Soil Erosion Plots
Measurement and analysis of soil erosion plot data for 2009
Report: SLMP-2010
National Soil Services Centre (NSSC), Semtokha
Tel No.: +975 2 351037, Fax No.: +975 2 351038
www.moa.gov.bt/nssc
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Table of Contents
1.
INTRODUCTION ........................................................................................................................... 3
2.
MATERIALS AND METHOD ....................................................................................................... 3
Table 1
3.
Overview of the main site characteristics of the erosion plots at five different sites. ........................................ 4
MEASUREMENTS OF SOIL EROSION PLOTS ........................................................................ 6
3.1
LOGCHINA MEASUREMENTS 2009 ............................................................................................................................ 6
Table 2 Logchina measurements 2009 ........................................................................................................................ 6
Table 3
Important rainfall events for Logchina in 2008 and 2009 ............................................................................... 7
3.2
NANGKHOR – BULI MEASUREMENTS 2009 ............................................................................................................... 8
Table 4 Buli measurements 2009 ................................................................................................................................ 8
Table 5
Important rainfall events for Buli in 2008 and 2009 ....................................................................................... 8
3.3
YANGNEER MEASUREMENTS 2009 ........................................................................................................................... 9
Table 6 Yangneer measurements 2009 ........................................................................................................................ 9
3.4
LUMANG MEASUREMENTS 2009 ............................................................................................................................ 10
Table 7
Lumang measurements 2009 ....................................................................................................................... 10
3.5
GOSHING MEASURMENTS 2009 .............................................................................................................................. 11
Table 8 Goshing measurements 2009 ........................................................................................................................ 11
3.6
ALL THE ACTIVITIES AND MEASUREMENTS WERE RECORDED ON A MONTHLY BASIS USING A STANDARD LOG BOOK. . 12
4.
OVERVIEW AND DISCUSSION OF THE MEASUREMENTS ............................................... 18
Table 9 Soil erosion classification, after Singh et al. (1992) as determined for India .................................................. 18
Table 10
Reduction in soil erosion rate on the SLM plots compared to the traditional practice ................................ 19
Table 11
Overview of soil erosion rates for 2009 for the 5 sites and 4 practices ...................................................... 20
5.
MAINTENANCE OF THE EROSION PLOTS ........................................................................... 21
6.
DISCUSSION AND CONCLUSIONS .......................................................................................... 23
7.
RECOMMENDATIONS ............................................................................................................... 25
8
REFERENCES .............................................................................................................................. 26
ANNEX 1A
SOIL ANALYSIS FOR THE BULI EROSION PLOTS ............................................ 27
ANNEX 1B
SOIL ANALYSIS FOR THE LOGCHINA EROSION PLOTS ................................ 28
ANNEX 1C
SOIL ANALYSIS FOR THE LOGCHINA EROSION PLOTS. ............................... 29
ANNEX 1D
SOIL ANALYSIS FOR THE GOSHING EROSION PLOTS ................................... 30
ANNEX 1E
SOIL ANALYSIS FOR THE LUMANG EROSION PLOTS ................................... 31
2
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
1.
Introduction
The Sustainable Land Management Project (SLMP) started soil erosion measurements in the SLMP pilot
Dzongkhags in 2008 after consultation and discussion with the World Bank Task Manager and Land Management
Advisor. The proposed soil erosion measurements replace the originally defined Global Outcome Indicator that
referred to “10% reduction in sediment flow in selected micro-watersheds in pilot geogs by project end”. It was
thought to be of more value, easier to measure and of more use to farming communities than relatively complicated
measurements of sediment flow, which not always can be directly linked to land management interventions.
Besides the direct benefits for the project it is thought that the establishment of permanent plots will enable the
generation of a long-term data series on soil erosion in Bhutan, information which is lacking at this very moment.
The goal of the installation of erosion plots is firstly to quantify the total amount of soil loss over a certain time
period (year) under specific slope and climatic conditions and secondly to have a validation of the introduced SLM
practices and to evaluate the difference with traditional cropping practices. An added goal is the demonstration
purpose to show local farmers and other visitors the benefits of SLM practices to reduce and limit soil degradation
and in particular to visualize the reduction in soil loss.
The guiding principle behind the establishment of the soil erosion plots is that they should be as simple as possible
to reduce the many measurement errors that can occur and to enhance the replicability of the erosion plots during
and after the life span of the project. It was therefore decided to focus on soil loss on a yearly basis and not to
measure runoff generated on the erosion plots. Although it is important to have quantification of runoff, the main
interest is to be able to quantify soil loss of areas under SLM practices compared to traditional farming practice and
a reference, bare plot. Runoff measurements require a more complex measurement set-up with tanks, dividers,
and/or data loggers. Besides these requirements, a number of errors can occur during sediment concentration
measurements. Furthermore, most of the erosion plots are far from the road head and this warrants simple set up of
soil erosion plots. The erosion plots therefore are intended to generate data on yearly soil loss per area under
different farming practices and different agro-ecological conditions. The set-up of the soil erosion plot is based on
earlier work by Hellin (2006) in Central America, Howeler (1987) in Colombia and Sombatpanit et al. (1992) in
Thailand. The erosion plots are sized 4m by 10m and have a catch pit lined with plastic at the bottom. The overall
intention is to establish a reliable series of data on soil erosion rates under different slope and agro-ecological
conditions and to compare traditional and SLM treatments. A baseline that is lacking at this moment and this longterm experiment aims at filling the existing knowledge gap, while at the same time advocating SLM practices on
steep dry land, typical for the Bhutanese farming conditions.
2.
Materials and Method
Soil erosion plots were installed at Yangneer RNR-EC, Trashigang Dzongkhag, Buli RNR-EC, Zhemgang
Dzongkhag and Logchina RNR-EC, Chhukha Dzongkhag in early 2008. Results from the first year of
measurements were discussed in a report by SMP-NSSC of 2009 named “Soil Erosion Plots, measurement of soil
erosion for 2008 and maintenance of plots for 2009.” In April 2009 additional erosion plots were established at
Lumang RNR-EC, Trashigang Dzongkhag and in May 2009 another series was installed at Goshing RNR-EC,
Zhemgang Dzongkhag. The five sites are located under different agro-ecological zones with different
environmental and slope conditions; see Table 1 for an overview of the site conditions. The sites are selected based
on the feasibility of setting up the erosion plots at one location, the representativeness of the pilot site and to obtain
a good spread of sites along the environmental and slope gradients of Bhutan.
Plot lay out and materials
The four plots (10m x 4m) are bordered by a solid strip of CGI sheets (6ft by 50mm) cut into two halves along the
length of the CGI sheet. At the bottom of the plot a catch-pit is made of 4m length (the width of the plots), ranging
from 0.7 to 1m deep and 1 to 1.5 m width, depending on the local conditions. This creates a volume of about 2 to
2.5m3, equal to 5 to 6.25% of the plot area (40m2), which is in line with international experiences and should be
enough to accommodate sediment and runoff of peak rain storms (with intensity of more than 100mm/day). The
plots are clearly smaller than standard USLE plots, but on steep Himalayan slopes establishment and management
of USLE plots would be quite difficult.
3
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Table 1
Overview of the main site characteristics of the erosion plots at five different sites.
Location of plots
Slope [°]
Soil texture
Altitude
Aspect
Climate
Rainfall
Yangneer
RNR-EC
Logchina
RNR-EC
Nangkhor
RNR-EC
16
Sandy clay loam
1835m
ENE (070)
800mm
28
Silty loam
1065m
NE (050)
9
Sandy loam
1645m
S (180)
Warm
temperate
Humid Subtropical
Warm
temperate
Lumang
RNR-EC
Goshing
RNR-EC
25
Silty clay loam
1600m
SSW (190)
24
Silty clay loam
900m
NW (330)
Warm
temperate
Sub-tropical
4000mm
1500mm
1400mm
3000+mm
The catch-pits are covered with a tarpaulin that is intended to collect the eroded soil particles. Small holes are made
to drain collected water from the catch-pits. Along the catch-pits small ridges are made to avoid soil and water,
other than from the erosion plot area, to fall into the catch-pit. Between the plots an area of at least 0.5m wide is
kept to facilitate easy access to the plots. The experience of the first years (2008 and 2009) has shown that the
durability of the tarpaulin sheets is a concern. In the more temperate sites as Yangneer and Buli the tarpaulins last
throughout the year without major problems, but in the more tropical sites like Logchina and Goshing high rainfall
and temperatures combined with solar radiation cause problems. In Logchina the sheets even had to be replaced
during the first year because of the “wear-and-tear”. Ultimately it is intended to replace the tarpaulin sheets with a
more durable solution by concreting the catch pits. In Buli a thick black plastic was used that seems to be more
durable and a reasonable alternative to the thinner tarpaulins.
