2.1. Study Area:
In this study, Jhimruk watershed situated in Pyuthan District is considered. Jhimruk watershed lies in the West Rapti River accounting 915 km2 of the 4500 km2 area of Rapti Basin. The river mainly covers Pyuthan District with the altitude of the basin reaching a height of 3200 m.The Jhimruk River originates in Gaumukhi and flows along the Mahabharat range until it merges into the Madi River and later downstream into the West Rapti River. The ArkhaKhola and Lung Khola are upper tributaries to the Jhimruk.The watershed contains sub-tropical and warm temperature climates, determined by elevation. The fertile alluvial soil of the watershed makes it suitable to paddy plantation, giving the area its nickname, “The Rice Bowl of Nepal”. In the northern part of the watershed, the Mahabharat Range comprised of metamorphic rock, such as slate, sandstone and limestone. The combination of rock types makes the zone susceptible to folding, faulting and thrusting.
The catchment area of the basin is diverse because of the difference in altitude, climate, geology, biological and land use conditions. Jhimruk River carries important value since the water from the river is used for agricultural purposes as well a 12 megawatt (MW) hydroelectric plant is being installed on the river since 1994.
2.2. Data collection:
Table 1 represents the data used for soil erosion modelling and the source from where it is taken.
Table 1: Data used for soil erosion modelling
Factor
|
Input data
|
Data source
|
Rainfall erosivity factor
(R)
|
Mean Annual Precipitation data
|
DHM, Government of Nepal
|
Soil erodibility factor
(K)
|
Digital soil map of Nepal
|
DSMW, FAO
|
Slope length and
steepness factor (LS)
|
ASTER 30m Digital Elevation data
|
USGS Earth Explorer
|
Land cover management
Factor (C)
|
LULC map from Sentinal 2 A(10m resolution) imagery
|
USGS Earth Explorer
|
Support Practice factor
(P)
|
Slope percentage from ASTER 30m DEM data
|
USGS Earth Explorer
|
2.3. The revised Universal Soil Loss Equation (RUSLE):
The revised Universal Soil Loss Equation (RUSLE), developed in 1987 by the Natural Resource Conservation Society NRCS which predicts the rill and inter-rill erosion was used for estimating the soil erosion rate in Jhimruk watershed. The equation is given as:
A = (R) (K) (LS) (C) (P) ………………………………… (i)
Where,
A = total soil loss, in tons per hectareper year
R = rain energy factor for the time period
K =the soil erodibility factor
LS = the length-slope factor
C = the degree of soil cover factor
P = conservation practices factor (for agricultural tillage and crop rotation operations, not generally applicable for construction site calculation
2.3.1. Development of model database for RUSLE
a. Rainfall erosivity Factor (R)
The rainfall erosivity factor quantifies the effect of rainfall impact at a particular location and also describes the runoff amount and rate of the rain (Men et al., 2008). To calculate the rainfall erosivity of Jhimruk watershed, 13 year mean annual rainfall data was taken from eight different meteorological stations of that area. Since, only one meteorological station was located inside the watershed, other stations were taken as reference stations. The rainfall map was then produced by kriging method in Arc GIS and then rainfall erosivity was calculated by using the formula given by (El-Swaify, 1997):
R=38.5 + 0.35*P……………………………………….. (ii)
Where,
R=Mean Rainfall Erosivity Factor (R)
P= Mean Annual Rainfall (mm)
b. Soil Erodibility Factor (K)
Soil erodibility (K) factor represents the susceptibility of soil or surface material to erosion, transportability of the sediment and the amount and rate of runoff due to rainfall. The main soil properties that affect the K factor are soil texture, organic matter, soil structure and permeability of the soil profile. For a particular soil, the soil erodibility factor is the rate of erosion per unit erosion index from a standard unit plot of 22.13 m long slope length with 9% slope gradient (Ganasri & Ramesh, 2016). The original equation given by the Wischmeir and Smith (1978) requires the soil structure and soil permeability values which were absent in the DSMW data. So, the equation provided by Sharpley and Williams (1990) was used for the estimation. The equation is given as:
![](https://myfiles.space/user_files/83400_b9e2661d18ef2d4b/83400_custom_files/img1631098307.png)
Where,
SAN, SIL and CLA are percent sand,silt and clay respectively; c is the organic carbon content and SN1 is sand content subtracted from 1 and divided by 100
Fcsand: It gives low soil erodibility factor for soil with coarse sand and high values for soil with little sand content.
Fsi-cl:It gives low soil erodibility factor with high clay to silt ratio
Forgc: It is the factor that reduce soil erodibility for soil with high organic content
Fhisand:It is the factor that reduces soil erodibility for soil with extremely high sand content
c. Topographic Factor (LS)
The (LS) factor is the ratio of soil loss per unit area from a field slope to that from a 22.13 m length of uniform 9% slope under otherwise identical conditions (Wischmeir & Smith, 1978). LS factor takes into the rill erosion. The topographic factor also known as Slope Length and Steepness Factor (LS) was prepared from two sub-factors: a slope length factor (L) and a slope gradient factor (S). For this an ASTER Digital Elevation Model (DEM) of 30*30 m resolution of the Jhimruk watershed was used. The LS factor is very important to estimate the soil loss as it helps in calculating the transport capacity of overland flow (Morgan et al., 1984). The ‘L’ represents the effect of slope length on erosion whereas the ‘S’ represents the effect of the slope steepness on erosion. There is greater soil loss in areas with greater soil steepness than that with the slope length (Koirala et al., 2019). The LS factor represents the effect of topography, hill slope length and steepness on soil erosion.
The slope gradient and slope length factors were calculated from the Dem and combined result in the topographical factor grid using the relation given by (Gao et al., 2012).
![](https://myfiles.space/user_files/83400_b9e2661d18ef2d4b/83400_custom_files/img1631098249.png)
![](https://myfiles.space/user_files/83400_b9e2661d18ef2d4b/83400_custom_files/img1631098268.png)
d. Cover management factor (C):
The cover management factor (C) represents the ratio of soil loss from land with specific vegetation to the corresponding soil loss from a continuous fallow (Morgan, 2005). It is used to reflect the effect of cropping and other management practices on erosion rates. Vegetation cover is also important factor for the erosion control. Here, the land use and land cover map of the Jhimruk watershed produced in Arc GIS 10.2.1 through supervised classification was used to prepare the C- factor map. The values were assigned based on the study of (Erencin et al., 2000). The values of c-factor ranges from 0 to 1 where higher values indicate no cover effect and soil loss as compared to that of the barren land. Whereas, the lower values of c represents the strong cover effect which has the effect to control the erosion.
e. Conservation support-practices factor (P)
Support practices like terrace, contour methods are the important factor to control the soil erosion. The P factor indicates the rate of soil loss according to the various cultivated lands on the earth. The value of support practice factor given by (Shin, 1999)was used for the computation. The contour farmland is taken into consideration as agricultural support practice factor in Jhimruk watershed.