The Natural Environment and Historical Water Management of Angkor,
Cambodia
Matti Kummu
Department of Water Resources, Helsinki University of Technology, Espoo, Finland
e-mail: [email protected]
Abstract: This paper gives an overall view to the present natural environment and historical
water management of Angkor; the ancient capital of the Khmer Empire, between the 9th and
15th centuries. The water system of the region may have played a key role in the city operation
but is not clearly understood how it worked and how and when it collapsed. The natural
environment and hydrology of the area and the key elements of the historical water
management will be introduced. The research methods and mathematical models applied to the
area are also briefly presented.
Key words: history of water, Angkor, Tonle Sap Lake, Cambodia, hydrological modelling,
hydrodynamic modelling
1. INTRODUCTION
The World Heritage site of Angkor, in Cambodia, is famous for its monumental
religious constructions, which have been studied in great detail over the last century.
Recent research has uncovered an equally impressive feature of this medieval capital:
an extensive hydraulic network stretching across a thousand square kilometres.
Although Angkor’s hydraulic nor hydrology is not nearly as well understood as its
religious architecture, this system may have played a key role in the city operation and
may also be implicated the demise of the urban complex at the beginning of the 15th
century.
Prior to an analysis of the historical water management, the present natural
environment and hydrology should be well understood. The second and third chapters
give an introduction to present hydrometeorological, geographical and hydrological
conditions of the area. The study area has been divided to three hydraulic zones by
elevation to understand better the hydraulics and hydrology of the area. Also, the
Angkor’s relation to Tonle Sap Lake and the history of the lake based on the current
knowledge, the key features of the historical water management and the methods that
have been applied to the research will be presented. The historical water system of the
area is investigated with the help of radar images, digital elevation model, aerial
photos, field studies and coring. That data will be used to create a three-dimensional
(3D) hydrodynamic model for investigating water levels and currents, inundation of
the flood plains and suspended sediments in the systems.
This study is a part of the Greater Angkor Project, a collaborative project between the
University of Sydney, the Ecolé française d’extreme-Orient (EFEO) in Siem Reap,
and APSARA, the Cambodian body responsible for overseeing the monuments at
Angkor. The study will be conducted in cooperation with MRCS/WUP-FIN Tonle
Sap Modelling Project (Mekong River Commission Secretariat/Water Utilization
Program) and is part of the Global Changes and Water Resources Project in Helsinki
University of Technology.
2. NATURAL ENVIRONMENT OF ANGKOR REGION
Angkor is situated north of the Tonle Sap Lake (also known as Great Lake), in the
northwest of Cambodia in Southeast Asia (Figure 1). In the north the Kulen Mountain
region sets a boundary to the area and watersheds while from the south area is
bounded by the Tonle Sap Lake, the biggest fresh water lake in Southeast Asia. The
plain terrain between Lake Tonle Sap and Kulen mountains is very shallow with the
average slope of 0.1 % while the elevation varies from 3 to 60 meters above the mean
sea level (a.m.s.l.). The Kulen Mountains rise to heights between 300 m and 400 m
from the mean sea level having the highest point is at the elevation of 487 m (or 494 m,
depends the source).
∴ Angkor
Figure 1
Map of Cambodia (Modified from Encarta, 2001: ref Keskinen, 2003)
The main temple area around Angkor Wat and Angkor Thom is situated
approximately 20 km north of the Tonle Sap Lake (Figure 2) and the elevation varies
between 20 and 30 m a.m.s.l. To the south of the temples, the area is bounded by the
lake and the 8-12 km wide floodplain. The floodplain reached up to some 11 m a.m.s.l.
The level of the lake ranges between 1 m at the end of the dry season and 6 – 10 m
a.m.s.l. at the peak of the rainy season.
The area of study region is around 2885 km2 and it is situated between latitudes 13˚04
- 13˚44 and between longitudes 103˚36 - 104˚13.
2
Figure 2 The situation of Angkor area related to the Tonle Sap catchment area (left) and
the study area in more detailed (JICA, 1999b; Evans, 2002; DMA, 1963; and
Certeza Surveying, 1964).
The climate in the study area is tropical, being dominated by seasonal winds or
monsoons. The wet southwest monsoon arrives around May with heavy clouds and
thundershowers, and usually continues until November, with rain occurring almost
daily during this season. The dry northeast monsoon normally starts from November
and continues until April (JICA, 1999a).
2.1. Hydrometeorology of the Angkor
The lack of data is the main problem for the hydrometeorological analysis. During the
Pol Pot and Khmer Rouge period no data were collected neither on the Angkor area
nor any other part of Cambodia. The systematic measuring work for e.g. temperature,
evaporation, humidity, and wind began as late as 1998. Prior to this only precipitation
was measured more or less systematically.
Average annual rainfall in the Tonle Sap basin ranges between 1,050 and 1,850
mm/year and can exceed 2,000 mm/year within the higher catchment elevations
(MRCS/UNDP, 1998). The mean value of annual station-averages has been 1370
mm/year for the period 1950-2000 and 1425 mm/year for 1980-2000. Data from a
total of five stations were used for the calculations (MRCS/WUP-FIN, 2002a).
Annual rainfall in Siem Reap town varies between 900 and 1,800 mm/year with an
average of 1,425 mm/year (data from 35 years, 1922-2002; MRCS, 2003).
The wet season (mid April to October) brings on average some 88 % of the annual
rainfall in the Siem Reap region. Occasionally annual exceptions occur particularly
bordering the traditional wet season in the months March and November, in which
heavy storms occur and large rainfall is recorded.
