Estimating groundwater recharge using deep vadose zone data under typical irrigated cropland in the piedmont region of the North China Plain

•Water content and potential in deep soil layers are used for model calibration.•Vertical recharge under irrigated cropland is both time and depth dependent.•The mean vertical recharge is approximately 200 mm/year from 1976 to 2013.

SummaryGroundwater recharge can be accurately estimated by understanding the soil water flow process in the deep vadose zone. In this study, soil water content and soil matric potential were measured in situ in the deep vadose zone (∼8 m) under typical irrigated cropland in the piedmont region of the North China Plain and were used to analyze the soil water dynamics and calibrate a transient matric flow model. Using the calibrated model, the long-period average groundwater recharge was estimated, and the influences of the lower boundary depth and time scale (length of study period) on the recharge were assessed. The study showed that the response time of the water table (with a buried depth of 42 m) to water input might be no more than 1 year because the velocity of the wetting front could be as high as 0.13 m/day below the root zone. However, the lag time could be more than 15 years because of the slower velocity of the soil water displacement. The variation in the recharge flux with depth was significant over shorter time scales. Therefore, for more representative estimated recharge with a maximum deviation less than 20% from the 38-year mean value, research should be conducted over a long period (>12 years). However, the average annual recharge showed almost no change with depth at the 38-year scale, and a depth of 2 m below ground surface could be used as an interface for estimating recharge at the 38-year scale. The simulated annual recharge at a depth of 2 m ranged from 59 mm to 635 mm with a mean value of 200 mm. The variation in water input (precipitation and irrigation) was the main reason for the variation in annual recharge at the depth of 2 m. This approach improves our understanding of the recharge process in the deep vadose zone in this region, and the results of this work could aid development of effective groundwater resources management.