Evalution Of Groundwater System In The North China Plain Using Groundwater Modeling

Intensive groundwater development in the North China Plain (NCP) is evidencedby large scale groundwater depletion. Spatiotemporal variability in groundwaterdepletion is evaluated using groundwater modeling. The groundwater model wascalibrated for both predevelopment (1960s) and post-development (1960s–2008)conditions. Initial recharge estimates and hydraulic parameters were derived from theliterature and recharge was modified through calibration. The steady-state flow modelrepresenting the natural conditions was calibrated using water level contours in1959.The transient flow model for the post-development period was calibrated using waterlevel contours in1975,1984,2001, as well as105and72annual water level timeseries from1993through2008in the shallow and deep aquifers, respectively. Modelresults indicate that mean groundwater depletion was~4km3/yr from1970through2008；however, depletion varied with time:~2.5km3/yr in the1970s,~4.0in the1980s,~2.0in1990–1996；and~5.0in1997–2008. Annual groundwater depletionrates are constrained by precipitation: they were lowest during high precipitationyears and highest during low precipitation years. Groundwater hydrograph data from117time series were used to detect seasonal variations in depletion, indicatinggreatest depletion during the spring and depletion is principally by groundwaterpumpage in seasonal scale. Mean annual calculated recharge is~120mm/yr and isfocused in the piedmont district (200–350mm/yr) and much lower in the central andcoastal plains (50–100mm/yr). The calibrated model provides the best available toolto assess potential sustainability strategies that can be applied to NCP.As a critical water discharge term in regional scale water balance, accurateestimation of evapotranspiration is therefore important for estimation of water storagechange and sustainable water resources management. Using meteorological data from23stations, three models based on the complementary relationship are evaluatedagainst of estimates of evapotranspiration derived from regional water balance over1993to2008in the NCP. The CRAE model is then used to evaluate the actualevapotranspiration variation for the period from1960to2008. The estimated evapotranspiration is highly correlated with precipitation in seasonal scale, and isgenerally higher than precipitation when annual precipitation is less than500mm.The long term decreasing trend in the actual evapotranspiration can be explained bydeclining in sunshine duration and wind speed. Calculated average annual waterstorage change, represented by the difference between evapotranspiration andprecipitation, was approximately36mm, or4.8km3in the NCP. Over past50years,the cumulative groundwater storage depletion was~1,700mm, or220km3in theNCP. The difference between precipitation and actual evapotranspiration indicates anannual precipitation threshold of approximately500–600mm and about8%ofprecipitation above this threshold will potentially become recharge.Accurate estimation of groundwater recharge is important for sustainablegroundwater utilization, and proper management of groundwater resources.Groundwater recharge remains one of the principal uncertainties in the water balanceof the North China Plain (NCP). Understanding impacts of climate, surface conditionssuch as soil properties and land use, and groundwater withdrawal on groundwaterrecharge is essential for accurate estimation of recharge, and characterization of thewater balance. In this study, spatial and temporal variability of recharge wereevaluated by integrating soil water balance and one-dimensional unsaturated flowmodels. Estimated recharge was further validated/assessed using a saturated zoneflow model. Soil hydraulic parameters were estimated using pedotransfer functions.Monthly simulation of recharge over16years (1993–2008) was performed across theentire NCP. Simulated mean annual recharge is~150mm across the plain,representing18%of mean annual precipitation+irrigation, and ranging from0to360mm in the piedmont area to0to260mm in the central and coastal plain. Theseamounts broadly agree with previous recharge estimates derived using differentmethods (e.g, environmental tracers). Variability in soil texture from clay to loamysand resulted in a large range of recharge rates, but this was not the primary factorcontrolling the amount of recharge；variation within soil classes was greater thanbetween soil classes. The modeling approach allowed assessment of the effect of unsaturated zone thickness (reaching10s of meters in many parts of the NCP) onrecharge. Increasing thickness of the unsaturated zone has little effect on the longterm mean annual recharge, however it has an important control on the lag-timebetween infiltration and reaching the water table (e.g, reaching up to8years for athickness of30m in loam soils)；and the delay varies with soil type. A sensitivityanalysis allowed assessment of model outputs dependence on particular parameters；initial water content was the most sensitive parameter, indicating that antecedent soilmoisture is a key control on recharge. This work provides information towards acomprehensive assessment of groundwater recharge in the NCP；insight into theimportant parameters controlling recharge and a further set of recharge estimates forcomparison with those derived by different methodologies.Groundwater in the deep aquifer beneath an intermediate, brackish flow zoneacross the central and coastal parts of the North China Plain (NCP) is the region’sprimary water resource for irrigation and domestic use. Accurate information aboutthe groundwater age, recharge and flow regime, and its alteration by extensivegroundwater abstraction is necessary for water resource sustainability andvulnerability assessment. This paper presents the application of direct simulation ofgroundwater mean ages to the NCP aquifer through the use of a solute transportmodel, calibrated with the aid of previously reported carbon-14data, and water levelobservations from the pre-and post-development period. This provides an effectivemeans to constrain the regional flow model and improve the estimation of flowparameters, including recharge rates. The mean recharge estimated for thepredevelopment period is85mm/yr, which compares reasonably well with previousestimates. The simulated age distribution and the calibrated flow model are then usedto characterize the flow regime under natural conditions and an altered state bygroundwater pumping. The model results indicate that simulated groundwater ages inthe NCP are affected by both paleo-hydrologic conditions and extensive groundwaterpumping. Flow path analysis, water budget calculations and simulated groundwaterage distributions show that lateral flow to the deep aquifer in the NCP is limited and that the primary inflow is downward leakage from the shallow aquifer, which isenhanced by the extensive development of the deep aquifer. Transient flow modelingreflecting the post-development conditions confirms the hypothesis that widelydistributed vertical recharge and resulting flow have become the dominant processesshaping the flow patterns both in the shallow and deep aquifers in the NCP.