3
Frequency analysis was conducted to the precipitation rates in Siem Reap weather
station. The Chi-Square and Kolmogorov-Smirnov tests showed that the annual mean
precipitation is normally distributed (Figure 3). Hence, the method for normally
distributed data can be used for the frequency analysis (Chin, 2000). Estimated
magnitudes for 50; 100; 1,000; and 10,000-year annual rainfalls are presented in
Figure 3.
Number of outcomes
8
7
6
5
4
3
2
1
>1800
17001800
16001700
15001600
14001500
13001400
12001300
11001200
0
Range (mm/a)
Year
50
100
1000
10000
Precipitation (mm/a)
Min
Max
1035
1816
983
1868
838
2013
718
2133
Figure 3 Precipitation distribution for Siem Reap station. Analysis data from 35 years were
used. Right: estimated magnitudes of the 50; 100; 1,000; and 10,000-year
minimum and maximum annual rainfalls.
The precipitation varies a lot inside the Angkor region. In total, eight measurement
stations are situated in the study area and four stations around the region like
presented in Figure 4 (one station not in the map, situates north from Angkor Chum).
Measurements in many stations began as late as 2001. Thus, only results from years
2001 and 2002 have been used for the analysis of the precipitation variation inside the
area. Highest rainfall occurs at the Kulen Mountain with an average of 1854 mm/year.
The lowest rainfalls occur on the Tonle Sap floodplain at Phnom Krom and on the
high plain at Banteay Srei with an average rainfall of 1183 mm/year and 932 mm/year,
respectively. Thus, the precipitations at the lake’s shore line and on high plain are only
64 % and 50 % (respectively) of the precipitation in the mountain region. The results
(Figure 4) indicate large variations in rainfall within the study area and show that the
rain falls down locally.
Presently only data from two years are available for the analysis. Also, the
measurement facilities and conditions vary between the stations. Therefore, the
analysis requires further work as more data collected. The present data give only an
approximation of the precipitation variation in the region.
4
Figure 4 Precipitation isolines for the study area (Yearly average precipitation data from
years 2001-2002 have been used. The measurement stations presented).
Hatched area indicates the peak of the year 2000 flood. (DMA, 1963; Certeza,
1964; MRCS, 2003; and Evans, 2002).
The annual average evaporation rate in the region is 1693 mm. The rate is highest in
March and April when the relative humidity is lowest, temperature highest, and dry
NE wind is blowing. The lowest rates of evaporation occur in August and September
when the temperature is around average but the relative humidity is very high.
Evaporation rates range between 119.3 mm (3.86 mm/d) and 186.7 mm (6.02 mm/d)
(Figure 5). The measured yearly evaporation rate is higher than the rainfall (1425 mm)
in Angkor area. But for getting the real evaporated rate, the evapotranspiration should
be calculated. Theory and equations can be found in any hydrological handbook (e.g.
Chow, 1963). For open water, the evapotranspiration can be calculated with pan
coefficient (the measured evaporation should be multiply this value) varying from 0.6
to 0.8.
Dec
Month Prec (mm)
Jan
4.5
Feb
3.8
Mar
26.8
53.5
Apr
May
134.1
Jun
199.3
Jul
197.3
Aug
212.5
Sep
283.2
Oct
226.7
Nov
79.1
Dec
4.6
Total
1425.5
Evap (mm)
128.8
129.5
180.0
161.9
156.8
136.6
141.5
137.1
115.1
111.1
112.4
114.8
1625.6
Jan
mm
300
Feb
200
Nov
Mar
100
Oct
0
Apr
Sep
May
Precipitation
(mm/month)
Aug
Jun
Jul
Evaporation
(mm/month)
Figure 5 Monthly average precipitation (data from 35 years, 1922-2002) and evaporation
(1998-2000) in Siem Reap. Data from Siem Reap Weather Station and MRCS.
5
Average temperature for the region is 28.2 ˚C and it varies from the coolest December
to the hottest April while the average temperature ranges between 25.3 and 30.6 ˚C,
respectively. Daily maximum and minimum temperatures vary from 30.5 to 35.7 ˚C
and from 20.1 to 25.3 ˚C, respectively.
Relative humidity varies in Siem Reap region from 63.24 % in Mars to 81.29 % in
September. The annual average humidity for years 1998-2002 was 73.16 %.
From December to Mars the polar winds prevail in Cambodia from NE - E. In April
the eastwards shift of the anticyclone causes the wind to suddenly change the
direction the east. During the wet season the wind blows from SW. From beginning of
October the polar NE – N winds start to dominate again in the area. The average
monthly speed of the wind varies from 1.93 m/s in January and March to 2.58 m/s in
August. The yearly average speed in the Angkor region is 2.52 m/s for years
1998-2002 (source: Siem Reap Weather Station).
2.2. Geology, soils, and ecology of the area
In the northern mountain area, sandstone and conglomerate strata form the main mass.
These were deposited in the Jurassic and Cretaceous periods, with the upper horizons
extending up to the early Tertiary Age (Garami & Kertai, 1993). The most widespread
geologic unit is situated on the high plain (between 25 and 70 m a.m.s.l.), is the Old
Alluvium composed mostly of sand, silt, clay, and laterite with a few layers of gravel.
On the shores of the Tonle Sap, and reaching elevation of 20 to 25 m above sea level,
the deposits are sandy silt and clay of Young Alluvium (Rasmussen and Bradford,
1977; and Friedli, 2003) (Appendix A: Figure 13).
Various soil types occur in the Angkor region. The most common soil type is
red-yellow podzols. The other soil types in the area are acid lithosoils, planosols,
plinthite podzols, cultural hydromorphics, grey hydromorphics, acid lithosoils,
alluvial lithosoils, and lacustine alluvial soils (Appendix A: Figure 14).
