Study On Groundwater Regime In Loess Plateau Irrigation Based On Visual MODFLOW

Loess irrigation area is located in the center of Shaanxi and north of Weihe River. Itplays a very important role in the national economy of the entire province. However, lack ofrainfall and surface water in the area had impacted seriously on the development of itsnational economy. Therefore, some water conservancy projects were constructed in order todevelop the economy and solve the water shortage problems. With the development of socialeconomy and the improvement of living standard, higher water quantity and water qualitywere required by people. Exploiting groundwater had been the main way to ease watershortages, which would lead to many environmental problems on groundwater. Thus it issignificant to study on groundwater regime and the relationship between groundwaterrecharge and discharge in loess irrigation area, which will relate to the whole nationaleconomic development and ecological environment safety.The latest development of groundwater numerical simulation and prediction werecollected and reviewed at home and abroad in this study. On the basis of predecessors’research achievements, Baoyang irrigation area (typical irrigation area of the loess) wasselected as the key study area. Hydrogeological conditions, social and economic development,groundwater dynamic field data and source sink term as well as boundary conditions of thestudy area were mainly collected and analyzed.Furthermore, groundwater regime types,relationship between recharge and discharge and influencing factors were all introduced andexplained in this study. Hydrogeological conditions and boundary conditions of the study areamust be generalized firstly to establish the hydrogeological conceptual model. Then mappingand vectorization of the control diagram of the study area conducted by Arcgis9.3softwarewas imported into the groundwater numerical simulation software(Visual MODFLOW.Three-dimensional numerical model of groundwater was established through the existing dataand then suitable hydrogeology parameters(permeability and water specific yield)werecalculated through model iteration. Finally, two different scenarios were assumed to predictthe groundwater table of2013. Consequences were shown as follows:(1)On the analysis of groundwater flow field,interannual and annual variationcharacteristics of groundwater in Baoyang irrigation area, the existing problems ofgroundwater regime in recent years were summerized, which could provide a reference forfurther research on groundwater development and utilization in this area. The results of this study included: simulated water level of unconfined aquifer ranged from440m to615m. Inaddition to northern groundwater level(piedmont diluvial fan)was higher than that of the south,groundwater flow field was from northwest to southeast direction. The study area was dividedinto9sub areas due to the different hydrogeological conditions.These nine areas had differentspecific yield and permeability coefficient. So the dynamic change of groundwater in thedifferent areas was not the same. The overall trend of groundwater level rose after the winterirrigation and then went down.It increased again after the spring irrigation in April.Thegroundwater level dropped dramatically in the summer while water levels began to ascend asa result of the rainy season arrivals. On the whole, irrigation infiltration, artificial exploitationand rainfall infiltration were the main three factors to influence the dynamic variation of thegroundwater in the irrigation area, but the influence of geological factors was not obvious.(2)The control chart vector quantized using Arcgis9.3was imported into VisualMODFLOW groundwater numerical simulation software after generalizing thehydrogeological conditions of study area.3D model of groundwater was build and thenmultiple iterations(run, tuning parameters and validation) were performed. A set of reasonableparameters of the model were obtained to prepare for predicting groundwater table in the nextstep.Measured groundwater flow field and simulated flow field were mapped by Arcgis9.3software. The comparison result indicated that the overall trend of both groundwater levelfields was consistent generally. The correlation coefficient, relative error and the mean squareroot between simulated water level and measured water level of the representative wells werecalculated.The maximum and minimum mean square error between measured and simulatedvalues of the selected seven representative wells was0.6342m and0.4021m respectively. Thecorresponding maximum and minimum relative error was9.7%and7.2%separately. Thecorrelation coefficient was from0.6950to0.9487. The measured and the simulated waterlevel values were close generally and the precision of the model was relatively accurate so itcould be used to predict the groundwater levels.(3) Assuming source sink term programs of two different scenarios, given values ofsource sink term were input into the3D groundwater numerical model which had beenestablished and calibrated before hand to predict groundwater regime variations of the studyarea in2013. The tendency of the two plans was basically accordant but the groundwaterdepth of the plan one was shallow. The overall trend of the groundwater level reduced forthe each plan. Specifically, the groundwater level of the plan two dropped slightly than that ofthe plan one but the overall range was not very large. It was concluded that groundwater level drops of Baoyang irrigation area had been effectively controlled.