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European Center for Medium‐Range Weather Forecasts Reanalysis (ERA), one of the most widely used precipitation products, has evolved from ERA‐40 to ERA‐20CM, ERA‐20C, ERA‐Interim, and ERA5. Studies evaluating the performance of individual ERA products cannot adequately assess the evolution of the products. We compared the performance of all ERA precipitation products at daily, monthly, and annual data (1980–2018) using more than 2100 Iran precipitation gauges. Results indicated that ERA‐40 performed worst, followed by ERA‐20CM, which showed only minor improvements over ERA‐40. ERA‐20C considerably outperformed its predecessors, benefiting from the assimilation of observational data. Although several previous studies have reported full superiority of ERA5 over ERA‐Interim, our results revealed several shortcomings in ERA5 compared with the ERA‐Interim estimates. Both ERA‐Interim and ERA5 performed best overall, with ERA‐Interim showing better statistical and categorical skill scores, and ERA5 performing better in estimating extreme precipitations. These results suggest that the accuracy of ERA precipitation products has improved from ERA‐40 to ERA‐Interim, but not consistently from ERA‐Interim to ERA5. This study employed a grid‐grid comparison approach by first creating a gridded reference data set through the spatial aggregation of point source observations, however, the results from a point‐grid approach showed no change in the overall ranking of products (despite the slight changes in the error index values). These findings are useful for model development at a global scale and for hydrological applications in Iran. Key Points: Estimations over Iran improved from ERA‐40 to ERA‐Interim but not from ERA‐Interim to ERA5ERA‐Interim performed better than ERA5 in statistical and categorical skill scoresERA5 has improved estimations of extreme precipitations over Iran [ABSTRACT FROM AUTHOR]

Population growth is increasing the pressure on water resource availability. For useful assessment and planning for societal water availability impacts, it is imperative to disentangle the direct influences of human activities in the landscape from external climate‐driven influences on water flows and their variation and change. In this study we used the water balance model, a gridded global hydrological model, to quantify and distinguish human‐driven change components, modified by interventions such as dams, reservoirs, and water withdrawals for irrigation, industry, and households, from climate‐driven change components on four key water balance variables in the terrestrial hydrological system (evapotranspiration, runoff, soil moisture, storage change). We also analyzed emergent effect patterns in and across different parts of the world, facilitating exploration of spatial variability and regional patterns on multiple spatial scales, from pixel to global, including previously uninvestigated parts of the world. Our results show that human activities drive changes in all hydrological variables, with different magnitudes and directions depending on geographical location. The differences between model scenarios with and without human activities were largest in regions with the highest population densities. In such regions, which also have relatively large numbers of dams for irrigation, water largely tends to be removed from storage and go to feed increased runoff and evapotranspiration fluxes. Our analysis considers a more complete set of hydrological variables than previous studies and can guide further research and management planning for future hydrological and water availability trends, including in relatively data‐poor parts of the world. Plain Language Summary: The global population is growing, increasing demands for water use. Knowledge of how human activities affect water resources is available for some areas and water cycle components, but more data are needed for solid scientific understanding of climate and human‐driven effects on water resources in the past and present, and to underpin management approaches to future trends in hydrology and water availability. This study examined human activity effects on different components of the terrestrial water cycle, distinct from the effects of climate change. Results showed that human activities alter movements of water worldwide, with changes, magnitudes, and directions depending on world region. The greatest human‐driven changes emerged in regions with the highest population density, which also have relatively large numbers of dams providing water for agricultural use. Overall, the water changes in these regions tend to include water removal from storage going to feed increased evapotranspiration and/or runoff. Regions with such change trends will require solutions to overcome future negative impacts on society and ecosystems. This study showed how different water cycle components and land regions have been affected by changes in human activities, and findings can guide research and management planning for hydrological and water availability in future years. Key Points: Human activities affected all four hydrological variables studied (runoff, evapotranspiration, soil moisture, and storage change)Human‐driven hydrological effects emerged as considerable over the northern tropical to temperate latitudes, where most people liveHigher runoff was seen in areas with dams of various types, with additional hydrological effects depending on dam type [ABSTRACT FROM AUTHOR]

