Identification and prioritization of critical sub basin using SWAT model in lower Son River basin, India

River basins are important geo-eco-hydrological units. Identifying problems in it provides the chance of delineating critical sub-areas which when managed in time can benefit the whole basin. The Lower Son river basin faces problems of pollution and soil erosion. The present study aims to locate critical sub-basins in the study area based on the output of Soil and Water Assessment Tool model. Analytic Hierarchy Process is used for recognizing the sub-basin needing immediate attention. The severity of the problem is calculated using six physical parameters on daily basis from 1991 to 2021 i.e. sedimentation, soil water content, surface run off, nitrate concentration, stream flow and evapotranspiration. In addition to this, land use/land cover map, soil map, slope map along with climatic parameters were prepared for set up of SWAT model. The study area spans in 4,839,797.7 ha and has been divided into 9 sub-basins. Results show that (1) sub basin 1 and 2 are severely degraded followed by sub basin 5 and 8. (2) Out of the 9 sub basins, two sub basins fall under very low degradation, one under low degradation, two in moderate, one in high and two in very high category. (3) The river basin's health can be substantially improved if sub basin 1 and 2 are managed properly.