The present study aimed at further improving the process-based WEP (Water and Energy Processes) hydrologic model, by coupling a soil erosion-transport model to introduce particle-bound pollutant component. The WEP model, initially developed for the extensive analysis of water and energy budgets at catchment scale, was later enhanced by incorporating material transport component (nitrogen and phosphorus; N and P), targeting integrated river basin management and decision support. The model involved a discharge-based process to simulate both N and P, and was validated using data in Yata River Basin, Japan, considering land use, plant acreage, fertilizer loading, plant nutrient uptake and crop harvest in respective administrative units. Although the model could adequately reproduce the dissolved N (DN) component as per the measured river N concentrations at selected observation points, particulate N and P (PN, PP), and dissolved P (DP) components were not satisfactorily simulated. In the present study, an attempt was made to incorporate PN and PP transport as soil absorbed constituents, by introducing a process-based sediment erosion, transport, deposition and associated pollutant load simulation procedures. The present modeling results indicate that PN and PP loads were better correlated with suspended solids (SS) in the stream and the model forecasting capabilities are noticeably enhanced, as verified based on the results obtained for simulation years 2001-2002 using pre-collected data (SS, PN and PP), signifying major pathways of nutrient losses in the basin.

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