.81
2.73
Intermediate
0.44
5.44
5.29
2.32
2.41
Gentle
0.92
1.45
3.66
1.07
1.37
Total
0.68
3.66
4.18
1.75
1.91
Figure 17: Distribution of forest within the Sesan and Srepok sub-basins 1975
32. As Figure 17 shows, forest cover was very extensive in the two sub basins in 1975, with deciduous forest in the lower
altitudes and on the flatter land, and evergreen forest on the hills and steeper slopes. Table 5 shows two trends: overall,
the rate of loss of forest cover is highest on the steepest slopes for the whole period, but for the first half of the period it
was highest on the flatter areas; the rate of forest loss increased dramatically during the period 1985-93 and overall,
continued to increase until 1997. Thus the evergreen forest has been subject to the greatest overall disturbance,
especially during the period 1985-93. However during the most recent period, the rate of loss decreased somewhat on the
steepest slopes and increased in flatter areas where deciduous forest predominates.
4.3. Climate and hydrology
33. The Sesan and Srepok sub-basins both rise in the Central Highlands of Vietnam, which has a monsoonal climate.
Most rainfall takes place during the period May to September with a tendency for peaks in July and September and
reduced rainfall during July. Figure 18 below shows the distribution of average rainfall for the months of January and July
across the Lower Mekong Basin. Figure 19 shows the impact of the seasonality of precipitation on the vegetation with
much of the Lower Mekong Basin carrying the deciduous forest characteristic of regions with a pronounced dry season.
Figure 20 below shows the location of the stations for which rainfall data (five stations) and river flow (six stations) are
available on a continuous basis for about a decade. The data fro these stations have been used in the study.
Figure 18: Seasonal rainfall patterns. TL: January ppt LL: July ppt (Source NESDIS MS digital atlas)
Figure 19: Map of Ecoregions in SE Asia showing area of tropical dry forest (beige colour). (Source Microsoft
2001 Digital Atlas)
Figure 20: Location of rainfall and river guage stations within the Sesan and Srepok sub-basins.
34. Figure 21 below shows the monthly precipitation at four stations in the upper Sesan and Srepok sub-basins, together
with the mean monthly run-off from the upper portions of the two sub-basins for the year 1994. The figure shows that the
rainfall peak in July has only a minor impact on the run-off, while the smaller peak in September produces much higher
mean run-off. This is most likely to be due to the condition of the soils. Early in the season they have not become
saturated and much of the precipitation remains in the soil, increasing its moisture content. By September the soil is
saturated and the higher precipitation results in a sharp rise in run-off.
Page 12 of 22
35. The figure 21 also shows that the precipitation in the Sesan sub-basin (Kontum and Pleiku) is higher than in the
Srepok (Buonmethuot). This difference shows up in the run-off with the Sesan showing more than double the run-off,
compared with the Srepok. The yellow line shows the run-off at Stung Treng, which is on the main channel of the mekong
just below the point where the two rivers enter it. It therefore measures the combined flow from the Mekong and the two
sub-basins. The figure shows that the flow of the combined rivers is more influenced by the Sesan than by the Srepok.
This has important implications when the impact of deforestation is considered.
36. Figure 22 shows the daily precipitation and run-off at Kontum and Dak Bla respectively in the upper Sesan for the
month of August 1986. The figure shows that the response of the run-off generally occurs on the same day as the peaks
of precipitation. Early in the month the peak of around 80mm produces run-off that raises the river level by about 1m,
whereas at the end of the month a similar peak raises the river level by about 1.8m. Thus peak rainfall events that occur
after the soil has become saturated are clearly much more important in producing floods than those that occur when the
soil is still dry. Since forest generally consumes more water than other forms of vegetation, especially grass and low
annual crops, the soil will tend to be drier under trees cover, and this is likely to have an impact on the propensity to flood.
Figure 21: Mean monthly precipitation and run-off for the Sesan and Srepok sub basins in 1994
Figure 22: Daily precipitation (Kontum) and river level (Dak Bla) in the upper Sesan sub-basin for August 1986
Page 13 of 22
37. Figures 23 and 24 below show the mean monthly precipitation, run-off and the rainfall/run-off ration for the upper
Sesan and upper Srepok respectively. A high value of the rainfall/run-off ratio indicates that run-off induced by a given
amount of rainfall is relatively low and vice versa. The figures show two features. The rainfall/run-off ratio in both basins
peaks at the beginning of the rainy season and drops off sharply as the season progresses. This is further evidence of the
role of soil moisture saturation since the lower rainfall/run-off ratio means more run-off from given amount of rainfall.
