Simulation And Regulation Of Groundwater Table Dynamics In The Minqin Basin Of The Downstream Shiyang River

The Minqin Basin is located in lower reaches of the Shiyang River Basin, which is a typical inland arid area in the Northwest China. As the surface water from upper reaches has been decreasing year after year, the groundwater has had to be over-exploited, which has made much decline of groundwater table, and has induced a series of eco-environment problems. The rapid deterioration of eco-environment is not only a threat to existing and development of the Minqin Basin, but also has significant influence on eco-environment of Gansu Province and even the Northwest China. Study on simulation and regulation of groundwater table dynamics of this area can provide guidance for reasonable exploitation and use and appropriate management of water resources, which is significant for sustainable development of social economy and rebuilding of eco-environment in this area and even northwest inland arid areas.This research analysed the mechanisms of groundwater table dynamics in the Minqin Basin. Based on the hydrogeological conceptual model and the mathematical model, the groundwater table Feflow numerical model was constructed, which simulated and predicted the space-time dynamics of groundwater table in the Minqin Basin. To effectively recover the groundwater table, the best scene of water resources exploitation and use was chosen, and the groundwater table was regulated, which can provide a theoretical guidance for water resources management in this area. The main research conclusions are as follows:(1) The Minqin Basin groundwater system is an open and compound system influenced by both nature and human. In the past twenty years, the groundwater table of this region influenced by intensive human activities declined year by year and fluctuated with seasons. The speed of groundwater table declination gradually got less and less from the oasis center to the deserts. The natural factors, including rainfall, evaporation and transpiration, had less influence on the variation of groundwater table, and the human activities, including ditch and field infiltration are the key driving factors in regulation of groundwater table dynamics.(2) Based on the hydrogeological conceptual model, the Feflow groundwater table numerical simulation model of the Minqin Basin was constructed. With the help of GIS plat, the in/out flow on top/bottom of the groundwater system and hydrogeological parameters were imported into the model. Via the simulation, not only the groundwater table space-time dynamics, but also the accurate hydrogeological parameters (conductivity and storativity) were acquired. Meanwhile, the viewdata of groundwater space-time dynamics could be acquired. In general, the model performed effectively with the mean error of 0.9768m and the correlation coefficient of 0.977 between the simulated and observed data.(3) In/out flow on top/bottom of the groundwater system was predicted with the time series analysis, which can provide important data for the Feflow model simulation. The method can effectively solve the problem of determining in/out flow on top/bottom of the groundwater system in the areas lacking data.(4) The groundwater table observed data of the Minqin Basin at the end of 2001 were set as flow initials, and the Feflow model and time series analysis were coupled to predict groundwater table dynamics in 2010. The groundwater table will have the same seasonal variation as that in 2001. The table will decline in exploitation and irrigation period, and will ascend in non-irrigation period, but in general, it will decline in the year. The water table in the oasis will all be lower than that out of the oasis with the same elevation, and their difference will be siginificant in 2010, which indicated that the water table in the oasis would be declining obviously faster and faster with time than that out of the oasis. Meanwhile, the groundwater depression cones would be expanding continuously, and their center depths would be increasing.(5) According to"the Current Key Fathering Layout of the Shiyang River Basin"and the reality of exploitation and use of water resources of the Minqin Basin, the period of 2002~2010 and 2011~2020 were respectively set as the phase of alleviating the water table declination and the recovery phase of water table, and the dynamic processes of water table in different periods under the different regulation schemes were predicted respectively with the built Feflow model. To alleviate the water table declination in 2002~2010, it is necessary to keep the output flux of the reservoir as 7476×104m~3/a and the exploitation of groundwater as 3.94×108m~3/a, and to take some water-saving measures by respectively increasing water use efficiency of ditch and well irrigation to 0.6 and 0.8. To recover the water table in 2011~2020, the control of water consumption and water saving are the prerequisite. Meanwhile, it is necessary to increase output flux by delivering water from the other river basins. Under the output flux of 2.7×108m~3/a, a balance state between recharge and discharge of groundwater will be possible; under the output flux with 3.5×108m~3/a, the water table of the Minqin Basin will rise again significantly, especially in the oasis.