Uncertainties In Estimating Groundwater Recharge With The Chloride Mass Balance Method

Groundwater recharge is sensitive to the response to land-use change(LUC)in arid and semi-arid regions.The difference of chlorine(Cl)records stored in the unsaturated zone under different vegetation types can quantify the impact of LUC on precipitation infiltration process.Numerous studies have estimated groundwater recharge by using various methods.However,the applicability and uncertainty of different Cl-based methods for estimating groundwater recharge are unclear.Based on this,this study selected Changwu tableland as the study area,and measured soil moisture,chloride and tritium contents under farmland,10-year-old and 20-year-old apple orchard to analyze the distributions in soil profile.Then,the Chloride mass balance method(CMB),the Chloride storage method(CS),the Chloride-Water mass balance method(CWMB),and the Chloride peak method(CP)were used to calculate the groundwater recharge under deep-and shallow-rooted plants.Taking the estimation from the tritium peak method(TP)as reference,the applicability and uncertainty of various Cl-based methods were systematically evaluated,and the optimal methods quantifying the recharge estimation under different plants would also be determined.Finally,we explored the impact of LUC on groundwater recharge.The main findings are as follows:1.The distribution of soil moisture and chloride content are significantly affected by LUC.The largest average soil moisture content appeared under farmland in Changwu tableland,followed by 10-year-old apple orchard and the lowest were in 20-year-old apple orchard.Soil water storage under farmland was similar to that under 10-year-old apple orchard,accounting for 80%of the field water holdings capacity,while the water storage under 20-year-old apple orchard accounted for about 69%of the field water holdings.After the conversion from farmland to orchards,the consumption of soil moisture is increased,especially soil water deficit increased with the age growing of the apple trees.The chloride concentration in the soil pour water under apple orchards was significantly higher than that under farmland,and it has a positive correlation with tree age(R~2=0.43).2.The groundwater recharge and infiltration rate calculated by different Cl-based methods were quite different.The recharge estimations using CMB under farmland were2.0-2.6 times higher than those using TP(P<0.05).Most results estimated using CS were less than those using TP.There was not significant difference of recharge estimation between CWMB and TP,however,the estimation using CP was significantly different from TP(P<0.001).Therefore,the best method on estimating groundwater recharge under deep-and shallow-rooted plants are CMB and CWMB,respectively.3.LUC can affects soil moisture and groundwater recharge.After the conversion from farmland to orchards,soil moisture had small change under 10 year-old apple orchard but had significant decrease under 20 year-old apple orchard,which caused a severe soil desiccation.Furthermore,such conversion also decreased the recharge rate with the values following the order of farmland(56.1±14.4 mm/a)>10-year-old apple orchard(53.8±32.1mm/a)>20-year-old apple orchard(36.4±16.1 mm/a).The time of precipitation infiltrating form surface to groundwater table was about 167 to 573 years in the Changwu tableland.This study analyzes the uncertainty in the estimation of groundwater recharge by the Cl-based methods.This study analyzed the uncertainties in estimating groundwater recharge with the Cl-based methods.It was found that the conversion of farmland into orchards increased soil water consumption;CWMB was more suitable for deep-rooted vegetation;and implying the potential risk of LUC effects on the sustainability of groundwater resources.The results can provide a scientific basis for management of water use structure and the groundwater sustainability in arid and semi-arid regions.