Study On Characteristics Of Groundwater Circulation In Canal-well Combined Irrigation Area Under Changing Environment Based On SWAT-MODFLOW Model

Studying on the characteristic of groundwater circulation in canal-well combined irrigation area can provide the scientific basis for solving present groundwater problems,rationally controlling and sustainably using groundwater resource.By taking Jinghuiqu irrigation area which is a representative canal-well combined irrigation area as the research area,the paper generalizes the water cycle system and analyses changing environmental factors that mainly influence the water circulation of the study area.For describing the water cycle process more truthfully,the study builts SWAT-MODFLOW model,a comprehensive hydrologic model.Under considering both the variation characteristics of changing environmental factors and future meteorological elements,two secnarios are set and input to the comprehensive model.Then,the paper obtains variation characteristics of groundwater level under two secnarios.The main conclusions of this study are:(1)By generalizing the water cycle system of Jinghuiqu irrigation area,the paper recognized changing environmental factors: precipitation,temperature,canal irrigation water,ground water exploitation,and land use types.Regression test and Spearman test were used to analyze trends of all factors from 1977 to 2011,and the results showed that precipitation had a less marked downward trend,temperature and canal irrigation water showed significant upward trends,groundwater exploitation had a significant downward trend,and the plantation structure of study area transformed mainly food crops into a new pattern which put food crops and economic fruits on a equal term.(2)For the difficult problem about the spatial discretization of SWAT model in plain irrigation area,the study concluded and estimated relative merits and application conditions of existing methods.By considering fully about the stream network,canal distribution,inverse confluence of manual collateral water circulation,and management characteristic of water transmission and distribution,the paper used ArcGIS to pre-define Watershed Dataset and Stream Dataset,and imported them into SWAT.Then SWAT model was built and checked using hydrological similarity principle and learning from existing results.It turned out that the monthly groundwater recharge rates were all between 15% to 32% in calibration and validation periods,and decisive coefficients of observed potential evaporation and simulated potential evapotranspiration in two periods respectively were 0.67 and 0.70.The SWAT model of Jinghuiqu irrigation area had a well simulating effect.(3)Based on the generalization of conceptual hydrogeological model for irrigation area,the paper built MODFLOW model by GMS software.ArcGIS and SWAT-MODFLOW softwares were used to build the SWAT-MODFLOW comprehensive model by setting up four text files to transfer variables between two models.The calibration and validation results of integrated model showed that the spatial distribution of the simulated water table was corresponding to the actual flow field of study area.The observed and simulated monthly groundwater levels of 7 observation wells were fitting well.The SWAT-MODFLOW model of Jinghuiqu irrigation area could generally reflect the characteristics of groundwater circulation.(4)Chose 3 typical years by frequency analysis of 30 years' precipitation sequences,and took them as based years to set two scenarios of 2020s(2011 to 2040).Scenario I: Processed the precipitation and temperature data of 3 based years according to existing research achievement.Decreased the canal irrigation water(10%)and increased well irrigation water of wet year(14%),normal year(13%)and dry year(11%).Changed the ground water exploitation in terms of the increased values of well irrigation water.The effective utilization coefficient of irrigation water was set to 0.65.Scenario II: Processed the precipitation and temperature data as well as scenario I.Decreased the canal irrigation water(5%)and increased well irrigation water of wet year(7%),normal year(6%)and dry year(6%).Changed the ground water exploitation in terms of the increased values of well irrigation water.The effective utilization coefficient of irrigation water was set to 0.65.(5)Put two scenarios into SWA-MODFLOW model and run the comprehensive model,the study obtained variation characteristics of groundwater level under two secnarios in 2020 s.The results showed that groundwater levels of both scenarios had significant downward trends,and drop rates of scenario I were greater than scenario II.For wet years,the minimum drop rates of observation wells were 1.25m(scenario I)and 0.75m(scenario II),and the maximum values were 12.93m(scenario I)and 6.91m(scenario II).For narmal years,the minimum drop rates of observation wells were 0.75m(scenario I)and 0.23m(scenario II),and the maximum values were 7.80m(scenario I)and 3.99m(scenario II).For dry years,the minimum drop rates of observation wells were 0.53m(scenario I)and 0.35m(scenario II),and the maximum values were 10.60m(scenario I)and 4.71m(scenario II).