| Groundwater recharge is a critical component in hydrologic cycle. It is pivotal to accurately analyze the groundwater recharge rules and to evaluate the groundwater recharge amounts, based on which the groundwater resources could be appropriately evaluated, managed and used. Vertical groundwater recharge is an important type of groundwater recharge to shallow groundwater in the plains; it can be affected by precipitation, irrigation, terrain, vegetation as well as the vadose zone. As a buffer zone between the land surface and the aquifer below, vadose zone has three states (i.e. solid state, liquid state and gas state) simultaneously, and therefore, leads to a complex recharge process and the uncertainty in the infiltration recharge evaluation.North China Plain is one of the areas that have the highest development and utilization of groundwater degree, In recent decades, long-term groundwater over-exploitation has caused a consistent descend of water level, leading to the vadose zone which is thicker, more complex and heterogeneous. It brings new unsure considerations to the research of vertical groundwater recharge in North China plain.Therefore, analyzing the mechanism of vertical groundwater recharge and figuring out the variation of infiltration recharge under the condition of water table decline are of great magnificence to elucidate the characteristics, spatial-temporal disparities and accurate amount of vertical groundwater recharge in North China Plain.In this thesis, based on summary and analysis of the previous achievements, following research work were carried out by the lysimeter infiltration test, artificial bromide tracer test, temperature tracer and numerical simulation using variably saturated flow model:(1) Analyzing the key issues related to the vertical groundwater recharge, such as the vertical zonation of groundwater recharge, effective recharge, the change of recharge response, and evaluating the recharge quantity during the unsteady stage of water table decline. According to the analysis above, the analysis pattern, which is based on the vadose zone, of vertical groundwater recharge on the condition of water table decline was generalized.(2) On basis of generalized analysis pattern, water balance of surface vadose zone, which determines the recharge quantity, was taken as the research object. Multiple methods were used to compare and evaluate the vertical groundwater recharge under the condition of declining water table, and to analyze the effects of different components of water balance on the recharge quantity, such as precipitation, irrigation, phreatic water evaporation and surface runoff.(3) Aiming at the lithologic characteristic of actual vadose zone, typical lithologic structures were generalized from the complex vadose zone, and the effect of soil lithology, soil hydraulic parameters, heterogeneous layers and lenses on vertical groundwater recharge were analyzed to reveal the infiltration recharge process in different typical structure of vadose zone.(4) The North China Plain could be divided into piedmont, central and coastal plain according to their sedimentary characteristics, and different water table dropping occurs in the three typical areas. In this background, two-dimensional variably saturated flow models were built based on the stochastic geological structure model to analyze the characteristic of vertical groundwater recharge in different typical areas during the past fifty years.Main conclusions of this research are as follows:(1) The analytical model about the variation characteristic of vertical groundwater recharge on the condition of declining water table.The decline of water table resulted in the change both of recharge quantity and the recharge process. Recharge quantity is determined by the water balance of vadose zone above the maximum evaporation depth. Thus, the effect of the water table declining on recharge quantity has the characteristics of vertical zoning. When the water table is above the maximum evaporation depth, the recharge quantity would obviously decreases with the water table depth increases, and the effective recharge, which is equals to the recharge minus phreatic water evaporation, would gradually increases. Evaporation will reduce the recharge but can only affect the vadose zone above the maximum evaporation depth, when water table is below the maximum evaporation depth, recharge quantity tended to be stable. When the water table drops, recharge process would be more stable and time-lag occurs between the recharge rate at the water table and the rainfall events at the surface. According to the vertical zonation of groundwater recharge, analytical model based on vadose zone was generalized to analyze the variation characteristic of vertical groundwater recharge on the condition of declining water table. When the water table declines but above the maximum evaporation depth, the variation of recharge quantity with the change of vadose zone and its water balance should be the key point. However, when the water table is below the maximum evaporation depth, research should focus on the variation of recharge process (includes peak rate of recharge, recharge time-lag, recharge period, etc), which is mainly affected by the thickness, lithology and the heterogeneity of vadose zone.(2) The water balance variation of vadose zone above the maximum evaporation depth under the condition of declining water table.The water balance of vadose zone above the maximum evaporation depth is consist of rainfall, irrigation, surface runoff, evaporation and the infiltration recharge. Results from several different evaluation methods (the lysimeter, bromide tracer and numerical simulation) show that the maximum evaporation depth is between 2.4m to 3.0m in the lysimeter. When the water table is above the maximum evaporation depth, recharge quantity decline with the water table due to the increase in water deficit of vadose zone which is caused by the evaporation.The analytical solution for one dimensional steady heat transfer with convection and conduction through a multi-layer system can be used to figure out the influence of spatial variability of vadose zone when use temperature as a tracer to estimate the percolation rate. However, the instability of vadose zone in time scale is likely to make the result deflected. Thus, taking the best advantage of the information from the stable moisture zone in the thick vadose zone will be the new direction for application of temperature tracing.Phreatic water evaporation is the main consumption item of the groundwater in areas with shallow water table. With the water table depth of lysimeter increases, the effect of the phreatic water evaporation gradually weakened, or even disappeared (>3m). The variation of phreatic water evaporation with water table can indicate the maximum evaporation depth and determine the effective