Impacts of nutrient management and decrease in paddy field area on ground water nitrate concentration: a case study at the Nasunogahara alluvial fan, Tochigi Prefecture, Japan

摘要

In order to help evaluate the trends in the NO_3-N concentration in groundwater with a view to preventing further degradation in water quality in the future, a distributed groundwater quality model was constructed for the Nasunogahara basin. The best fit for the groundwater table elevations by the flow component of the model was achieved with average mean absolute errors (MAEs) of 0.92 m for the calibration period and 0.83 m for the validation period. Moreover, the best fit for the NO_3-N concentration by the water quality component was achieved with average mean relative errors (MREs) of 29.8% for the calibration period and 30.3% for the validation period. After developing a robust model, various change scenarios were tested; specifically, the effects of effluent load control and a decrease in paddy field area on the NO_3-N concentration in groundwater were predicted. The most intensively farmed area contributed about 40% of the total effluent load because of livestock farming in the basin. When the effluent load from this area was decreased by 50%, the average NO_3-N concentrations at sites S1, S2 and S3 were reduced by about 15%; however, the average concentrations at S4 and S5 were reduced by only 1%. Furthermore, when the total effluent load from the concentrated livestock area was removed completely, the average groundwater NO_3-N concentrations at S1, S2 and S3 were reduced by about 30% as compared with the original calculated results. In contrast, decreasing the area of the paddy fields in the basin did not greatly influence the groundwater NO_3-N concentration. In the case of a 70% reduction in paddy field area, average NO_3-N concentrations increased by about 7% at S1, S2 and S3.