Study On Spatial And Temporal Variability Of Groundwater Regime Under Variational Environment In Irrigation Area

Baojixia irrigation area is one of the top ten irrigation areas, it has been 30 years since the irrigation area was founded.With the development of the irrigation area, because of the impact of the external environmental factors, groundwater system circulation condition has changed a lot which directly influence the evolution trend and the utilization pattern of the groundwater. Therefore, the study of the impact of groundwater dynamic by the external environmental.factors and the spatial and temporal variability of the groundwater has a great theoretical significance and application value for the rational development of groundwater resources and the sustainable use of water resources.This paper embarking from the groundwater resources transformtion, analyzed the groundwater system and the trend of the external environmental factors, combining with the evolution of the groundwater dynamic, analyzed the main factors of groundwater dynamic; established multiple regression model and BP neural network model of the groundwater level and predict the groundwater level; combined geostatistics and GIS together to study the temporal and spatial variation of the groundwater level.The main conclusions obtained are as follows:(1) Generalized the groundwater system of Baojixia irrigation area and maked clear the climatic factors and human activities were the external environmental factors of groundwater systems, analyzed the dynamic characteristics of groundwater, the results showed that: the groundwater level of the loess tableland area has gone through four phases from 1982 to 2007 :the rising period,the steady period,the declining period and the steady period; the groundwater level of the Wei river terrace area showed a downward trend.The hardness of the groundwater has showed a rising trend since 2002 and the upward trend is obvious; alkalinity didn't change significantly in recent years and shows a steady trend; mineralization is rising, but the trend is not very obvious.(2) Using Mann-Kendall method, Spearman test and regression test to analyze the trend of precipitation, evaporation, surface water irrigation and groundwater exploitation in the area from 1982 to 2007 .The results show that :the precipitation shows a downward trend, but not significant and the ample water and low water alternating; the evaporation show upward trend, but the rise was not significant and the rise and fall periodically alternating; surface water irrigation significantly decreased; groundwater exploitation was rising, but the trend was not significant.(3) Using the gray correlation analysis method to analyze the main driving factors of the groundwater level. The results show that: the order of factors of groundwater level in the loess tableland area was: evaporation, precipitation, surface water irrigation, groundwater exploitation; the order of factors of groundwater level in the Wei river terrace area was: evaporation, precipitation, groundwater exploitation, surface water irrigation. Through the comparision of the loess tableland area and the Wei river terrace area, it can be found that the influence caused by the factors in the Wei river terrace area is greater than the influence in the loess tableland area.(4) Using the groundwater level dynamic and the external environmental factors, established the multiple regression model and BP neural network model of the groundwater level, we can find that BP neural network has a higher accuracy than the multiple regression model by comparison. Combined the trend analysis of the impact factors of groundwater, using the model, predict the dynamics of the groundwater level which shows that the groundwater level will show a downward trend within the next few years and the rate of groundwater level decline in the loess tableland area is greater than the rate in Weihe rive terrace area.(5) Using the geostatistical analyst module of ARCGIS to anlyze the spatial variability of the groundwater level,established the spherical, exponential and Gaussian model ,we can find that Gaussian model has a highest accuracy through comparison and cross-validation. Derived from the Gaussian model: groundwater level has a strong spatial correlation and show increasing trend; groundwater level has obvious anisotropy and has good spatial continuity in the irrigation area.The angle of the major axis is 65°direction which is the same with the direction of the Weihe River.The results of Kriging show that: groundwater level decreasing from the northwest to southeast and had small amplitude from 2002 to 2007.Standard error map shows that the model predicted water table space high precision irrigation groundwater monitoring wells within the distribution is more uniform and rational.