Relating rainfall, runoff, and sediment to phosphorus loss in northern rocky mountainous area of China

As a notoriously ecologically fragile mountainous region, the northern rocky mountainous area of China is experiencing serious phosphorus loss due to its numerous sloping cultivated lands and abundant rainfall. Therefore, determining the relationships between rainfall, runoff, sediment, and the different components of phosphorus loss is essential for developing effective measures to control phosphorus loss. To address this issue, this study aimed to investigate the relative importance of rainfall, runoff, and sediment factors in the loss of different fractions of phosphorus using 32 natural rainfall events in a typical watershed in the northern rocky mountainous area of China. Because of the high interdependence among these influencing factors, a partial least square regression (PLSR) model was employed to quantify the relationship between phosphorus loss and its 15 influencing variables. First-order controls were identified by calculating the variable importance of projections (VIP). The results indicated that dissolved inorganic phosphorus (DIP) losses were the highest under heavy rain conditions, whereas dissolved phosphorus (DP) losses peaked during torrential rain. The trends in particulate phosphorus (PP) and total phosphorus (TP) were consistent, with large fluctuations in concentration under heavy rainfall conditions and an increase in concentration 30 min after rainfall under torrential rainfall conditions. Rainfall, runoff, and sediment characteristics exerted a substantial influence on the different components of phosphorus loss (93.33 %, 91.73 %, 94.2 %, and 95.3 % of DIP, DP, PP, and TP loss variability, respectively). From the VIP values, sediment, runoff, rainfall erosivity, rainfall kinetic energy, and 60 min maximum rainfall kinetic energy were the main first-order factors controlling the loss of the four phosphorus fractions. The PLSR method eliminated the interdependence among variables and facilitated the determination of factors influencing phosphorus loss. This study provides scientific support and decision-making references for formulating effective water resource and phosphorus pollution management strategies.