Effects of apple orchards converted from farmlands on soil water balance in the deep loess deposits based on HYDRUS-1D model

Land use change (LUC) impacts on the soil water balance is important for effective water resources management and land use planning. The Loess Plateau of China has loess deposits up to 350-m depth and constitutes large reservoirs of soil water storage. In recent decades, areas within these reservoirs have been depleted of their water storage. LUC impacts on soil water storage have been previously investigated in this region; however, LUC impacts on other components of soil water balance such as evapotranspiration and deep drainage have received limited study because of difficulties in direct measurement of these components. Using continuously monitored 10-m soil water profiles under farmland and apple orchards converted from farmlands for 10, 20, and 30 years for the period 2011-2013, the HYDRUS-1D model was calibrated and then employed to evaluate long-term LUC impacts on different components of the soil water balance in a typical loess tableland based on climate data for the period 1960-2013. Compared with farmlands and young apple orchards (stand age< 10 years), the measured soil water storage under mature apple orchards (stand age> 20 years) was significantly decreased over time. The simulated deep drainage was 12.1 mm year(-1) under farmland and accounted for 2% of the annual average precipitation, but this value was reduced to near zero under mature apple orchards. The simulated average annual actual evapotranspiration was 565.8 mm and represented 98% of the average annual precipitation under farmlands, but the evapotranspiration was increased under mature apple orchards. The LUC-induced decrease in soil water storage and groundwater recharge threatens the sustainability of water resources and agriculture on the Loess Plateau. The balance between economic development and agriculture ecosystems and environmental sustainability are, therefore, important considerations in future land use planning.