Deep rooted apple trees decrease groundwater recharge in the highland region of the Loess Plateau, China.

Unlike recharge in shallow rooted ecosystems, estimating the groundwater recharge beneath deep rooted plants that absorb water from deep soil remains difficult. The purpose of this research is to develop an approach to estimate the groundwater recharge beneath deep-rooted vegetation by combining water mass balance and chloride mass balance (CMB) and to quantify how the conversion of shallow-rooted cropland to deep-rooted apple orchards changes groundwater recharge. The proposed groundwater recharge rate under deep-rooted vegetation in this study is the difference between the groundwater recharge rate in a cropland (obtained using CMB) and the mean annual soil water storage deficit beneath an adjacent deep-rooted vegetation. The results show that the conversion from cropland (shallow-rooted) to apple orchard (deep-rooted) decreased soil water storage by 776, 1106, and 1117mm, corresponding to 19, 20, and 26-year-old apple orchards, respectively. Groundwater recharge beneath cropland, on average, was 58mmyr-1 , which amounts to 10% of the average annual precipitation. Groundwater recharge beneath the apple orchards were variable, but all being <3% of the average annual precipitation. The conversion of cropland to apple orchards lead to a substantial decrease in groundwater recharge, potentially threatening the sustainability of the land use change.