Spatial and temporal estimation of runoff in a semi-arid microwatershed of Southern India

Abstract

In a semi-arid microwatershed of Warangal district in Southern India, daily runoff was estimated spatially using Soil Conservation Service (SCS)-curve number (CN) method coupled with GIS. The groundwater status in this region is over-exploited, and precise estimation of runoff is very essential to plan interventions for this ungauged microwatershed. Rainfall is the most important factor governing runoff, and 75.8 % of the daily rainfall and 92.1 % of the rainy days which occurred were below 25 mm/day. The declines in rainfall and rainy days observed in recent years were 9.8 and 8.4 %, respectively. The surface runoff estimated from crop land for a period of 57 years varied from 0 to 365 mm with a mean annual runoff of 103.7 mm or 14.1 % of the mean annual rainfall. The mean annual runoff showed a significant reduction from 108.7 to 82.9 mm in recent years. The decadal variation of annual runoff from crop land over the years varied from 49.2 to 89.0 % which showed the caution needed while planning watershed management works in this microwatershed. Among the four land use land cover conditions prevailing in the area, the higher runoff (20 % of the mean annual rainfall) was observed from current fallow in clayey soil and lower runoff of 8.7 % from crop land in loamy soil due to the increased canopy coverage. The drought years which occurred during recent years (1991–2007) in crop land have increased by 3.5 %, normal years have increased by 15.6 %, and the above normal years have decreased by 19.1 %. This methodology can be adopted for estimating the runoff potential from similar ungauged watersheds with deficient data. It is concluded that in order to ensure longterm and sustainable groundwater utilization in the region, proper estimation of runoff and implementation of suitable water harvesting measures are the need of the hour.