Water Budget in Sub-Watershed Scale - A Case Study on A Representative Basin in Shilabati- Kangsabati Interfluve

Abstract

River basin is considered as a fundamental hydro-geomorphic unit, used for assessing water availability through constructing water budget for better management of water resource. Availability of water in a place may be considered as a function of a numbers of interacting factors as they vary spatially in a basin system. The present study area is small representative watershed of 20.61Km² situated on a large interfluve between mighty Silabati and Kangsabati. Estimation of rain input, infiltration, evapotranspiration and runoff from the basin unit applying field techniques and appropriate models seem to be important for preparing a water budget and rational management proposals for augmenting water supply. Daily rainfall data since 1992 collected from the surrounding climatic stations are analysed following Theisson Polygon. Assessing the duration of extreme droughts using log probability law reveals that a maximum length of 303 days of no rain at a stretch and may occur at a recurrence interval of 20 years at 5% probability. Infiltration is measured in the field following Goudie,’l990 and the result is used as the input of Horton's model (1939). The infiltration rate over the entire basin is estimated to be 4.396 inch / hr following Rawls et.al (1976) and Bedinent et. al. (2008). For the better estimation of runoff, the basin under study is again divided into four subwatersheds and SCS Curve Number technique of USDA is used. Runoff is corisidered as an out put of systematic interaction between Landuse and hydrologic soil condition. Annual runoff for each of the subwatersheds at the lower catchment is estimated to be 994.395 m³ 894.434 m³ 896.279 m³ and 895.152 m³ respectively. Evaportranspiration for each subwatershed are also estimated following Pennman (1948, 1956 and 1963), Jensen et.al. (1990) and Schwab et. al. (2002) and is estimated to be 31604328 m3. Monthly evaporation as well as runoff from every subwatershed is calculated to understand the seasonal availability of water. The mass curve technique is applied to determine the rational storage volume to ensure introduction of irrigation and double cropping in the water scarcity region. Rational management for addressing the water scarcity of the entire interfluve region may use these empirical data as an input to decision making system.