Presently the ecology of the Angkor area can be divided into three groups. Firstly, the
flood plain consisting of occurs rice fields and flood forest. Secondly, the plain which
consists of mostly shallow water rice cultivation and bush land and finally, the hills
and forest area which starts north of the gentle plain area and extends up to Kulen
Mountains waterfalls and thick forest (Appendix A: Figure 15).
3. HYDROLOGY OF THE ANGKOR AREA
To complete a comprehensive hydrological analysis of the area, much more data (e.g.
flow, water stage, sediment concentration, etc.) is needed. There is presently only
hydrological data for the very last years and in some areas, e.g. Puok River no data
have been collected. This chapter introduces the watersheds of the area and the
changes of the river lines due to the anthropogenic modifications. Ground water
characteristics will also be presented.
3.1. Watersheds
At the present time the Angkor hydrological area consists of three watersheds: Puok,
6
Siem Reap and Roluos. Before the Angkor period, the region was divided into only to
two main watersheds, which were Puok (including most of the present Siem Reap
watershed) and Roluos. The total area of the hydrological region is 2885 km2.
Angkorean changed the whole natural water way by making offtake channels to the
Puok River in 10th and/or 11th century.
One of the offtakes was the present Siem Reap River (point F in Figure 11 shows the
place of the offtake). The most probable reason for this channel was to take more
water down to East Baray. Before the Siem Reap offtake existed, the baray was fed by
one of the tributaries of Roluos River. Other of the major offtakes was the Great North
Channel which most probably was built to get water to West Baray.
The areas of watersheds have been calculated using a combination of digital elevation
model (DEM) produced by DMA (1963), and cartographic and field surveys. Due to
the very shallow slope of the plain area (order of 1/1000), inaccuracies in the used
DEM, and the modifications to the natural water ways made by the Angkoreans the
calculated watershed borders may include some inaccuracies especially in the main
temple area. More detail study will be performed with the help of new DEM created
by NASA/JPL (JPL, 2002; and Evans & Kummu, 2003) in the near future.
Figure 6
Ancient (left) and present watersheds (right) of the Angkor area (DMA, 1963;
Certeza, 1964; JICA, 1999b; and Evans, 2002)
Puok
At the present time the catchment area of Puok is 961 km2. The Puok River is
modified in many places by anthropogenic activity. The watershed used to extend all
over the Kulen Mountains, which belongs to Siem Reap catchment now as presented
in Figure 6. The ancient Puok watershed covered an area of 1652 km2. Angkoreans
changed the whole structure of the watersheds constructing the Siem Reap Channel. It
was first of the many offtakes of Puok River but over time it increased in size to the
main stream due to erosion. Nowadays in many places the river is very disjointed and
unnatural.
For the Puok catchment no flow or sedimentation data are available. The watershed
models calibrated for the Siem Reap catchment will be applied for the river later on.
7
Siem Reap
The catchment area of Siem Reap River at Tonle Sap Lake is 697 km2. For Prasat Keo
Bridge, where the water stages and flows have been measured, the area of watershed
is 525 km2. The Siem Reap River, originally an artificial channel now captures most
of the flow of the former Puok River. It has been hypothesised that the heavy bed
erosion of the river made it increasingly difficult to divert water to barays and
possibly other storages. This may have dramatically affected the whole water
management in Angkor area.
The mean annual runoff to Siem Reap River is 435 mm with a standard deviation of
112 mm (Table 1 and Appendix B: Figure 16). Data from years 1999-2001 were used
in the calculations.
Table 1
Average monthly flow (m3/s), runoff (mm) and percentage distribution of runoff
within the year (%) (years 1999-2001) for Siem Reap River.
Jan Feb
Flow (m3/s) 2.40 1.60
Runoff (mm) 12.14 7.30
Runoff
Distribution 1-4% 1-3%
Mar
1.27
6.42
Apr May
Jun
1.57 2.40 6.62
7.67 12.15 32.39
Jul
7.84
39.60
Aug
10.38
52.45
Sep
11.48
56.13
Oct
23.88
120.70
Nov
Dec
12.94 4.90
63.29 24.76
1-3% 1-2% 1-4% 3-11% 8-11% 11-13% 10-18% 22-34% 9-22% 5-7%
Roluos
The catchment area of Roluos River at the Tonle Sap is 801 km2. For Kompong Thkau,
where the water stages and flows were measured in the early 1960’s (Carbonnel &
Guiscafré, 1963) the watershed area is 424 km2. Roluos River has had approximately
the same watershed throughout its history from pre-angkorian times until today. Some
modifications for the river line were done by Angkoreans especially around the
Phnom Bok, eastern part of the watershed, where some embankments and channels
were made to lead the water to East Baray. How the water was controlled and how it
moved in the system is still unclear but will be declared during the project. Naturally,
the Siem Reap Channel affected also to Roluos Catchment by taking part of its water
(Figure 6). The average flow and sedimentation concentrations of the river are
presented in Table 2.
Table 2
Sedimentation and flow rates in Roluos watershed (Carbonnel & Guiscafré, 1963).
Sed (g/s)
Sed (t/month)
Flow (m3/s)
Flow
(1000 m3/month)
Oct-62 Nov-62 Dec-62 Jan-63
Jul-63 Aug-63 Sep-63
Total
127.45 49.27 10.52
7.31 - - - - 1.87 304.48 227.20
10.01
341.37 127.70 28.17 19.58 - - - - 5.02 815.53 588.90 1926.26
4.28
1.86
0.31
0.21 - - - - 1.57
4.28
5.69
2.60
11461
4975
827
560 - - - - -
4200
11472
15227
6960
3.2. Ground water
Ground water is a very important hydrological feature in the study area. The Angkor
area has very good sources of ground water. It is easily accessible as the water table
during the wet season and dry season lies between depths of 0 and 5 m below the
ground level as presented in Figure 7 (JICA, 2000; JSA, 1996; and JSA 2002).
