Development of web-based WERM-S module for estimating spatially distributed rainfall erosivity index (EI30) using RADAR rainfall data

Abstract

Despite technological advances, soil erosion modeling is a very complicated process as the amount and rate of soil erosion vary considerably over space and time. Universal Soil Loss Equation (USLE) is one of the oldest and popular models used for soil loss estimation worldwide. USLE R-factor is one of the six input parameters accounting for the impact of rainfall amount and intensity on soil erosion in USLE. The USLE R factor is calculated by averaging annual long time rainfall erosivity index (EI30) values, computed by multiplying maximum rainfall intensity during 30min periods and the kinetic energy of the rainfall. The gage rainfall data are used for the determination of such EI30 index, and one representative value is given for the entire area. Due to the spatial and temporal variability of rainfall pattern, the value may vary considerably over space and time. It is required to obtain the rainfall data over a surface (heterogeneous) rather than at a point (homogeneous) so that spatially distributed erosivity index values can be calculated. Even though RADAR can provide spatially and temporally distributed rainfall data, the process of manual erosivity index calculation for each raster pixel is very tedious, time-consuming and practically not feasible. To overcome these limitations, the web-based WERM-S module was developed to compute a spatial EI30 index from the 10-min interval spatial rainfall data. The WERM-S consists of three different Fortran modules (Convert Module, R-factor calculation module, and R-factor ASCII module). The Jaun-ri watershed was selected as the study area to test the module since the RADAR rainfall data was available for 2015. June, July, and August were found to be the months receiving the maximum amount of rainfall and the average erosivity indices for June, July and August were found to be 2096, 1002, and 993MJ·mm/ha-hr-month, respectively. The maximum erosivity index for a pixel within the study area was observed to be 9821MJ·mm/ha-hr-month for June 4382MJ·mm/ha-hr-month for July and 6093MJ·mm/ha-hr-month for August respectively. The higher value of standard deviations of 1850, 950 and 1115MJ·mm/ha-hr-month for June, July, and August were observed respectively representing that the erosivity index of individual space widely deviated from the mean monthly erosivity index. Thus spatial erosivity index is suggested to be used over average annual R factor values to calculate soil loss using USLE. Furthermore, the WERM-S module can be a very useful tool to automatically calculate the spatially distributed rainfall erosivity index from 10-min interval RADAR rainfall data.