Figure 23: Mean monthly precipitation, run-off and rainfall/run-off ratio for Kontum/Dak Bla in the upper Sesan
sub-basin
Figure 24: Mean monthly precipitation, run-off and rainfall/run-off ratio for Buonmethuot/Ea Krong in the upper
Srepok sub-basin.
38. The second point of interest is the difference in the trend of the rainfall/run-off ratio in the two sub-basins over the
decade. The trend in the Sesan sub-basin is for the ratio to increase, though it seems to decrease again towards the end
of the period. This indicates that there is more run-off from a given amount of precipitation, which coincides with the loss of
forest cover. However in the Srepok sub-basin the trend is the reverse, showing a slight decline overall. In the case of the
Srepok, the trend seems to reflect the general rainfall pattern with the lower ratio occurring during two dry seasons when
the soils may not have become saturated. The rainfall pattern in the Sesan is generally much more uniform and 1989 and
1992 were not especially dry seasons.
Page 14 of 22
39. The two rivers, Sesan and Srepok join just above Stung Treng before they disgorge into the main Mekong channel.
The gauging station at Stung Treng therefore provides a picture of impact of the run-off from the two river basins on the
flow in the main river. Figure 25 below shows the daily river levels on the upper Sesan at Dak Bla, the upper Srepok at Ea
Krong and the combined flow at Stung Treng, for the period 10th October to 10th November 1992. On 23th October there
was a major rainfall event in the upper part of both sub basins. Around 100mm of rainfall fell in 24 hours in the upper
Sesan and around 30mm during the same period in the upper Srepok. On the 29th October, there was a second event
with 67mm in 24 hrs in the Sesan and 85mm in the same period in the Srepok. The Sesan showed a sharp response to
both events, and the combined flow at Stung Treng showed a similar response three days later. In contrast the Srepok
rose slightly following the first event, but continued to rise gradually until the day after the second event and then the river
level declined gradually. However the impact of the increased flow in the Srepok on the combined river flow was very
small. The rise in river level in the Sesan of 3m produced about 1.5m rise in the combined flow, and so the second rise of
1.5m at the second peak might have been expected to raise the combined flow by about 0.75m. In fact it rose by about
only 0.9m despite a rise of 3m in the Srepok. This suggests that the combined flow is about ten times more sensitive to
the flow in the Sesan than in the Srepok.
Figure 25: Daily river levels at Dak Bla (Sesan), Ea Krong (Srepok) and Stung Treng (confluence) 10 October-10
November 1992 in response to 98.5mm rainfall in the Sesan and 29.3mm in the Srepok on 23rd October and
67.3mm in the Sesan and 85mm in the Srepok on 28th October.
4.4. Population and settlements
40. Population data for the two sub-basins within both Cambodia and Vietnam is available for the District level, but is not
readily available by village. The latter would be necessary in order to make estimates of the numbers of people likely to be
affected by floods of different levels of severity. In the upper parts of the river basins the major impact of deforestation is
more likely to be erosion on the steep slopes, with loss of top-soil and damage to crops and infrastructure. On the lower
ground flooding is likely to be more serious, but limited to those communities on low ground near the rivers.
Figure 26: Density of towns and villages of study area, centred on Pleiku
41. Figure 27 below shows the districts in Cambodia and Vietnam that lie within the Sesan and Srepok basins. Data for
Cambodia and Vietnam on population by districts was obtained from the GMS (see list of files in Table 1 above). However
there is considerable difference between the two sets of data in terms of the data items. The Cambodia data includes
population and other demographic information as well as a number of important indicators relating to poverty for each
district. These include access to different sources of energy, water and sanitation as well as rates of literacy. In contrast
the Vietnam data only provide some basic poverty indicators without the necessary demographic data to enable
interpretation of the information. Vietnamese data is available from the national Statistical Office, but the files are too large
to send by email, and staff at the office do not have time to make summaries.
42. Using demographic data for Vietnam at Province level some approximate estimates have been made of district
populations from a combination of average population density and area of district. These data have been compared with
Page 15 of 22
actual data for two districts in Gai Lai Province that were the subject of a special Rural Appraisal study and show
reasonably good agreement, differing by less than 5%.