Figure 7 shows the ground water level at Bayon, Angkor Thom. The seasonal
variation is clearly visible in this data and it represents the average behaviour of the
8
ground water in the region. The results are relatively similar to the measurements
taken from Angkor Wat (JSA, 2002) and Banteay Srei temple (Dr. Friedli1, personal
comm.)
Jan-98
Jan-99
Jan-00
Jan-02
500
-0.5
450
-1.0
400
-1.5
350
-2.0
300
-2.5
250
-3.0
200
-3.5
150
-4.0
100
-4.5
50
-5.0
0
Ground water stage
Figure 7
Jan-01
Precipitation (mm/month)
Ground water stage (m)
Jan-97
0.0
Precipitation
Ground water stages (m) at Bayon temple area (JSA, 2002) plotted against the
precipitation (mm/month) in Siem Reap.
The area has large ground water storage with bedrock situated about 50-60 meters
below the ground surface (JICA, 2000). The upper two layers, younger and older
alluvium aquifer, have very good ground water potential while the third layer (30-50
m below the ground surface), called Pleistocene formation, seems to have a very poor
ground water potential. Thus, the aquifer is some 30 – 50 m deep in the lower plain
(JICA, 2000). The average water content of sandy-soils of the upper most recent
deposits is about 11 %. The average water content of clayey soils varies from 14.2 %
in the Uppermost Recent Deposits Layer to 16.8% (CL) and 25.7 % in the Natural
Deposits Layer. (JSA, 1996).
Based on the results of the drilling program carried out by Department of Hydrology
of the Siem Reap Province the ground water table is shown to vary a lot depending on
the area. In Kralanh District, west from the Siem Reap, for example the aquifer was
penetrated at depths greater than 40 m. Compared to the east of Siem Reap Province,
in Sautre Nikum District where the groundwater lies at a depth of between 11 and 12
m. (Garami & Kertai, 1993). Around the Tonle Sap Lake, the ground water stages
vary dramatically (order of tens of meters) even in very short distances depending on
the depth of the bedrock and the characteristics of the soil layers.
4. HYDRAULIC ZONES
The Greater Angkor area can be divided hydrologically into watersheds and
sub-watersheds as described above. However, in the case of Angkor it is essential to
1
Dr. Roland E. Friedli did a comprehensive study of geophysics in the beginning of the year 2003 for
the Banteay Srei Conservation project.
9
divide the study area into hydraulic zones based on elevation to understand better the
hydraulics and hydrology of the area. There are three main hydraulic zones: upper,
middle and lower. Two of the zones can be divided into two sub-zones: the upper
comprising of upper plain and mountain areas and the lower comprising of floodplain
and upper drainage zone. Each zone has its typical natural hydrological and cultural
hydraulic characters which are irrespective of the watershed borders. The
characteristics for each zone will be described briefly later on in this chapter.
In the upper zone (also known as Collector zone), the water was taken from natural
rivers which ran from the Kulen Mountains and then spread to the North-South
aligned channels. In the middle zone (also known as Temple zone), the water was
collected mainly in large barays, water reservoirs, which were most probably built for
multifunctional purposes. The lower zone (also known as Drainage zone) operated as
a drainage system for the temple area and dispersed the water down into the Tonle Sap
Lake. The total area of the study region is 2885 km2 and it is presently made up of
three watersheds (previously described in Chapter 3).
The elevation data used for the zoning of the floodplain were taken from Certeza
(1963). That data give the most accurate elevation for the lake and its floodplain
(MRCS/WUP-FIN, 2002a). For the other areas the elevation model made by DMA
(1963), digitalized by MRCS was used.
Figure 8
Hydraulic zones of Angkor region. From north to south: Collector zone (Mountain
area and upper plain), Temple zone, and Drainage zone (Upper drainage zone
and floodplain). (DMA, 1963; Certeza, 1964; JICA, 1999b; and Evans, 2002).
4.1. Collector zone
The collector (upper) zone includes the Kulen Mountains and the upper plain. The
elevation varies from 28 to 487 m a.m.s.l. and the total area is 1218 km2, making it
largest zone containing some 42.2 % of the total study area. The area is divided into
two sub-zones: upper plain (elevations between 28 and 60 m a.m.s.l.) and mountain
areas (elevation up from 60 m a.m.s.l.).
10
The most dominant part of the upper zone is the upper plain. The main water source is
the Kulen Mountain region (mountain area) from where all three rivers of the study
area (look Chapter 3) are sourced. The precipitation rates are almost double in the
mountain region compared to the upper plain (Figure 4). From the 10th or 11th
centuries the water was taken from the natural rivers running from the mountains and
spread out into N-S aligned channels, such as the Great North Channel and Siem Reap
Channel (present River) (Figure 11).
4.2. Temple zone
The temple (middle) zone includes all the main temple area excluding Roluos Group.
It is situated between the contours of 18 m and 28 m a.m.s.l. The Temple zone is 465
km2 which is 16.1 % of the total area.
The water was directed from the Collector zone’s N-S channels to the Barays using a
wide channel network (Figure 11, page 17) constructed using ground embankments.
Barays were the main storages but also large amount of water was collected to the
temple moats and trapeangs (ground water and rain fed reservoirs), mainly from
ground and rain water sources. The total volume of the barays was approximately
80,000,000 – 120,000,000 m3. The total calculated volume varies a lot between the
references (e.g. Lustig, 2001 – 78.4 million m3; Acker, 1998 – 227.2 million m3; and
Garami and Kertai, 1993 – 155 million m3). It is also very highly unlikely that all the
reservoirs were working at the same time.