Treatments or practices
Various SLM treatments or practices are used in the erosion plots to measure how effective these measures are in
controlling soil erosion. The SLM treatment used in each erosion plot is as follows:
Plot 1 Reference plot kept bare throughout the year (no crops and no tillage)
Plot 2 Traditional practices with local cropping method
Plot 3 Traditional practices with 2 hedgerows at 1m and 6m from the catch pit
Plot 4 Traditional practices with legume intercropping along with 2 hedgerows at 1m and 6m from the
catch pit
Plot 5 Natural vegetation. Since 2009 a new treatment has been introduced to evaluate the impact of a
complete vegetation cover on erosion rates. It is expected that a good soil cover combined with
considerable biomass to intercept rainfall will lead to minimal runoff and minimal erosion rates. A
plot is kept to validate this hypothesis in the new sites of Goshing and Lumang.
Erosion plots at Yangneer RNR-EC
4
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Erosion plots at Logchina RNR-EC
Erosion plots at Buli RNR-EC
Erosion plots at Lumang RNR-EC
Erosion plots at Goshing RNR-EC
Treatments and replications
In the set-up of the erosion plots each treatment, as specified in the previous section, is only practiced once at every
site. From a scientific and geo-statistical perspective it would be preferable to have a number of replications of the
same treatment, in a random lay-out. The sites of the soil erosion plots do not allow for many plots as the slopes are
steep and space is limited on the RNR extension centre terrains. Instead of having many plots at one site it was
preferred to have a series of plots at different sites (at the end of 2009 5 sites with in total 22 plots have been
established and in early 2010 another site, Bongo, with 5 plots is installed, resulting in a total now of 27 plots). As
these sites are located under different agro-ecological conditions (altitude, climate, precipitation) and have different
slope characteristics (ranging from 9 to 30+ degrees) and soil types the impact of all these different factors can be
evaluated. The objective is aimed at quantifying a range of soil erosion rates under different agro-ecological, slope
and soil conditions and to quantify and demonstrate the effectiveness of SLM treatments to reduce soil erosion. The
present set-up is thought to be able to achieve that. Another site is being developed at this moment at the Wengkhar
RNR Research Division in Mongar Dzongkhag with financial support of SLMP where a number of replications (4)
of different treatments will be made at one location. This will generate an added data set and lead to geostatistically more reliable results, excluding some of the influences of spatial variability. Mountain slopes and their
soil however are inherently variable and measurements will always be disturbed by this heterogeneity.
Soil samples
Soil samples were taken of top- and sub-soil during establishment and at the end of each year to document soil
properties and to monitor changes in soil fertility and other soil characteristics over time.
5
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
3.
Measurements of soil erosion plots
A standard log book was used to record any activities or events on a monthly basis including precipitation,
maintenance practices (sowing, weeding etc.), harvesting (expressed in kg/volume) and unusual events during the
year (hail storm, full catch pit etc.). The actual measurement of sediments in the catch pits was done in December
2009. The soil was removed from the catch pits and air dried by laying them on plastic sheets for 2 to 3 days. For
some locations the soil was already air day and could therefore be weighed immediately. Soils from different plots
were kept and weighed separately using a spring balance.
3.1
Logchina measurements 2009
Table 2 Logchina measurements 2009
Logchina
Soil [kg, air dry]
Erosion rate [kg/ha]
Plot 1
Plot 2
Plot 3
Plot 4
447.7
81.0
34.7
51.14
111925
20250
8675
12785
The measurements for Logchina give a consistent
high erosion rate for the different plots. The erosion
rate for the reference plot is extremely high with
almost 112 t/ha, even surpassing the already high rate
of 2008, 85.55 t/ha. The rates for the other plots are
also significantly higher than in 2008, but reflect the
same trend with higher erosion rates on the plots with
traditional practice and reduced rate on the SLM
practices plots. On average the soil erosion rate on
the SLM plots is 1.89 times (53%) less than on the
plot with traditional practice. The high rainfall
amount, 3789mm, and the very erodible soils on a
step slope gradient (28 degrees) result in. The erosion
rate of 112 t/ha equals a soil depth loss of more than
20cm! (1t/ha equals 0.18mm uniform depth of soil).
[t/ha]
Rainfall [mm]
111.925
20.250
8.675
12.785
3789
Erosion rate Logchina Plots 2008 and 2009
120
100
[t/Ha]
80
2008
60
2009
40
20
0
The very full catchment pit of the control plot at Logchina
6
Reference
Traditional
SLM
SLM+
2008
85.55
12.9875
2.425
6.6375
2009
111.925
20.25
8.675
12.785
Note the clear step of more than 60cm developed
below the Napier hedgerows in the SLM+ plot
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Less than 2 years after establishment of the erosion plots at Logchina EC, the effect of the Napier grass hedgerows
is very striking. Not only show the measurements that erosion rates are reduced with almost a factor 2 compared
with the rates on the plot with traditional practice, but on the plot itself the slope profile has changed dramatically.
Below the hedges a clear step has formed of more than 60cm and above the hedges soil accumulation has resulted
in a sharp decline of the slope angle and formation towards a bench terrace. The demonstration effect aimed at is
reached in a dramatic fashion! The reduced slope angle has also increased the ease of workability.
Plot 4 SLM+ in December 2008
Plot 4 SLM+ in December 2009
The photographs shown above give a good indication of how quickly the establishment of the Napier grass
hedgerow as a vegetative barrier changes the slope profile and results within two monsoon seasons in the formation
of a bench terrace like stepped profile. The high precipitation of 3917mm in 2008 and 3789mm in 2009 is very
much condensed in the monsoon months of June to August and during this period in 2009 there were 7 events of
more than 100mm and 17 days with more than 50mm and less than 100mm, see Table 2. In 2009 there were three
days with more than 150mm precipitation compared to only 1 day in 2008, which could be a possible explanation
for the higher erosion rate in 2009 compared to 2008. It reflects the high erosivity of the precipitation with very
intense rainstorms leading to generation of runoff and intense splash and sheet erosion.
The soil collected in the catch pit of plot 4
Compared to the soil accumulated in the pit of plot 1
Table 3 Important rainfall events for Logchina in 2008 and 2009
Number of rainfall events 50mm<P>100mm P>100mm P>150mm
2008
17
8
1
2009
17
7
3
7
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
3.2
Nangkhor – Buli measurements 2009
Table 4
Buli measurements 2009
Nangkor
Soil [kg, air dry]
Erosion rate [kg/ha]
[t/ha]
Rainfall [mm]
7
10.4
6.1
9.6
1750
2600
1525
2400
The measurements for the erosion plots in Buli
reflect a consistent low soil erosion rate in line
with the very gentle slope gradient of only 9
degrees. The erosion rates for the reference plot are
lower than initially expected, due to incorrect
management as the plot was not kept bare
consistently as intended. The resulting grass cover
has clearly reduced the erosion rate to levels lower
than the traditional practice and SLM plots. The
erosion rates on the SLM plots are less than the
plot with traditional practice, on average 2.5 t/Ha
compared to 3.64t/Ha, a reduction with 31%. The
lower erosion rates of 2009 coincide with a lower
amount of precipitation in 2009 with 1500m
compared to 1888m in 2008, with 9 events of more
than 50mm in 2008 compared to only 6 in 2009,
see Table 3.
1.75
2.6
1.525
2.4
1500
Erosion rates Buli Plots 2008 and 2009
5
4.5
4
3.5
[t/ha]
Plot 1
Plot 2
Plot 3
Plot 4
3
2008
2.5
2009
2
1.5
1
0.5
0
Reference
Traditional
SLM
SLM+
2008
2.55
4.675
3.625
2.4375
2009
1.75
2.6
1.525
2.4
Table 5 Important rainfall events for Buli in 2008 and 2009
Number of rainfall events 50mm<P>100mm P>100mm P>150mm
2008
9
0
0
2009
6
0
0
During the establishment of the erosion plots in Buli it
was decided to make only a single Napier grass
hedgerow because of the very gentle slope of the
plots, only 9 degrees. This could be a possible reason
why the effectiveness of the hedgerows is relatively
less compared to for instance the Logchina plots. In
literature studies it is mentioned that grass hedgerows
become more effective with increased slope angle,
which apparently is confirmed by the measurements.
To make the measurement more consistent it has been
decided to add another hedgerow to have an identical
SLM plot set-up for all SLM plots.
