Simulated water balance of forest and farmland in the hill and gully region of the Loess Plateau in China

Abstract

Large-scale vegetation establishment has not only helped to prevent serious soil and water loss on the Loess Plateau of China but also led to reduced water availability and soil desiccation. Examining the water balance of forested and farmed land allowed us to determine water consumption of vegetation and understand the mechanisms of soil desiccation, important factors for the sustainable conversion of farmland to forest in this area. The effects of forests and crops on the water balance within the soil–vegetation–atmosphere system were studied using a coupled water and heat flow model known as “CoupModel”, which was calibrated on the basis of field measurements of soil water content, surface runoff, throughfall, stemflow, leaf area index, vegetation cover, canopy height and depth of root system in the hill and gully region of the Loess Plateau. Data were collected between 27 May 2006 and 31 October 2007. Two types of planted vegetation (acacia forest and farmland) in the Yan'gou watershed in the northern part of the Shaanxi Province were chosen to examine water transfer in the absence of soil texture (silt loam) differences and to explore the importance of vegetation type in relation to water balance. The simulations indicated that vegetation type and slope aspect significantly influenced the magnitude of every water balance component in the soil–vegetation–atmosphere system. Compared to forest, farming the land reduced interception (69.1–78.3%) and transpiration (37.3–40.4%), while increasing surface runoff by a factor of 5.0–40.2 and soil evaporation by 3.1–32.0%. Thus, farming resulted in 1.1–1.2 times more soil water storage than afforestation. Therefore, canopy interception and transpiration are responsible for soil desiccation in plantation forests. Simulation differences in water balance between acacia forest and farmland highlight that when converting farmland to forest on the Loess Plateau of China, tree species should be selected with care; this is particularly important on south-facing slopes.