The purposes of the barays are still unclear. A lot of discussion has been written and
many theories have been advanced (e.g. Groslier1974, and 1979; Stott, 1992; van
Liere, 1980, and 1982; Pottier, 2000; Bernon, 1997; and Goodman, 2000).
Comprehensive studies will be carried out during this project but it is still too early to
convict or accept any of the presented theories.
4.3. Drainage zone
The drainage (lower) zone includes the area lower than 18 m a.m.s.l. The area can be
divided into two areas: Floodplain (1-10 m a.m.s.l.) and Upper drainage zone (10-18
m a.m.s.l.). The total area of the lower hydraulic zone is 1202 km2 and the areas of
sub-zones are for floodplain and upper drainage zone 717 and 485 km2, respectively.
The floodplain comprises some 24.9 % of the total study area and upper drainage zone
16.8 %. Hence, the drainage zone is 41.7 % of the total area.
Some main components of the area are e.g. SE channel and SW channel running from
the SW corner of the West Baray to SE and SW, respectively. The ancient Siem Reap
Channel (present river) and Angkor Wat Channel are the main structures for
transporting the water to the lake from the Temple Area.
5. HYDRAULIC FEATURES IN ANGKOR PERIOD
This chapter briefly describes the hydraulic features of Angkorean water management.
Additional information about the features can be found in studies by e.g. Groslier
(1974 and 1979), Acker (1998), Garami and Kertai (1993), Goodman (2000), Pottier
11
(1999 and 2000), van Liere (1980 and 1982), Fletcher (2001) and Evans (2002).
5.1. Water reservoirs
Due to the long dry season, the collection and storage of the water is very important
for the people of the area. At the household level this is done by building small ponds
adjacent to the house where then rainwater and groundwater can collect. For village
use, a bigger pond, trapeang was built. The sizes of trapeangs vary from 2 to 20 ha.
Normally trapeangs were related closely also to the temples.
In the urban area, in Temple zone, the water was collected in the big barays by
channels. Angkor area has four barays: Indratataka (Baray of Loley), East Baray, West
Baray, and Jayatataka (North Baray). The vast majority of these features, barays and
trapeangs, have a length width ration of 2:1 and are generally aligned E-W (Evans,
2002). Trapeangs were dug into the ground, fed by rain and ground water, and are
unrelated to the channel network, while the barays were basically not dug and fed by
channels and rainfall (Fletcher, 2001). Almost every temple has its own moat into
which water was normally collected from rain and ground water sources. The moats
are square or rectangular and are generally oriented E-W and N-S (Garami and Kertai,
1993).
The reservoirs, barays, trapeangs and moats probably had multifunctional purposes
for Angkorean. The uses could have included ritual uses, aesthetics, water supply for
humans and animals, transport, irrigation, drainage, defence, health, fishery, and
washing.
5.2. Channels, roads and other structures
Channels were crossing over the whole landscape. The channels were generally very
shallow and wide, some 1-2 m deep and 30 – 40 m wide. Normally the road(s) were
lined the channel on an embankment(s) about 1 - 2 m above natural land surface. The
sides of the embankments were probably used for living as well as it is possible to get
the house above the floods in rainy season. Some channels had only one embankment
on the side of downward slope and those probably have the main purpose as a road.
Thus, the linear hydraulic features were normally for multipurpose uses as well.
Normally the water was controlled in Angkor area by earth embankments. Not many
evidences have been found that Angkorians used laterite constructions or sluice gates
for controlling flows. However, there are couple of laterite constructions which
probably has served as a water control feature. One is situated at the Ban Penh Reach
channel, which has a low-level offtake from the Siem Reap just downstream from the
junction of the Siem Reap with the Puok. Other one is Krol Romeas (misleading
having a same name as the animal compound next to Preah Kanh temple) which is
situated at the east end of East Baray and probably functioned as an inlet or outlet for
the baray.
6. TONLE SAP LAKE
Tonle Sap Lake is the largest freshwater lake in Southeast Asia and an important
12
component in the Mekong water system. Its size varies from approximately 160 km
long and 35 km wide during the dry season up to 250 km long and almost 100 km
wide at the height of the flooding (e.g. Keskinen, 2003).
The Tonle Sap River connects the lake to the Mekong River and joins it at Chaktomuk
junction near Phnom Penh (Figure 1, page 2), after which the river immediately splits
into the smaller Bassac River and the larger Mekong River. (Lamberts, 2001). The
area is globally unique and the lake has an extraordinary hydrological system. In the
wet season, the Tonle Sap River reverses its direction of flow to the Tonle Sap Lake,
instead from the lake due to the heavy flooding of the Mekong River. The lake
functions as a natural overflow basin for the Mekong system.
Figure 9
Tonle Sap catchment plotted with the coverage of year 2000 flood. (DMA, 1963;
JICA, 1999b; and Evans, 2002).
6.1. Water stage
The area of the lake varies between the dry and wet season from 3500 km2 up to 14
500 km2 and the depth of the lake increases from 0.5 m up to 6-9 m. The bottom of the
lake lies approximately 0.5-0.7 m a.m.s.l. During the year the surface of the lake
varies between 1 m and 9 m a.m.s.l, respectively. Figure 9 shows the peak of the flood
in year 2000 (possibly the biggest flood during the last 50 years) when the water level
reached 9.8 m and 10.3 m a.m.s.l at the Kampong Lung station and Prek Dam station,
respectively. During the wet season, the volume of the lake increases from about 1.3
km3 to 50-80 km3 depending on the flood intensity. (The area and volume of the lake
during the floods vary in different sources. The reason is that the area calculated as a
lake and its floodplain varies between the researches).
