The Changing Relationship Between Rainfall and Surface Runoff on the Loess Plateau, China

Lack of hydrological data of high spatiotemporal resolution has been a major challenge for understanding the rainfall-runoff relationship on the Loess Plateau of China. In this study, we examined this relationship across 82 watersheds on the plateau for two time periods: P1 (1971–1987) and P2 (2008–2016). We found linear relationships between the annual rainfall series and annual surface runoff series in P1 that were stronger in the southeast and that declined toward the northwest, but this spatial pattern was slightly disrupted during P2. The mean annual runoff coefficient ranged from 0.056 to 0.126 for the 25th and 75th percentiles, respectively, during the P1 period, but the corresponding values were smaller in P2 (0.035 and 0.088, respectively). In total, the runoff coefficient decreased for 87% of the watersheds, with an average reduction of 0.033. For most watersheds of the plateau, we found a stable threshold value for the monthly runoff coefficient when monthly rainfall reached a certain value (i.e., a critical point). Between P1 and P2, the threshold value of the runoff coefficient decreased for 57 watersheds as a result of soil conservation measures. The rainfall critical point represents the minimum monthly rainfall to maintain the stable relationship between monthly infiltration and monthly rainfall in the process of runoff production. This value became greater on the middle regions of the plateau during P2 (increasing by 28% on average). Check dams, vegetation, and land use all have profoundly influenced the relationship between rainfall and surface runoff in this region.