Evaluating potential groundwater recharge in the unsteady state for deep-rooted afforestation in deep loess deposits

Groundwater recharge reduces due to high transpiration from shallow-rooted to deep-rooted afforestation. However, reaching a steady state in recharge process is challenging and no methods are available for assessing potential groundwater recharge under unsteady state. Hence, this study developed a new method to quantify groundwater recharge in the unsteady state by (1) calculating the water age (A₂) at maximum root depth (D₂) for deep-rooted afforestation using the chloride accumulative age method; (2) determining the soil depth (D₁) corresponding to A₂ under shallow-rooted vegetation using the multi-year average pore water velocity multiplied by A₂; (3) calculating the reduction in groundwater recharge (∆R) from shallow- to deep-rooted afforestation as the depth difference multiplied by the average water content between D₁ and D₂, divided by stand age. The average groundwater recharge for deep-rooted afforestation is equal to the average annual groundwater recharge under shallow-rooted vegetation minus ∆R. Soil cores with >25 m soil profiles below four land-use types of Hippophae rhamnoides Linn. (H. rhamnoides), Platycladus orientalis (L.) Franco (P. orientalis), Robinia pseudoacacia L. (R. pseudoacacia), and grassland were collected to measure soil water content, root distribution, and chloride and tritium contents. The results revealed that: (1) maximum root depths were 11.0 ± 0.5, 20.2 ± 1.2, and 22.6 ± 0.8 m, with soil water deficits of 373.48, 823.65, and 1847.92 mm under H. rhamnoides, P. orientalis, and R. pseudoacacia, respectively; (2) groundwater recharge following land-use change has not reached a steady state; (3) an average annual groundwater recharge was 89.12 mm yr^-1 under grassland, amounting to 16 % of the average annual precipitation; deep-rooted afforestation did not significantly differ, with 83.55, 84.91, and 85.65 mm yr^-1 under H. rhamnoides, P. orientalis, and R. pseudoacacia, respectively. This study contributes to a rational assessment of groundwater resources under unsteady state during land-use change.