Topographic Modelling of Soil Wetness in a River Basin using Digital Elevation Models

Abstract

Soil wetness or soil moisture, highly variable in both time and space, has a major influence on a range of hydrological processes including flooding, erosion, solute transport and land—atmosphere interactions, as well as a range of geographic and pedogenic processes. Topography greatly affects the spatial distribution of soil moisture, and groundwater flow often follows surface topography. The topographic wetness index (TWI) is now widely used for predicting the spatial distribution of wetness conditions (Le. the position of shallow groundwater tables and soil moisture) and is usually calculated from digital elevation model (DEM). The resolution and information content of a DEM has a great influence on the computed topographic indices. Hence, in the present study, the topographic wetness index was estimated for the Puma River Basin in Akola and Amravati districts of Maharashtra using ASTER (30 m resolution) and SRTM (90 m resolution) DEMs, as the topography of the basin is well suited for this type of study. The specific catchment area (SCA), A, has the highest variation in the maximum values of SCA, i.e., 15109.0 and 2044.8 m2 in for SRTM and ASTER DEMs, respectively. This variation in SCA is due to the cell size and flow accumulation values. Similarly, there was significant difference in slope derived ASTER DEM (slope: <1 — 12.50%) and SRTM DEM (slope: <1 —2.33%). However, there was significant difference only in the minimum TWI values derived from ASTER DEM (minimum TWI: 2.59) and SRTM DEM (minimum TWI: 4.45). The non-significant difference was observed in the maximum TWI values (maximum TWI values derived from ASTER: 24.58 and SRTM: 24.60). The difference in the minimum TWI values may be attributed to the difference in resolution of the DEMs used. The information on TWIs, thus generated, will be useful in predicting the spatial distribution of soil moisture content, assessment of soil erosion and understanding the hydrological processes in a river basin.