| The Jinghui Canal Irrigation District(JCID)in Shaanxi Province,is one of the representative canal and well irrigation districts in China.It is also an important food production base in the Guanzhong area.Due to the rapid development of social economy in the irrigation area,the demand for water resources increases.At present,some problems such as shortage of surface water resources and overexploitation of groundwater have appeared in the irrigation district.Influenced by climate and human activities,the hydrological elements of JCID have changed profoundly.Thus,it is necessary to analyze the change characteristics of hydrological factors,reveal the water cycle change law of the irrigation district and adjust and control the water resources effectively,which can provide a scientific proof for legitimate allocation and sustainable exploitation of water resources in JCID.Based on the hydrometeorological data,groundwater monitoring data,land use data,soil data,and hydrogeological survey data,this paper studied the evolution characteristics of water cycle elements in the irrigation area.In addition,a surface water-groundwater coupling model was established to reveal the law of water cycle transformation in the study area.At the same time,the reasonable surface water-groundwater regulation plans were determined based on the coupled model.The main results are as follows:(1)The variation characteristics of water cycle elements such as precipitation,evaporation,runoff and infiltration of JCID were analyzed based on cumulative anomaly method,sliding t test method,and Mann-Kendall test method et al.The results show that the annual precipitation and the annual runoff of Jing river had an obvious decreasing trend after1996,while the annual evaporation showed an insignificant upward trend after 2003.The infiltrated water quantity via precipitation,field irrigation and the canal systems from 2001 to2018 showed an increasing trend.The intra-annual distributions of precipitation,evaporation and runoff of Jing river were uneven,and that of the runoff of Jing river and evaporation of JCID tended to be uniform.(2)Based on the groundwater depth monitoring data,linear regression,moving average,cumulative anomaly method,and two-way regression analysis methods were used to analyze the spatio-temporal evolution law of the groundwater depth.Principal component analysis was applied to extract the main influence factors of groundwater depth,and grey relational degree method was used to evaluate the effect degree of these main factors on groundwater depth.The results showed that the groundwater depth showed a significant increase from1981 to 2018,and a sudden change occurred in 2003.The decrease of canal irrigation water and the increase of groundwater exploitation were the main reason for the increase of groundwater depth after 2003.The distribution of groundwater depth during the year showed a unimodal pattern,with the minimum groundwater depth in March,April,and May,and the maximum groundwater depth in July,August,and September.The variation of groundwater depth in different hydrogeological zones were different.Moreover,The spatial difference of groundwater depth was large.(3)A method for determining the SWAT model based on the MOD16 global evapotranspiration data set product is proposed to validated the SWAT model due to the lack of hydrological runoff data.The model calibration and validation results show that R~2,NSE,and PBIAS reached 0.74 and 0.79,0.72 and 0.77,-6.8%and-3.7%respectively during the calibration period and the verification period,indicating that the adjusted model can be applied in this area.The model results show that during the surface hydrological cycle of the irrigation area,precipitation and irrigation were the main water input items,while evapotranspiration was the largest output item,followed by surface runoff,soil recharge to groundwater,and finally,the lateral runoff.With the decrease of water input,surface runoff,lateral runoff,and soil recharge to groundwater were significantly reduced.The annual distribution of each runoff component were different.In different hydrological year,the changing trends of the percentages of the various components in the total river runoff in the process of confluence were also different.(4)Based on the determination of the corresponding relationship between hydrological response unit(HRU)of SWAT model and finite difference grid(CELL)of Modflow model,the coupled SWAT-MODFLOW model was established.The measured data of 8 typical groundwater level monitoring wells were selected to identify and verify the coupled model.The results showed that during the simulation period,the simulated values and the measured values had a good fit.The fitting degree of simulated and measured groundwater flow field was relatively good.These indicated that the SWAT-MODFLOW model can objectively reflect the characteristics of groundwater level changes in the irrigation area.According to the results of the coupled model,during the simulation period,the groundwater in the irrigation area was in a negative equilibrium state,with a difference of 7243.86×10~4m~3.The water equilibrium state of the groundwater system in different hydrological years differed greatly.The main reason for the imbalance of groundwater recharge and discharge in the irrigation area was that the discharge items on the upper boundary were much larger than the recharge items,which was related to irrigation and extraction.Therefore,the relationship between irrigation and extraction was the key to maintaining groundwater balance in irrigation areas.(5)The effects of different combined control schemes of surface water and groundwater on water cycle in JCID were researched based on the SWAT-MODFLOW model.The results show that in the surface hydrological cycle,with the increase of the irrigation water ratio of canals and wells,the changes of lateral runoff and evaporation were not obvious,while the surface runoff and soil water storage increased and the soil recharge to groundwater decreased.In the process of river confluence,with the increase of the irrigation water ratio of canals and wells,the surface runoff and lateral runoff increased,while the groundwater flow decreased significantly.In the groundwater circulation,with the increase of the irrigation water ratio of canals and wells,the groundwater system equilibrium difference gradually changed from negative to positive,and the area of groundwater level rised increased,while that of groundwater level dropped decreased.In order to ensure the balance of groundwater recharge and discharge,the suitable irrigation water ratio of canals and wells for typical dry years,normal years and wet years should be maintained at 1.71～1.73,1.7 and 1.17～1.2,respectively.The groundwater extraction should be reduced by 23%,16%and 4.6%in typical dry years,normal years and wet years,respectively,which will be beneficial to alleviate the decline of groundwater level of the irrigation area. |
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