Groundwater is an important part of the global ecosystem. It strongly influences ecosystems in arid and semi-arid regions of northwestern China and is the primary source of water for human use in northern China. This paper investigates a typical expansive groundwater basin- the lower reaches of the Liaohe River Plain- as a study area, which continues to experience rapid economic growth along with rapid development of the region’s groundwater resources. The groundwater has been and is being seriously depleted. Some hydrogeological problems in the system are becoming apparent and the area is experiencing significant water shortages. The ecological health of groundwater systems has become a serious issue in ecology and is receiving considerable attention from the general public.
In consideration of the evaporation characteristics in this area, the requirements needed to maintain a healthy state of the water table for some ecosystems such as natural grasslands, natural wetlands and rivers are demonstrated; then an ecologically appropriate level of the groundwater table was determined based on hydrogeology and ecology. An ecological water table can be defined as the range of water table depths that can maintain a water supply for both human uses and for normal growth of natural vegetation. For example, in natural grassland the appropriate depth of an ecological water table in spring, summer and autumn is 3-4m, 2-3m and 3-4m, respectively; and in winter, the appropriate depth of an ecological water table only needs to meet the needs of the hydrogeological limit. As for natural wetlands, the depth of the ecological water table in spring, summer, autumn and winter is 0.5-0.8m, 0.3-0.5m, 0.5-0.8m and 1.0m, respectively. Based on a comparison of the ecological water table and the actual water table, Golden Surfer software was applied to calculate the ecological water demand for the restoration of the groundwater system on a monthly scale.
This paper adopts the guarantee rate method to delimit five levels of ecological water demand for the restoration of groundwater systems, which have the guarantee rates of 90%, 80%, 70%, 60% and 50%. Calculating the probability density of ecological water demand using the 12 datasets applied here is impossible using a traditional statistic model, so the information diffusion technique was applied; with this technique, each data point is regarded as a set of data. This technique has been proven to be successful and is widely used. Then, the ecological water demand under different guarantee rates can be obtained. Under each guarantee rate, the ecological water demand can be divided into five grades, which are 100%, 80%, 60%, 40% and 20% of the ecological water demand corresponding to the related guarantee rates. The results show that with a decrease of guarantee rate or an increase of grade, the ecological water demand increased.