No abstract

The natural oases in the plain area of the northwest inland basin strongly depend on the groundwater depth. With the overexploitation and utilization of groundwater, natural oases are faced with the problems of serious degradation and rehabilitation. How to evaluate the degree of the degeneration crisis of groundwater ecological function has become one of the key scientific and technological problems to be solved. In this paper, the Shiyang River basin of Gansu Province was selected as a typical research area. The remote sensing interpretation, groundwater–soil ecology comprehensive investigation, and groundwater in situ monitoring were adopted to carry out the research. Based on the correlation analysis method of natural ecology and groundwater, the interactive relationship between the natural ecological environment and groundwater depth in different ecological types of the region were studied: (1) under the arid climate condition in northwest China, the relationships between the ecological situation and the groundwater depth in different ecological types of the region were obviously different, and as a result, the optimal or limit ecological water level of groundwater in different ecological types was also different; (2) in the natural wetland area, the suitable ecological water level of groundwater was between 0.5 m to 1.5 m, and the limit ecological water level was 8.0 m; in the natural vegetation area, the suitable ecological water level was between 3.0 m to 5.0 m, and the limit ecological water level was 10.0 m; and in the farmland area, the suitable ecological water level was between 2.0 m to 5.0 m, and the limit ecological water level was 2.0 m; (3) in order to effectively protect the natural ecology in different ecological types, a five-level early warning and control index system should be established for the ecological function degeneration crisis of groundwater. It may be beneficial to promote restoration and protection of the groundwater ecological function and natural ecology in the inland area of northwest China.

The natural vegetation in arid areas of northwest China is strongly dependent on the availability of groundwater. Significantly, capillary water plays an essential role in regulating the ecological groundwater level in the multilayered structure of the vadose zone. The soil-column test and field survey in the lower reaches of the Shiyang River Basin were conducted to investigate the influence of the multi-layered structure of the vadose zone on maintaining the ecological effect of groundwater. Based on the field survey, the results show that the depth of groundwater is 3.0 m, and the rising height of capillary water is 140 cm. In the soil-column test, the height of the wetting front of the column was 125 cm. During the water releasing test, the water held by the vadose zone was 182.54 mm, which would have maintained Haloxylon’s survival in a growing season. Therefore, the multi-layered structure of the vadose zone extends the ecological groundwater depth and consequently enhances the ecological function of groundwater. Importantly, with a lower groundwater level, the clay soil layer within the rising height range of the original capillary water would hold more water and maintain a higher water content for a certain period to supply surface vegetation.