43. Based on the data for Cambodia, and the estimates for Vietnam, the total population living within the Sesan and
Srepok basins in 1997 is about 2.46 million with an average population density of 39 persons/km2. In 1975 the total
population in the two basins was around 1.04 million with a population density of about 16 persons/km2. This population
growth is equivalent to an annual rate of about 3.8%. The combination of population growth and loss of forest area
reduced the per capita area of forest (closed and open) from 3.4 ha in 1975 to 1.05 in 1997.
Figure 27: Districts of Cambodia (left) and Vietnam (right) that are within both the Se San and Srepok Catchments
4.5. Socio-economic conditions
44. The data on population obtained from GMS for Cambodia includes information on the number of households that have
access to electricity for power and lighting, various sources of water for drinking and household use and toilets as well as
the main type of fuel used for cooking. The data also contains details of adult literacy. For power, water, toilets and literacy
the District is given an average ranking as shown in the table 6 below.
Table 6: Rank of poverty indices according to the percentage of the population not having access to the facility or
not considered as literate
Rank
Power
Water
Toilet
Literacy
0
<81
<60
<86
<29
1
81-85
60-65
90-95
29-33
2
85-95
65-85
90-95
33-40
3
95-99
85-95
95-97
40-60
4
>99
>95
>97
>60
45. The basis for the ranking is not clear, but the choice of the range of values represented by one rank indicates that
there is considerable variation in access to the different facilities. It seems that more households have access to safe
water than have access to power and toilet facilities. The average value of the overall rank for the four indices for
Cambodia is 2.27. A summary of the average rank by District shows a sharp difference between the urban and rural
areas, with those Districts having a high population density, because they include substantial settlements, generally
having an average rank of less than 1, while the sparsely populated Districts have an average rank of 3 or 4. This shows
up when the population within the Sesan and Srepok basins is considered. The Districts concerned are largely rural and
the average rank of the poverty indices is 3.28.
5. RESULTS
5.1. Changes in forest cover and land-use
46. The extent of forest cover in the Greater Mekong basin in each of the assessment years has been has been discussed
in section 4.2 above. The comparison suggests that almost 16 million ha of forest were lost throughout the GMB during
the 22 years from 1973-1997. This represents about 46% of the forest that existed in 1973 and an annual loss of almost
2% annually. The rate of loss was greatest on the steepest sloping land. This loss of forest area does not take account of
the degradation of the remaining forest. Because of changes in the definition of the classes of forest identified at each
survey, and the fact that deciduous forest appears to have been omitted from the earlier surveys, it is difficult to give a
precise measure of the degree to which the residual forests are degraded. However the 1973 survey suggests a total area
of forest of 34.5 million ha. of which 27.1 million ha. was classed as "evergreen" and 26.9 million ha. was classed as
"closed". The balance of the area was apparently made up of deciduous forest which was probably considered as "open"
forest. By 1997, the total area of forest had been reduced to 18.7 million ha. but only 3.2 million ha. of that was considered
as "closed" and the balance of 15.5 million ha. was classed as "Medium and low density", probably comparable with the
earlier "open". Thus the area of forest classed as "open" or "medium or low density" has more than doubled from around
7.6 million ha. to 15.5 million ha. In other words around 83% of the forest that remains now is degraded to an extent that
led to it being classed as "medium and low density".
47. The Sesan and Srepok basins show a similar trend to the GMB as a whole. Table 7 below shows the change in
"closed" and "open" forest areas in the two basins between 1975 and 1997. The loss of forest area as a proportion of the
total was only about 27%, compared with the 46% referred to above for GMB as a whole, but as the table shows, there
were very heavy losses of closed forest (87% in both basins). The Sesan basin showed a small net increase in the area of
"open" forest, suggesting that much of the formerly closed forest has actually been degraded so that it is now classed as
"open.
Page 16 of 22
Table 7: Change in forest cover between 1975 and 1997 in the Sesan and the Srepok river basins.
