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Nitrogen and phosphorus loads from agricultural non-point sources based on grid-based geographic information systems (GIS)

The overall objective of this study is to quantify the contributions of nitrogen and phosphorus loads in surface water and identify the critical source areas in a medium-sized scale watershed in the southeast of China. The method of integrating grid-based Geographic Information Systems with empirical models is applied. The curve numbers method is used to estimate surface runoff. The Universal Soil Loss Equation is employed to predict the soil loss in the Jiulong River watershed. Empirical methods for calculating nitrogen and phosphorus discharge are further utilized to calculate loads based on the spatial analysis of a grid system. Spatial distribution of nitrogen and phosphorus loads in terms of sub-watersheds, land-use type and counties/districts in the Jiulong River watershed is represented. Various contributions and sources of nitrogen and phosphorus loads in surface water for each county or district in the Jiulong River watershed are calculated and evaluated. Study results show that excessive fertilizer use and livestock breeding contribute significant nitrogen and phosphorus loads for many counties. The contribution of livestock breeding to these nutrient loads for the Zhangzhou and Xinluo districts were 40 to 47% and 51 to 62%, respectively. Critical source areas and major contributors of non-point source nutrients are identified. The quantitative data of nutrient loads from agricultural non-point sources in the Jiulong River watershed estimated from the study could provide the scientific basis for management and control of nutrients and pollution.