The gentle slope of the erosion plots in Buli
8
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
3.3
Yangneer measurements 2009
Table 6
Yangneer measurements 2009
Yangneer
Soil [kg, air dry]
Erosion rate [kg/ha]
Plot 1
Plot 2
Plot 3
Plot 4
17.5
25
16.2
15.3
4375
6250
4050
3825
[t/ha]
4.375
6.25
4.05
3.825
Rainfall [mm]
802
[t/Ha]
The erosion rates of the Yangneer plots are
Erosion Rates Yangneer 2008 and 2009
intermediate in value (moderate erosion) compared to
the Buli plots (slight erosion). The Yangneer plots
have a slope angle of 16 degrees, but the precipitation
16
amount for Yangneer is clearly less than for the other
14
sites, in 2009 only 802mm, which reduces overland
12
2008
flow and the occurrence of erosion. Compared to 2008
10
2009
the erosion rates are consistently lower for all 4 plots,
8
but as there are no precipitation records for 2008 it is
6
not clear if this is related to a decreased rainfall
4
2
amount in 2009 or due to disturbance of the plots
0
during the initial establishment in 2008. The erosion
Reference Traditional
SLM
SLM+
rate for the reference plot has reduced considerably in
14.9
8.0375
5.425
9.875
2008
2009 compared to 2008 probably due to incorrect
4.375
6.25
4.05
3.825
2009
management as the plot was not kept bare all the time
and a grass and herb cover developed that reduced the erosion. This year a pattern develops as anticipated, except
for the reference plot, with lower erosion rates for the SLM plots compared to the traditional practice plot (6.25
t/Ha versus an average of 3.94 t/ha for the SLM plots) representing a reduction in erosion rate of 37%. The data of
2009 for the SLM plots are more consistent than in 2008 when the SLM+ plot had a soil erosion rate higher than
the traditional plot. Now the hedges are well established, helped by gap filling in 2009, the erosion rate has come
down considerably and the erosion rates of both SLM plots are very similar.
The Yangneer plots with well established Vetiver hedgerows
9
The reference plot with too much vegetation cover
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
For the whole year 2008 there were only 3 rainfall events of more than 50mm and no event of more than 100mm,
which contrasts sharply to Logchina with 24 events of more than 500mm and to lesser extent to Buli with 6 events.
3.4
Table 7
Lumang measurements 2009
Lumang measurements 2009
Lumang
Plot 1 natural vegetation
Plot 2 reference plot, bare
Plot 3 traditional practice
Plot 4 SLM
Plot 5 SLM+
Soil [kg, air dry]
4.9
17.25
8.5
7.25
Erosion rate [kg/Ha]
1225
4312.4
2125
1812.5
[t/Ha]
1.225
4.3124
2.125
1.8125
5.95
1487.5
1.4875
Rainfall [mm]
1297
[t/Ha]
The first measurements of the Lumang erosion
Erosion Rate Lumang Plots 2009
plots, after establishment in April 2009, show a
consistent trend that is in line with the expectation.
5
The erosion rate on the bare reference plot is
4.5
double that of the plot with the traditional practice
4
and the SLM plots have a lower erosion rate than
3.5
the traditional practice plot (on average 1.625 t/ha
3
compared to 2.135 t/ha, a reduction with 23%). As
2.5
2009
the hedgerows were only established in May 2009
2
they are still thin and the efficiency in soil erosion
1.5
reduction is less and expected to increase in 2010.
1
Striking for the Lumang measurements was the
0.5
relatively high percentage of stones fallen into the
0
catch pits (23% for the traditional plot, 7% for the
Reference Traditional
SLM
SLM+ Vegetated
SLM and traditional plots). This can be explained
2.135
1.8125
1.4875
1.225
2009 4.3125
because of the Lumang soils that are very stony
and the combined effect of tillage and soil erosion
by water bringing down stones on all plots, except for the plot with the natural vegetation. The high percentage of
stones in the catch pit on the traditional practice plot, 23%, might be strongly influenced by tillage. The hedgerows
on the SLM plots will retain rolling stones and gravel and thus reduce the stone content measured for these plots
and the stone percentage on the reference plot is equal to the SLM plots with 7%, reflecting the absence and
influence of tillage.
The Yangneer plots after establishment in April 2009
SLM+ plot with relatively thin Napier hedgerow
10
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
The erosion rate on the vegetated plot is higher than one would expect, but is expected to reduce over time as the
vegetation cover of this plot is not complete and with increasing biomass erosion rates should come down.
Measurement of the air dry soil in Lumang
3.5
Table 8
Goshing measurments 2009
Goshing measurements 2009
Goshing
Soil [kg, air dry]
Plot 1 natural vegetation
Plot 2 reference plot, bare
Plot 3 traditional practice
Plot 4 SLM
Plot 5 SLM+
Erosion rate [kg/ha]
[t/ha]
Rainfall [mm]
600
675
875
756
875
0.6
0.675
0.875
0.756
0.875
1297
2.4
2.7
3.5
3.0
3.5
[t/ha]
The first series of measurements for the
Goshing plots do not yet reveal a clear trend
Erosion Rate Goshing Plots 2009
and are not entirely in line with expectations.
The erosion rate for the natural vegetation plot
1
is lowest as expected (0.6 t/ha), but the
0.9
measurements for the SLM plots are just a
0.8
little less or equal to the traditional practice
0.7
plot (SLM 0.75 t/ha, SLM+ 0.875 t/ha and for
0.6
the traditional plot 0.875 t/ha). As the plots
0.5
2009
were only established in May and the
0.4
hedgerows were planted only in July there was
0.3
only a short time span for the grass hedgerows
to develop. By the time they were starting to
0.2
grow and form a physical barrier the rainy
0.1
season had ended and no real impact could be
0
Reference Traditional
SLM
SLM+
Vegetated
observed on the erosion rates. Rainfall records
0.675
0.875
0.75
0.875
0.6
2009
indicate a high precipitation amount with only
a partial coverage of 2009 more than 3000mm
was recorded (July to September 3057mm). Another factor to consider why the erosion rates on slopes of 24
degrees are limited could be the fact that the Goshing soils are relatively heavy, silty clay loam, giving good
cohesion and binding to the soil particles and reducing erodibility. It is striking that the erosion rates are so limited
for a slope of 24 degrees and rain fall amounts and intensity as witnessed in Goshing.
11
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
3.6
All the activities and measurements were recorded on a monthly basis using a standard log book.
All the activities and measurements were recorded on a monthly basis using a standard log book
Month
Activity/measurement
Buli
Lokchina
Yangneer
January
Precipitation (Total & Max.)
0mm
32.5mm
NR
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Nil
NR
Activities / Management
Nil
Hail storm (2)
Catch pit was half
full with hail!