Year
Sesan
Srepok
Total
Closed
Open
Closed
Open
Closed
Open
1975
880,178
345,759
693,096
1,642,577
1,573,274
1,988,336
1997
113,703
789,947
85,811
1,605,728
199,514
2,395,675
Difference
-766,475
444,188
-607,285
-36,849
-1,373,760
407,339
Table 8: Change in forest area by slope class for the Sesan basin
Year
1975
Closed
1993
1997
Open
Total
Closed
Open
Mosaic
Total
Closed
Slope 1 (0- 377,013
10
301,459
678,472
47,681
491,718
123,918
663,317
47,388
464,665 132,202
644,255
Slope 2 (1021)
333,221
27,426
360,647
44,097
220,674
61,794
326,565
43,962
215,129
60,483
319,574
3 168,235
4,338
172,573
22,001
104,730
32,885
159,616
21,958
103,213
32,420
157,591
1,211,692 113,779 817,122
218,597
Slope
(>21)
Total
878,469
333,223
Open
Mosaic
Total
1,149,498 113,308 783,007 225,105 1,121,420
48. Table 8 shows clearly that if areas of "open" forest and "mosaic" of forest and cultivated land, such as is found where
shifting cultivation is practised, are taken into account, the total forest area in the Sesan basin has only declined by about
100,000 ha. or about 10% between 1975 and 1997, but clearly the quality of the forest both for production and
conservation has been greatly diminished, with only 10% of the area remaining as "closed" forest. The rate of loss of
closed forest was about 42,500 ha annually between 1975 and 1993, and seems to have almost ceased since then. This
rate of loss represents almost 5% annually, on the steep and moderate slopes and is similar to the pattern found in the
Greater Mekong basin, as shown in Tables 4 and 5. Figures 28 and 29 below show the change in forest cover in the upper
Sesan basin between 1975 and 1997 and the areas of closed forest that were lost during the same period in greater detail
for the Province of Kon Tum in the central Highlands of Vietnam respectively.
Figures 28 (left) and 29 (right): Change in forest cover in the upper Sesan basin between 1975 and 1997 with
closed evergreen forest 1997 (green), closed evergreen forest in 1975 lost by 1997 (red) and deciduous forest
1997 (brown) and deciduous forest lost by 1997 (pink). Figure 28 shows a topographic map of Kon Tum Province
in Vietnam enlarged with the areas of closed forest lost between 1975 and 1997 shown in green
49. Figures 30 and 31 below show how the land-use has changed on a transect along the upper 21 km of the Srepok
basin. Figure 30 shows that in 1975 closed forest covered the land down to an elevation of about 700m. By 1997 the
closed forest is restricted to the highest sample point only at around 1300m and no closed forest is found below that
altitude. A small area of forest mosaic has appeared at about 50% of the distance, but a substantial proportion of the open
forest at the lowest part of the transect has become forest mosaic.
Figure 30: Land-use in 1975 along a 21 km transect of the upper Srepok river
Page 17 of 22
Figure 31: Land-use in 1997 along a 32 km transect in the upper repok basin
50. Figure 32 is a perspective view of the topography of the Sesan basin from the south west showing the relatively limited
extent of the high ground in the eastern margin of the basin. Figure 33 is the same view showing the distribution of the
remaining areas of forest, especially the closed forest shown in dark green, which is now very limited and mainly found on
higher ground.
Figure 32: Perspective view of Se San catchment from the Southwest showing elevation. Loss of forest appears
to have occurred most on the Vietnamese side, and on more steeply sloping land.
Figure 33: Perspective view of Se San catchment from the Southwest showing 1997 land cover: Closed forest =
Dark green, Open Forest = Light green, Forest mosaic = Blue, non forest = pink
5.2. Changes in rainfall and run-off
51. The rainfall records that were obtained for this study are insufficient to show whether or not rainfall patterns have
changed. The rainfall records for some stations are available in the annual reports of the Mekong River Commission which
go back to the early 1960s, but they will need to be digitised. A selection of the records from the Mekong Hydrological
Journal were scanned and digitised for this study, but more time and a larger budget would be needed to select and scan
Page 18 of 22
sufficient data to establish long term rainfall patterns. Data was obtained for rainfall and precipitation for stations in the
upper Sesan and upper Srepok basins for six of the ten years between 1985 and 1994. These show that there was
considerable variation over the decade both in the total rainfall, and its distribution. This is shown in figures 23 and 24
above. Figure 34 below shows the Rainfall/Run-off ratio for the period August-October for 1985 and 1994 according to the
daily rainfall ranked from highest (left) to lowest (right). The figure shows that both years have the overall trend of
decreasing rainfall/run-off ratio with decreasing rainfall, but the trend-line is lower in 1994 than in 1985. The lower value of
the ratio implies that a given amount of rainfall will result in higher run-off, and the figure suggests that over the decade
run-off has increased in the upper Sesan basin. Over the same period, forest cover was reduced, but it is not possible to
say conclusively that the change in run-off is a direct result of deforestation.
