Simulation of Soil Water Dynamics in a Black Locust Plantation on the Loess Plateau, Western Shanxi Province, China

Soil moisture plays an important role in vegetation restoration and ecosystem rehabilitation in fragile regions. Therefore, understanding the soil water dynamics and water budget in soil is a key target for vegetation restoration and watershed management. In this study, to quantitatively estimate the water budget of the GFGP forests in a dry year and a wet year and to explore the recharge in deep profiles, the vertical and temporal soil moisture variations in a black locust (Robinia pseudoacacia) plantation were simulated under typical rainfall events and two-year cycles in a loess area between April 2014 and March 2016. We calibrated and tested the HYDRUS-1D (Salinity Laboratory of the USDA, California, USA) model using the data collected during in situ field observations. The model’s performance was satisfactory, the R2, Nash efficiency coefficient (NSE), root mean square error (RMSE), and mean absolute error (MAE) were 0.82, 0.80, 0.021, and 0.030, respectively. For the four rainfall events of 9.1 mm, 25 mm, 71.1 mm, and 123.6 mm, the infiltration amounts were 8.1 mm, 19.3 mm, 65.2 mm, and 95.3 mm, respectively. Moreover, the maximum infiltration depths were 30 cm, 100 cm, 160 cm, and >200 cm, respectively. Additionally, in the two-year model cycles, the upward average water flux was 1.4 mm/d and the downward water flux was 1.69 mm/d in the first-year cycle; the upward average annual water flux was 1.0 mm/d and the downward water flux was 1.1 mm/d in the second-year cycle. The annual water consumption amounts in the two-year cycles were 524.6 mm and 374.2 mm, and the annual replenishment amounts were 616.8 mm and 401 mm. The amounts of percolation that recharged the deep soil were only 28.1 mm and 2.04 mm. A lower annual rainfall would cause a water deficit in the deep soil, which was not conducive to the growth of Robinia pseudoacacia vegetation. To ensure the high-quality sustainable development of the forest land, it is suggested to adjust the stand density in a timely manner and to implement horizontal terraces to increase the infiltration and supply of precipitation. Our study provides an improved understanding of the soil water movement in Robinia pseudoacacia plantations and a simulated temporal moisture variation under different time scales. The results of our study provide a feasible approach for the sustainable management of Robinia pseudoacacia plantations during vegetation restoration.