Nil
Weeding (15)
21.9mm
5mm
-
Weeding and
cleaning of plot (26)
Sowing of maize
(0.5kg yangtsipa and
0.5kg Suphala) (26)
8kg seed potato sown
Weeding (15)
Plot 1
Plot 2
Plot 3
Plot 4
Remarks
February
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Activities / Management
Plot 1
Plot 2
Nil
Plot 3
Plot 4
Remarks
Nil
Sowing of potato
(6kg), maize (300gr)
Suphala 250gr
Soya bean 500gr
Field preparation
(21)
Application of FYM,
6 loads per plot (22)
Sowing of maize and
kidney beans (23)
Field preparation
(21)
Application of FYM,
6 loads per plot (22)
Sowing of maize and
kidney beans (23)
Field preparation
(21)
Application of FYM,
6 loads per plot (22)
Sowing of maize and
kidney beans (23)
Maintenance of catch
drain above plots
Lumang
Goshing
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Month
Activity/measurement
Buli
Lokchina
Yangneer
March
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Activities / Management
Plot 1
Plot 2
55.4mm
Nil
0mm
-
NR
NR
Weeding
Nil
Weeding (15)
Nil
Land preparation
Potato plantation
(Suphala and Urea)
Land preparation
Potato plantation
(Suphala and Urea)
Plot 3
Sowing of potato
(6kg), maize (300gr)
Suphala 250gr
weeding
Plot 4
Nil
Land preparation
Potato plantation
(Suphala and Urea)
Remarks
April
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Activities / Management
Plot 1
Plot 2
82.4
65.2 (7)
142.5mm
75mm (22)
Weeding (16)
Weeding (16)
Weeding (14)
Land preparation for
resowing of maize
(14)
Resowing of maize
and beans (15)
Land preparation for
resowing of maize
(14)
Resowing of maize
and beans (15)
Land preparation for
resowing of maize
(14)
Resowing of maize
and beans (15)
Plot 3
Weeding (16)
Plot 4
Weeding (16)
Lumang
Added Suphala and
Urea fertilizer
NR
NR
Maize sowing
13
Plot set up (10)
Plot set up (10)
Leaf litter collection
and application and
maize sown
Maize sowing
Plot set up (10)
Leaf litter collection
and application and
maize sown
Maize sowing
Plot set up (10)
Leaf litter collection
and application and
maize sown
Suphala added
Remarks
38mm
Goshing
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Month
Activity/measurement
Buli
Lokchina
Yangneer
Lumang
May
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
230.8mm
93.3mm (26)
64mm (27)
477.5mm
64mm (25)
135mm (26)
78.5mm (28)
70mm
197mm
1125mm (30)
Activities / Management
Weeding
Nil
Goshing
1st weeding
Weeding (22)
Establishment of
Plots (22-22)
idem
1st weeding
top dressing
1st weeding
top dressing
1st weeding
top dressing
Weeding (22)
idem
Weeding (22)
idem
Weeding (22)
Soya bean sown
idem
537.5mm
76mm
199mm
75 (19)
160.5mm (26)
68.5mm (30)
50mm (30)
Weeding (5)
Weeding (21)
Clearing of bushes
around the plot
Harvested Napier (3)
Clearing of bushes
(21)
Weeding
Harvested Napier (3)
Clearing of bushes
(21)
Soya bean plantation
Plot 1
Weeding (21)
Weeding (6)
Weeding (23)
Plot 2
Weeding (21)
Plot 3
Weeding (21)
Harvested Napier
Plot 4
Weeding (21)
Harvested Napier
77.5mm
Remarks
June
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Activities / Management
Plot 1
Weeding
Weeding (19)
Plot 2
Weeding (19)
Plot 3
Weeding (19)
Plot 4
Weeding (19)
Nil
Remarks
14
Weeding (15)
Weeding (15)
Urea application
Weeding (15)
Napier Hedgerow
establishment
Urea application
Weeding (15)
Napier Hedgerow
establishment
Urea application
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Month
Activity/measurement
Buli
Lokchina
Yangneer
Lumang
Goshing
July
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
433.7mm
65.0mm (2)
50.5 (3)
65.1 (29)
871mm
143mm (2)
80.5mm (3)
61mm (4)
51.5mm (9)
50mm (25)
87mm (28)
72.5mm (29)
115mm (30)
180mm
76mm (29)
503
97mm (3)
52.5mm (4)
63.5mm (27)
1455m
77mm max
Activities / Management
Weeding & Potato
harvest
Weeding (3)
Weeding (4)
Weeding (23)
2nd Weeding
Weeding (4)
Weeding (5)
2nd Weeding
Weeding (4)
Maize sown (5)
2nd Weeding
Weeding (4)
2nd Weeding
Weeding (4)
Napier planted (1)
Maize sown (5)
Napier planted (1)
Maize sown (5)
Beans sown (5)
331mm
66.5mm (7)
62.5mm (19)
233.5mm
53.5mm (15)
1290mm
57mm max
Weeding (13)
Weeding (11)
Nil
Weeding (11)
Plot 1
Plot 2
Plot 3
Plot 4
Weeding & potato
harvest (5.4kg) (3)
Weeding & potato
harvest (7.85kg) (3)
Weeding & potato
harvest (6.85kg) (3)
Remarks
August
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
408.4mm
98.7 (19)
Activities / Management
Weeding
Weeding (28)
Plot 1
Plot 2
Maize harvested
(2.8kg) rat and crow
damage (28)
1115mm
61.5mm (3)
57.5mm (5)
90.5mm (8)
55.0mm (9)
50mm (11)
50mm (12)
157.5mm (15)
59.5mm (17)
105mm (18)
160mm (19)
Weeding (6)
Weeding (23)
Maize harvest (6)
Land preparation (7)
Application of FYM,
6 loads per plot (8)
Finger millet
transplanting (9)
15
Potato harvest
Nil
Potato harvest
(59.6kg)
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Month
Activity/measurement
Buli
Lokchina
Yangneer
Lumang
Goshing
Plot 3
Maize harvested
(3.34kg) (28)
Potato harvest
(39.5kg)
Nil
Weeding (11)
Plot 4
Maize harvested
(4.21kg) (28)
Maize harvest (6)
Land preparation (7)
Application of FYM,
6 loads per plot (8)
Finger millet
transplanting (9)
Maize harvest (6)
Land preparation (7)
Application of FYM,
6 loads per plot (8)
Finger millet
transplanting (9)
local variety millet
(Katic kodo)
383mm
85.5mm (18)
80.5mm (27)
Potato harvest
(39.5kg)
Nil
Weeding (11)
39.5mm
127mm
312mm
Weeding (8)
Weeding (21)
Nil
Weeding (27)
Weeding (16)
Harvest (27)
Dug the field for
winter crop
Harvest (27)
Dug the field for
winter crop
Harvest (27)
Dug the field for
winter crop
Weeding (16)
0mm
0mm
Nil
Weeding (26)
Mustard seed
sowed (3)
Mustard seed
sowed (3)
Weeding (26)
Remarks
September
Precipitation (Total & Max.)
Unusual weather events >50mm (e.g.
storms, hail, etc.)
Activities / Management
Plot 1
82.6mm
Nil
Nil
Plot 2
Nil
Plot 3
Hedgerow cutting
(7kg)
Napier harvested (11)
Plot 4
Hedgerow cutting
(9kg)
Napier harvested (11)
99.5mm
81.4mm (8)
219mm
50mm (6)
166mm (7)
Weeding (16)
Weeding (7)
Weeding (22)
Weeding of finger
millet (11)
Weeding of finger
millet (11)
Weeding (16)
Weeding (16)
Remarks
October
Precipitation (Total & Max.)
Unusual weather events >50mm
(e.g. storms, hail, etc.)
Activities / Management
Plot 1
Plot 2
Weeding (16)
Plot 3
Weeding (16)
16
105mm
Maize harvest
(14kg)
Maize harvest
(6kg)
Weeding (26)
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Month
Activity/measurement
Plot 4
Buli
Logchina
Yangneer
Lumang
Goshing
Weeding (16)
Weeding of finger
millet (11)
Cleaning of plots
surrounding (10)
0mm
Maize harvest
(2.2kg)
Mustard seed
sowed (3)
Weeding (26)
0mm
0mm
0mm
Weeding (9)
Weeding (23)
Nil
Weeding (8 and 16)
Remarks
November
December
Precipitation (Total & Max.)
Unusual weather events >50mm
(e.g. storms, hail, etc.)
Activities / Management
Plot 1
2.1mm
Nil
Plot 2
Nil
Nil
Weeding and
cleaning (22)
nil
Plot 3
Nil
Nil
nil
Plot 4
Nil
Soybean harvest
(2kg)
nil
Remarks
Precipitation (Total & Max.)
Unusual weather events >50mm
(e.g. storms, hail, etc.)
Activities / Management
Plot 1
Nil
3.6mm
6mm
Nil
NR
NR
0mm
0mm
7kg
Measurement (9)
Replaced tarpaulin
(10) 447.7kg
Measurement (9)
Replaced tarpaulin
(10) 81kg
Measurement (9)
Replaced tarpaulin
(10) 34.7kg
Measurement (9)
Replaced tarpaulin
(10)
51.14kg
Finger millet harvest:
Plot 2: 0.5kg
Plot 3: 0.9kg
Plot 4: 0.3kg
17.5kg
4.9 kg (nat. veg)
17.25kg reference
Weeding (24)
25kg
8.5kg
Weeding (24)
16.2kg
7.25
Weeding (24)
15.3kg
5.95kg
Weeding (24)
Plot 2
10.4kg
Plot 3
6.0kg Napier
12.5kg bio-mass
Plot 4
9.6kg Napier
14.3kg bio-mass
Remarks
Maize harvest (8)
Maize sown (16)
Maize harvest (8)
Maize and radish
sown (16)
Maize harvest (8)
Maize and beans
sown (16)
17
Stray cattle browsed
the hedge rows in
plot s 3 and 4
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
4.
Overview and discussion of the measurements
Looking at the results for the 5 different sites it is eye catching that the results for Logchina are much higher and
represent an outlier compared to the range of the values of the other 4 sites. If the measured erosion rates are
classified according to soil erosion classes (Singh et al. 1992) the erosion rates for Logchina are very severe (>80
t/ha) whereas the erosion rates for Yangneer are moderate (5-10 t/ha) and the values for Buli are of a slight soil
erosion class (<5 t/ha). The results for both Lumang and Goshing are too preliminary to classify the erosion rate. If
one takes the average of the 5 sites for the different treatments or practices a clear trend emerges with a very high
erosion class for the reference or bare plots (24.61 t/ha), a moderate erosion class for the traditional practice plots
(6.42 t/ha) and only slight erosion on the SLM and vegetated plots.