Figure 34: Rainfall/Run-off ratio for the upper Sesan during the period August-October in 1985 and 1994
according to daily rainfall ranked in descending order.
52. The higher run-off that appears to result from a given amount of rainfall will mean that the risk of flooding in the lower
reaches of the Mekong is increased under normal rainfall regimes, and may be substantially increased with an exceptional
rainfall event late in the season. Data for rainfall and the rainfall/run-off ratio during the period August to October was
subjected to a multiple regression analysis in order to determine whether there were significant differences between 1985
and 1994, and if so estimate the magnitude of any change in run-off. The following regressions were computed for the
Rainfall/Run-off ratio (RRoR) for 1985 and 1994 respectively:
RRoR1985 = 0.2358 – 0.0031Dperiod + 0.0481Drain + 0.0571R . . . . . . . . . . . . . . . . . . .(1)
RRoR1994 = 0.3281 – 0.0025Dperiod - 0.0056Drain + 0.0228R . . . . . . . . . . . . . . . . . .(2)
(Dperiod = number of days since the start of the period 1st August. Drain = number of preceding days without rain. R =
daily rainfall (mm)
53. Although the intercept in the equation for 1994 is slightly higher than in 1985, the difference is not significant, but the
difference between 0.0571 and 0.0228 for the Rainfall coefficient is significant, and results in a substantially higher
estimate of run-off for a given amount of rainfall in 1994 compared with 1985 as shown in Table 9 below.
Table 9: Comparison of estimated run-off from the upper Sesan for different amounts of daily rainfall at different
times during the period August-October for 1985 and 1994
Rainfall (mm)
Estimated run-off (mm) 1985
Estimated run-off (mm) 1994
End August
End September
End October
End August
End September
End October
10
13.14
14.97
17.39
21.03
24.97
30.72
30
15.76
16.57
17.47
32.21
35.03
38.39
50
16.42
16.94
17.49
36.04
38.10
40.40
70
16.72
17.10
17.50
37.97
39.58
41.33
100
16.95
17.22
17.50
39.56
40.77
42.06
Page 19 of 22
54. The table shows clearly that substantially higher run-off appears to be occurring in 1994 compared with 1985, and in
addition it suggests that heavy rainfall late in the season has a particularly big impact.
5.3. Changes in population and economic activity
55. In section 4.4 and paragraph 43 above, it is shown that population within the Sesan and Srepok basins grew by about
3.8% between 1975 and 1997. Even with this relatively rapid population growth the overall population density has only
reached about 25 persons per km2 in the Sesan basin and 41 persons per km2 in the Srepok, well below the national
average for Vietnam of 235 persons per km2. The area of forest lost in relation to the local increase in population or
Household varies considerably between districts. It seems that those Districts that had the lowest forest area per capita in
1975 experienced the greatest loss of forest cover between then and 1997. This was found to be the case in both basins,
but the Sesan basin showed slightly higher rates of loss of forest for each additional Household. The major change in
land-use is from forest to cropland, but the result suggest that about 10 ha of forest is lost for each additional ha of
cropland per Household.
CI = 0.1065 L + 0.4456 (3)
(where CI = increase in cropland per Household (ha) and L= Loss of forest area per additional Household (ha))
56. Several studies of rural households and income in Vietnam have been carried out and according to Thang et. al. the
GDP per capita in Dak Lak Province grew by over 10% during the 1990s. Much of this growth is attributed to the
cultivation of coffee. Prior to 1981 very little coffee was grown in the Central Highlands of Vietnam and the main expansion
in the cultivation of coffee took place between 1987 and 1997. There is no direct correlation between the increase in area
used for coffee growing, and the loss of forest area at the District level. However according to Thang et. al. the impact of
coffee growing on forest clearance is indirect, in that coffee cultivation is expanded on the best soils, and this pushes the
cultivation of food crops onto more marginal land and especially onto cleared forest land. The same study suggests that
households have expanded their landholdings by between 1 and 2 ha. Much of this expansion is due to coffee growing so
that households expand their landholding in order to be able to continue growing food crops while also growing coffee.
