Over past decades, terrestrial water fluxes have been affected by humans at an unprecedented scale and the fingerprints that humans have left on Earth’s water resources are turning up in a diverse range of records. In his PhD thesis, Yoshihide Wada developed a state-of-the-art global hydrological model (GHM) and global water demand model. These were eventually coupled to quantify and distinguish human and climate impacts on surface freshwater and groundwater resources. Yoshihide Wada’s thesis is composed of three major parts: Part 1. Human and climate impacts on surface freshwater resources; Part 2. Global assessment of groundwater resources; Part 3. Integrated modeling and indicators of global water resources. Yoshihide Wada was promoted cum laude!
The thesis first explores the human and climate impacts on seasonal surface freshwater resources by forcing the global hydrological model PCR‐GLOBWB with daily meteorological fields and by calculating global monthly water demands with the effects of socio-economic and land use change. Increased water demand was found to be a decisive factor for heightened water stress in various regions, while climate variability is often a main determinant of extreme events. Over Europe, North America and Asia, severe hydrological drought conditions are driven by increasing consumptive water use rather than to be merely induced by climate variability; the magnitude of droughts intensified by 10–500%.
Next, the thesis assesses global groundwater resources by estimating groundwater recharge and abstraction. Global groundwater depletion was found to triple in size over the last 50 years, and contributes ~20% to irrigation water supply. Groundwater stress was then assessed using newly developed indicators considering groundwater contribution to environment. The global groundwater footprint was found to be 3.5 times the actual area of aquifers driven by a few heavily overexploited aquifers. The aquifer stress indicator revealed that ~8% of transboundary aquifers are currently stressed due to human overexploitation. Importantly, groundwater depletion was found to be an important contributor to sea-level rise and is likely to dominate over those of other terrestrial water sources. The contribution of groundwater depletion to sea-level increased by more than ten-fold over 1900–2000, and is projected to increase further by 2050.
In the final part of this thesis, an improved modeling framework that dynamically simulates daily water use per source per sector was developed. Human impacts on terrestrial water storage signals were evident in the validation with GRACE satellite observation, altering the seasonal and inter-annual variability over heavily regulated and intense irrigated basins. The newly developed model together with other six state-of-the-art GHMs was applied to simulate future irrigation water demand using the latest CMIP5 climate projections. The increase in irrigation demand varies substantially depending on the degree of global warming and associated regional precipitation changes. GHM dominates the uncertainty throughout the century, but GCM uncertainty substantially increases from the mid-century. To comprehensively assess global water resources, an improved approach was introduced. The Green Water Stress Index is capable of reproducing varying degrees of green water stress conditions, reflecting a multi-decadal climate variability. The Blue Water Sustainability Index revealed an increasing trend of water consumed from nonsustainable surface water and groundwater resources (~30%) worldwide.
Human and climate impacts on global water resources (2013), PhD thesis (cum laude!), Utrecht University, Utrecht.