Soil Erosion Rate NSSC Plots 2009
120
Buli
100
Yangneer
[t/Ha]
80
Goshing
Lumang
60
Logchina
40
AVERAGE
20
0
Reference Traditional
SLM
SLM+
Vegetated
2.6
1.525
2.4
0
Buli
1.75
Yangneer
4.375
6.25
4.05
3.825
0
Goshing
0.675
0.875
0.75
0.875
0.6
Lumang
4.3125
2.135
1.8125
1.4875
1.225
Logchina
111.925
20.25
8.675
12.785
0
AVERAGE
24.6075
6.422
3.3625
4.2745
0.9125
The erosion rate on the SLM plots is reduced on average by about 41% compared to the traditional practice plot
(3.82 t/ha for the SLM plots compared to 6.42 t/ha for the traditional plots). The first measurements for the
vegetated plots (or natural vegetation plots) show the lowest erosion rates (less than 1t/ha) and this rate is expected
to decrease over time as the vegetation cover will recover after initial disturbance during establishment.
Table 9
Soil erosion classification, after Singh et al. (1992) as determined for India
Soil erosion rate range (t/ha/yr)
0-5
5-10
10-20
20-40
40-80
>80
Soil erosion class
Slight
Moderate
High
Very High
Severe
Very Severe
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Table 10 Reduction in soil erosion rate on the SLM plots compared to the traditional practice
Site
Logchina
Yangneer
Buli
Lumang
Goshing
Reduction in soil erosion rate:traditional practice versus SLM
plots
53%
31%
37%
23%
7%
If we disregard the outlier of Logchina the trend is less conspicuous, mainly because of a relatively low erosion rate
value for the reference or bare plots, due to management problems. Because of the high weed pressure the
extension staff often does not have enough time to keep the plots bare and grasses come up so that the soil is
covered and the erosion rate reduced. Erosion rates for the traditional practice plot are consistently higher than for
the SLM plots, with an average of 2.97 t/ha for the traditional plots and 2.09 t/ha for the SLM plots, a reduction of
about 30%. The sites that were established in 2009, Lumang and Goshing, show a smaller reduction in soil erosion
rate compared to the 3 sites established in 2008. The hedgerows at these sites are now grown into real physical
barriers without gaps and are more effective in catching soil compared to the newly established hedgerows.
Erosion Rate NSSC Plots 2009
8
Buli
6
Yangneer
[t/Ha]
Goshing
4
Lumang
2
AVERAGE
0
SLM
SLM+
Vegetated
Buli
1.75
2.6
1.525
2.4
0
Yangneer
4.375
6.25
4.05
3.825
0
Goshing
0.675
0.875
0.75
0.875
0.6
Lumang
4.3125
2.135
1.8125
1.4875
1.225
2.965
2.034375
2.146875
0.45625
Rainfall Data 2009 and 2009
4500
4000
3500
3000
2500
2000
1500
1000
500
0
19
2008
Lu
m
an
g
G
os
hi
ng
2009
Ya
ng
ne
er
Lo
gc
hi
na
The rainfall data for 2009 show that precipitation
amounts differ greatly between the 5 sites with
very high amounts for both Logchina and
Goshing (3789 and 3057mm), intermediate
values for Buli and Lumang (1500 and 1297mm)
and a relatively low amount at Yangneer
(802mm).
[mm]
AVERAGE 2.778125
Bu
li
Reference Traditional
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
“The normal rate of soil erosion under natural vegetation is thought to be in approximate equilibrium with the
rate of soil formation. It is helpful to consider some numbers. One ton of soil per acre is equal to uniform depth
of .18 mm. At one ton lost per acre per year, it would take 43 years to lose an inch of topsoil. This is
approximately the rate of erosion on uncultivated cropland. Even this rate, however, may be faster than the rate
at which new soil is formed.” Cited from http://www.mackinac.org/2836.
The above citation is in line with other research on the soil erosion and agricultural sustainability, e.g. Montgomery
(2007), finding that erosion rates from conventionally plowed (tilled) agricultural field average 1-2 orders of
magnitude greater than rates of soil production, erosion under native vegetation, and long-term geological erosion.
The erosion rates found for the soil erosion plots over the last 2 years are much higher than the rate of soil
production and should be considered as not sustainable over time to practice agriculture. The soil profiles found on
the steep slope plots of Logchina, Yangneer and Goshing are typically truncated with an AC profile with sometimes
a remnant of a (truncated) B horizon, indicative of the intense hill slope erosional and denudational processes.
Table 11 Overview of soil erosion rates for 2009 for the 5 sites and 4 practices
Site
Logchina
Nangkor
Yangneer
Lumang
Goshing
Total
Average
Plot 1 (ton/ha)
Reference / bare
111.92
1.75
4.38
4.31
0.68
123.04
24.61
Plot 2 (ton/ha)
Traditional practice
20.25
2.6
6.25
2.13
0.88
32.11
6.42
Plot 3 (ton/ha)
SLM
8.67
1.53
4.05
1.81
0.67
16.73
3.34
Plot 4 (ton/ha)
SLM+
12.78
2.4
3.83
1.49
0.88
21.38
4.28
The amount of top soil lost due to soil erosion will lead to a rapid decrease of the nutrient and organic matter base
and therewith reduce agricultural yields, reduce water retention capacity, reduce soil aggregate stability and
decrease workability of the soil because of increasing stoniness. The results of the first two years of soil erosion
plots in Bhutan confirm the urgent need for application of SLM or soil conservation measures to reduce the
prevalent high erosion rates on the steep Bhutanese slopes and to make agriculture more sustainable and productive.
Soil properties: a first interpretation of apparent changes
As stated in the Materials and Methods section, Chapter 2, soil samples have been collected of top- and subsoil of
every plot during establishment and at the end of every year during measurement. These samples have been
analysed in the NSSC soil laboratory for pH, available P, K, organic C%, Total N (%), exchangeables (Ca, Mg, Na
and K),CEC, TEB, CEC, BS and texture. See the annexes 1A to 1E for the analyses results. The soil analysis for the
Logchina samples gives the most striking result. The amounts of available P, K and organic C and total N have all
decreased dramatically. The same accounts for the amounts of Ca, Mg, K and Na and this results in a sharp change
in CEC and BS% (CEC falls from 25.5 on average to 8.5 on average, BS increases from 10 on average to 17.2).
The very severe erosion rate, due to high rainfall amounts and intensity, combined with a steep slope angle and
erodible soil result in truncation of the top soil and reduction of the available nutrients and organic matter content in
the topsoil. The soil analysis for Goshing reveals relatively high pH values, up to pH6.63, which is most likely the
result of the carbonate rich parent material. CEC values are relatively high for Bhutan, probably due to the silty clay
texture. In the samples of Buli one can see a distinct increase of Ca and Mg and related BS%. As Suphala, a
compound fertilizer with N, P and K, is applied the increase in Ca and Mg is surprising and unexpected. Also in
Yangeer Suphala is applied, together with Urea (N), which is reflected in a sharp increase in the P and N values, but
a decrease in available K. The sharp increase of P and N values, together with a sharp increase in the organic C %
(which is very unexpected) result in a considerable increase of CE values from 4.5 on average to 37.4. The soil
analyses results for December 2009 should be useful to see if these trends are consistent or related to analysis
inconsistencies or sampling issues.
20
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
5.
Maintenance of the erosion plots
During the period December-January measurements were made of the erosion plots by removing the collected soil
from the catch pit. The soil was dried in the sun and weighed with a spring balance after it was air dried. The old
tarpaulin sheets were removed and replaced by new ones. Additional maintenance work of the plots and catch pits
was carried out.
Preparing the SLM plot for the new tarpaulin
Drying the wet soil until it is air dry
The new tarpaulin for the pits in Logchina
The catch pits after the tarpaulin is cut out
Maintenance and preparation of the soil erosion plots is relatively easy compared with the initial establishment,
which is laborious. The most critical work is the replacement of the tarpaulins and fixing of the tarpaulin edge into
the soil. It means a recurrent disturbance of a small strip of the plots, about 40-50cm, but as it is uniformly done for
all plots it is not thought to make a serious impact on the measurements. Some of the problems encountered during
the measurements and the preparations of the plots are the growth of grasses under the tarpaulin and the risk of
small animals entering under the tarpaulin sheets for those catch pits where there is too little space to construct
proper ridges. Wear and tear of the tarpaulin sheets is greatest in Logchina with high temperatures and high rainfall
amounts and intensity. It has to be seen if the black plastic used for the 2010 season will be more durable.
