Research On Estimating Methods Of Groundwater Recharge

Groundwater recharge evaluation is an important component of groundwater resource evaluation.Accurate evaluation of groundwater recharge is the basis of analyzing hydrology cycle law,making water resources plans and sustainable utilization project of groundwater.It is of great strategic significance to achieve the sustainable development of local social economy,especially,for arid-semiarid region,where the water resources is shortage.In recent decades,meteorology,hydrology,land use and irrigation conditions in the North China Plain have undergone major changes because of natural and human beings factors,and groundwater recharge change accordingly.As for rainfall infiltration coefficient and irrigation infiltration coefficient which are the main parameters for calculating rainfall or irrigation recharge only take lithology and water table depth into account generally,and few consider land use and water-saving measures.Therefore,the influence of water-saving measures(such as irrigation,straw mulch,etc.)and land use on groundwater recharge is particularly important. With environmental conditions(such as water table depth,irrigation,land use,weather,etc.) changing,how to evaluate groundwater recharge timely and dynamically is urgent.In addition, lateral groundwater recharge is determined by Darcy cross-section method,and it needs many parameters such as hydraulic gradient,conductivity and aquifer occurrence.But these parameters are not easy to determine.What's more,they exist uncertainty.So,it is extremely important to develop a new evaluation method to avoid these uncertain parameters.At present there are many evaluation methods for groundwater recharge,different evaluation methods for different study objects(vadose zone,the unsaturated zone),and different evaluation methods are applicable to different spatial and temporal scales,scope and reliability. So,it is very important to choose the appropriate method according to the research.Therefore, to provide reliable theory and method of groundwater recharge evaluation,it is urgent to analyze the applicability of different evaluation methods and establish temporal and spatial scale,scope and reliability of different evaluation methods.So,it has a great theoretical and practical significance to choose the reliable method for evaluating groundwater recharge accurately,which is propitious to reveal the change law of temporal and spatial of groundwater recharge and improve the theory and method of groundwater resource evaluation. Because of the uncertainty of groundwater recharge and estimation methods,in this paper the comprehensive experiments on groundwater recharge evaluation were carried out on the typical area in the North China Plain.The temporal and spatial changes information of vadose zone and saturated zone was collected.The lateral groundwater recharge was estimated by single well technique of the radioactive isotope in the piedmont plain and the vertical groundwater recharge rates were evaluated by using multiple methods such as natural chloride method,tritium and bromide tracers technique,Darcian approach,the lumped parameter model EARTH and the variably saturated flux model HYDRAUS-1D.The temporal and spatial change law of groundwater recharge is revealed,and high reliability evaluation methods are provided according to the research.The main conclusions are as follows:1.The seepage velocity through the 46km long cross-section from Luquan to Lingshou is in the range of 0.11～1.16m/d by single well technique of the radioactive isotope.The total recharge through the 46.2km long cross-section from Luquan to Lingshou is 5073.19×10~4m~3/a,in which Luquan city occupied 3555.61×10~4m~3/a,and this result is in good agreement with Darcy cross-section calculation(3972.9×10~4 m~3/a)from Water Resources Bureau of Luquan in 1992. The result indicates that the lateral groundwater recharge can be determined by the seepage velocity(Darcian velocity),flow direction and aquifer occurrence.This method can overcome the difficulties of parameters(such as hydraulic gradient,conductivity)obtaining and uncertainty. It improves the credibility and is a very important method supplement for lateral recharge evaluation.It has significant pratical value for water recources investigation and estimation.2.Average recharge rates in Luquan and Luancheng are 63.72mm/a and 60.8mm/a respectively determined by chloride mass balance method,which account for 12.3%and 12.5% of the even annual precipitation,and are 45.19mm/a and 45.46mm/a determined by the chloride cumulative method.The method is fit to deeper water table depth and fresh water zone in piedmont plain.But its application is limited to the area for saline water,salinization,irrigation (or fertilization),as well as shallow water table depth.3.Applied tritium and bromide method was used to evaluate the groundwater recharge in different irrigation regime,lithology,land use,and straw mulch or not.Temporal and spatial change law of groundwater recharge rate and integrated recharge coefficient was analyzed.It shows that the recharge rate varied from 0.00 to 1.05 mm/d,with a mean of 0.35 mm/d,and the integrated recharge coefficient varied from 0.0 to 42.5%,with a mean of 15.9%.It also indicates the relative recharge rate for the following paired influences(items within each pair are listed with the influence producing greater recharge first):check-irrigated cropland(0.29-0.65 mm/d) and non-irrigated non-cultivation land(0.15-0.32 mm/d),check-irrigation(0.42-0.58 mm/d)and sprinkling irrigation(0.17-0.23 mm/d),non-irrigated non-cultivation(0.22-0.32 mm/d)land and sprinkling irrigation,no stalk mulch(0.56-0.80 mm/d)and stalk mulch(0.44-0.60 mm/d), vegetable(0.70 mm/d)and wheat-maize(0.38 mm/d),peanut(0.51 mm/d)and peach(0.43 mm/d).And the