Dynamic Evolution Of Groundwater In Lhasa River Valley Under Changing Environment

At present,the research on the mechanism of dynamic evolution of groundwater under changing environment is still in the exploratory stage.Due to its frontier nature,it has attracted extensive interest of many scholars,and the research in this field is being accelerated.The versatility of groundwater is an important support for the economic development of the local society and the health of the ecosystem.Therefore,it is of great significance to study the law of dynamic evolution of groundwater driven by changing environment to ensure the sustainable development of the region.Based on the public welfare project of China geological survey,this paper takes the Lhasa river valley as a typical research area and analyzes the dynamic spatiotemporal evolution of groundwater in this area under changing conditions by using mathematical statistics,numerical simulation and other technical methods.The main results are as follows:(1)The overall groundwater level in the study area showed a downward trend,with an average decline rate of about 1.1728m/10 a and a trend coefficient of 1.1638.There were significant differences in the dynamic changes of groundwater on different geomorphic units.The groundwater level decline rate was > floodplain area in the diluvial sector,and the water level of observation holes in local areas showed a great decline.The Hurst index of underground water level in observation hole is all greater than 0.5.The groundwater depth may continue to increase,that is,the water level may continue to decline.By exploring the abrupt change year and periodic characteristics of each observation hole,the abrupt change year of groundwater dynamic change was determined to be 2006.The first main period of groundwater change is 13 a,and the second main period is 3a(2)Through the analysis of meteorological data collected,it is concluded that the climate of the study area is changing to the direction of warming and humidification,in which the annual average temperature and evaporation volume have a significant rising trend.The rising rates of the meteorological elements(temperature,rainfall and evaporation volume)are 0.42366℃/10 a,7.984mm/10 a and 78.9402mm/10 a respectively.The mutation years of each meteorological element sequence were 1994,1998 and 1978,respectively.All meteorological elements have the first principal period of 33 a.(3)Analyze the main driving force of groundwater dynamic change,and conclude that the dynamic characteristics of groundwater level in the study area are controlled by both natural factors and human factors,and mining is the main reason for groundwater level decline in this area;By studying the hysteresis of groundwater dynamics to rainfall,it is concluded that the rainfall in observation holes lags from 1-4 months.(4)Comprehensively analyzed the geological and hydrogeological conditions of the study area,constructed the hydrogeological conceptual model and the groundwater flow numerical simulation model of the study area,and used GMS software to solve the problems.The model is identified and verified by the groundwater level dynamics,flow field and water balance in the observation hole in the region.The groundwater level fitting is good and the accuracy meets the requirements.The flow field at the end of the calculation is basically consistent with the measured flow field.(5)Based on the output results of climate model,a variety of scenarios are set up to predict the evolution rules of groundwater flow field.The results show that: in the reference scheme,the groundwater flow field and water level do not change obviously,and the groundwater system is in equilibrium.Under the climate change plan,groundwater dynamics and precipitation show a strong positive correlation,and fluctuate with rainfall fluctuations.In the scheme of human activities,without changing the pattern of exploitation,the significant increase of exploitation amount will change the balance of the original groundwater system,and the groundwater level will decrease significantly.In some areas,the original funnel scope will be further expanded and new funnel will appear in some areas.The negative effect of increasing production volume can be effectively improved after rational optimization of mining layout.