Estimating regional losses of soil water due to the conversion of agricultural land to forest in China's Loess Plateau

Afforestation on the Chinese Loess Plateau (CLP) has been extensively implemented by the central government over the past decades to control soil erosion. The conversion of agricultural land to forest, however, has led to decreases in soil-water storage (SWS), which may in turn limit tree growth and threaten the health of ecosystems in the region. This study estimated the regional patterns of losses of soil water (Delta SWS) following conversions across the CLP. Soil-water content at 0-5.0 m was measured in 169 forests on the plateau, and the initial preafforestation SWS at each sample site was then estimated using stepwise regression. The mean.SWS in the 1.0-to 5.0-m profile across the study area was 203.7 mm, with an estimated annual average Delta SWS rate of 16.2 mm/ year. Delta SWS and its main contributing factors varied amongst 3 rainfall zones. Delta SWS generally increased with mean annual precipitation (MAP), Delta SWS depended primarily on tree age in the > 550 mm MAP zone and on slope gradient and initial SWS in the < 450 mm MAP zone. This result suggested that the vegetation might be more important than soil or topographic properties for estimating Delta SWS following the conversion of farmland in the wettest area of the CLP. Our study also suggests that MAP, tree age, slope gradient, and initial SWS have important effects on Delta SWS, which vary with rainfall. Understanding the regional hydrological effects of afforestation is necessary for the efficient management of soil-water resources on the CLP and in other water-limited regions.