Hotspots for social and ecological impacts from freshwater stress and storage loss

Human societies and ecosystems are intricately linked through the hydrological cycle. Changes in water availability significantly impact both social and ecological systems, yet global-scale assessments rarely consider these impacts concurrently. This study focuses on the potential for combined social and ecological damage stemming from freshwater stress (the ratio of freshwater withdrawal to streamflow) and storage loss (decreasing groundwater, soil moisture, surface water, and snowpack). The research builds upon existing literature on freshwater scarcity and security, acknowledging limitations in previous approaches. Freshwater scarcity studies often employ globally consistent classification schemes that may not adequately capture regional variations in social and ecological sensitivity. Similarly, freshwater security assessments, while incorporating multiple indicators, can obscure the specific impacts of scarcity by aggregating diverse factors. This study integrates concepts from social-ecological systems research to address these limitations. The core objectives are to: (1) assess the global co-occurrence of freshwater stress and storage trends; (2) analyze the relationship between social adaptive capacity, ecological sensitivity, and freshwater conditions; (3) map the global social-ecological vulnerability gradient and identify hotspot basins; and (4) evaluate integrated water resources management (IWRM) within these hotspots. The analysis is conducted at a large basin scale (n=1204), with data primarily from 2015, summarized using area-weighted averages or basin sums depending on data type.

The paper reviews existing literature on global freshwater scarcity and security. It points out limitations in previous research, namely that freshwater scarcity assessments often use globally uniform classifications that don't account for variations in social and ecological sensitivity. Water security assessments, while incorporating numerous indicators, often aggregate data in ways that make it difficult to isolate the impact of water scarcity. This study aims to overcome these limitations by integrating social-ecological systems research.

The study uses a basin-scale analysis (HydroBASINS level 4, n=1204) with data primarily from 2015. Data were harmonized to a 0.5-degree resolution. Area-weighted averages or basin sums were calculated for intensive and extensive properties, respectively. Basins in Greenland, northern Canada, and some islands were excluded due to inconsistent data coverage. A key aspect of the methodology is the development of indicators to represent vulnerability. Basin freshwater status is a composite indicator combining normalized freshwater stress (W/0.4Q, bounded at 1) and normalized storage trends (ΔS/Q, bounded at ±1, and flipped for consistency). A social-ecological sensitivity indicator is created by combining (through a fuzzy sum operation) data on ecological sensitivity (derived from de Graaf et al. and Seddon et al. datasets, transformed to percentiles and averaged) and inverted social adaptive capacity (from Varis et al.). Vulnerability is then calculated as the product of basin freshwater status and social-ecological sensitivity. The Head/Tail Breaks method is used to classify basins into vulnerability classes, identifying hotspots. Uncertainty and sensitivity analyses are conducted to assess the robustness of the results. The study also compares national IWRM implementation levels with vulnerability results.

The study found a significant overlap between freshwater-stressed basins and those experiencing storage loss. Approximately 201 (42%) of the 478 stressed basins are simultaneously drying. These basins are concentrated in regions such as the southwestern USA, northeastern Brazil, and parts of the Middle East, India, and China. These regions are often agriculturally significant and heavily irrigated. Conversely, 98 (21%) stressed basins are wetting. The study then shows the social and ecological implications of freshwater stress and storage loss. It revealed that approximately 2.2 billion people, 27% of global food crop production, and 28% of global GDP are located in stressed and drying basins. When considering social adaptive capacity, 73 basins show low adaptability and severe freshwater status, concentrated in Northern and Eastern Africa, the Arabian Peninsula, and parts of Asia. These basins have approximately 1.2 billion people, 12% of global food production, and 6% of global GDP. The study then introduces a vulnerability indicator which incorporates basin freshwater status, ecological sensitivity, and social adaptive capacity. The global gradient in social-ecological vulnerability is mapped, identifying 168 hotspot basins (14% of all basins and 11% of global land area). These hotspots contain over 1.5 billion people, 17% of global food crop production, and 13% of global GDP. The analysis reveals no clear relationship between vulnerability and IWRM implementation, highlighting the need for improved IWRM, particularly in transboundary basins.

The findings highlight the significant social and ecological risks associated with the co-occurrence of freshwater stress and storage loss. The identified hotspots represent regions particularly vulnerable to ecological damage (transgressed environmental flows, increased drought frequency, harm to groundwater-dependent ecosystems) and social challenges (water scarcity impacting domestic, industrial, and agricultural demands, potential for conflict). The lack of correlation between vulnerability and IWRM implementation emphasizes the need for proactive, integrated strategies. The study's focus on the combined impact of stress and storage loss provides a more nuanced understanding than previous research, which often treats these factors separately. The identification of hotspots allows for targeted interventions and resource allocation.

This study provides a crucial global assessment of the combined social and ecological vulnerability to freshwater stress and storage loss. The identification of 168 hotspot basins, representing significant social and ecological assets, underscores the urgent need for improved integrated water resources management, hydro-diplomacy, and investments in building adaptive capacity in vulnerable regions. Future research should focus on refining vulnerability indicators by incorporating more comprehensive ecological models and assessments of societal responses to specific freshwater stresses, as well as indirect impacts like virtual water trade.

The study acknowledges limitations in data availability and the need for more sophisticated ecological sensitivity indicators. The use of a single year's data (2015) limits the analysis of temporal trends. Sub-grid variability in both ecological sensitivity and social adaptive capacity is not addressed. The definition of social adaptive capacity is relatively general. The analysis does not consider indirect and non-local impacts of freshwater stress such as those related to virtual water trade.