Climate and land-use change are the two main driving forces that affect watershed hydrological processes. Separately assessing their impacts on hydrology is important for land-use planning and water resource management. In this research, the SWAT (Soil and Water Assessment Tool) and statistical methods were applied to evaluate the effects of climate and land-use change on surface hydrology in the hilly-gully region of the Loess Plateau. The results showed that surface runoff and soil water presented a downward tendency, while evapotranspiration (ET) presented an upward tendency in the Yanhe watershed from 1982 to 2012. Climate is one the dominant factors that influence surface runoff, especially in flooding periods. The average contribution rate of surface runoff on stream flow accounted for 55%, of which the flooding period accounted for 40%. The runoff coefficient declined by 0.21 after 2002 with the land-use change of cropland transformed to grassland and forestland. The soil water exhibited great fluctuation along the Yanhe watershed. In the upstream region, the land-use was the driving force to decline soil water, which reduced the soil water by 51%. Along the spatial distribution, it converted from land-use change to climate variability from northwest to southeast. The ET was more sensitive to land-use change than climate variability in all sub-basins, and increased by 209% with vegetation restoration. To prevent the ecosystem degradation and maintain the inherent ecological functions of rivers, quantitative assessment the influence of climate variability and land-use change on hydrology is of great importance. Such evaluations can provide insight into the extent of land use/cover change on regional water balance and develop appropriate watershed management strategies on the Loess Plateau.

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Extreme rainfall is an important driver of soil erosion and land damage. The Gully Land Consolidation program (GLCP) was first launched in 2011 as a major land reclamation practice to increase farmland in the Loess Plateau of China. Studying the impact of artificial projects on hydrology can help humans to respond to the various water issues, but the assessment of the effects of the GLCP on extreme rainfallinduced water runoff at watershed scale is currently lacking. Our study used the soil and water assessment tool (SWAT) to evaluate the influence of the GLCP at different locations and areas on water runoff under extreme rainfall events in the Yanhe watershed. Results showed that: (1) the GLCP can improve the interception of surface runoff, with interception efficiency in downstream of the watershed approximately twice that at midstream and entire watershed as well as seven-times that at the upstream; (2) when GLCP measures are evenly distributed in a watershed, as the area of GLCP increases from 76.40 km 2 (1% of watershed area) to 382.01 km 2 (5%), the interception of surface runoff increases by 0.77 mm; (3) and the GLCP can increase soil infiltration and groundwater recharge. This research is expected to provide insights into the optimized layout of the GLCP at watershed scale. Correspondingly, policymakers can refer to this information in developing policies on the sustainable use of land.

<p><strong>Abstract: </strong>Climate and land-use change are the two main driving forces that affect watershed hydrological processes. Separately assessing the impacts of climate and land use change on hydrology is important for water resource management. In this research, the SWAT (Soil and Water Assessment Tool) and statistical methods were employed to evaluate the effects of climate and land-use change on surface hydrology in the hilly-gully region of the Loess Plateau. The results showed that both the temperature and potential evapotranspiration (PET) had significant upward trends (p < 0.05), while the precipitation presented a slightly downward trend in the Yanhe watershed during 1982-2012. The contribution of precipitation to streamflow is concentrated in the flooding periods (from July to September), the average contribution rate of surface runoff on stream flow accounted for 55%, of which the flooding period accounted for 40%. With the 2.17% of slope farmland transformed to the forest and grassland, the average runoff coefficient decreased from 0.36 to 0.15 during 1982-2012. The impact of land use change on soil water content is mainly happened in the upstream stream, while the dominated factor converted to climate from northwest to southeast in the Yanhe watershed. The Evapotranspiration was more sensitive to land-use change than climate variability in all sub-basins, and increased by 209% with vegetation restoration in the Yanhe watershed. Therefore, the impacts of climate variation and land use change on surface hydrological processes were heterogeneity in different geographical regions, climate is the main factor to influence the runoff, while the land use is the dominated factor to evapotranspiration. The quantitative assessment the influence of climate variability and land-use change on hydrology can provide insight into the extent of land use/cover change on regional water balance, and develop appropriate watershed management strategies on the Loess Plateau.</p><p><strong>Keywords: </strong>climate shift, human activities, hydrological processes, SWAT, the Loess Plateau</p><p><strong>Funding:</strong> This study was funded by the National Natural Science Foundation of China (No. 41877078, 41371276), Key research and development project of Shaanxi Province (2020ZDLSF06-03-01), National Key Research and Development Program of China (No. 2017YFC0504703) and Knowledge Innovation Program of the Chinese Academy of Sciences (No. A315021615).</p><p> </p>