Soil Erosion and Hydrological Study of the Bakun Dam Catchment Area, Sarawak Using Remote Sensing and Geographic Information System (GIS)
4.0 Methodology
The methodology adopted in the generation of the R,K, LS and C digital raster layers for soil erosion modeling and hydrological studies was done in MICSIS (Micro-computer Spatial Information Special system for soil erosion modeling based on the parameters of the USLE was incorporated in MICSIS.
The Universal Soil Loss Equation (USLE) (Wichmeier and Smit, 1978) is an erosion model designed to predict average soil loss from specific tracks of land under different land use management systems. It computers soil loss as a product of six major factors, whose values are expressed numerically. The USLE is represented by:
A = R* K* L*S*C*P
Where :-
A - Annual Soil Loss in Kg m
-2y
-1
R - Rainfall Erosivity
K - Soil Erodibility
L - Slope Length
S - Slope Gradient
C- Land Cover
P - conversation Practice
The USLE was adopted in this study with Minor modifications in estimating the R and K parameters to suit malaysin conditions.
Rainfall Erosivity (R)
Soil loss is related to rainfall through the detachment power of raindrops striking the soil surface and the entrainment of the detached soil particles by run-off water down slope. The R values were computed from the product E. I
30 (E- Total kinetic energy of rain, I
30 -peak 30 min intensity), which reflects the potential ability of raindraops to cause erosion. E values were computed using the regression equation E = 9.28 P - 88367.15 (Morgan & Davison, 1986) developed to suit Malaysian conditions (P - Mean Annual Precipitation). As I
30 values were not available from most rainfall stations located in the study, area, an average I
30 value of 75 mmhr-1 was recommended. Mean annual precipitation data were obtained from 16 rainfall stations located in Bakun and its vicinity. The R values for each station were computed using the equation 9.28P-8836.15 *75 and input in the GIS as a discrete point file with location and R value map was generated using the triangulation method of interpolation.
Soil Erodibility (K)
Soil erodibility (K) defines the inherent resistance of the soil to both detachment and transport. The soil map of Sarawak at scale 1:500,000 (1968) was referred. K was determined only from the most predominant soil type in an association. Soil texture, structure, permeability and organic matter content, the parameters that determine the value of K, were obtained from soil profile descriptions, provided by the Department of Agriculture, Sarawak. K values, were estimated from soil erodibility nomograph (Wichmeiser and Smith , 1978) derived from the equation below.
k=(2.1x10-4(12-O.M%)(N1xN2)1.14+3.25(S-2)+2.5(P-3))
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Where :-
O.M. =Organic Matter Content
N1 = % Silt + % Fine Sand
N2 = %Silt +%Fine Sand +%sand
S =Level of Soil Structure
P = Permeability
A polygon vector file of the soil map was digitized. This was then rasterised and converted into a K value map by reclassing each soil polygon into its corresponding K value.
Topographical Factor (Slope Gradient and Length)
Soil erosion increase with increases in slope gradient (S) and slope length (L) resulting from respective increase in velocity and volume of surface run-off water. LS values were calculated using:
LS = ÖL/22 (0.065 + 0.045 S + 0.0065 S2)
Where :-
L = slope length in m
S = slope gradient in percent
Contour data were extracted from existing topography maps (38 sheets at scale 1:50000) through scanning (32 sheets ) and manual digitization (6 sheets). A DEM was generated from these data and converted to an image file. It was used to generate the slope layer and the slope length.
The Cover Factor (C)
Vegetation cover intercepts raindrops dissipating their kinetic energy before reaching the ground surface. C values were extracted from Morgan & Finney (1982) for cover types found in the area. Four landsat scenes (1988 and 1994) were used to output the land cover layer. Each of the scene was geo-referenced using map control points to conform to the Malaysian Map Projection. Colour balancing was then done on a band basis to reduce radiometric anomalies between adjacent scenes. The four scenes were mosaiked into one image, which encompassed the whole of the Bakun catchment area.
Supervised classification (VI) classifier, was carried out to generate the land cover/use layer. This layer was converted to C layer through reclassification of each cover type into its corresponding C value.
The Conservation Factor (P)
In general the protection offered by crops cultivated on slopes against erosion should be supported by soil conservation practices, which slow down the run-off water. This factor was not accounted in the study area as shifting cultivation commonly conservation practices that did not contribute significantly to erosion protection.
Current Soil Erosion Map
The current soil erosion map was generated. This depicts the extent of soil erosion in the Bakun area in tons/km
2/yr, given the present vegetative cover (Figure 1).
Figure 1. Soil Erosion Risk Map - Present Status
Soil Erosion Risk map
Generation of this map excluded the C layer. It represents a worse case scenario where the soil is depleted of its vegetative cover. It is obvious that the LS factor influences greatly the results depicted (Figure 2).
Figure 2. Soil Erosion Risk Map - Selective Logging
Computation of Inundation Extent and Water Volume
Using the Digital Elevation Model (DEM) of Bakun, the area coverage of surface inundation (km
2) and associated water volume storage (km
3) of the Bakun area were estimated at three flood levels designed for the reservoir at Bakun dam - probable maximum flood level (233
m.a.s.l), maximum operational flood level (228 m.a.s.l) and minumum operational flood level (195 m.a.s.l).