6.2. Sedimentation
The first theory postulated was that the lake was been infilled with sediment at the rate
13
of 2-4 mm/year (e.g. Carbonnel & Guiscafré, 1963; and Garami & Kertai, 1993); but
the resent studies by Mildenhall (1996), Tsukawaki et al. (1997), and Penny (2002)
show that the sedimentation rate during the last 5000 years has been approximately
0.1 mm/year. Thus, no significant sedimentation is occurring in the dry season lake
area. Sedimentation is heavier on the flooded forest and flood plains in the vicinity of
the lake proper and the rivers (MRCS/WUP-FIN, 2003a). Hence, during the
Angkorean time the bottom of the dry season lake was only 10 cm below the present
bottom level. Thus, no significant changes have occurred in the lake’s morphology
during the last thousand years. Therefore, it can be assumed that the present
conditions of the lake are similar to present during the Angkorian time.
Figure 10
Estimated sedimentation rates of the northern part of the Tonle Sap Lake. (“This
study” means here the results of Penny, 2002). (Penny, 2002).
Sediment flux to Tonle Sap Lake during the wet season is manifold compared to the
outflow flux during the dry season (MRCS/WUP-FIN, 2003a). The average annual
input to the lake is about 5.7 million tons of sediment (TSS) while the outflow is about
1.2 million TSS. Thus, Tonle Sap Lake is retaining more than 80 % of the annual
amount of sediments it receives from Mekong River. The TSS data, secchi depth
measurements, and the model calculations indicate that efficient sedimentation takes
place in the flood season in the vicinity of Tonle Sap River and the tributaries, in the
delta area and in the flooded forest around the lake proper (MRCS/WUP-FIN, 2003a).
6.3. History of the Lake
Analysis of the sediment cores extracted from the lake bed suggests a dramatic change
in the rate of sedimentation in the lake around 5100 - 5600 B.P. It has been suggested
by Tsukawaki (2002) that the lake was not connected with the Mekong River prior to
this time, and the lake (or series of lakes) received sediments and water from its own
catchment area only and the Tonle Sap River flowed directly to the South China Sea.
If this is the case, then the current ecosystem of the lake only started to develop some
5500 years ago.
14
The probable cause for the environmental change described above is a rapid rise of
sea-level after the Last Glacial Maximum (LGM: about 18 000 years B.P.). This was
followed by a high stand period at the Holocene Climatic Optimum (about 6000 years
B.P.) around the Gulf of Thailand (Tsukawaki, 2002; and Nguyen et al., 2000). The
risen back water effect, because of the higher sea level, affected to the Mekong floods,
shifted to upstream direction, and finally cased the diversion of Mekong flood water
to the Tonle Sap area about 5500 years ago. This change started to create the present
hydrological and ecological conditions in the Tonle Sap Lake and its floodplains
(MRCS/WUP-FIN, 2003a).
6.4. How the Angkor was related to the Tonle Sap Lake
The Tonle Sap Lake had a very important role to play in the history of Angkor.
Initially it was an important source of nourishment. The Tonle Sap ecosystem is
believed to be one of the most productive inland waters and one of the most
fish-abundant lakes in the world (Bonheur, 2001). At the present time the lake
provides about 60% of the annual commercial fisheries production of Cambodia and
its annual fish catch is recently estimated to be approximately 235 000 tons (Van
Zalinge et al., 2001) being 40% of Cambodia’s total supply of protein.
The lake was also a very important part of the transportation network for Angkor, e.g.
goods, food and people were easy to move from point A to B by boats. The location of
Angkor is optimum for the city. It is safe from the flood but at the same time very
close to the lake even during the dry season (Figure 9, page 13). The floodplain is
relatively narrow in the Angkor region (averagely some 12 to 14 km) compared to the
other parts of the lake where it can be quadruple (e.g. Battambang region, west side of
the lake). Hence, it was possible to come easily and fast by boat up to the city by the
present Siem Reap River (Angkor period, channel) probably during the dry season as
well.
The annual flood creates an optimum condition to cultivate floating rice (called also
deepwater rice) during the rainy season and flood recession rice during the dry season
when water starts to recede. Floating rice is grown in low-lying areas and depressions
that accumulate floodwater at a depth of 0.50 m or more, up to 3-4 m. According to
Chou (2001), the floating rice was growth naturally without sowing during the
Angkor period. The flood recession rice is grown in the fertile soils on the floodplains.
The sowing is conducted as water recedes, starting from the upper fields (Javier, 1997;
ref. Lustig, 2001).
During the pre and early Angkor period, the floods might have been lower than they
are nowadays. The facts which indicate to that are the early temple areas (e.g. Roluos
group, Pr. Chedei and Pr. He Phka) which are much lower (between 8 to 12 m a.m.s.l.)
than the main Angkor temple area (Evans, 2002). Hence, the higher floods may have
forced to move the capital further from the lake.
The most probable reason for that would be the different climate conditions on the
Mekong, including Tonle Sap watershed area, or different morphological conditions
between the Mekong River and Tonle Sap Lake. Clear evidences have not yet been
15
found to support either of the theories. More investigation will be made concerning
the issue of flood levels.
7. METHODS FOR STUDYING THE WATER SYSTEM
The methods used for the Angkor water system studies are field studies, coring,
Ultra-Light plane, aerial photos, AIRSAR-TOPSAR data, two hydrological models
and 3D hydrodynamic model. In this chapter, those methods are briefly presented.
7.1. Field methods
Field studies are an essential part of the study. They offered a lot of information that is
not possible to get from anywhere else. To get a good view of the hydraulic network,
individual structures and features, as well as to understand better the local nature,
hydrology, and habits of the people the comprehensive field studies are necessary to
do.
