The Changes Of Vadose Zone And Its Impact On Groundwater Recharge

Groundwater is the main fresh water resources in the North China Plain (NCP for short), where precipitation makes up the majority part of recharge. For decades, the water table keeps going down and develops cone of depression due to agriculture and domestic over-exploitation, which leads the infiltration recharge processes becomes even more complex. The NCP is one of the most significant agricultural production base of cotton and grain, which consuming up to70%of the total water use. Irrigation water supply is always beyond the crop growth requirement at plenty of farmlands in the NCP, making the irrigation return flow becoming a large portion of recharge. Many studies have been conducted to estimate groundwater recharge at (semi) arid region at home and abroad (including the NCP). However, it’s still a challenge due to the uncertainties of recharge esimation and the complexity of unsaturated water flow. Thus, it is badly necessary to carry on an accurate estimation of groundwater recharge in the NCP, which would prevent water-table decline, help to establish reasonable exploitation plan and irrigation regime, and better preserve groundwater resources. At the same time, the recharge rates obtained in this study will be used as input in a mathematical modeling effort designed to simulate the regional groundwater system in the NCP.The objective of this article is to research on the changes caused by long-term over-exploitation of groundwater and its impact on groundwater recharge through previous research collection, field investigation, field and laboratory experiments and instrumental analysis, based on vadose zone hydrology and solute transport theory. We carried out the following works.1. A tracer test, lasted for more than7months, was conducted at an unsaturated-saturated field. Water flow and solute transport were visualized three-dimensionally by Tecplot software using the monitoring data. Moment analysis method was also applied to quantitative analyze how water infiltrate and solute transport vertically in heterogeneous vadose zone.2. Estimation of vertical groundwater recharge was carried out at both piedmont plain (Shijiazhuang) and alluvial and lacustrine plains (Hengshui) in the NCP, where the groundwater is over-exploitation for decades. Mass balance method, applied bromide tracer method, Darcy’s method and one-dimensional unsaturated flow numerical models were chosen as recharge evaluation methods. In addition, plenty of previous research results were obtained as comparison. All of the above estimations were mutually authenticated to enhance the credibility of the results.3. The impact factors of vertical groundwater recharge were discussed from the points of vadose zone changes (thickness and structure) and external factors (land use and extreme climate) based on the above results.Here come the main conclusions:1. Tracer tests conducted at unsaturated-saturated heterogeneous fieldDuring the infiltration, both chloride and tritium would transport around the low-permeability lenses, and the phenomenon was more obvious for chloride in a heterogeneous field. Temporal moment analysis of six control locations (including24points) shows:1) tritium had shorter arrive time than chloride due to slight adsorption of chloride.2) Longer tailings were observed at the tritium’s elution curve than chloride’s curve.3) From an overall view, water flow and solute transport could keep as piston flow if the high-permeability medium had a good enough connectivity.4) The velocity of tritium was lower than recharge rate due to the rejection phenomenon between the negative charged soil particle and chloridion.2. The estimations of vertical groundwater recharge rates were mutually authenticated by multi methods.The results of mass balance method at Shijiazhuang indicated that F-, Cl-and SO42-were suitable tracers beneath the non-irrigated woodland, yielding recharge rates of16.9,18.8and19.4mm/a, respectively. Recharge estimation was not straightforward when taking account of crop type, irrigation and/or fertilizer use. After comparing with previous research, conclusions were drawn:Cl-was an appropriate tracer for irrigated farmland when taking account of Cl-input from irrigation and absorption by crops; recharge rates were65.9-126.8mm/a.The results of applied bromide method shows:the recharge rate ranged between54.5-466.4mm/a beneath wheat-maize (3of the subsites calculated during wet season),110.1-564.3mm/a beneath orchard, and0-20.5mm/a beneath woodlands with an average recharge coefficient of0.173for the Shijiazhuang sites, while the recharge rate ranged between28.9-164.8mm/a beneath wheat-maize,11.9-114.6mm/a