The phosphorus export coefficients variability of specific land-use and the threshold response relationship with watershed characteristics in a subtropical hilly region.

The magnitude of land-use phosphorus (P) export in subtropical hilly watersheds is subject to the collective influence of various watershed characteristics. However, the spatiotemporal dynamics of key watershed characteristics and their impacts on P exports remain unclear. Total phosphorus export coefficients (TPECs) can quantify the contribution of different land-use types to P exports. Here, we employed continuous observation data of typical subtropical hilly areas in the Laodao River watershed, China, from 2012 to 2019, as well as an enhanced export coefficient model combined with Bayesian statistical methods, to quantify the spatiotemporal variability and uncertainty of TPECs under various hydrological conditions in different land-use types (forest, cropland, and tea plantation). We then determined the key watershed characteristic factors influencing TPECs and threshold responses of TPECs to these factors. The three land-use types exhibited significant spatiotemporal differences in TPECs, i.e., under different hydrological regimes and within each catchment. Watershed characteristic factors explained 68.5-74.1 % and 33.3-50.4 % of TPEC variations under medium- and low-flow regimes, respectively, but only 1.9-3.8 % under the high-flow regime. Compared to topography and soil factors, landscape patterns had a higher individual impact on TPECs. Threshold response relationships existed between TPECs and key influencing factors under both medium- and low-flow regimes. Moreover, the thresholds corresponding to abrupt TPEC change points were comparatively consistent under medium- and low-flow regimes. These findings have practical applications for rapidly characterising critical P pollution source areas and formulating basin-scale land-use plans for water quality protection.