The net sedimentation in the ancient channels can be determined with the help of
coring (Player, 2002) but the exact sedimentation rates can be defined only from the
features which have been wet since they were constructed using radiocarbon dating
(Penny, 2002).
Palaeo-environmental data have been collected from features, such as moats and
trapeangs, which have been wet since they were constructed. The work has been
conducted by Dr. Penny. With the radiocarbon method it is possible to date the
sediment layers and thus calculate the net sedimentation for each periods.
The Ultralight plane is used to carry out precise checks on problem locations quickly
and more safely than is always possible on the ground. The size of the area and areas
to where it is difficult or even impossible to go because of the land mines and lack of
paths make the use of ultra-light plane very useful. With the plane it is possible to get
a view over the landscape from the height of 1 000 m or then a very close look to the
details from the height of couple of tens of meters. The potentials of the Ultralight are
incredible for the areas like Angkor where the hydrological and hydraulic features are
spread over 1000 km2 and the access to the places is difficult or even impossible from
the ground.
7.2. AIRSAR-TOPSAR data and aerial photos
Aerial photos have been used for identifying the hydraulic features during the field
work. Pottier (1999) produced the archaeological maps of southern Angkor (Figure 11)
with the help of aerial photos taken by FINNMAP (1992).
Airborne synthetic aperture radar (AIRSAR-TOPSAR) data have been used to create
a GIS database of the network of canals, reservoirs and embankments used to manage
water in Angkorean times in the Northern Angkor (Evans, 2002; and Fletcher et al,
2002). Together with Pottier’s (1999) mapping an exhaustive hydraulic map over the
Greater Angkor area have been created (Figure 11).
The TOPSAR digital elevation model (DEM) is critical for understanding the
16
dynamics of the fluvial environment. The data have a 5x5m ground resolution and can
resolve sub-metre differences in elevation in areas of low topographic relief. The data,
radar images and DEM have proven particularly useful in the discovery and analysis
of Angkorean hydraulics and hydrology.
Study Area
Radar Strips 1-4
Kulen Hills
Foothills
(C)
Great
North
Channel
(Ends)
Northern Angkor
(Mapped in Current Study)
Foothills
Kulen Hills
Foothills
Banteay Srei
Great
North
Channel
(F)
Siem Reap
River
Puok
River
Route 6 to
Sisophon
(B)
Baray (Jayatataka)
Banteay Sra
Bayon
Prei Khmeng
West Baray
(E)
(D)
East Baray
Angkor Thom
Angkor Wat
Roluos
River
Kuk Svay Thom Area (A)
Chau Srei Vibol
Comparative Test Area
(D)
Baray (Indratataka)
Siem Reap
Southeast Channel (D)
Prasat He Phka
Phnom Krom
High Water Mark
(Dashed Line)
(D)
Roluos
Prasat Trapeang Phong
(D)
Southern Angkor
(Mapped by Pottier (1999))
Damdek
Canal
Low Water Mark
±
Tonle Sap (Great Lake)
0
2.5
5
Study Area
10 Kilometers
Water
Figure 11. Structure of Angkorean archaeological and hydraulic features showing the locations
mentioned in this paper (Evans, 2002 – the current study in the map means the study made
by Evans).
7.3. Mathematical models applied to Angkor area
The hydrology and hydrodynamic of the Angkor area are investigated with the help of
mathematic modelling. Based on a GIS database and TOPSAR elevation model, a
three-dimensional (3D) EIA Flow Model (Virtanen et al., 1998) has been set up for
simulating water levels and currents, inundation of the flood plains and suspended
sediments. Distributed hydrological model, VMOD (Lauri & Virtanen, 2002), applied
to the watersheds have been used for the boundary flows of 3D Flow Model
(MRCS/WUP-FIN, 2003a). The models have been calibrated for the present
hydrological and meteorological conditions of the study region and have been applied
for the ancient land use, meteorological, and morphological conditions of the
watershed as accurately as possible within the bounds of current knowledge. Using
17
the model, sedimentation and erosion rates can be defined for the area using different
historical scenarios for landuse and climate, allowing archaeologists to test
hypotheses about the decline of Angkor against rich and complex datasets.
Some of the most valuable uses of the model in the study area are (Evans & Kummu,
2003):
•
•
•
•
•
•
•
flows and currents in complex channel network
reconstruction of the water system
influence of the land use changes scenario analysis
sediment accumulation and filling of the channels, both in short term and
centuries timescale
erosion rates in the channels and canals, especially in Siem Reap River and
Great North Channel
sedimentation rates in the Barays
the river floodplain vegetation effects to the currents and sediment
concentration
These results will be used for the archaeological, hydraulic and hydrological analysis
of the area. With the analysis, it is possible to get more information about the water
system, an estimation of how significant that system was in the city’s operation, and
to indicate whether or not the demise of the hydraulic network implicated its urban
environment in the middle of the second millennium CE.
Figure 12.
Surface (left) and bottom (right) sediment concentrations calculated with 3D Flow
Model, West Baray.
8. DISCUSSION
In this chapter, the benefits of the Angkor area from water management point of view
have been discussed. Also, the possible problems which might have been occurred in
the area are briefly presented.
8.1. Advantages of the area from water management point of view
The situation of Angkor is optimal from a hydrological point of view. The Tonle Sap
Lake was very close to the city but still it was safe from its floods. The lake offered an
18
excellent source of nourishment and potential for transportation. The floodplain was
also very fertile for growing rice.
Ground water in the area is close to the surface even during the dry season, unlike in
many other places around the lake. Thus, the Khmers had easy access to water all year
round, guaranteeing water for drinking and bathing even during the driest dry seasons.