beneath orchard,87.3-186.3mm/a beneath cotton, and0-32.2mm/a beneath woodlands with an average recharge coefficient of0.100for the Hengshui sites.The infiltration recharge rates at sampling time were25-240mm/a at the depth of8.0-21.0m at Zhengding, Hebei site according the Darcy’s equation.The boreholes used by mass balance method and Darcy’s method were selected to establish one-dimensional unsaturated flow numerical models. The estimated recharge rates and coefficients were similar with the other methods, which inferred the credibility to use Hydrus1D. The recharge rates (recharge coefficients) were48-89mm/a (0.053-0.096) beneath wheat-maize,12.3mm/a (0.024) beneath non-irrigated woodlands and83.9mm/a (0.165) beneath natural grass ecosystem. In this study, the recharge rates beneath irrigated farmlands ranged between65.9-126.8mm/a at Shijiazhuang sites calculated by chloride mass balance method,54.5-466.4mm/a at Shijiazhuang sites and11.9-238.8mm/a at Hengshui sites calculated by applied bromide method and48.0-89.0mm/a at Shijiazhuang sites by one-dimensional unsaturated flow numerical models. As comparison, the recharge rates gained through this study were believable.3. The impact factors of vertical groundwater recharge①Vadose zone changes impact on groundwater recharge.The results beneath winter wheat and summer maize farmland, gained by mass balance method, showed that the recharge seldom changed with varying thickness (ranged between5and10m) of vadose zone, which indicated recharge would stay stable as the thickness of vadose increased below a certain depth.Bromide tracer tests obtained the recharge coefficients of piedmont plain and alluvial/lacustrine plains were0.173and0.100, respectively. The results showed that recharge rates were lower for the alluvial and lacustrine plains sites, which comprise finer-textured soils than those present in the piedmont plain.Soil hydraulic parameters and infiltration capacities would change because of water table fluctuated-declining and drainage consolidation, which, of cause, would impact on vertical infiltration recharge. If the velocity of water table decline was fast at a certain depth historically, the unsaturated conductivities with same water content should also have large values, indicating the soil having large infiltration capacity.②External factors impact on groundwater recharge.The tracer concentration beneath irrigated winter wheat and summer maize (DH01) was larger than that beneath non-irrigated woodland (DH02) due to the higher tracer concentration in irrigated water. Irrespective of the tracers, the obtained recharge rates of DH01were larger than DH02. This inferred that irrigation enlarge recharge rate at certain extent.Irrigation provided the primary contribution to recharge, with precipitation providing a minor contribution. In general, the irrigation was larger than that required by the crops. Thus, improving the irrigation regime to match crop requirements would be helpful to better preserve groundwater resources and prevent water-table decline.Land use was changed in the one-dimensional unsaturated flow numerical models to investigate how this factor impacted on recharge. The irrigation amounts were260,130,130,0and0mm for winter wheat, summer maize, cotton, natural grass and poplar, respectively during their growth periods. The results showed that the recharge rate was the largest when only grew winter wheat, then came rotated winter wheat and summer maize, and the recharge rate was the smallest when it was natural grass land, the. In general, it was the worst to groundwater storage if grew rotated winter wheat and summer maize as the irrigation water was pumped from the groundwater. In addition, how historical extreme climate between the year of1960and2010impacted recharge was discussed. It indicated maximum daily precipitation had greater impact on recharge than continuous several days’precipitation. It could not impact the recharge if the maximum daily precipitation or the year’s precipitation was small even though the continuous rainning days reached the extreme value. Extreme high or low temperature seldom impacted the recharge.The major innovations are as follows:1. Research on vertical three-dimensional water flow and solute transport in the unsaturated and saturated zone using an experimental data set conducted in a heterogeneous field site.2. Estimated vertical groundwater recharge using mass balance method, combining three environmental tracers chloride, fluoride and sulfate to mutually authenticate the reliability of the results.3. Research on how vadose zone (including thickness and structure), climate and land use changes impacted on vertical groundwater recharge.