Siphoning and emptying of the catch pits after heavy downpours had to be done at some of the sites. The staff try to
be careful not to throw away too much sediment in solution, but that is probably unavoidable.
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
The Yangneer plots after
the new tarpaulin sheets
have been fitted
The Lumang plots after plot preparation
22
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
6.
Discussion and Conclusions
To quantify soil erosion rates in Bhutan under different slope and agro-ecological conditions in total 5 sites with 22
erosion plots established over the last 2 years. In 2008 the first three sites were set up and in 2009 another 2 sites
were added. The soil erosion plots established are 10m by 4m and are bound with CGI sheets and have a catch pit at
the bottom lines with a plastic tarpaulin. Only soil loss over a whole year is measured without recording the amount
of overflow to keep the measurement set-up as simple as possible. Additionally precipitation is recorded on a daily
basis. Erosion rates on steep slopes, the majority of agricultural land in Bhutan, are relatively unknown as most of
the research worldwide focuses on much gentler slopes. Many farmers have to cultivate land of more than 30
degrees and it is not rare to see farmers work on slopes of over 30 degrees. The erosion plots established vary
between 9 and 30 degrees and are thought to be a representative selection of SLMP geog conditions and indicative
for overall Bhutanese conditions. The plots have 4 different practices or treatments: plot 1 is the reference plot
which is kept bare throughout the year, plot 2 has the traditional agricultural practice of that particular area, mostly
maize and potatoes, plot 3, the SLM plot, has 2 grass hedgerows at 6m and 1m above the catch pit combined with
the traditional practice and plot 4, the SLM+ plot, has the treatment of plot 3 added with legumes for extra ground
cover and enhanced soil fertility. The newly established plot sites at Lumang and Goshing have and additional Plot
5 with natural vegetation. These plots are intended to measure erosion rates on soil with complete vegetation cover
and with soil protected from precipitation by biomass. Erosion rates expected are very low, but it is thought to be
interesting to establish a baseline for natural vegetation conditions and to be able to quantify the difference with
agricultural practices. Apart from providing basic information on soil erosion rates under different agricultural
practices and the effectiveness of SLM measures to reduce erosion rates, the plots are also instrumental in
demonstrating the rate and impact of soil erosion on steep farming land to the local farmers and visitors. Also the
benefits of SLM measures as hedgerows can be shown in practice, which ultimately is the best way to convince
farmers of the usefulness of new interventions to control surface erosion.The soil erosion measurements indicate a clear trend. The bare reference plots for the year 2009 have on average
the highest erosion rates (24.6 t/ha) followed by the traditional practice plot with an average rate of 6.42 t/ha. The
SLM plots have an average soil erosion rate of 3.36 t/ha, whereas the SLM+ plots average is 4.27 t/ha. The average
results of the 5 sites are strongly influenced by Logchina, which has by far the highest erosion rates of the 5 sites,
although the trend is similar to the other sites. The extremely high rates of Logchina are determined by the high
precipitation amounts and intensity (with numerous rainstorms of more than 100mm), the steep slope angle of the
plots (28 degrees) combined with very erodible phyllitic soils. If one excludes the Logchina data, the trend remains
still the same with average soil loss on the SLM plots distinctly less than on the traditional practice plots. The
difference with the reference plots is less conspicuous as management problems have left the reference plots not
completely bare throughout the year, reducing the erosion rate considerably.
The results for the newly established plots in Lumang and Goshing are preliminary as the grass hedgerows have not
yet reached a full size and are less effective as a physical barrier to arrest erosion. The late establishment at the
onset of the monsoon seasons makes the data also less reliable. The trend though for the Lumang plot is identical to
the older sites. For the Goshing plots though the trend is yet to emerge.
The effect of the hedgerows is starting to emerge clearly after 2 years. Especially in Logchina with very high
erosion rates and high precipitation amounts on very steep slopes the Napier hedgerows have resulted in the start of
the formation of terraces, with a clear step developing below the hedgerow and leveling out of the slope surface
above the hedgerow. From the original homogeneous slope profile the plot slope has transformed into a stepped
profile. The rate of this development is very fast and the impact of the hedgerows is dramatically demonstrated on
the Logchina SLM plots. The rate of the soil erosion in Logchina is a concern for the set-up of the erosion plots,
especially the reference plot and to lesser extent the traditional practice plot, as so much material has eroded that a
surface is clearly visible and the CGI sheets in some sections are totally exposed.
The effect of legumes as cover crop on the SLM+ plots has not yet emerged as a factor reducing the soil erosion
rate. Most likely the effect of the hedgerows is so significant that the impact of more ground cover of the legumes
(beans, soya bean) is hardly noticeable in the soil erosion rates between the SLM and SLM+ plots. The effect on
soil fertility status is not yet defined, but samples have been taken and analysis of these samples might show impact
23
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
on soil fertility status. Impact on yield cannot be defined, based on the limited crop cut data collected and some
disturbances by wild boar, crow and rat.
Comparison of the results for the older plots sites with the data series for 2008 gives a mixed image. The Logchina
soil erosion rates are very much comparable between 2008 and 2009 with slightly higher values for 2009, probably
due to a higher number of extreme precipitation events (>150mm), namely 3 in 2009 compared to only 1 in 2008.
These heavy downpours are thought to be responsible for a considerable fraction of the total soil loss. The trend
between the plots is very comparable. The results for the Buli plots are also comparable between 2008 and 2009
with overall a lower erosion rate in 2009. Unfortunately the reference plot was not kept bare throughout the year
and therefore the result for the reference plot is lower than expected. The Yangneer plots show consistently lower
erosion rates for 2009 compared to 2008. The SLM plots are now giving considerable lower erosion rates than the
results of 2008 when the grass hedges were not yet fully grown and wild boar damaged the SLM plots. Due to
improper management of the reference plot the erosion rates for 2009 are considerably lower than expected.
Based on the positive experiences of the 2008 and 2009 data series it has been decided to add one more series of
plots, namely in Bongo geog. These plots have been established in February 2010. RNR research Centre Wengkhar
will also establish a series of erosion plots, in total 20 numbers, following the same erosion plot set-up to make the
results comparable.
The soil erosion rate found over the years 2008 and 2009 shows clearly that the present agricultural practice on the
steep Bhutanese slopes in not sustainable as it leads to accelerated loss of top soil (on average 6.42 t/ha and in
extremis 20.25 t/ha (Logchina)) and reducing the nutrient and organic matter base, affecting yields and workability
and ultimately also impacting off-site and downstream by increased sediment loads in the Bhutanese rivers linked
to agricultural induced erosion. Introduction of Napier grass hedgerows has lead to a considerable decrease of soil
erosion rates, on average 40% for well established hedgerows, compared to the traditional practice. The average
soil loss rate on the SLM plots is 3.81 t/ha compared to the 6.42 t/ha for the traditional practice plots. On the steeper
erodible plots hedgerows reduce soil erosion rates up to 53% (Logchina).
Soil analysis data of soil samples of all the plots, taken during establishment and during the first measurement at the
end of 2008, give some striking preliminary results. For the Logchina samples the nutrient availability, organic
matter percentage and amount of exchangeables have all reduced dramatically after the first year of plot
establishment, resulting in a sharp drop in CEC. These changes in soil properties and soil fertility status can only be
related to the high soil erosion rates measured for the Logchina plots and the resulting truncation of the top soil. For
the Yangneer soil samples the most remarkable is the increase of P, K and N and even the organic C%. Suphala and
urea are added as treatment in Yangneer, which seems to be the explanation for the sharp increase in the available
P, K and N. The increase in organic C% cannot be explained. The result for the Buli soil samples show a clear
increase in Ca and Mg, which is difficult to explain as Suphala was applied, which does not contain much of Mg
and Ca. The Goshing soil samples give a high pH value, indicative of the carbonate-rich substrate. The monitoring
of the soil sample analysis over the years will be important to evaluate the impact of the measured soil erosion on
the soil fertility status and the effect of the different management practices, especially the legumes ground cover of
the SLM+ plots.
24
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
7.
Recommendations
To further improve the quality of soil erosion data a number of recommendations is presented here. Besides
following the management protocol as compiled for the erosion plots and monitoring the plots on a regular basis it
is recommended to:
 To keep the control or reference plot (Plot 1 or control plot) bare throughout the year and to do this without
tilling. The experience over the least 2 years has been that it is not easy to keep away grasses and weeds
from the plot, especially for the sub-tropical sites with high weed pressure. The extension staff is
encouraged to hire local farmers to weed if deemed necessary. Insufficient weeding has resulted in cover
with grasses and weeds and reduced erosions rates on the bare plots for some sites below that of the
traditional practice plot or even the SLM plots.
