Simulation Of Water Cycle In Typical Region With A Deep Groundwater Table In North China Plain

Due to the long-standing groundwater overexploitation,larger and larger area has been being with a deep groundwater table.Deep vadose zone changes the conditions and characteristics of the local water cycle.Luancheng County,located at the piedmont region of North China Plain,is a typical regions of irrigated agriculture with deep groundwater table.The interactions between surface water and groundwater depends much on well irrigation and the percolation among the deep vadose zone,thus the water cycle is dominated by the vertical soil hydrological processes.This study,based on a field experiment,focused on the soil hydrology,and simulated the water cycle to discover its features in regions with deep groundwater table.Significant variations can be observed at the depth between 2 and 4 meter during the rainy season,July and August;The vadose zone below 5 meter maintains relatively stable soil moisture and matric potential with single peak of matric potential,whose scope is small.Simulation results also show that the water potential gradients at deep vadose zone are not always steady.Based on a field experiment carried out with cooperation of the Luancheng Station,Chinese Academy of Science,the study investigated the soil moisture dynamics and explored the features and pattern of variation of vertical hydrological fluxes in deep vadose zone.Further,a Richards' equation based soil moisture dynamics simulation model was established,which details the processes of the root uptake in typical farmlands in North China Plain.In typical regions with deep groundwater table,the soil moisture in root zone is mainly discharged by evapotranspiration.Though,large deep percolation also exist in wet years,and it showed that the deep percolation reach about 27%of the total water input,including precipitation and irrigation,during April and September in 2012 and 2013.In addition,the root zone did not get water supplyment from the vadose zoen below the root zone.Root uptaking in deep layers of the root zone is enhanced when the upper layers are dry.The soil moisture dynamics model was adopted to simulate the hydrological fluxes under several scenarios,to reveal their variations under different hydrolgocial years,potential changes under the strict water-saving measures and climate change,respectively.And the results showed that average of annual ET is 712mm,while the average of annual deep percolation is 141 mm,amounting to 16.5%of the sum of precipitation and irrigation?Under strict water saving irrigation measures,the annual irrigation amount decreased from 392 mm to 220 mm,and the ET will decrease by 15.2%?21.7%,and the deep percolation will decrease by 12%?45%.Though much uncertainties exsit in the changes of hydrological fluxes under climate change,the increase of ET caused by the changes of meteorological factors such as temperatures may be more than the increase of increase of precipitation.And this may lead to the annual average of deep percolation decreasing from 141 mm between 2001 and 2015 to 128mmbetween 2036 and 2050,making the gap between the groundwater exploitation and groundwater recharge increase by 10.8%,reaching 278 mm.The soil moisture dynamics model is also incorporated into a distributed hydrological model,WACM 4.0,to reconstruct its physical mechanism and applicability in study area with a deep groundwater table.The revised WACM 4.0,applied with the MODFLOW model,realized the coupling simulation of surface water,soil moisture and groundwater.And the coupling model was calibrated and validated by the observation data of 2012 and 2013.Results showed that the specific yield of the unconfined aquifer is about 0.03.The simulation results of hydrological fluxes,including evapotranspiration and deep percolation,at the regional scale and the local scale,show significant difference,which may be due to the existence of different land use types.In addition,the recharge to the aquifer is relateively stable after the long percolation processes in the deep vadose zone,and the recharge to the aquifer at a certain year may differ significantly from the potential recharge,or deep percolation at the bottom of the root zone of the same year.Also,the flucations of groundwater tables indicates a regular pattern with the irrigation pumping.The groundwater table decreased rapidly in the periods of irrigation,reached its nadir at June,and recover in the latter half of years.