| As the largest city in the northwest China,Xi'an has suffered severe water shortage and groundwater was the major source of water supply.Over the past few decades,because of rapid development of urbanization,such as land use/cover change(LUCC),construction of water conservancy projects and urban waterscape engineering,and over-exploitation,the supplement,runoff and draining of groundwater have profoundly changed.As well as the healthy and coordinated development between urbanization and groundwater utilization was seriously affected.With the support of China Geological Survey's Project“The Influence of Urbanization on Water Cycle of Xi'an City”and based on the groundwater monitoring data,hydrometeorological data,land use data,social economic data,field reconnaissance data and remote sensing image data,this paper studied the groundwater flow field evolution characteristic and its driving forces under the influence of urbanization of Xi'an,by adopting the method of urbanology,groundwater hyrology,mathematical statistics,remote sensing and geographic information system.Main conclusions are as follows:Population and social economic urbanization level of Xi'an were evaluated respectively according to a dozens of indicators,such as urbanization population,the population of secondary and ternary industry,economic density,and per Captia GDP.Then based on the remote sensing image data,the land use dynamic model and transition matrix were employed to quantitatively analyze the land-use changing process of Xi'an.Results showed that the population and social economic urbanization level of Xi'an exhibited spatial variations,the city centre is the most urbanized area which radiated to the surrounding areas.The growth of urbanization in northeastern area was much faster than that in the south area.Meanwhile,from1995 to 2015,there showed a dramatic change in land use in Xi'an,the urban area expanded greatly while the farmland reduced.In a word,the space fragmentation in Xi'an goes deeper.The evolution stages of groundwater flow field during 1965 to 2013 was identified by using the Low-frequency Signal Reconstruction Method of Wavelet Transform.Results indicated that the 3-year reconstruction signals could better represent the evolution process of groundwater flow field,thus the evolution trend of groundwater flow field in Xi'an could be divided to five stages,the natural stage,the gradually increasing depth of groundwater stage,the rapidly increasing depth stage,the slowing down stage and the recovery stage.Futhermore,the characteristics of each stage were studied by Geostatistical Analysis Method,and it was found that the groundwater level showed a regional decline during last fifty years,but with significant spatial variation.The northwest region,the urban area and riverside area presented a greater mean magnitude for groundwater level decrease than the loess platform in the southeast,and there formed groundwater depression cones in the urban groundwater sources.Groundwater levels of the southern counties varied within a smaller range because of closing to Qinling Moutain,which were rich in surface water.Meanwhile,the Geo-statistical Analysis showed that the spatial correlation of groundwater level was getting worse while the anisotropy of groundwater level was enhanced.In order to analyzing the effect of urbanization on groundwater evolutionary mechanism,regional precipitation change and river hydrological process below urbanization infuences was studied.Besides,a quantitative relationship between climatic factors and urban construction was established,and the impacts of climate change and urbanization on streamflow were quantified by applying Budyko water-energy balance model.The results indicated that both the yealy uneven degree of precipitation distribution and the frequency of extreme heavy precipitation event exhibited a trend of increasing,which may carry serious urban waterlogging risk.The correlation between precipitation and streamflow of Bahe River from 1990 to2015was weaker than that from 1970 to 1989,as well as the fluctuation of streamflow within a year became moderate during the urbanization period,especially in the flood seasons.Furthermore,it was found that the streamflow will reduce 24.4%while the precipitation reduces 10%,and the streamflow will increase 11.2%while the potential evaporation reduces10%.The results of Budyko Model demonstrated that the contribution degree of urbanization to streamflow was 58.3%,which was larger than that of the climate change(41.7%).The effects of precipitation,streaflow and exploition on groundwater were quantificationally and qualitatively analyzed.The results indicated that the sensitivity of groundwater level to precipitation varied with hydrogeological conditions,and the precipitation in flood seasons was more important to groundwater recharge.Influenced by urbanization,precipitation had a weaker effect on groundwater level,but still is important to groundwater dynamic.Furthermore,there existed a temporal lag of groundwater from river water,and the temporal lag was varied with the distance between monitoring well and river.The reduction of streamflow caused by urbanization would cut down groundwater recharge.Besides,the groundwater level had an exponential relationship to the exploitation before 1997 while there existed a linear relationship between groundwater level and exploitation after 1997,and the response of groundwater level to exploitation was more sensitive in urban groundwater sources.The results of PSO-PPM method suggested that the exploitation was the major factor in causing regional groundwater level decline of Xi'an.The congribution degree of each factor was varied with the hydrogeological condition,as well as the evolution stages.Based on the Analytical Method and the Groundwater Numerical Simulation method,this study investigated the influence of exploitation near the river and the construction of water scape engineering on groundwater flow field.Results showed that the groundwater level would dropped at a rate of 0.67m/a and the depression would expand when streamflow of Bahe river reduced by one-third,which may have the same effect with over-exploitation.The groundwater level near Qujiang Lake increased first then decreased after the Qujiang Lake was impounded,the leakage for groundwater recharge would reached to 327214m~3 in one year if there were no seepage prevention measures.Aimed at evaluating groundwater vulnerability below urbanization influence,this paper proposed a DREAMSLIC evaluation model which was modified by traditional DRASTIC model,as well as established a weight determin system on the basis of Game Theory,Analytic Hierarchy Process and Entropy Weight Method.Then the Fuzzy Mathematics Theory and GIS Technique were adopted to calculation.Results showed that the highly vulnerable areas were mainly distributed in city center and flood plain of the Bahe River,where had frequent and intensive human activities,seriouse pollution and large area of water impermeable interface.On the contrary,due to small groundwater exploitation and less human activities,areas in the southeast has a low groundwater vulnerability.The achievements of this thesis may contribute to improving groundwater environment and revealing the changing law of groundwater influenced by urbanization.As well as could provide theoretical references for analyzing the evolution mechanism of groundwater flow field.Moreover,the study has practical significance in coordinating the development between groundwater system and urbanization construction. |
PDF Full Text Request