Long-term response of runoff and sediment load to spatiotemporally varied rainfall in the Lhasa River basin, Tibetan Plateau

Fluvial runoff and sediment play vital roles in channel evolution, material cycling, water resource utilization and ecological environment. In the context of global climate change, the assessment of runoff and sediment in response to climate change is of great significance for water and soil conservation, especially in alpine regions such as the Tibetan Plateau with limited availability of long-term observed data. In this study, based on the available records for the period 1980–2018, the spatiotemporal variations in rainfall-related parameters (rainfall and rainfall erosivity (RE)), and their impacts on the runoff–sediment load (SL) process were investigated in the Lhasa River Basin (LRB) by cross-coherence analysis. In addition, the individual and combined effects of other regional environmental factors (e.g., temperature, snow water equivalent (SWE) and vegetation) on runoff and SL were quantitatively identified using partial least squares structural equation modeling (PLS-SEM). The results showed that the changes in rainfall-related parameters experienced an upward trend at a rate of 3.59 mm/decade (rainfall amount) and 46.05 MJ·mm·ha−1·h−1/decade (RE), respectively. The regions with the most obvious increase occurred in downstream of LRB. However, runoff displayed an insignificant decreasing trend, together with increasing water inputs from the wetter climate in the middle and lower reaches. The contrasting SL trends were found in the middle (significantly descending: 5.5 × 104 t/decade) and downstream reaches (slightly increasing: 19.27 × 104 t/decade). Rainfall-related parameters generally played a positive role in runoff and SL at all hydrological stations, and their action gradually weakened after 2005 despite still being the dominant factor. Other environmental variables, such as vegetation, did not limit runoff but did reduce the SL with an increase in vegetation coverage in the LRB, where large-area vegetation restoration projects were implemented and mainly distributed on the sandy land and floodplain of the lower reach of the basin. Snow, however, had slightly positive promoting effects on runoff and SL under the scenario of SWE reduction with increasing temperature. All above-mentioned variables jointly explained 73.9–76.7% and 39.2–52.4% of the total variability in runoff and SL, respectively. These findings provide valuable insights into future studies on water and soil conservation planning and ecological restoration in cold and high-altitude regions.