Impact of Afforestation on Atmospheric Recharge to Groundwater in a Semiarid Area

While the global forest area is generally decreasing, various afforestation projects have been implemented, including the large-scale Three-North Afforestation Program (TNAP) and the Conversion of Cropland to Forest Program in China, under which 0.63 million square kilometers of trees have been planted. The large-scale land use and cover change (LUCC) would affect the redistribution of precipitation and change the water cycle, especially groundwater recharge. The chloride mass balance (CMB) between precipitation and soil water/groundwater is the most widely used technique to estimate the recharge in arid and semiarid areas. However, in the case of decreased recharge following LUCC, a new equilibrium of water and chloride flux is not easily reached and the identification of steady state is the premise to use CMB. This study provided a method to assess the steady state by comparing the history of LUCC and chloride cumulative age at sampling depth and by checking the breaks in the slope of the line for cumulative chloride and soil water. The case study in the Mu Us Sandy Land, affected by the TNAP, shows that soil profiles beneath sparse grassland, shrubland, and woodland have reached steady state. However, new equilibrium has not been reached in the soil profiles beneath the dense shrubland. The estimated recharge rates beneath the plantations represent reductions from 33% to >90% relative to the surrounding bare sandy land (50-54 mm/year). The results highlight the unfavorable effects of some afforestation and ecological rehabilitation approaches in arid and semiarid areas on regional groundwater resources.