Large‐scale covariations of freshwater fluxes and storages on land can critically regulate the balance of green (evapotranspiration) and blue (runoff) water fluxes, and related land‐atmosphere interactions and hydroclimatic hazards. Such large‐scale covariation patterns are not evident from smaller‐scale hydrological studies that have been most common so far, and remain largely unknown for various regions and climates around the world. To contribute to bridging the large‐scale knowledge gaps, we synthesize and decipher hydroclimatic data time series over the period 1980–2010 for 6,405 catchments around the world. From observation‐based data, we identify dominant large‐scale linear covariation patterns between monthly freshwater fluxes and soil moisture (SM) for different world parts and climates. These covariation patterns are also compared with those obtained from reanalysis products and Earth System Models (ESMs). The observation‐based data sets robustly show the strongest large‐scale hydrological relationship to be that between SM and runoff (R), consistently across the study catchments and their different climate characteristics. This predominantly strongest covariation between monthly SM and R is also the most misrepresented by ESMs and reanalysis products, followed by that between monthly precipitation and R. Comparison of observation‐based and ESM results also shows that an ESM may perform well for individual monthly variables, but fail in representing the patterns of large‐scale linear covariations between them. Observation‐based quantification of these patterns, and ESM and reanalysis improvements for their representation are essential for fundamental understanding, and more accurate and reliable modeling and projection of large‐scale hydrological conditions and changes under ongoing global and regional change. Key Points: The closest large‐scale monthly covariations across different regions and worldwide are between soil moisture and runoffMonthly covariations in blue‐water precipitation‐runoff and green‐water precipitation‐evapotranspiration are mostly weakerSoil moisture‐runoff and precipitation‐runoff relationships are the most misrepresented by studied reanalysis products and Earth System Models [ABSTRACT FROM AUTHOR]

Wetlands provide multiple ecosystem services of local and global importance, but currently there exists no comprehensive, high‐quality wetland map for the Arctic region. Improved information about Arctic wetland extents and their vulnerability to climate change is essential for adaptation and mitigation efforts, including for indigenous people dependent on the ecosystem services that wetlands provide, as inadequate planning could result in dire consequences for societies and ecosystems alike. Synthesizing high‐resolution wetland databases and datasets on soil wetness and soil types from multiple sources, we created the first high‐resolution map with full coverage of Arctic wetlands. We assess the vulnerability of Arctic wetlands for the years 2050, 2075, and 2100, using datasets on permafrost extent, soil types, and projected mean annual air temperature from the HadGEM2‐ES climate model for three change scenarios (RCP2.6, RCP4.5, and RCP8.5). Our mapping shows that wetlands cover approximately 3.5 million km2 or roughly 25% of Arctic landmass and 99% of these wetlands are in permafrost areas, indicating considerable vulnerability to future climate change. Unless global warming is limited to scenario RCP2.6, robust results show that large areas of Arctic wetlands are vulnerable to ecosystem regime shifts. If scenario RCP8.5 becomes a reality, at least 50% of the Arctic wetland area would be highly vulnerable to regime shifts with considerable adverse impacts on human health, infrastructure, economics, ecosystems, and biodiversity. The developed wetland and vulnerability maps can aid planning and prioritization of the most vulnerable areas for protection and mitigation of change. Plain Language Summary: Wetlands play an important role in the Arctic; they cool the global climate, hold freshwater for animals and plants, regulate water, carbon, and nutrient cycling, and are biologically diverse and of great importance for indigenous human activities. Large areas lie on frozen ground (permafrost), but this could thaw out under expected future global and regional warming. Permafrost thaw can lead to wetland change leading to ecosystem as well as societal problems since the permafrost also acts as the foundation for roads and buildings in the Arctic. Through combined multi‐variable mapping, we estimate that wetlands cover 25% of land in the Arctic region, almost entirely on permafrost areas. Air temperatures above −2 degrees Celsius may lead to permafrost thaw and associated wetlands drainage, and this is enhanced by higher temperatures and longer durations of temperature elevation. If global temperatures increase by more than 2 degrees Celsius from preindustrial levels to year 2100, 30%–50% of Arctic wetlands are vulnerable to change. Limiting global warming is critical for preserving Arctic wetlands and reducing societal and ecosystem impacts of their changes. Key Points: Multi‐data synthesis shows that wetlands cover 25% of the Arctic landmassAround 50% of Arctic wetlands are vulnerable to permafrost thaw by the year 2100 in climate scenario RCP8.5Much of the permafrost underlying Arctic wetland areas can remain stable up until the year 2100 under climate scenario RCP2.6 [ABSTRACT FROM AUTHOR]