Quantitative simulation of hydrological elements based on RHESSys model in Yanhe River basin
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Abstract:
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 the effects between 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 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, the typical basin of the Grain for Green project - the Yanhe River basin was selected 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 to 2019, the temperature and precipitation in the Yanhe River basin showed an increasing trend, with an increase rate of 0.01 ℃/a and 4.44 mm/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). It can be seen that the contribution of land use change to hydrological elements in the Yanhe River basin decreased after the implementation of the Grain for Green Project, but land use change was still the main influencing factor of hydrological elements. 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.
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