Optimizing land use systems of an agricultural watershed in China to meet ecological and economic requirements for future sustainability

The contradiction between economic development and water pollution is ever so obvious in most agricultural watersheds in China. The continuous popularization of commercial crops in hills, especially tea, has intensified water eutrophication, which has attracted close attention of the Chinese government. Under this circumstance, local authorities have proposed the returning of tea plantations to forests, which would distinctly inhibit the development of rural economics. Hence, we employed a calibrated SWAT model to determine the total nitrogen/total phosphorus (TN/TP) loads in the reservoir, nutrient contribution of each land use type, and landscape pattern in a representative agricultural watershed. Aimed at the synchronous development of the economy and environment, this research study investigated the correlation between landscape pattern and nutrient output to accordingly propose alternative management strategies based on the correlation and actual watershed situation. Correlation analysis suggested that the landscape pattern prominently affects nutrient output in farmlands, tea plantations, orchards, and grasslands, and that the impact varies with the land use types and landscape parameters. Furthermore, a stepwise multiple regression model revealed that the landscape pattern could effectively predict nutrient output in farmlands, tea plantations, orchards, and grasslands. The aggregation index (AI) (R2 = 0.996, p < 0.05) and contiguity index (CONTIG) (R2 = 0.963, p < 0.01) are the primary landscape drivers for the export of TN and TP in farmlands, respectively, and the percentage of landscape (PLAND) is the key driver for TN loss in tea plantations (R2 = 0.841, p < 0.05). The fractal dimension index (FRAC) is the pivotal driver both for TN (R2 = 0.969, p < 0.01) and TP (R2 = 0.845, p < 0.05) export from orchards. According to the actual watershed situation and the relationship between nutrient output and landscape pattern, six alternative management strategies were performed: planting tea along contour lines (S1), constructing filter strips downstream tea plantations and orchards (S2), changing the shape of orchards (S3), controlling the nutrient emissions of residential areas (S4), returning farmlands to forests (S5), and a combination of S1S5 (S6). All these measures are superior to the current government strategy (returning tea plantations around reservoirs to forests) in terms of both cutting down reservoir nutrient loads and reducing economic loss. Among them, S6 is the optimal strategy; using this, TN and TP loads are expected to be reduced by 51.64% and 58.96%, respectively, compared with those in 2019, and the economic loss will be far lower than that upon implementation of the current government strategy. This study has provided detailed suggestions for watersheds plagued by the contradiction between agricultural economic development and water pollution.