Simulation of water flow and shallow groundwater recharge in coarse-textured soils on the Loess Plateau, China

The Loess Plateau suffers serious soil and water loss, resulting in a fragile ecological condition with intense water resources deficit. Groundwater is crucial not only to guarantee regional social and economic development, but also to maintain ecosystem balance. However, the process of groundwater transformation and recharge on the Loess Plateau is still not entirely clear. The objective of this study was to investigate water movement in deep soil profile and test the possibilities of precipitation recharge of shallow groundwater in the water-wind erosion crisscross region on the Loess Plateau. Considering the limitations in current equipment and techniques for deep soil profile moisture observation, the HYDRUS-1D model was used to calculate solute soil hydraulic parameters and simulate deep profile (0-1 500 cm) soil water movement. The datasets used in the simulation included meteorological data and soil water content in the 0-600 cm soil layer in a willow field plot with aeolian sandy soil in the Liudaogou catchment in Shenmu City of Shaanxi Province, China for the period 2013-2016. The upper boundary condition was set as atmosphere boundary conditions with inputs of rainfall and potential transpiration (T_p) and potential evaporation (E_p) partitioned from crop evapotranspiration (ET₀) by using measured leaf area index (LAI) in different growing periods. The lower boundary condition was set as free drainage. The optimized hydraulic parameters were derived from a repeated inverse solution process and used to simulate soil water movement in the 0-1 500 cm soil layer. Then the depth of soil water infiltration and the possibility of shallow groundwater recharge were analyzed. The results showed a slight soil water content deficit in the 0-600 cm soil layer at both the end and start of growing season in 2014 (439 mm, normal precipitation year) and 2015 (371 mm, dry year). However, there was surplus soil water content at the end of growing season, compared with the start of growing season in 2013 and 2016 (both were wet years with 669 mm and 704 mm precipitations), and rainfall infiltration exceeded observed 600 cm depth. From the simulation results, soil water content of deep soil profile did not significantly change in 2014 and 2015 with slight and slow water movement into the deeper soil layer, while rainfall infiltration depth reached 1 100 cm in 2013 and 1 200 cm in 2016. This exceeded rooting zone at the end of 2013 and 2016, which raised the possibilities of rainfall recharge of shallow groundwater in the study area. For the 4-year simulation period, average soil evaporation was 14.87 cm·a^-1 and average plant transpiration was 33.70 cm·a^-1, soil water contents were mainly consumed in the form of transpiration. Due to high infiltration rate of the coarse-textured soil in wet years, a large proportion of rainfall was transformed into shallow groundwater. Dry and wet years alternated on the Loess Plateau with intensive rainfall variations. Rainfall infiltration in wet years was one important way of shallow groundwater recharge in the region.