IIASA water-quality modeling has contributed to the World Meteorological Organization’s (WMO) State of Global Water Resources 2025 report. The assessment shows that the global water cycle is becoming increasingly erratic, with serious implications for people and economies worldwide.
The annual report provides a comprehensive, science-based assessment of the global water cycle and extreme events in 2025, while also summarizing trends over the past five years. It brings together observations, model outputs, and satellite data covering river discharge, groundwater, terrestrial water storage, snow and ice, evapotranspiration, and water quality.
Key messages
- 2025 was one of the driest years for global river discharge in 35 years, with below-normal flows across 36% of the world’s river basin area.
- The past seven years have seen abnormally low numbers of rivers with “normal” flows. In 2025, normal conditions were recorded in only a minority of river basins.
- Water stored on Earth’s land surface has declined over the past decade. Terrestrial water storage, including groundwater, lakes, rivers, soil moisture, vegetation, snow, and ice, has shown a persistent negative trend since the mid-2010s.
- Glaciers lost ice across all major glaciated regions for the fourth consecutive year. In 2025 alone, glaciers lost an estimated 408 gigatonnes of mass.
- Asia and Africa were the regions most affected by water-related disasters. Together, they accounted for more than 80% of reported deaths associated with extreme water-related events over the past five years.
- IIASA modeling contributed to the report’s assessment of water quality, including water temperature. Global simulations from the CoSWAT-WQ hydrological and water quality modeling framework were developed through collaboration between the IIASA Water Security Research Group and the Department of Water and Climate at Vrije Universiteit Brussel.
“The inclusion of water quality, including water temperature, in the WMO’s global assessment provides a more comprehensive picture of freshwater conditions,” says IIASA researcher Albert Nkwasa, who contributed to the report. “Water temperature is a key water-quality parameter that influences physical, chemical, and biological processes in freshwater systems. By combining observations with modeling, we can help address monitoring gaps and strengthen our understanding of global water resources. We are pleased that simulations from our CoSWAT-WQ global water-quality model contributed to this important assessment.”
Although water data availability has improved, substantial gaps remain, particularly in Africa and Asia, despite these regions being among the hardest hit by water-related extremes. The report highlights how combining observations and modeling can support better monitoring, early warning, and water management in a changing climate.
Adapted from a press release by the World Meteorological Organization.
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