recharge rate of bromine is the tritium's 123%on average,by comparing the tritium and bromide tracing results,which may be attributed to anions exclusive effect.Based on the above,the applicability of this method is analyzed.The applied tracer can be put in timing, positioning and quantitative,which can overcome the influence of roots and preferential flow. But this method is based on the diffuse flux mechanism,and the recharge of preferential flow is excluded,which maybe results in less recharge.When preferential flow is strong,the method may be not appropriate.The method is useful when water table depth is deeper,but it is limited when water table depth is shallow.So,it's necessary to do the tracing experiment further to discuss the possibility on determining the recharge and evaporation of phreatic quantitatively.4.In this paper,soil water flux method was used to evaluate groundwater recharge rate in different water table depth.The result shows:when water table depth is deep,the recharge rate in cropland with no straw mulch is bigger than that with straw mulch,but the amount of evapotranspiration is smaller.It indicate that straw mulch makes against the infiltration of precipitation and irrigation,but restrains evaporation of soil water.When water table depth is shallow,if the amount of precipitation and irrigation is small,there is no recharge nearly,and so the precision and reliability of result calculated by this method is reduced greatly.Based on the theory of water balance,straw or velum mulch can reduce the evaporation of soil water and use soil water effectively,which reveal the mechanism of different soil water control.Compared to the tritium tracing results,they are basically identical,which can prove each other.5.The lumped parameter model(EARTH)was established to simulate the recharge rate and recharge process in typical areas by the observation datum of weather,soil water and groundwater synthetically,and the spatial and temporal variation law of groundwater recharge was revealed.Recharge rate and recharge coefficient is gradually reduced from piedmont plain to coastal plain,annual precipitation and irritation is the determinants of recharge rate.Based on the simulation result,the method of stepwise regression analysis was used to determine the determinants that influence the recharge rate and leakage of root layer,and the correlative model was founded between recharge rate or leakage of root layer and the determinants.It shows that the main determinants of the recharge rate(R_d)are the potential evapotranspiration(ET_p), precipitation(P),groundwater table(H)and irrigation(I)for deep water table depth,which can be expressed as R_d=1.754-0.065ET_p-0.005P-0.045H-0.002I;and precipitation(P)and irrigation(I)are the main determinants of the leakage of root layer(R_p),which can be expressed as R_p=-0.287+0.463P+0.224I.For shallow water table depth,the main determinants of the recharge rate(R_d)are the soil moisture in 2 meters(S),irrigation(I)and groundwater table (H),which can be expressed as R_d=14.142+0.029S-0.016I-0.068H;and all variables are the main determinants of the leakage of root layer(R_p),which can be expressed as R_p=-8.34+0.222P+0.081I+0.021S-0.152H-0.059ET_p.6.The rate and coefficient of groundwater recharge in typical areas was estimated by one dimensional variably-saturated water flow numerical model(HYDRUS-1D),which are 258.22 mm/a(31.1%)in Luquan,215.25 mm/a(23.4%)in Luancheng,47.95 mm/a(9.4%)in Hengshui, 220.35 mm/a(20.0%)in Dezhou,209.34 mm/a(24.5%)in Cangzhou,respectively.The results are accorded with the EARTH model.The recharge rate gets smaller obviously when there is no irrigation by comparing the simulate results from irrigation and nonirrigation.The percent of the irrigation recharge accounts for 13.86-41.68%of the total recharge from precipitation and irrigation,and Luquan is biggest(about 42%).It also indicates the present irrigation quota (750-900 m~3/ha)is not reasonable and should reduce to 450-675 m~3/ha according to the test result from Hengshui,which almost results in no leakage.7.By comparing the results of different methods,the application conditions and limitations of each method are analyzed.The paper also points out that the objective,climatic regions, recharge range,temporal and spatial scales of different approaches,accuracy and reliability of recharge estimates,time and expense should be considered on choosing a appropriate method to estimate groundwater recharge.Based on the above,the application of different method in North China Plain was discussed.The innovations are:(1)The single well techniques of the radioactive isotope was used to evaluate the lateral groundwater recharge in piedmont plain,which can conquer parameter(such as hydraulic gradient and conductivity)uncertainty problems and can improve the reliability.It improves the credibility and is a very important method supplement for lateral recharge evaluation.It has significant practical value for water resources investigation and estimation.(2) The multi-techniques such as natural chloride ion,applied tritium and bromide technique,soil water flux method,the EARTH model,and numerical model were used to estimate groundwater recharge of the typical area in North China Plain,which proved each other.It's an innovation in method.(3)The recharge rate of bromide is the tritium's 123%on average,by comparing the tritium and bromide tracing test.And the bromide may be used as a dependable tracer for evaluating groundwater recharge.(4)Reveal the soil water controlling mechanism by stalk or velum mulching for different water table depth(5)Put forward the determinants that influence the recharge rate(and leakage amount of root layer),and build the correlative model between recharge rate(leakage amount of root layer)and its determinants for different water table depth.