Predicting non-point source of pollution in Maithon reservoir using a semi-distributed hydrological model

Non-point source (NPS) pollution has been emerged as a major cause for reduced water quality of a lake due to increased human interference and disturbances in the natural condition of the surrounding catchment. The impact is, even more, worsening in the monsoon season when there is increased surface runoff. In the present study, an attempt has been made to predict the seasonal (monsoon) NPS loading in terms of sediment, nitrogen, and phosphorous in Maithon reservoir using Soil and Water Assessment Tool (SWAT) hydrologic model. The SWAT model was initially calibrated using monthly runoff and sediment yield data of monsoon period for the year 1998–2005 using observed data of Rajdhanwar station followed by its validation for the observed monthly runoff and sediment data from Giridih and Santrabad for the same duration. The calibrated SWAT model was used to predict the sediment, total nitrogen, and phosphorous influx in the Maithon reservoir. It has been observed that average sediment yield from different micro-watersheds varies from 0.231 to 7.458 ton/ha, while average monthly nitrogen and phosphorous yields vary from 0.224 to 1.377 kg/ha and 0.073 to 0.363 kg/ha, respectively, during the monsoon period. On the other hand, the net monthly average sediment yield and total nitrogen and phosphorous yields in the reservoir were found to be 1.53 M ton, 1834.2 kg, 191.1 kg, respectively. The results indicate there is a substantial influx of nutrients and sediments into the Maithon reservoir. The study not only provides insights on the potential NPS pollutant loading in the reservoir but also enables to identify the hotspot of NPS pollution where immediate mitigation measures have to be taken at priority basis.