27.-Occurrences of droughts and floods associated with rainfall pattern of India

Abstract

A study on runoff and soil loss was conducted (1986-99) to study the effect of rainfall, degree and length of slope in vertisols of Bellary (Karnataka) on runoff, soil loss and to determine optimum length of runoff plots. Runoff and soil losses as influenced by rainfall, degree and length of slopes were assessed from plots of four lengths i.e., 22.13, 44.26, 66.39 and 88.52 m having uniform width of 1.83 m at three slopes namely 0.5%, 1.0% and 2.0%. The smallest length (22.13m) has produced maximum runoff and soil loss. As the length of the slope increases runoff percentage as well as soil loss per unit area have decreased considerably. Runoff and soil loss increased with increased slope of percentage. The runoff percentage and soil loss were found to be negatively correlated with length while it was positively correlated and linear with degree of slope. The runoff and soil losses were higher up to 50m length and thereafter the runoff and soil losses were almost constant in all the slope groups. Thus optimum length of runoff plot was found to be 50m. Analysis of hydrographs obtained during 1998 and 1999 have indicated that there is a low peak and less volume in case of 66.39 m plot in comparison to 22.13m plot for all three slopes. The rising time or time to peak from initiation of runoff is less in the case of shorter length than longer lengths. It may be attributed to less time of concentration and opportunity time for infiltration with plots of shorter length when compared to longer runoff plots. This may be useful in planning and designing urban drainage system which usually having small catchments.