Subwatershed Prioritization for Watershed Management
using Remote Sensing and GIS
3.2 Assessing Resources
By using these data, the general methodology was followed as presented in Fig. 1 Land cover map of 1978 was obtained by using visual interpretation of aerial photograph with the help of land use map of 1982, while supervised digital image processing was adopted for 1991 satellite data. After comparison of land cover of 1978 and 1991, the forest loss had been obtained.
Fig. 1: General Methodology
Widely adopted USLE model was taken to estimate the soil loss (Schwab et. al, 1993). The equation is written as
E = RKLSCP
Where,
E= Mean annual soil loss (tons/ha/yr.),
R = Rainfall erosivity index,
K = Soil erodibility,
L = Slope length,
S = Slope steepness,
C= Crop management and vegetation cover,
P = Erosion control practice factor.
From the average annual rainfall (1942-mm) and the maximum 30-munute intensity (100-mm) of the year 1991, the R-value was calculated (Foster, 1988 and
Morgan, 1986. the K Factor for 20 soil units and the P factor for different land cover were determined by using Schawab et al. (1993). The slope length for different land cover were adopted from the DSCWM/HMG, Nepal 1992, while the slope inclination factor was determined by using the digital Terrain Model (DTM) which was interpolated
After digitizing the 20 meter contour interval lines. The C value was adopted from Wischmeier and Smit (1978) for forest, shrubs and grazing lands, while for agriculture land, it was calculated as suggested by Morgan, (1986). The Soil samples were analyzed for fexture for texture and permeability at RARS, Tarahara, Sunsari, Nepal. The layers of USLE model were created and integrated with the help of GIS (Fig. 2).
Fig. 2: USLE Model in Conjunction with RS & GIS to Estimate Soil Erosion
4. Result and Discussion
4.1 Overall Situation
Overall land cover change and soil loss increment for the duration of 13 years; between 1978 and 1991 are given in Table 1. the rate for forest loss of the study area was 0.57 per cent per year and is too high for sustainable use of resources (Sah et al, 1997). The increment of the soil erosion rate from 0.95 to 1.37 mm/yr is significant for 13 years of time interval as the tolerance level of soil erosion in Nepal is in the range of 0.5 mm/yr (Sharma, 1981)
Table 1: Forest Loss and Soil Loss Status of Watershed
| Land Cover Types |
Land Cover (%) |
Soil Loss (mm/yr) |
| 1978 |
1991 |
Change |
1978 |
1991 |
4.33 |
| Shrubs |
2.27 |
3.45 |
1.18 |
9.40 |
13.73 |
4.33 |
| Degreded forest |
4.19 |
8.90 |
4.71 |
0.84 |
0.80 |
-0.04 |
| Forest |
65.21 |
57.86 |
-7.35 |
0.13 |
0.14 |
0.01 |
| Agriculture |
22.49 |
23.66 |
1.17 |
2.74 |
3.14 |
0.40 |
| River |
5.84 |
6.14 |
0.30 |
0.00 |
0.00 |
0.00 |
| Total |
100.0 |
100.0 |
0.00 |
0.95 |
1.37 |
0.42 |
| Degreded forest <40% canopy cover, forest=>40% canopy
cover Forest Loss Rate=7.35/13=0.57 percent per year
|
4.2 Delineation of Subwatershed:
From the topographic map, twenty-two subwatersheds were delineated and after the main river of that subwatshed (Map 2). The forest cover of the subwatershed and the soil loss status were taken as the basis for their prioritization. Furthermore, the contribution form each subwatershed to the total amount of soil loss from whole watershed area was also considered for such evaluation. Table 2 shows the name list of delineated subwatershed along with their forest and soil loss status.
Map 2: Subwatershed Map of the Trijuga Watershed, Nepal
Table 2: Forest Loss and Soil Status in Subwatershed
4.3 Defining the Parameters and Formulating Indices (Condition Indicator)
4.3.1 Degradation Speed Index (DSI)
The soil and forest and main resources of the watershed. Their amount of change in specified period of time is the indication of the status changing speed. So by assessing the forest and soil loss change between 1978 and 1991 and contribution to the total soil loss form each subwatershed. DSI has been formulated (Sah et al, 1997). The DSI is defined as the degradation speed of the subwatersheds.
DSI = 0.3*forest change (%) + 0.45 rate of soil loss change (t/ha/yr.) +0.25contribution to soil loss change (%)
The weight of individual factor has been decided on the basis of their importance to the land degradation. The location DIS value is given in Table 2.
4.3.2 Sensitivity Analysis and Sensitivity Index (SI)
Impact of forest loss subwatersheds causes various level responses, which is soil loss increase in this case. It depends on the characteristics such as steepness of subwatersheds of way of cutting tree. For example forest loss in steep slope is more critical than in flat area. To assess this characteristic, land sensitivity had been proposed and SI is defined (Sah et al, 1997). The land sensitivity analysis (Fig. 3) shows that some subwatersheds are more sensitive as slight loss of forest produced tremendous amount of soil. The SI for subwatershed is given in Table 2.
Fig. 2: USLE Model in Conjunction with RS & GIS to Estimate Soil Erosion
SI = Soil loss increment (t/ha/yr.)/Forest loss (%)
4.3.3. Present Condition (PC):
The soil erosion rate of the 1991 is considered as PC of the subwatersheds. Among several indicators, the higher rate of soil erosion can be considered as on indicator of the conditions of the subwatershed (Table 2).
