A GIS-based Upscaling Estimation of Nutrient Runoff Losses from Rice Paddy Fields to a Regional Level

Nutrient runoff losses from cropping fields can lead to nonpoint source pollution; however, the level of nutrient export is difficult to evaluate, particularly at the regional scale. This study aimed to establish a novel yet simple approach for estimating total nitrogen (TN) and total phosphorus (TP) runoff losses from regional paddy fields. In this approach, temporal changes of nutrient concentrations in floodwater were coupled with runoff-processing functions in rice ( L.) fields to calculate nutrient runoff losses for three site-specific field experiments. Validation experiments verified the accuracy of this method. The geographic information system technique was used to upscale and visualize the TN and TP runoff losses from field to regional scales. The results indicated that nutrient runoff losses had significant spatio-temporal variation characteristics during rice seasons, which were positively related to fertilizer rate and precipitation. The average runoff losses over five study seasons were 20.21 kg N ha for TN and 0.76 kg P ha for TP. Scenario analysis showed that TN and TP losses dropped by 7.64 and 3.0%, respectively, for each 10% reduction of fertilizer input. For alternate wetting and drying water management, the corresponding reduction ratio was 24.7 and 14.0% respectively. Our results suggest that, although both water and fertilizer management can mitigate nutrient runoff losses, the former is significantly more effective.