Estimating impacts of water-table depth on groundwater evaporation and recharge using lysimeter measurement data and bromide tracer

The presented study evaluates the effect of water-table depth on groundwater evaporation (E-g) and recharge (R-g) when they occurred alternately. A lysimeter experiment incorporating a 1-year-long bromide tracer test was conducted under conditions with a range of maintained water-table depths. The results revealed that both the R-g and E-g decreased as the water table fell, until it was down to the extinction depth of groundwater evaporation (EDGE, 2.4 m). The annual quantity of R-g started to be stable at 100 mm when the water table was below the EDGE, since the maximum soil-water deficit no longer increased. When the water table was above the EDGE, R-g and E-g restricted each other and thus occurred alternately; in the wet season, >68% of the annual R-g occurred, with only E-g. The fast response of the soil-water potential to irrigation and soil evaporation tended to make the gradient of the whole potential profile unidirectional when the water table was shallow, which promoted both R-g and E-g. Taking soil evaporation, R-g and E-g into account, the inversely calculated position of the bromide concentration peak was close to the actual position, suggesting that bromide tracer is effective for tracing the complicated processes of the unsaturated zone flow when R-g and E-g occur alternately. The study improved understanding of the way that the water table affects R-g and E-g in the shallow groundwater area and proved that bromide tracer would be an innovative technique to estimate E-g.