Research On Uncertainty Analysis Of Groundwater Ecological Water Level Threshold In Desert Margin

The groundwater level is important to maintaining regional ecological environment in desert oasis region,and because of its special geographical location and complicated hydrogeological conditions,it is difficult to survey the groundwater system of the oasis region,so there are many uncertainties in the determination and simulation of the groundwater ecological level.In this paper,taking a study area in Pishan as an example,through establishing groundwater numerical simulation model,determine the ecological groundwater level threshold value in the study area,analyze the uncertainty factors during determining the ecological water level threshold value,constructs the ecological water level fuzzy random risk model,and aiming at the uncertainty of groundwater system model structure,the probabilities of different models reflecting actual conditions are estimated.Firstly,the relationship between ecological groundwater level and crop vegetation growth,soil salinization,evaporation limit depth and groundwater salinity was analyzed.Combined with the numerical model,the threshold of ecological groundwater level in the study area was determined to be 3?11.3m.At the same time,based on the modeling process and actual hydrogeological conditions,the uncertain factors affecting the of groundwater ecological water level threshold were divided into random,fuzzy,gray and unascertained uncertain factors.Secondly,considering the widely existing randomness and fuzziness of groundwater system in the study area,the ? cut set method is used to calculate the fuzzy random risk when the parameters obey different membership functions.The results show that the risk rate calculated by the cut set method is a series of interval values.When the membership function of the fuzzy random variable has a normal or quasi-normal distribution,the difference in the fuzzy risk is smallest.When ?<0.67,the fuzzy risk interval of the triangle membership function is the largest,and its risk interval maximum value of 98.36% can be obtained at ?=0.1;however,when ?>0.8,it has the smallest risk interval of all the membership functions.If the skewness of the design parameters is taken into consideration,the lognormal membership function is a better choice.When ?=0.5,for membership functions that follow the triangle,normal,quasi-normal,and log-normal distributions,the fuzzy random risks are 57.57%,61.15%,61.26%,and 57.68%,respectively.Finally,the model structure uncertainty is studied in the study area.The results of observation point analysis show that the model error caused by the change of aquifer structure is much larger than that caused by the partition of model parameters.From the results of regional water level analysis,the three alternative models can describe the real water level distribution to a certain extent on the whole,and the error ranking between them and the real distribution is M4>M3>M2.The model probability calculation results show that the probability of M2,M3 and M4 reflecting the actual hydrogeological conditions is 0.83,0.10 and 0.07,respectively.The uncertainty analysis results of the ecological water level threshold model show that the reliability of the maximum threshold of water level depth is limited by the reasonability of the model generalization.