Groundwater Recharge History And Its Responses To Climatic Change

In the Earth system, the hydrological cycle in terrestrial water systems is the most critical and active link among the interactions of the atmosphere, the Hydrosphere, the lithosphere and the biosphere. The hydrological cycle in the continent, which affected by the climatic and environmental changes and human activities, controls the formation of water resources and regional environmental changes. In China, the temporal and spatial distribution of precipitation is uneven, resulting the many acute environmental problems, such water shortages, floods and soil erosion, and degradation of the ecosystem. In the context of global change, understanding the variability and future trends of the hydrological cycle in continent is not only a major issue in the national need for the water security, but also a basic scientific question in the Hydrology and Earth system Sciences.Vadose zone located in the interfaces of the atmosphere, lithosphere, hydrosphere and biosphere. It plays a key role in transferring the precipitation into groundwater; and is the key to understand the transforming of matter and energy among the different spheres. Global climate change not only causes the variation in groundwater level and discharges, but also leads to the re-distribution of groundwater resources in space. These changes not only involve all kinds of the water exchange within the aquifer, but also affect vadose zone water rapidly and strongly. Therefore, vadose zone is the critical entry points for studying the regional groundwater recharge history and the important linkage between the global climate change and groundwater evolution.Desert-loess transition zone in northern Shaanxi is located in the fringe area of the East Asian summer monsoon, and in the vulnerable arid and semi-arid zone in China. The precipitation in this region is from 200 to 400 mm, while the evaporation is more than 2000mm. Thus, this region is characterized by the shortages in water resources, fragile ecological environment, and high sensitivity to climate change. Since this area is rich in oil, gas and mineral resources, it has become a new energy and chemical base in China. However, due to severe shortage of surface water, and great potential for groundwater exploitation, water requires in energy development will inevitably depend on the limited groundwater resources. For this reason, the sustainable developing and utilizing of groundwater resources is a major issue that related to the ecological and environmental protection, local economic development. Therefore, the study of the groundwater recharge history in this area has important theoretical and practical significance in understanding the historic background and the potential of groundwater resources; in mastering the storage of the water resources and its dynamics; and in the rational development, utilization and management of groundwater resources.On the other hand, since the area is located in the desert-loess boundary and the thickness of the unsaturated zone are quite different, i.e., from 1 meter to over 20 meters, relatively thicker vadose zone can record the groundwater recharges on a longer time scale. Thus this region is one of the best areas to study the history of climate change, groundwater recharge, and their relationship as well.In this study, we drilled four cores in this desert-loess transition zone in northern Shaanxi, and systematically measured chloride ion concentration, water content and other indicators. Based on these acquired data, we extracted the hydrological and climatic information documented in the vadose zone, reconstructed the history of the groundwater recharge, and revealed the possible influencing mechanisms of climate change on the groundwater recharge. We also reconstructed the regional climatic and environmental changes by using the proxy index of chemical elements in vadose zone water.This thesis focuses on four aspects of the study:(1) Base on groundwater water chemistry analysis of four drilling cores from the desert-loess transition in northern Shaanxi, we obtained the records of Cl" concentration in these four profiles and reconstructed the recharge history of the study site by using the traditional and generalized method of the chloride mass balance; (2) Compared with the local instrumental meteorological records, we discussed the responses of the groundwater recharge to climate change; (3) Reconstruct the history of climate changes over the last hundred years in the study region (4) suggest the future direction and focus of the water resources development and utilization in the context of global climate change.The folio wings are the main conclusions:1. Validating that the reliability of chloride ion as a tracer of the groundwater recharge, and reconstructing the recharge history in the study site with the unsaturated zone chloride profiles records.The chemical analyses of water in the four unsaturated profiles show that the average Cl" concentration is similar, but that of SO42-is quite different. Considering both Cl'and SO42-in the vadose water are of atmospheric origin, it may be concluded that the Cl" concentration in the atmosphere is largely controlled by the distance from the land to sea, but the SO42- concentration is affected by smaller spatial factors. Thus Cl" is proved to be used as a tracer to record groundwater recharge. But may not be a suitable tracer because of its variability in space.Based on the Generalized Mass Balance method, the recharge history can be obtained from the three unsaturated profiles. GZ profile records three relatively stable period of groundwater recharge, from the year of 1990 to the year of 1991,1992 to 1996, and 1997 to 2000; XSG profile also records three stable periods, that is 1981 to 1991,1992 to 1995,1996 to 2001. And GQH profile records four stable stages. They are 1967 to 1970,1971 to 1975, 1976 to 1995, and 1996 to 2003. The average fraction of precipitation that infiltrated into the subsurface during the longest stable stage recorded by the three profile are 14.5%,12.6%, 12.4% respectively, and corresponding recharge rate are 53.4mm,47.2mm and 47.1mm.2. Testifying that the precipitation dominate the regional groundwater recharger, and at the same time, revealing that temperature, vegetation and other factors may also have influence on the groundwater recharge. We also proposed that chloride ion concentration of vadose zone water, which faithfully record changes in regional climate and environment, can be used as a reliable proxy of climate and environmental change.The comparison of the groundwater recharge history recorded by the three chloride profiles and the meteorological records (precipitation and temperature) indicated that yearly rainfall is the key factor to control the groundwater recharge. The similar variation of the Cl" concentration for the three profiles also proves that the same recharge history recorded by the three profiles is attributed to the climatic change.Apart from yearly rainfall, however, there are other meteorological factors that could affect groundwater recharge, such as the distribution of rainfall, temperature and vegetation variation resulted from climatic change. All of these factors make the groundwater recharge respond to yearly rainfall in some anomaly ways. In the other words, temperature and vegetation variation may increase or decrease the responses of groundwater recharge to yearly rainfall. At some times, groundwater recharge could have an opposite trend to that of yearly rainfall.3. Reconstructing the history of climate changes over the last hundred years in the study region. Conventional CMB method was used to obtain the groundwater recharge history kept in HS profile during last nearly a hundred years. The comparison between the groundwater recharge rate indicated by the Cl" concentration and actual precipitation record shows that groundwater recharge and yearly rainfall have some but not absolutely linear relationship. The trend of groundwater is mainly affected by yearly rainfall. So it is reasonable to get information on precipitation variation during the times without actual precipitation record. The record in HS profile tells us that from late 1930's to early 1940's it should be arid with scarce rainfall. But during the middle 1940's 1950's, the precipitation increased substantially, during which the climate is the wettest in the last nearly one hundred years.4. Putting forward the future direction and focus of the water resources development and utilization in the context of global climate change.Since the desert-loess transition is an area sensitive to the climatic change, the groundwater there is influenced greatly by climate. Groundwater responds to climatic change at different scales, from seasonal, decadal to millennia. Deep water in the area is palaeowater, which is recognized as irreversible water resources. For this reason, shallow groundwater should be the main source to supply water. Shallow water responds more quickly to climatic change and vulnerable to the opposite effects caused by climate change. In order to explore the groundwater resource in a sustainable way, more attentions should be attuned to the effects of climate.This thesis is an attempt to integrate both the hydrological cycle and climate changes study in this typical arid and semi arid area in China. It is topic is also a new field of Earth System Science study in the world. As the beginning of this new exploration, more in-depth research from different angles of view is necessary in the future.