An integrated air-water modeling framework for nitrogen and phosphorus losses: a case study for the Lake Erhai basin

Human activities cause nitrogen (N) and phosphorus (P) losses to the air and water in plateau lake basins such as Lake Erhai. However, the spatial distribution of N and P losses to air and water, and their sources remains insufficiently understood, hindering effective pollution control. Here, we aim to better understand the impact of current N and P management practices in agricultural production and sewage systems on nutrient losses from land to air, rivers, and Lake Erhai in a spatially explicit way. We developed a modeling framework by integrating the substance flow analysis and MARINA-Lakes model, taking the Lake Erhai basin as a case study. Results show that seven out of 26 sub-basins are hotspots for nutrient losses to air (N) and/or water (N and P). For air, hotspots contribute to around 2900 tons, accounting for around 85% of agricultural N losses, of which two-thirds are from livestock production. For water, hotspots account for over 80% of the total nutrient exports. Agricultural land covers only 24% of the total drainage area but contributes to nearly 75% of the total river exports of N (2200 tons) and P (112 tons). Less than 50% of the total dissolved N (45%) and total dissolved P (30%) in rivers reach the lake due to in-stream retention. Hotspots based on nutrient losses (tons year -1 ) are in the northern and eastern regions, while the high emission intensity areas (kg km -2 year -1 ) are in the western regions. This calls for source-specific and region-specific reductions to manage the nutrient losses in the Lake Erhai basin. • Seven sub-basins account for over 85% of nitrogen losses to the air in Lake Erhai basin, largely from livestock production. • Seven sub-basins account for over 80% of river exports of nutrients to Lake Erhai. • Agricultural land covers 24% but contributes to around 75% of nutrients exported to Lake Erhai. • In-stream retention removes more than 50% of nutrients before it reaches Lake Erhai. • Source- and region-specific reductions are needed for air and water emission control.