In the studies of runoff prediction, the Variable Source Area (VSA) methods are becomingincreasingly more pronounced especially for the humid areas, such as the North-Easternareas of India. Such methods of physically-distributed modelling system are greatly dependenton land surface characteristics (slope gradient, hydrological soil types and land use/ land coverpatterns) that are stable spatially, and on the temporarily fluctuating pattern of precipitationintensity. Considering grid cells of 10 m*10 m size in a micro-areal watershed of 103 ha calledUm-U-Lah that is situated in the extremely humid area of Cherrapunji, having an average slopevarying from 4 to 30 percent, and with sandy-loam porous soils with grass land dominating vegetalcover, the mapping of the saturated area adopting the Topographic Wetness Index (TWI)was made to measure the location and extent of saturated areas, which contract and expand inproportion to the changes in intensity of effective rainfall. After calibrating the Curve-Numberbased VSA model at effective available soil storage of 5.88 mm (AMC-III Soil conditions) and atInitial Abstraction of 1.746 mm, the rainfall simulation were conducted. It is found that increasingdepth of effective rainfall quickly expands the saturated areas over the flat lands of hill-topsand slopes in the watershed. At 8.25 mm of effective rainfall depth, about 85.03 percent areas ofwatershed becomes saturated directly contributing to runoff.
Surendra Singh, epartment of Geography, North-Eastern Hill University, Shillong-793014, India
Jones Hiambok, Department of Geography, North-Eastern Hill University, Shillong 793 022, India