Assessment of nitrogen and phosphorus contributions from different agricultural non-point sources to waterbody in watershed

Assessment of nitrogen and phosphorus contributions from different agricultural non-point sources (ANPS) to watershed waterbody is the key to preventing its pollution. However, the precise assessment remains challenging owing to the absence of reliable method to (1) allocate district ANPS data accurately to watershed accounting units of ANPS pollution, and (2) account each type of ANPS pollution load according to its consumption pathways. We propose a method in which (1) district statistical data for livestock and poultry breeding number, aquaculture output, population number and fertiliser input are allocated to accounting units using the area ratio of facility agricultural land, aquaculture ponds, residential land and each type agricultural land in accounting unit and that in its district, respectively; and (2) pollutant contributions from different ANPS in accounting units to watershed waterbody are quantified by coupling Output Coefficient Model with Soil and Water Assessment Tool according to their consumption pathways. The results of the Luoyang River Watershed revealed from 2010–2022 that (1) agricultural land inputs, livestock and poultry breeding, rural life and aquaculture average annually accounted for 76.6 % and 53.6 %, 16.8 % and 34.7 %, 4.3 % and 11.7 %, and 2.3 % and 0 % of the total TN and TP entering waterbody from ANPS, respectively, and (2) the average TN and TP amounts entering waterbody from ANPS in watershed upstream, midstream and downstream areas were 5.53 and 0.16, 8.67 and 0.41, and 9.54 and 0.49 kg ha−1 yr−1, respectively. The results obtained using our method show that (1) agricultural land inputs and livestock and poultry breeding were the main nitrogen and phosphorus contributors to watershed waterbody, and (2) the ANPS contribution from the midstream and downstream areas to watershed waterbody is greater than that from upstream area. Our method can be useful for precise assessment of nitrogen and phosphorus contributions from ANPS to watershed waterbody.