 Maintenance of the Napier hedgerows is important: for the newly established site gap filling is required and
for the older sites timely cutting of the grass and weighing of the grass cut should be done based on the
growth and local conditions.
 Avoid too much tillage in the plots to prevent scouring under the CGI sheets and induce more soil erosion.
Especially on these plots with stony soils tillage will easily move stones down slope as was recorded for the
Lumang plots.
 Empty the run-off water from the catch pit if it is completely full using a water pipe (by siphoning). Care
should be taken not to remove any soil particles along with the water, although this will be hard. Another
option is to use a bucket to lower the water level if it threatens to overflow.
 Properly fence and guard the erosion plots from wild boar and cattle. Although almost all plots are fenced it
is still needed to monitor if any stray cattle is able to enter the EC’s compound and browse the hedgerows.
 Measurements of any kind should be made as accurately as possible.
 The SLM plots in Buli should get an additional hedgerow to make the results comparable with the other
sites. The gentle slope angle of only 9 degrees justifies the establishment of only 1 hedgerow, but to
facilitate comparison this needs to be adjusted.
 Sowing of seeds should be done at the right season and with good quality seeds.
 Clear bushes and trees around the erosion plots to avoid any interference through shading or dripping of
intercepted rain from the trees/plants.
 Properly adjust the spring balance before making the measurement.
 Make sure that soils are properly air dried before making the measurement.
 Proper maintenance of the erosion plots should be done throughout the year.
 After harvest of the crops and cutting of the hedgerows please weigh the biomass and measure the yield
(expressed in kg).
 The geog RNR staff are encouraged to approach the NSSC staff for guidance if they have any doubts or
queries.
25
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
8
References
Hellin, J. (2006). Better Land Husbandry. From Soil Conservation to Holistic Land Management. Land
Reconstruction and Management, Vol. 4, 2006. Science Publishers, Enfield, USA, 315 pages.
Montgomery, D.R. (2007). Soil erosion and agricultural sustainability.
www.pnas.org/cgi/doi/10.1073/pnas.0611508104
NSSC-SLMP (2008a)
BTOR Establishment of Soil Erosion Plots at Yangneer EC, January 2008
NSSC-SLMP (2008b) BTOR Establishment of Soil Erosion Plots at Buli and Logchina ECs, March 2008
NSSC-SLMP (2009) Soil Erosion Plots. Measurement of soil erosion plots for 2008 and maintenance of plots
for 2009. Technical report, SLMP 2009/1.
NSSC-SLMP (2009a)
BTOR Establishment of Soil Erosion Plots at Lumang EC, April 2009
NSSC-SLMP (2009b) BTOR Establishment of Soil Erosion Plots at Goshing EC, May 2009
NSSC-SLMP (2010)
BTOR Establishment of Soil Erosion Plots at Bongo EC, February 2010
Singh, G., Babu, R., Narain, P., Bhushan, L.S. and Abrol, I.P. (1992). Soil erosion rates in India. Journal of Soil and
Water Conservation 47 (1): 97-99.
Shrestha, R., Eiumonoh, A. and Baimoung, S. (1996). Soil erosion assessment and its policy implications: A Case
Study of RS and GIS applications in Uthai Thani,,Thailand.
http://www.gisdevelopment.net/aars/acrs/1996/ss1004pf.htm.
Wangchuck, T., Schouten, Chira and Yeshey (1997). Comparison of soil erosion control methods at Kangma.
Technical Document 9, RNRRC Khangma, east Bhutan, November 1997.
26
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Annex 1A Soil analysis for the Buli erosion plots
Top soils
Series
Site
Buli(job 1842)
(21/3/2008)
No of
pH
pH
Avail P
Avail K
Org C
Total N
Exchangeable meq / 100g
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
AK1782TS
5.74
4.49
2.36
69.03
4.5
0.1
1.56
0.35
0.4
0.04
TEB
CEC
BS%
Lab texture
AmOAc
AmOAc
2.35
8.69
27.14
SL
AK1783TS
5.65
4.43
2.17
55.07
4.2
0.12
1.5
0.36
0.32
0.02
2.2
11.78
18.75
SL
AK1784/TS
5.48
4.45
2.93
22.08
3.9
0.09
1.24
0.28
0.43
0.04
1.99
12.39
16.06
SL
AK1785/TS
5.77
4.45
1.06
81.13
3.4
0.1
1.37
0.26
0.48
0.04
2.15
6.74
32.04
SL
Hand texture
Buli(job2012)
Plot-1TS
5.58
4.25
6.69
61.17
1
0.05
3.49
0.66
0.33
0.04
4.52
9.63
46.82
LS
(1/4/09)
Plot-2TS
5.38
4.38
1.73
89
0.9
0.07
3.43
0.7
0.48
0.06
4.67
6.99
66.82
LS
Plot-3TS
4.97
4.06
1.11
53.2
0.9
0.06
3.51
0.79
0.29
0.04
4.63
7.65
60.44
LS
Plot-4TS
5.06
3.99
0.92
40.72
0.8
0.06
3.02
0.55
0.23
0.04
3.84
5.84
65.64
LS
Lab texture
Sub soils
Series /
No of
pH
pH
Avail P
Avail K
Org C
Total N
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Buli(job 1842)
AK1782SS
5.72
4.45
1.46
5.28
3.4
0.08
0.89
0.26
0.31
0.03
(21/3/2008)
AK1783SS
5.71
4.47
0.05
22.08
3.4
0.06
0.47
0.1
0.14
AK1784SS
5.76
4.43
0.81
54.55
3.3
0.06
0.53
0.11
AK1785SS
5.79
4.44
0.05
30.81
2.4
0.09
0.57
0.08
Site
Exchangeable meq / 100g
TEB
CEC
BS%
AmOAc
AmOAc
1.49
5.95
25.01
SL
0.04
0.75
10.4
7.31
SL
0.3
0.03
0.97
8.39
11.69
SL
0.21
0.03
0.89
6.2
14.42
SL
Hand texture
Buli(job2012)
Plot-1SS
6.09
4.71
0.05
32.11
0.3
0.05
4.14
1.24
0.32
0.06
5.76
7.68
75.02
SC
(1/4/09)
Plot-2SS
5.67
4.57
0.05
27.64
0.6
0.05
5.27
1.29
0.27
0.07
6.9
11.75
58.74
SC
Plot-3SS
5.53
4.36
0.08
34.09
0.7
0.05
6.88
1.2
0.29
0.05
8.42
11.68
72.11
SC
Plot-4SS
5.61
4.3
0.06
24.12
0.6
0.05
4.61
1.18
0.21
0.05
6.05
9.58
63.19
SC
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Annex 1B Soil analysis for the Logchina erosion plots
Top soils
Series
No of
pH
pH
Avail P
Avail K
Org C
Total N
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Logchina 1
EP1TS
5.47
4.31
29.09
52.27
6.7
0.3
1.83
0.5
0.31
0.03
Job 1837
EP2TS
5.32
4.28
19.21
40.08
5.4
0.21
1.15
0.34
0.24
EP3TS
5.27
4.23
29.54
56.65
6.8
0.38
1.85
0.38
0.33
EP4TS
5.07
4.09
104.28
68.51
6.6
0.31
2.16
0.55
0.37
Site
(10/3/08)
Exchangeable meq / 100g
TEB
CEC
BS%
Lab texture
AmOAc
AmOAc
2.67
24.73
10.8
CL
0.04
1.77
21.61
8.24
CL
0.05
2.61
30.78
8.5
CL
0.05
3.13
24.96
12.54
CL
Hand texture
Logchina2
P1TS
5.14
4.35
0.44
18.68
2.2
0.12
0.56
0.14
0.2
0.04
0.94
9.03
10.38
LS
Job 2006
P2TS
5.08
4.43
1.96
35.04
2.8
0.15
1.55
0.34
0.31
0.13
2.33
12.22
19.07
LS
(25/3/09)
P3TS
5.19
4.42
1.04
37.43
2.2
0.12
0.93
0.16
0.2
0.01
1.3
4.82
27.11
LS
P4TS
5.07
4.37
1.41
34.7
1.8
0.15
0.62
0.13
0.21
0.01
0.97
7.99
12.25
LS
pH
pH
Avail P
Avail K
Org C
Total N
TEB
CEC
BS%
Lab texture
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
AmOAc
AmOAc
Logchina 1
EP1SS
5.44
4.34
15.02
40.69
5.6
0.19
1.49
0.39
0.25
0.04
2.17
22.05
9.87
CL
Job 1837
EP2SS
5.31
4.42
19.18
21.58
5.3
0.22
0.25
0.06
0.16
0.02
0.47
20.86
2.34
CL
EP3SS
5.31
4.28
41.76
40.52
6.2
0.32
0.82
0.15
0.25
0.03
1.25
23.07
5.47
L
EP4SS
5.08
4.19
78.28
38.26
6.2
0.23
1.37
0.25
0.26
0.05
1.93
23.13
8.39
C
Sub soils
Series /
Site
(10/3/08)
No of
Exchangeable meq/100g
Hand texture
Logchina 2
P1SS
5.1
4.15
0.05
4.69
1
0.08
0.44
0.05
0.09
0.02
0.6
10.62
5.7
LS
Job 2006
P2SS
5.04
4.31
0.05
4.36
1.2
0.06
0.5
0.06
0.09
0.02
0.67
8.4
7.99
LS
(25/3/09)
P3SS
5.09
4.39
0.05
13.71
1
0.06
0.62
0.09
0.13
0.01
0.85
4.85
17.52
S
P4SS
5
4.42
0.58
10.32
0.8
0.04
0.31
0.04
0.09
0.02
0.46
2.7
17.09
S
28
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Annex 1C Soil analysis for the Logchina erosion plots.
