Land use change impacts on the amount and quality of recharge water in the loess tablelands of China

Exploring how land use change (LUC) influences the amount and quality of recharge water is important for groundwater sustainability and land use planning. With loess of up to 200 m in thickness and unsaturated zones up to 100 m below the surface, the loess tablelands in China can store abundant groundwater resources. However, groundwater depletion and substantial LUC have been simultaneously observed. It is thus necessary to investigate the relationship between LUC and groundwater. We sampled 10-m soil profiles for three land use types (farmlands, apple orchards of 10 and 20 years old). After measuring the chloride and nitrate concentration in soil pore water, the LUC effects on the amount and quality of recharge water quality were quantified based on the mass balance method. Results showed that soil moisture in aged (20-year-old) apple orchards was significantly reduced relative to that measured in farmlands and younger (10-year-old) orchards, where measured soil moistures were roughly equal. The accumulated nitrate nitrogen and the depth below which nitrate is stable was smallest in farmlands, intermediate in 10-year-old apple orchards, and largest under 20-year-old apple orchards. The diffuse recharge was 33.0 +/- 17.9 mm year(-1) accounting for 7.3 +/- 1.8% of mean annual precipitation under farmlands; however, the conversion from farmlands to 20-year-old apple orchards reduced recharge by 42%. The nitrate infiltrating to groundwater annually was 4.9 +/- 2.9 kg hm(-2) and 4.1 +/- 3.1 kg hm(-2) under farmlands and 20-year-old apple orchards, respectively. The impacts of LUC over the past decades have not yet reached groundwater because of low recharge rates; further, the primary factor influencing groundwater quality is recharge rate rather than pollutant concentration. As such, the LUC from farmland to apple orchard has little impact on shortterm groundwater recharge and quality; long term impact, however, may be significant. (C) 2018 Elsevier B.V. All rights reserved.