为量化气候和土地利用变化对水文过程的影响，引入RHESSys模型对退耕还林（草）工程实施前后延河流域水文要素（蒸散、径流和土壤水）进行模拟，并基于不同模拟情景量化土地利用变化和气候变化对不同水文要素的影响。结果表明：退耕还林（草）工程实施以后，延河流域蒸散发呈现显著上升趋势，径流和土壤水呈现下降趋势，但趋势并不显著。与基准期（1990—1999年）相比，P2时期（2000—2009年）人类活动引起的土地利用变化对蒸散、径流和土壤水变化的贡献率分别为82.66%，-51.87%和-55.13%，随着退耕还林工程的继续实施，P3时期（2010—2019年）贡献率下降到57.64%、-51.63%和-52.54%。由此可见，退耕还林（草）工程实施后，土地利用变化成为影响流域水文过程的主导因素。该结果对于黄土高原退耕还林（草）工程的实施前景和可持续发展管理有重要意义。

Since the last century, the global water cycle processes have been constantly changing, and it is generally believed that the main factors causing this change are climate change and human activities. The impact of this change on the fragile ecological environment of the Loess Plateau is more prominent. In order to improve the ecological environment of the Loess Plateau, the government has implemented a series of ecological construction with planting trees and grasses as the main measures since the last century, especially the implementation of the Grain for Green project, which has increased the vegetation cover of the region by about 25%, the underlying surface conditions of the basin have changed significantly, and under the joint influence of climate change, the mechanism and characteristics of runoff have changed significantly, the uncertainty of hydrological processes has increased significantly. Therefore, distinguishing between the effects of climate change and human activities on hydrological processes is significant importance to understand hydrological changes and to achieve sustainable water resources management in the future. At present, there are three main types of methods to distinguish the impacts of human activities and climate change on hydrological processes, which are the comparative watershed, the empirical equation and the model simulation. The first two-methods have some limitations (high similarity requirement between two watersheds and difficulty in elaborating the interaction mechanism between hydrological processes and influencing factors), the model simulation method, which analyzes the effects of changing environments on hydrological processes through hydrological models driven by remote sensing data, has been widely used worldwide. Based on this, this study selected the typical basin of the Grain for Green project — the Yanhe River Basin as the study area. In view of the complex underlying surface conditions of the Yanhe River Basin, the ecological hydrological model RHESSys that considering the dynamic growth of vegetation was used to simulate the hydrological elements (evapotranspiration, runoff and soil moisture) of the Yanhe River Basin before and after Grain for Green project, and the impacts of human activities and climate change on different hydrological elements were quantified based on different simulation scenarios. The results showed that from 1990-2019, the temperature and precipitation in the Yanhe River basin showed an increasing trend, with an increase rate of 0.01°C/a and 4.44mm/a, respectively, and gradually increased from northwest to southeast in space. The land use characteristics of the basin were the transfer of farmland to other types of land. Considering other land use flows, the final area of farmland decreases by 899.31 km2, while the area of grassland and forestland increases by 535.80 km2 and 214.32 km2, respectively. In terms of changes in hydrological elements, the actual evapotranspiration in the basin showed a significant upward trend with an increasing rate of 6.13mm/a after the implementation of the Grain for Green project, while the runoff and soil water showed a downward trend, but this trend was not significant. Compared with the base period (1990-1999), the contribution rate of land use change caused by human activities to evapotranspiration, runoff and soil water change in P2 period (2000-2009) was 82.66%, -51.87% and 55.13%, respectively. With the continuous implementation of the Grain for Green project, the contribution rate of land use change decreased to 57.64%, -51.63% and -52.54% in P3 period (2010-2019). The result showed that land use change was the main factor of hydrological factors change after the Grain for Green project. This study is critical to the implementation prospect and sustainable development management of the Grain for Green project on the Loess Plateau, and can provide theoretical support for water resources planning and management in the Loess Plateau.

“十四五”国家重点研发计划课题（2021YFD1900701）；流域水循环模拟与调控国家重点实验室开放研究基金（IWHR-SKL-KF202101）