This observation is consistent with the analysis of the land-use data in this study, which shows that the cropland increased
by around 2 ha per household in most Districts. The expansion in landholding has contributed to economic growth and
consequent reduction in poverty, but there is insufficient data to assess the net economic benefit after taking account of
the loss of asset value and the possible environmental costs resulting from the loss of forest.
5.4. Rainfall and flooding events
57. According to the MRC flood warning a river level of 12m above the datum of 36m at the gauging station at Stung
Treng will produce flooding in the lower Mekong. Figure 35 below shows that almost half of Cambodia is liable to flooding
when the water level at Stung Treng rises just from 46 to 48m. Above this level the additional area flooded with further
rises in river level is comparably small. During the period for which data was obtained this critical flood level was reached
only in 1994. It did not come within a metre of the critical level in any of the previous years.
Figure 35: Map showing the extent of flooding in Cambodia when the level of the river Mekong at StungTreng
reaches 12m above the datum (48m asl), shaded in light blue. The pinkish margin around the blue area is the
additional area flooded when the water level rises by a further 8m
58. Twice during August and September 1994 the river level at Stung Treng rose above the flood level. Looking at the
rainfall and river levels in the upper Sesan and Srepok suggests that these two rivers contributed substantially towards a
first period of flooding in August, while they had a much more variable impact on the second. This is probably due to the
distribution of rainfall, with relatively more rainfall occurring in Laos and affecting the flow in the main river channel in
September, while the heavy rainfall in the Central Highlands of Vietnam had a bigger impact on the August event. Figures
36 and 37 below illustrate the situation during the two events.
Figures 36 and 37: Daily river levels at Stung Treng and gauging stations in the upper Sesan and Srepok basins
during August and September 1994
Page 20 of 22
59. In August 1994 the pattern of water levels in both the Sesan and the Srepok generally shows the same pattern as the
Mekong at Stung Treng, except after the floods have subsided the flow of the tributary of the Sesan that is gauged at
Krong Po Ko shows a major rise, that does not show up at Stung Treng. This is probably because of heavy rainfall locally,
but as the catchment above the gauge station is only 3,090 km2 the impact on the main river is negligible. In September
the high flows at Dak Bla in the upper Sesan are followed by floods at Stung Treng but the second smaller peak seems to
be more related to the flow in the Srepok, though after the 2 day flood on 19th, the level at Stung Treng declines despite a
continued rise in the level of the Srepok.
5.5. Economic impact
60. It has not been possible to obtain suitable economic data to make a quantitative assessment of the impact of
deforestation and possible flood damage. It is fairly clear that there is little risk of flood damage within the Sesan and
Srepok basins, because of their shape, which allows rapid discharge of water from the uplands. The impact of any
flooding that might be induced by land management in these basins will be felt in Cambodia and the Mekong Delta in
Vietnam. Data on the population distribution by District within Cambodia suggests that only about 13,000 people within the
Sesan and Srepok basins are likely to be affected by floods, and these are the people living near the confluence with the
main Mekong channel around Stung Treng. However the population that is affected by a 1m flood, when the water level at
Page 21 of 22
Stung Treng rises to 47m asl is about 9.3 million throughout Cambodia, or 81% of all people. Taking a Poverty Index of >3
as an indication of serious poverty, the number of poor people affected by such a flood is around 500,000 out of a total of
960,000. Thus proportionally the poor are less affected by flooding, and this is probably because poverty is mainly
associated with the remoter rural areas where people do not have access to social and public services.
6. CONCLUSIONS
61. Since 1975, there has been severe loss of forest area within the Greater Mekong Basin and serious degradation of the
forest areas that remain. These losses of forest cover have been most severe on the steeper slopes, but this is almost
certainly due to the fact that little forest remained on the flatter land in 1975. The basins of the Sesan and Srepok rivers
that rise in the Central Highlands of Vietnam and flow through Cambodia to join the main channel of the Mekong River at
Stung Treng have suffered a similar fate to the GMB as a whole. Very little closed evergreen or deciduous forest remains
throughout the GMB, and most of the remaining forest is now classed as "open or "mosaic of forest and cropland".
62. The loss of forest cover in the Sesan and Srepok basins is associated with changes in the pattern of run-off in the
rivers. Over the decade from 1985 to 1994, the rainfall/run-off ratio decreased, so that a given amount of rainfall was
associated with a greater run-off. The additional run-off from heavy rain, especially when it occurs late in the season
contributes to an increased risk of flooding in the main Mekong channel.
