| The distributed-parameter Water and Energy transfer Process (WEP) hydrologic model was further improved incorporating a process-based biogeochemical sub-model to simulate basinwide nitrogen (N) and phosphorous (P) dynamics. The enhanced model was applied to the Yata River Basin in Ibaragi Prefecture, Japan, to evaluate the detrimental effects of fertiliser N and P surplus loading on stream and subsurface water quality. According to the nutrient budget analyses in the watershed, out of the total fertiliser N and P input loadings of 998 ton (with a rate of application of 12-180 kg ha-1 yr-1, in each sub unit) and 456 ton (5-42 kg ha-1 yr-1), only 64% and 52%, respectively, were absorbed by crops and removed via harvesting in 2001/2002. The total amount of dissolved N and P loading transported to the river network through runoff and leaching processes were as high as 14% and 9%, implying major pathways of N and P losses and resultant water quality degradation in the ecosystem. |
