Groundwater is a vital resource for drinking water and is crucial for maintaining healthy ecosystems, particularly in regions with limited water availability. Karst areas, known for their distinctive geological formations, host some of the world's most valuable groundwater resources and are characterized by high levels of biodiversity. With global warming and intensified human activities, there are significant effects on hydrological cycles, natural ecosystems, and socio-economic development, which have also led to changes in the spatial and temporal patterns of global groundwater resources. At the same time, karst region is highly susceptible to the impacts of climate change and human activities. Nevertheless, most existing studies have focused on analyzing the effects of environmental changes on groundwater at the basin or regional scales, lacking a comprehensive global perspective on groundwater characteristics and spatial distribution. The advancement of satellite remote sensing technology in recent years has enabled large-scale, long-term monitoring of hydrological processes. This thesis investigates the complex relationship between groundwater resources and the overarching influence of global environmental changes, with a particular focus on karst regions and trends in global groundwater storage.
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To clarify the impact of environmental changes on global groundwater storage and the driving forces behind these changes, and land-use changes in global karst regions. This study focuses on the following aspects: First, the study utilizes GRACE data from 2003 to 2022, combined with ERA5-Land dataset, to derive global groundwater storage trends, and analyzes the spatial and temporal distribution characteristics and patterns of global groundwater storage changes over the past 20 years. Next, by integrating meteorological data and human activity data, the study discusses the driving forces behind specific regions with significant changes in groundwater storage. Finally, using Climate Change Initiative-Land Cover (CCI-LC) and World Karst Aquifer Map (WOKAM) datasets, it examines the current status and spatial changes of land use in global karst regions over multiple years. It also proposes factors that can quantitatively assess global karst land-use changes in spatial terms. Based on this analysis, three karst regions with high land-use change intensity are identified. The main conclusions of this study are as follows:
(1) The study's results indicate that global groundwater storage between 2003 and 2022 has shown spatial heterogeneity, with most depletion occurring in the Earth's mid-latitudes. Notably, regions such as northern India, eastern Brazil, and areas around the Caspian Sea experienced significant declines in groundwater storage, exceeding 20 mm/a. It is estimated that around 3.4 billion people live in areas where groundwater storage has substantially decreased over the past two decades.
(2) Spatial analysis revealed that the most significant groundwater declines occurred in arid and semi-arid regions, particularly where the aridity index (AI) ranges from 0.1 to 0.5, peaking at 0.1 to 0.2. Contrary to common belief, although there is a correlation between precipitation and groundwater storage, changes in precipitation do often not directly affect groundwater storage. Compared to precipitation, changes in the Standardized Precipitation-Evapotranspiration Index (SPEI) have a more significant spatial impact on groundwater storage.
(3) Human activities, particularly irrigation and excessive groundwater abstraction, are the major drivers of groundwater storage decline. Among human activities, agricultural activities have a more significant impact on groundwater storage than population density. There are two main causes of groundwater rise: wetter climate conditions and dam construction.
(4) The analysis of land use in global karst regions in 2020 reveals that forests constitute the largest land-use type, covering 31.78% of these areas, followed by bare areas (27.58%), cropland (19.02%), grassland (10.87%), shrubland (7.21%), wetlands (1.67%), ice and snow (1.16%), and urban areas (0.71%). From 1992 to 2020, the total change in land use in global karst regions is estimated to be 1.30 million km², accounting for approximately 4.85% of the global karst surface.
(5) The primary trend identified is afforestation, which is supplemented by localized urbanization and agricultural reclamation, particularly in regions with tropical climates where land-use change intensity is higher. Furthermore, areas undergoing agricultural reclamation are closely aligned with regions of high population density, indicating the significant role of human activities in driving these changes.
This thesis emphasizes the urgent need for long-term dynamic observation and the development of sustainable groundwater management policies. Such policies are crucial for regions facing severe groundwater depletion, as they aim to ensure the long-term sustainability of freshwater resources, which are essential for both human survival and ecological health.
