Effect of land use on water yield from small agricultural watersheds of western ghats

Abstract

Effect of deforestation and other land use changes brought about by human activities of hydrologic cycle continues to be of great concern. A study was conducted to assess the effect of land use on water yield from small agricultural watersheds of Western Ghats of Kerala.

Four small watersheds planted with cashew, rubber, coffee and tea were selected for the study. To get information regarding rainfall, temperature, humidity and daily evaporation; raingauges, thermometers and USWB class A Pan evaporimeters were installed in each watershed. The runoff which is of specific concern was measured using stage level recorders along with weirs and flumes. Infiltration measurements were done using double cylinder infiltrometers. Soil samples were collected from each watershed and were analysed for grain size distribution, soil pH and organic carbon content. Different geomorphological characteristics of the watersheds were also worked out.

Analysis of rainfall and runoff data indicated that nearly 50 per cent of the total rain fall leaves these watersheds as runoff except in the case of rubber watershed. The infiltration studies indicated that all these watersheds have high infiltration rates even after saturation, thereby absorbing even the most intense storms of the study period. From the hydrograph analysis of these watersheds it was observed that the hydrographs attain a sharp peak immediately as the rainfall and there is a baseflow which is folowing through the 3 watersheds, except in the case of rubber watershed. The rubber watershed is very small and lies adjacent to the Kuttiadi reservoir. The interflow from the rubber watershed was observed to join the reservoir avoiding the measuring channel. From the soil profile analysis of Western Ghat region it was observed that there is an impermeable clay layer lying below the laterite having an average thickness of 3.5 m located at 7 to 10.5 m below the ground surface.

The results of the study leads to the conclusion that the infiltrated rain water meets the impermeable layer and there it flows laterally through the soil. This lateral interflow reaches the valley portion of the watersheds where it saturates the soil. This saturated area acts like an impervious layer producing 100 per cent surface runoff and it is responsible for the sharp peak of hydrographs. Runoff is generated from these source areas and Hortanian overland flow is a rare phenomenon in these watersheds. Thus from the study it was concluded that land use has no significant effect on water yield from the selected small agricultural watersheds of Western Ghats of Kerala.