Dependence of riverine total phosphorus retention and fluxes on hydrology and river size at river network scale

Current basin-scale patterns in riverine total-phosphorus (TP) retention and flux for predicting water quality remain unclear, when considering TP as a key water quality indicator. We modeled TP retention and fluxes from the largest Changjiang River network in China based on high-frequency monitoring data of TP concentrations at 55 monitoring stations during Jan.1, 2021-Dec.31, 2022. We emphasized basin-scale control of the TP loss rate (also called the first-order rate constant) in response to variations in discharge and total suspended solids (TSS) under climate change. We found that the TP loss rates ranged from 0.008 h−1 to 0.032 h−1 and declined with water discharge but increased with the TSS content. The ratio of TP retention is negatively related to Strahler river orders, and was ∼ 0.55 for the streams with low orders (1–3) and ∼ 0.25 for the highest order river (8). TP concentrations at 55 stations ranged from 0.008 ∼ 0.145 mg·L-1 for annual average and 0.03 ∼ 4.89 mg·L-1 for daily maximum. TP fluxes demonstrated significant spatial pattern from the source area to the estuary with an increasing trend, with the highest flux of 64150 t·year−1 at Datong. We also found the urban disturbance index (UDI) was significantly positively correlated with the TP input load and output flux (input: r = 0.78; output: r = 0.62), suggesting that basin-scale urbanization, together with hydrology and climate change, controls the river TP concentration and retention in the entire Changjiang River network. Our results can help to understand TP retention process, and are useful for TP water quality planning and management.