Top soils
Series
No of
pH
pH
Avail P
Avail K
Org C
Total N
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Yangneer
P1TS
6.05
4.52
2.85
60.08
1.1
0.08
2.69
0.79
0.45
0.07
Job 1835
P2TS
5.5
4.25
1.33
70
1.8
0.06
2.99
1.16
0.26
(7/3/2008)
P3TS
5.73
4.29
0.6
51.66
2.9
0.06
2.19
0.64
0.37
P4TS
5.36
4.15
0.44
46.5
2
0.06
2.03
0.63
0.35
Site
Exchangeable meq / 100g
TEB
CEC
BS%
Lab Texture
AmOAc
AmOAc
4
4.52
88.51
SCL
0.07
4.48
4.14
74.77
SCL
0.06
3.26
4.62
70.55
SCL
0.05
3.06
4.85
62.97
L
Hand Texture
Job 2011
P1TS
4.5
4.12
35.18
24.12
6.4
0.46
2.42
0.47
0.19
0.01
3.09
36.78
8.4
L
(1/4/09)
P2TS
4.89
4.45
47.45
56.4
6.3
0.39
3.5
1.46
0.4
0.01
5.37
30.51
17.63
L
P3TS
4.72
4.25
87.58
25.15
6.6
0.51
3.72
1.1
0.18
0.01
5.01
41.13
12.2
L
P4TS
4.7
4.22
102.6
37.73
6.8
0.44
4.45
1.45
0.27
0.01
6.18
41.17
15.01
L
pH
pH
Avail P
Avail K
Org C
Total N
TEB
CEC
BS%
Lab Texture
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
AmOAc
AmOAc
Yangneer
P1SS
6.36
4.85
0.27
26.16
1.4
0.01
2.59
1.29
0.38
0.09
4.35
4.35
92.4
SCL
Job 1835
P2SS
6.27
4.81
0.05
12.64
1.3
0.03
2.99
1.16
0.26
0.07
4.48
4.48
88.33
CL
(7/3/2008)
P3SS
6.2
4.77
0.05
21.81
1.4
0.05
3.42
1.4
0.34
0.06
5.22
5.23
73.55
CL
P4SS
5.97
4.67
0.05
16.18
1.3
0.05
3.24
1.37
0.31
0.05
4.97
4.97
64.06
CL
Sub soils
Series /
Site
No of
Exchangeable meq / 100g
Hand texture
Job 2011
P1SS
4.64
4.19
20.6
23.26
4.7
0.34
2.34
0.2
0.11
0.04
2.69
23.95
11.31
L
(1/4/09)
P2SS
4.65
4.22
21.78
16.07
5.4
0.31
2.08
0.16
0.07
0.04
2.35
24.97
9.38
L
P3SS
4.52
4.18
47.2
13.42
5.6
0.38
2.51
0.21
0.09
0.05
2.86
28.48
10.01
L
P4SS
4.5
4.05
98.12
20.29
5.9
0.39
3.41
0.36
0.12
0.07
3.96
28.44
13.91
L
29
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Annex 1D Soil analysis for the Goshing erosion plots
Top soils
Series
No of
pH
pH
Avail P
Avail K
Org C
Total N
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Goshing
AN001/TS
5.48
4.58
10.86
113.63
2.7
0.23
4.59
1.68
0.67
0.04
(job 2052)
AN002/TS
5.61
4.61
1.98
52.81
2.8
0.21
3.94
1.67
0.42
AN003/TS
5.91
4.72
1.29
62.29
2.6
0.21
4.2
1.81
AN004/TS
6.22
5
0.23
37.07
2.5
0.19
5.14
AN005/TS
6.63
5.48
1.01
38.88
2.8
0.22
7.12
Site
(3/8/09)
Sub soils
Series /
No of
Exchangeable meq / 100g
CEC
BS%
AmOAc
AmOAc
6.98
22.08
31.58
ZiCL
0.03
6.06
19.04
31.84
ZiC
0.44
0.03
6.48
20.52
31.6
ZiC
2.19
0.34
0.06
7.73
20.01
38.6
ZiC
3.13
0.36
0.04
10.65
19.68
54.14
ZiCL
Lab texture
pH
pH
Avail P
Avail K
Org C
Total N
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Goshing
AN001/SS
5.57
4.6
4.97
50.74
2.4
0.21
4.25
1.55
0.39
0.03
(job 2052)
AN002/SS
5.74
4.6
0.41
22.57
2.1
0.17
3.21
1.38
0.23
(3/8/09)
AN003/SS
5.92
4.69
0.32
35.91
2.1
0.19
3.91
1.71
AN004/SS
6.38
5.13
0.05
19.54
1.7
0.15
0
AN005/SS
6.87
5.67
0.94
20.1
1.3
0.13
4.13
Site
Exchangeable meq / 100g
30
TEB
TEB
Lab texture
CEC
BS%
AmOAc
AmOAc
6.22
19.91
31.22
ZiC
0.03
4.85
17.36
27.99
ZiC
0.31
0.03
5.96
20.11
29.68
ZiC
0.02
0.29
0.44
0.75
17.5
37.07
ZiC
2.05
0.24
0.03
6.45
11.89
54.24
ZiC
Measurement and analysis of 2009 data and preparation of the plots for the 2010 season
Annex 1E
Top soils
Series
Soil analysis for the Lumang erosion plots
No of
pH
pH
Avail P
Avail K
Org C
Total N
Site
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Lumang
Plot-1-T
5.15
4.02
0.93
68.45
4.7
0.16
14.65
1.42
0.37
0.05
Job 2033
Plot-2-T
4.85
3.98
0.05
32.3
1.5
0.15
0.25
0.04
0.31
Plot-3-T
4.6
3.94
0.3
37.27
2.9
0.19
0.25
0.08
0.28
Plot-4-T
4.67
3.96
0.35
25.91
3.1
0.16
0.61
0.2
Plot-5-T
4.41
3.92
0.35
39.64
3.4
0.23
0.76
0.25
pH
pH
Avail P
Avail K
Org C
Total N
Sub soils
Series /
No of
Exchangeable meq / 100g
CEC
BS%
AmOAc
AmOAc
16.49
27.32
60.35
0.06
0.66
10.74
6.21
L
0.02
0.63
13.11
4.86
ZiL
0.31
0.03
1.15
16.02
7.14
L
0.26
0.04
1.31
18.25
7.15
ZiL
TEB
CEC
BS%
Lab texture
AmOAc
AmOAc
Exchangeable meq / 100g
TEB
Lab texture
L
Site
Samples
H2O
KCl
ppm
ppm
%
%
Ca
Mg
K
Na
Lumang
Plot-1-S
5.05
4.12
0.31
26.39
ND
0.05
0.25
0.1
0.52
0.04
0.91
15.72
5.79
L
Job 2033
Plot-2-S
5.08
4.02
0.05
20.92
2.3
0.11
0.47
0.15
0.35
0.03
1
16.97
5.84
ZiL
Plot-3-S
4.85
4.02
0.05
13.75
1.7
0.1
0.61
0.26
0.38
0.03
1.28
17
7.49
CL
Plot-4-S
5.07
4.03
0.33
16.05
1.9
0.07
0.32
0.1
0.14
0.03
0.59
11.34
5.26
L
Plot-5-S
4.73
4.05
0.05
16.73
1.4
0.06
0.32
0.03
0.11
0.02
0.48
10.87
4.5
ZiL
31