recharge to the shallow aquifer. Theoretically, it’s a feasible plan to use the bromide as a tracer to evaluate the quantity of phreatic water evaporation and the key of it is to avoid the infiltration water.Precipitation and irrigation are the main source of vertical groundwater recharge. The annual and seasonal change of rainfall will lead to the variation law of recharge quantity, which can be exemplified by the research in Hengshui area. The annual rainfall varied obviously in recent 60 years in the research area (262.8～934.5mm, coefficient of variation is 0.28). The annual recharge quantity has significant positive correlation with annual precipitation; in dry year, the annual recharge quantity is 30mm/a, much lower than the 120～150mm/a in wet year. The average annual recharge is 66.6mm/a and the average recharge coefficient is 0.13. Agricultural irrigation increased the water input at the surface, and the recharge quantity increases with irrigation volume. In this research area, the recharge coefficient of each single irrigation during the growth of winter wheat and summer maize (separately in October, February, March, April, May and July) is 0.54,0.45,0.2,0.01,0.17 and 0.73, respectively. The large recharge coefficient in October and July indicate that the irrigation volume of the experimental field is too large. The reasonable irrigation quota and frequency should be based on the actual precipitation.As an important discharge item of the water balance of surface vadose zone, surface runoff controls the water volume infiltrated into vadose zone at the land surface. It is mainly influenced by the feature of rainfall and the lithology of vadose zone. The relationship between the single rainfall intensity and the soil saturated hydraulic conductivity determines the formation and quantity of surface runoff. There is no significant impact on surface runoff from the water table depth.(3) The law of the vertical groundwater recharge influenced by the lithology structure of vadose zone.The complex lithology characteristics of vadose zone can be generalized to three lithology structure modes:the homogeneous vadose zone, vadose zone with heterogeneous lenses and heterogeneous layered vadose zone. For the homogeneous vadose zone, the fine lithology will prevent the surface infiltration and conduce to cause the surface runoff. The finer the lithology is, the more lagging the recharge process is, and the recharge rate tends to be stable. The soil hydraulic parameters are the quantitative expression of the soil lithology. The analysis of linear sensitivity shows that θs, n, and Ks have a high sensibility to the variation of vertical groundwater recharge, and n, Ks positively correlated with recharge quantity and recharge process. However, the θs and Ks will change at the same time with soil porosity actually. The smaller the soil porosity is, the θs and Ks will be smaller, leading to a poor permeability. The maximum evaporation depth is significantly affected by the n and a. The reasonable analysis of parameter sensitivity needs to combine the study of their physical meaning.The lenses in vadose zone can be divided into two types:lenses with low-permeability and high-permeability, both of which will change the infiltration pathway, leading to the change of recharge process. The low-permeability lenses will restrict vertical infiltration and form faster flow around the lens relative to flow inside of the lens; while the high-permeability lenses are the preferential infiltration pathway, through which the water can recharge the shallow aquifer as soon as possible.The permeability, quantity and thickness of the heterogeneous layer in vadose zone are the main factors that affect the recharge process. The low-permeability layers will restrict the infiltration and make the recharge process more lagging, while the high-permeability layers are conducive to infiltration. However, the buried depth and combination of layers have no significant impact on the recharge process.The potential recharge quantity is determined by the relationship between the buried depth of lens or layer and the maximum evaporation depth. When the water table is above the maximum evaporation depth, the low-permeability lens or layer will make the evaporation increase and reduce the potential recharge; while the effect of the high-permeability lens or layer is exactly opposite. But if the lens or layer stands below the maximum evaporation depth it will not influence the potential recharge quantity. Meanwhile, the shallow high-permeability lens or layer will increase the permeability of the surface and reduce the surface runoff, finally increasing the infiltration quantity.(4) The variation of vertical groundwater recharge in the typical areas of the North China Plain.The three typical areas of the North China Plain (west piedmont plain, central plain and east coastal plain) show different recharge feature in the past fifty years due to the difference in sedimentary characteristics and water table dynamic. In 1960s, the water table depth has already been larger than the maximum evaporation depth in the piedmont, which means the recharge quantity there is stable; the sharp decline of water table mainly affects the recharge process. When the water table is shallow, there is a positive correlation between the annual precipitation and the annual recharge. And the decline of water levels will lead to lagging recharge process. What’s more, the heterogeneous lenses structure of the vadose zone in the piedmont plain will make the recharge process from showing the single recharge peak to more peaks. In 1980s, the water table had fall below the maximum evaporation depth in central plain, the sharp decline of water table hereafter only can change the recharge process as well rather than the recharge quantity, In the coastal plain, the water table has been dropping from the zone of variable recharge quantity to the one of stable recharge quantity during the past fifty years. The decline of water table has weakened the phreatic water evaporation, and then made the effective recharge increase. It is seems more meaningful for the renewal of the groundwater resources only from the perspective of recharge quantity.The major innovations are as follows:(1) The analysis mentality about the effect of water table decline on vertical groundwater recharge was proposed to reveal the variation of vertical groundwater recharge and to evaluated the recharge quantity.(2) The "effective recharge" was put forward. It enriches the understanding of vertical groundwater recharge in shallow gourndwater areas.(3) Analyzing the effect of multipile structures of vadose zone on vertical groundwater recharge, and elucidating the spatial-temporal disparities of recharge characteristics in the North Chian Plain during the past fifty years. |
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