Spatial variability of free porosity in the groundwater level fluctuation zone in the Baoding Plain area

Scientific determination of geological parameters of the groundwater level fluctuation zone is an important step for water resources evaluation and management. In the past 40 years, over-exploitation of groundwater in the Baoding Plain led to a serious deficit of aquifers, thus forming a huge deep vadose zone. After the South-to-North Water Diversion Project was put into operation, with the progress of ecological water supplement of rivers and lakes and groundwater limited-over-exploitation, the groundwater levels in some areas of the Baoding Plain stopped falling and rose. The quantitative relationship between the amount of water supplement and the change of groundwater levels has become an important scientific issue in the management of over-exploitation of groundwater. However, under the condition of groundwater level recovery, the calculation results will be deviated if the specific yield parameter of water release process is still used. Therefore, the free porosity parameter study of groundwater level recovery process is the key to solve this problem. Based on 67 engineering geological boreholes in the Baoding Plain, the comprehensive free porosity during groundwater level recovery is determined according to the lithologic characteristic parameters of the groundwater level fluctuation zone. The semi-variogram model is selected by the trend analysis and cross validation, and the spatial unknown points are interpolated by the Ordinary Kriging interpolation. The results show that (1) the best semi-variogram model of the comprehensive free porosity in the groundwater level fluctuation zone of the Baoding Plain is the 1 order index model. The spatial autocorrelation of the data is obvious, which is mainly affected by structural factors such as spatial location of the groundwater level fluctuation zone and stratigraphic type. (2) The comprehensive free porosity distribution shows that the southwest and northwest are the high value areas, and the extreme value can reach 0.25. The parameter gradually decreases to the central and eastern regions, and the minimum value is 0.02. (3) Compared with the value of the conventional specific yield, the value of the free porosity increases by about 0.03 in the north and south regions, which is about 1.2 times that of the conventional specific yield. In the central region, it is reduced by about 0.06, which is about half that of the conventional specific yield value. The research results are of important research value and great significance for ecological water supplement and water resources regulation in the benefited regions of the South-to-North Water Diversion Project.