Quantifying the effects of climate variability, direct and indirect land use change, and human activities on runoff

Abstract

Much attention has been recently focused on the direct hydrological effects (DHEs, such as runoff yield and concentration) of climate variability and land use/cover change (LUCC); however, the influence of LUCC on regional climate change and corresponding runoff change (indirect hydrological effect, IHE) have rarely been assessed quantitatively. This study employed the Mann-Kendall test and ensemble empirical mode decomposition (EEMD) method to analyze linear and nonlinear trends in potential evapotranspiration (PET), precipitation and runoff and their correlation in typical basin (Yihe River) in eastern China during 1951–2013. This study presents a framework to quantify runoff change in a detection and attribution study based on the Soil and Water Assessment Tool (SWAT) and Weather Research and Forecasting (WRF) model, and it quantifies the effects of climate variability, DHE and IHE of LUCC (1990–2010) and other human activities on runoff. The results indicate that (1) the annual PET in the Yihe River basin had an increasing trend, with a rate of 6.3 mm/decade, while the precipitation and runoff during the period 1951–2013 exhibited decreasing trends, at rates of 26.6 mm/decade (a = 0.05) and 0.16 × 108 m3/decade, respectively. (2) The runoff, PET and precipitation had similar periodic at inter-annual scales (approximately 3 years and 6 years) and inter-decadal scales (approximately 13–16 years). The contribution rate of runoff to total change from the inter-annual scale was the largest, reaching 58%, and those from the inter-decadal scale and multi-decadal scale were 6% and 11%, respectively. (3) The results also revealed that climate variability was likely to be the principle cause of an obvious decrease in runoff, and the contribution rate was as high as 90% before the 1980s while declining to 66.3% in 1990–1999. Furthermore, DHE, IHE and other human activities were responsible for 18.5%, 9.2% and 6.0%, respectively, of the runoff change in the 1990s. It is worth noting that the percentage of downward runoff accounted for 35.8% and 21.3% from DHE and IHE, respectively, during 2000–2013, which implied that human activities have gradually become the dominant factors affecting runoff change, with a contribution of 77.1%.