Also the significant public health benefits of high water tables cannot be ignored
(Himel, 2001; ref Goodman, 2001). Regular bathing in the family ponds was noted to
have taken place by Chou Ta Kuan (Chou, 2001) at least twice a week.
The very gentle slope of the plain created an excellent terrain for managing the water
with artificial channels and reservoirs. The Khmers were exceptionally good at
managing the water and moving it from one place to another. The channel network
was built with care and spread over a 1000 km2 area (Evans, 2002) – some of the
channels and reservoirs (e.g. West Baray, thought the water is not flowing to the Baray
its original route) are still in use today.
8.2. Disadvantages and problems in the Angkor area from hydrological point of view
Although, the closeness of the Great Lake had its advantages it also had disadvantages,
for example, it decreased the area of flooded fertile fields. Hence, the potential of the
rice cultivation using the flood water was limited in relation to the closeness of the
main temple area. The floating and flood recession rice form nowadays only some 4
% of the total rice production in the Siem Reap Province (see Appendix C: Table 3
and Figure 17) which might have been the case also during the Angkor period.
Even though the Khmers were excellent water engineers, they could not avoid normal
problems relating to erosion and sedimentation. In Barays and some channels,
sedimentation was a major problem. Over time, the settled sediments decreased the
capacity of the channels and barays and made the supplying water to the city more
difficult.
The possible climate changes may have affected a lot to the Angkor area. Goodman
(2000) proposed that climate played a role in the emergence of Angkor. The possible
changes in temperature, precipitation and evaporation may have affected to the local
hydrology in many ways. Too much water may caused floods in the city area. On the
other hand, it advanced rice growing as a lack of water may have decreased the rice
crops, making the water supply more difficult, and lower the ground water level
dramatically.
Thung (1999) proposed that the tectonic shifts have affected on local hydrology and
are responsible for down cutting in the various watercourses on the Angkor plain.
However, no clear geophysical evidences have been found to support this theory.
Thus, more information about the tectonic shifts in the area is needed for further
discussion about this argument.
Also, the changes in land use may have affected the hydrology of the area. The more
people the more rice is needed. Thus, probably large areas of forest had been turned to
rice fields to feed the growing population in Angkor. This affected to the local
19
hydrological cycle in many ways. The surface flow increased with the sediment flux
to the rivers; causing several problems on downstream channels and reservoirs.
9. CONCLUSION
This paper gives an overview of the present natural environment and hydrology of the
Angkor region. The precipitation varies significantly inside the Angkor region being
at the lake’s shore line and on high plain only 64 % and 50 % (respectively) of the
precipitation in the mountain region where the highest rainfall occurs. The changes of
the river lines due to the anthropogenic modifications have been presented with the
current and ancient watershed borders. The area is also divided to three hydraulic
zones to understand better the hydraulics and hydrology of the area. Each zone has its
typical hydrological and hydraulic characteristics: in the upper zone the water was
taken from natural rivers and then spread to the North-South aligned channels, in the
middle zone the water was collected mainly in large barays, and the lower zone
operated as a drainage system.
The Tonle Sap Lake is not filling up with sediment against the earlier belief. Angkor
was related very closely to the lake. It was an important source of nourishment and
one of the key features of the transportation system. The annual flood creates an
optimum condition to cultivate floating and recession rice. The Angkor area was
perfectly situated from hydrological point of view: the lake was close but the city was
safe from the flood, the high ground water table offered a secured water supply also
during the dry season, and the shallow slope of the terrain offered good possibilities of
manage the water. Even thought the situation was optimal Khmers had problems with
sedimentation and erosion in their hydraulic network. It is likely that the climate
changes during the periods played a role in the history of Angkor.
20
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24
APPENDIX A GEOLOGY, SOIL AND ECOLOGY MAPS FOR STUDY AREA
Figure 13
Geological features of the Angkor area
Figure 14
Soil types for the Angkor area
Figure 15
Ecological map (present) from Angkor area
25
APPENDIX B SIEM REAP FLOW
160.00
140.00
Aver
Min
Max
Runoff (mm)
120.00
100.00
80.00
60.00
40.00
20.00
Figure 16
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
Nov-01
Sep-01
Jul-01
Mar-01
May-01
Jan-01
Nov-00
Sep-00
Jul-00
Mar-00
May-00
Jan-00
Nov-99
Sep-99
Jul-99
May-99
Jan-99
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Mar-99
Flow (m3/s)
JAN
0.00
Average monthly runoff (mm) and daily flow (m3/s) for Siem Reap River at
Prasat Keo.
26
APPENDIX C RICE PRODUCTION IN SIEM REAP PROVINCE
Table 3
Average rice productions and cultivated areas for Siem Reap Province, years
1997-2001 (MRCS/WUP-FIN, 2003b: source Ministry of Agriculture, Mr.
Rathana).
Siem Reap Province
(1997-2001)
Wet season rice
Dry season rice
Floating rice
Recession rice
Cultivated Harvested
Area (ha) Area (ha)
178,395 173,033
10,038
9,804
6,446
4,457
2,749
2,732
Yield Production
(T/ha)
(T)
1.37
233,520
2.43
23,806
1.19
5,250
1.95
5,316
2000-2001
1999-2000
1998-1999
1997-1998
1996-1997
Recession rice
Floating rice
Dry season rice
Wet season rice
0
50,000
100,000
150,000
200,000
250,000
300,000
Production (T)
Figure 17
Distribution of rice production among different varieties at the area of Siem
Reap Province (MRCS/WUP-FIN, 2003b: source Ministry of Agriculture, Mr.
Rathana).
27