Understanding the main causes of runoffchange by hydrological modeling: A case study in Luanhe river basin, North China

Abstract

In the traditional point of view, if there is a significant decreasing trend for a runofftime series, while no significant trend for a precipitation series is present, then an unreliable conclusion will be made that the land surface change is the main contributor to the runoffchange. To test it, we selected four sub-watersheds in the Luanhe river basin as the study areas where land use has changed severely. We first detected the long-term rainfall and runofftrend by the Mann-Kendall test, Sen's slope, and the moving average method, and found that the runoffhad a decreasing trend at the 0.05 significance level, while the rainfall had no significant trend in all sub-watersheds. Then an orderly cluster analysis and moving T test method were used to detect the change point of the runoffseries. We quantified the contributions of the land surface change and climate variability based on Soil and Water Assessment Tool (SWAT), and the contribution of climate variability accounted for more than 50%, which implies that climate change is the main factor of runoffdecrease in the study areas. To further test this, a trend analysis of a reconstructed annual runofftime series under undisturbed conditions has been done. The results showed that in some sub-watersheds, although rainfall series had no significant decreasing trend, the runoffseries had significant downward trend. This can be explained by the nonlinear relationship between rainfall and runoff. This study came to a different conclusion from the common view, which observes that runoffdecrease is mainly caused by land surface change if rainfall series lacks a significantly decreasing trend.