63. The topography of the lower Mekong basin in Cambodia is such that a very small rise in the water level above a critical
point, results in very extensive floods. The Tonle Sap Lake can absorb large volumes of water and so prevent the water
level rising above the critical point. However, when the lake is full a small additional flow produces a flood that covers
about 40% of the area of the country. During the period for which river flow and rainfall data was obtained, the water level
only rose above the critical level in one year, 1994; the most recent year for which data was obtained. The flow in the
Sesan and Srepok rivers appears to have contributed to the overall high level, but flow from the upper Mekong also
contributed. It is difficult to say how much the general loss of forest cover both in the Mekong as a whole and in the Sesan
and Srepok basins in particular, contributed to this high level of flow in the river. The evidence from the Sesan and Srepok
basins suggests very strongly that loss of forest cover would have increased the flow for a given amount of rainfall, and so
turned a rainfall event that might have been within the capacity of the lower Mekong basin to absorb, into a flood.
64. The loss of forest cover in the Sesan and Srepok basins is related to a combination of population growth and indirectly
to the cultivation of coffee. The latter has generally been expanded on the best soils in the Central Highlands and has
resulted in a demand for more land for cultivation of food crops. The population growth has been around 3.8% in the upper
Sesan and Srepok basins, partly due to immigration, and newcomers and new households have been forced onto more
marginal land as a result of the coffee boom that led to the occupation of most of the best land. The loss of forest cover
was most severe in the more densely populated Districts, where 20 to 60 ha of forest were cleared for each additional
household. In the less densely populated Districts the area cleared per additional household was generally less than 10
ha. However despite these relatively large areas that have been cleared in relation to additional households, the increase
in cropland per additional household appears to be only around 2 ha. Part of the difference between forest area cleared
and additional cropland, may be due to the use of some of the land for coffee and other plantation crops, that do not yet
show up clearly in satellite imagery, but it seems that much of the land, having been cleared is then abandoned.
65. It is not possible to assess the economic impact of the deforestation from the available data. This is partly due to lack
of detailed socio-economic data on the population living within the Sesan and Srepok basins that would enable the
economic benefits to local communities to be estimated, partly due to lack of data that would allow the full forest values to
be estimated, and partly due to lack of data on the social infrastructure within Cambodia that might be adversely affected
by flooding. However, with more than 9 million people being affected whenever the water level in the Mekong rises past
the critical level, even quite small costs for each person or household will be a very large sum in aggregate terms.
66. The results of the study suggest that the value of forest for watershed protection depends very much on the local
conditions, and that some specific areas will have very high value, while other areas will provide relatively little benefits. It
is therefore important to better understand the specific attributes of forests that have a positive impact on water
conservation and river flow so that such areas can be protected and managed properly. This will depend on the
topography and geology of the basin, the pattern of rainfall, the soil conditions and the state of the human communities
and settlements in the lower reaches of the catchment. This study has shown that in a monsoonal climate with very
seasonal rainfall, the rainfall is mainly absorbed by the soil in the early part of the rainy season, and that run-off from a
given amount of rainfall increases as the season progresses. The removal of forest appears to have an impact on the soils
ability to absorb and store water, and so gives rise to increased run-off.
67. In the Mekong basin, a serious flood event affects about 80% of the population, but this effect falls relatively more on
the better off. Only about 500,000 of those affected by flooding have an aggregate poverty rank of >3 out of a total in the
country of about 960,000. However, it is not clear how the ranking relates to other definitions of poverty, since only 8% of
the population have an aggregate rank of more than 3. Proportionally flooding affects the poorest less than the better-off,
mainly because the poorest live in the remoter uplands that are not affected by flooding. However, the data on poverty
rank is only available at the District level, and it is not possible to say whether flooding affects those that are poor within
the flood prone areas, more severely, as might be expected, if they tend to live along the river banks.
68. The season of 2001 has also proved to be one of serious flooding, though it was not possible to collect the rainfall or
Page 22 of 22
river flow data so soon after the event. Data could not be readily obtained on historical flood events, but the data is likely
to be present in the Mekong Hydrological Journal and other sources. However, the former needs to be digitised to
facilitate the type of analysis necessary, and the latter is difficult to access according to the Starbuck report. (see para 13).
