70 Study on Effect of the Yellow River Irrigation Water Volume Change on Groundwater Environment in Hetao Irrigation District in Inner Mongolia

With the big change that the irrigation water from Yellow River is facing sharp drop situation in Hetao irrigation area, the ecological environment which has shaped for a long time and has stabilized to some extent will be changed inevitably. In this paper, we researched the groundwater environment impact factors because of the irrigation water volume change, based on the typical experiment station —Longsheng in Hetao irrigation district. According this study, we found that the groundwater table is continuously declining during the crop growth period. It showed that crops need to consume part of groundwater during the crop growth period and autumn irrigation can effectively make up the groundwater. Water-saving transformation of the former water table is higher than most of the time-saving transformation of the water table subsequent years, irrigation is main supply source of ground water in the Inner Mongolia Hetao irrigation district. Water levels of the different forecasting results show that when the Yellow River water in the 4.0 billion m³/a when the average depth of groundwater at 1.90m about crops suitable for the growth limit has been reached, different water level of the prediction results show that, when the yellow water in 4.0 billion m3/a, annual average depth of groundwater is about 1.90 meter, it has reached the crops being suitable growth limiting value; when the yellow water in 3.64 billion m³/a, annual average depth of groundwater is about 2.50 meter, it has surpassed the suitable crops growth limiting value, the two levels has been reduced in groundwater freshwater area, but the yellow water in 3.64 billion m3/a has decreased significantly, from the angle of ecological safety, the threshold water volume would be 4 billion m³/a in order to keep the water environment benign cycle.