Precipitation (P) and potential evaporation (E_p) are commonly studied drivers of changing freshwater availability, as aridity (E_p/P) explains ∼90% of the spatial differences in mean runoff across the globe. However, it is unclear if changes in aridity over time are also the most important cause for temporal changes in mean runoff and how this degree of importance varies regionally. We show that previous global assessments that address these questions do not properly account for changes due to precipitation, and thereby strongly underestimate the effects of precipitation on runoff. To resolve this shortcoming, we provide an improved Budyko-based global assessment of the relative and absolute sensitivity of precipitation, potential evaporation, and other factors to changes in mean-annual runoff. The absolute elasticity of runoff to potential evaporation changes is always lower than the elasticity to precipitation changes. The global pattern indicates that for 83% of the land grid cells runoff is most sensitive to precipitation changes, while other factors dominate for the remaining 17%. This dominant role of precipitation contradicts previous global assessments, which considered the impacts of aridity changes as a ratio. We highlight that dryland regions generally display high absolute sensitivities of runoff to changes in precipitation, however within dryland regions the relative sensitivity of runoff to changes in other factors (e.g., changing climatic variability, CO₂-vegetation feedbacks, and anthropogenic modifications to the landscape) is often far higher. Nonetheless, at the global scale, surface water resources are most sensitive to temporal changes in precipitation.

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Precipitation (P) and potential evaporation (Ep) are commonly studied drivers of changing freshwater availability, as aridity (Ep/P) explains ∼90% of the spatial differences in mean runoff across the globe. However, it is unclear if changes in aridity over time are also the most important cause for temporal changes in mean runoff and how this varies across regions. Here, we resolve shortcomings of previous Budyko-based global assessments on the relative role of aridity for changes in water availability. We argue that previous assessments do not properly account for precipitation effects. To resolve this issue, the effects of changes in Ep and P need to be considered separately. We present a new global assessment of the elasticity of runoff to changes in precipitation, potential evaporation, and other factors. The global pattern suggests that for 83% of the land surface runoff is most sensitive to precipitation changes, while other factors dominate for the remaining 17%. Runoff elasticity to changes in potential evaporation is always lower than elasticity to precipitation, and in many arid regions this difference can reach an order of magnitude. Although surface water resources in dryland regions are highly sensitive to precipitation changes, their sensitivity to changes in other factors (e.g. changing climatic variability, CO2 – vegetation feedbacks and anthropogenic modifications to the landscape) is often far higher. Nonetheless, at the global scale we find precipitation changes have the greatest impact on water availability, which contrasts markedly with recent assessments.@en

Analyses of trends in observed floods often focus on relatively frequent events, whereas changes in rare floods are only studied for a small number of locations that have exceptionally long observational records. Understanding changes in rare floods is especially relevant as these events are often most damaging and influence the design of major structures. Here we provide an assessment of changes in the largest flood events (∼0.033 annual exceedance probability) observed during the period 1980-2009 for 1744 catchments located in Australia, Brazil, Europe and the United States. The occurrence of rare floods in spatial aggregate shows strong decadal variability and peaked around 1995. During the 30-year period there are overall increases in both the frequency and magnitude of extreme floods. These currently unattributed increases are strongest in Europe and the United States, and weakest in Brazil
and Australia.@en