7. RECOMMENDATIONS FOR POLICY AND FOLLOW-UP INVESTIGATIONS
69. The apparent severity of the deforestation that has taken place within the Mekong basin as a whole is cause for
serious concern. It is recommended that ADB works with national governments and other agencies to seek to have
existing ground inventory data for the region collated. In order to clarify the uncertainties over definition of forest types,
some standard definitions need to be established and some ground surveys conducted to verify the extent to which the
findings of this study reflect the reality on the ground, and the degree to which they are affected by inconsistencies in the
interpretation of remotely sensed data. Since the major loss of forest cover appears to be in Laos, it is desirable that some
priority be given to ground checking, especially in the mountainous and hilly regions where accessibility may be a
problem.
70. The results of the study that show an apparent link between deforestation and increased run-off is sufficiently strong to
justify further more detailed analysis within the watersheds in order to elucidate more precisely the conditions under which
forest removal is having an impact. This would enable critical areas within watersheds to be identified, and for a value to
be attributed to them for water conservation, so that they can be given priority for conservation and protection. It could
lead to instruments to internalise the benefits of water conservation for forests. These conclusions should be discussed
with the MRC and consideration given to using the Katoomba Group as a means of promoting private sector involvement
in financing conservation measures where the benefits can be internalised. Whenever ADB is considering investment in
upland areas within the Mekong basin the conditions on the ground should be checked against the findings of this study.
In such areas, consideration should be given to developing a soil and water conservation investment package as part of
the overall development plan. Such a package would include the identification of critical areas, where forest cover is the
most efficient form of ground cover and investment in the protection and management of existing areas and reforestation
of degraded areas.
71. The full economic costs and benefits of the deforestation within the Mekong, Sesan and Srepok basins could not be
estimated because of lack of data. In view of the development goals of poverty reduction, and the indication that the
poorest Districts are generally those in the remoter uplands where most of the forest remains, it is important to examine
the relationship between poverty and deforestation. The results suggest that the poorer communities with little access to
social services may also lack access to other resources, and so have little alternative to clearing forest to survive. This
would imply that investment in the Central Highlands of Vietnam and other upland areas in Laos and Cambodia
associated with forest management and conservation could have environmental benefits beyond the immediate locality of
the investment. When planning new investment aimed at poverty reduction, especially in upland areas, the environmental
costs of continued deforestation should be assessed, and where appropriate, some of the investment should be
channelled into supporting local communities in forest protection and conservation activities.
72. With at least 40% of Cambodia being at risk of flooding, further studies are needed of forest cover and management in
the upland areas throughout the Mekong basin to ensure that critical areas are either not deforested or those that have
been deforested are restored. Any social and economic development within Cambodia must take a holistic approach and
include measures to reduce the risk of damage from exceptional floods in the future. This may include a combination of
local measures and measures in the upper parts of river catchments. When promoting rural development care must be
taken to achieve a balance between the expansion of cash crops such as coffee and oil palm, that bring short-term
benefits and the maintenance of forest cover on critical areas, especially the steeper slopes, where soil and water
conservation may be important. The "knock on" effect where expanding the area used for cultivation on better soils may
force people onto marginal areas needs to be balanced against investment in intensification of management on existing
sites. This may be particularly important where population growth is exerting pressure to clear more forests.
73. Climate change may bring about changes in rainfall patterns. If the result is greater variation in rainfall, with heavier
downpours, the risk of flooding in Cambodia could become greater. Further, more detailed assessment should be made of
the possible changes in the risks of flooding that may arise as a result of the combined effects of deforestation and climate
change in order to assess the need for any investment in preventative measures.
Deciduous
Evergreen
Figure 2: Top Landsat TM dated 03 April 2000. Below GtZ/MRC vegetation
map of part of upper Srepok catchment.
Figure 18:
Seasonal rainfall
patterns.
TL: January ppt
LL: July ppt
(Source
NESDIS
MS digital atlas)
Page 1 of 2
Figure 26: Districts of Cambodia (left) and Vietnam (right) that are within both the Se San and
Srepok Catchments
Page 2 of 2
Figures 28 (left) and 29 (right): Change in forest cover in the upper Sesan basin between 1975 and
1997 with closed evergreen forest 1997 (green), closed evergreen forest in 1975 lost by 1997 (red)
and deciduous forest 1997 (brown) and deciduous forest lost by 1997 (pink). Figure 28 shows a
topographic map of Kon Tum Province in Vietnam enlarged with the areas of closed forest lost
between 1975 and 1997 shown in green
Area show at right