干旱区生态环境知识资源中心

The processes and rates of groundwater recharge in arid and semiarid environments are highly related to local climate parameters, particularly precipitation. The chloride profile of an unsaturated zone in an arid and semiarid region can be used to infer the recharge history and past changes in climate, by extension. In this study, a 17-m chloride profile was collected from the sandy loess in the northwestern Chinese Loess Plateau, which also lies in the transition zone between desert and loess. A 71-year groundwater recharge history was reconstructed using the chloride mass balance method with an annual Cl- input of 0.84 g/m(2)/year. The reconstructed history revealed a long-term decline in recharge with multiple shorter-term oscillations. Five recharge stages between 1938-1946, 1947-1955, 1956-1975, 1976-2000, and 2001-2008 AD were identified, where the lowest average recharge value was 25.1 mm/a in 1976-2000 AD and the highest was 71.7 mm/a in 1947-1955 AD. Climate conditions during these five periods were also inferred based on the reconstructed recharge rates with the knowledge that high recharge corresponds to more humid climates. The climate over the past 71 years generally became drier in the study area, despite some fluctuations. The reconstructed recharge rates, calculated from 1/Cl- in the profile, exhibited the same variability as annual precipitation measured in the region, both in long-and short-term oscillations over the period from 1955 to 2008. The chloride concentration variations in the profile, indicating changes in recharge flux, also well correlated with annual precipitation anomalies in the region to the east of 100 degrees E in China for the whole study period. These comparisons verified that it is feasible to study not only groundwater recharge, but also past climate change using a chloride profile from the sandy loess area. The results suggested that unsaturated zones of sandy loess may be valuable archives for reconstructing recharge history and regional paleoclimate changes in the region.