ATMOSPHERIC PRECIPITATION RECHARGE ON GROUNDWATER IN DESERT AREAS AND ITS ENVIRONMENTAL IMPLICATIONSA CASE REVIEW OF THE LATE-HOLOCENE RECORDS FROM THE ALASHAN PLATEAU
A difficult but key question for hydrologists is concerned with the choice of the right methods for quantitatively evaluating the groundwater recharge and its related environmental response in arid lands. In this paper, we reviewed the literature to assess the current research work and existing problems related to the tracing methods and theories selected to study the unsaturated (vadose) zone recharge in desert. A lot of information about the regional groundwaters,their recharge and evolution was obtained from case studies on the Alashan Plateau in Northwestern China,aiming to shed light on the history of related palaeoenvironmental change in this desert during the Late Holocene. Based on chronological sequences and calculated recharge rates in multiple unsaturated zone profiles from the desert using the chloride mass balance theory,results indicate that the Badanjilin desert in the central part of the Alashan Plateau and surrounding Gobi areas have experienced several centennial-scale fluctuations in groundwater recharge during the Late-Holocene in the last 700 to 2000 years, which were closely related to the wet-and-dry fluctuations in regional climate. Four relatively humid (1330 ~ 1430A.D.,1500 ~ 1620A.D., 1700 ~ 1780A.D. and 1950 ~ 1990 A.D.) and three relatively arid phases (1430 ~ 1500A.D.,1620 ~ 1700A.D. and 1900 ~ 1950A.D.) are discernable across these unsaturated profiles. These records are consistent with other palaeoclimatic records from the northern fringes of the Qinghai-Tibetan Plateau and are somewhat corresponding to the climatic records from Eastern China, indicating that the groundwater recharge history on the Alashan Plateau broadly reflect centurial timescale precipitation changes in the large areas of Northwestern China. It also suggests that variations in the East Asian Summer Monsoon intensity could have affected the desert recharge rates. The evaluated average groundwater recharge rate is about 1. 3~2. 6mm/a on the Alashan Plateau over the last thousand years. This evaluation provides a new geological evidence for the understanding of the sources of the regional groundwater recharge but is somewhat conflicting with previous conclusions derived from other environmental proxy records from the plateau. We point out that the environmental records derived from unsaturated profiles have many uncertainties,caused mainly by the assumptions about the atmospheric origin of the Cl input in the chloride mass balance theory and the selection of homogeneous structure of soil unsaturated zone (piston-flow profile). We suggest that a broader test is needed to examine the uncertainties of the atmospheric Cl-input and their error ranges through comparison between the deterministic data (such as regional base weather station) and the large-scale random Cl input settings of dust and precipitations (dry and wet deposition) in Northwestern China. Sedimentological, hydraulic and geochemical evidences should also be provided for the potential constraints on the structure homogeneity of unsaturated profiles, the relative importance of soil moisture transport mechanisms between horizontal and diffuse processes, and the chloride-bulge-shaped curves of salt sequences indicating characteristic periods in unsaturated zone profiles.
1.中国科学院地理科学与资源研究所, 中国科学院陆地水循环及地表过程重点实验室, 北京 100101, 中国 2.中国科学院地质与地球物理研究所, 中国科学院新生代地质与环境重点实验室, 北京 100029, 中国 3.Centro de Investigaciones en Geografia Ambiental, Universidad Nacional Autonoma de Mexico, Campus Morelia, Mexico