Subsurface Drainage and Storage Properties in the Western Ghats – A Study in the Basin of Netravati

One of the reasons for the highly subdued response of the catchments, even to rainfalls of very high daily intensity, in the wet tropical areas of the Western Ghats in Karnataka is their soil mantle, which is very thick, well drained and very stable. An attempt is made in this work to understand the factors leading to such features, through a field study of the hydrological properties of the soils in the region. Cores extracted at road cuttings, from three different depths, at ten sampling sites scattered over the upland areas of the Kumaradhara basin, selected with due regard for the type of land covers commonly associated with hydrology, have been subjected to laboratory experiments to determine the soil texture, the porosity and the hydraulic conductivities. Ring infiltration tests have also been conducted on the field to cross verify the laboratory results. Storage capacity of the soils is estimated and compared with the rainfall magnitude in two small catchments in the area. The results indicate that the surface soils are sandy, while even at great depths, soils are Silty sands or Sandy silts, with high porosities and low drainage rates. The results are interpreted to mean that preferential pathways of macro-pores and soil piping would be an important feature in these areas. It is emphasised that more elaborate studies need to be taken up in order to understand the subsurface runoff processes in this hydrologically unique tropical region.