Marko Kallio

Approximately 500 million people globally still rely on surface water or unimproved sources for drinking water, even as surface water quality deteriorates in many regions, often the same regions where safe access is lacking. Yet this co-exposure is not quantified globally. We constructed the global harmonised subnational drinking water service estimates (G-HSDW), the first fully global subnational dataset of all four lower WHO/UNICEF Joint Monitoring Programme service rungs (basic, limited, unimproved, surface water). G-HSDW covers 3,040 admin 1 units across 240 countries over 1990–2023. We combined it with gridded surface water quality estimates (salinity, organic pollution, faecal coliforms) over 1990–2019. In 2019, only 1 in 10 people relying on unimproved or surface water sources lived in areas with low water quality risk, while 52% of people using unimproved and 60% using surface water sources faced high to severe risk. We identify dark spots where action is most urgent and bright spots where progress provides models for effective intervention.

The assessment of flood risk is crucial for sustainable planning, policymaking, and emergency management. Even with recent methodological advancements, flood risk assessment in data‐scarce regions still encounters significant uncertainties. Despite the multitude of available methods and data sources, no context‐specific guidance exists for selecting a suitable combination of methods to map risk in a given data‐scarce area of interest. To address this gap, we conducted a systematic review of 264 studies on flood risk assessment in data‐scarce regions. First, our study reveals diverse interpretations of “risk” and “data scarcity”, deviating from well‐established definitions. We highlight the use of a standardized definition of flood risk to enhance the comparability of findings and improve the reliability of assessments. Second, we conceptualize data scarcity in the flood risk context to support an informed selection of assessment methods. Third, we systematically compile risk indicators, methods, and data sources applied in the context of data scarcity, and we analyze the interplay between methods for mapping the three key risk components, that is, hazard, exposure, and vulnerability. This interplay enables the design of a three‐step approach for identifying an appropriate methodological combination for flood risk assessment in any data‐scarce application context. Through these three key contributions, our review paves the way for more reliable and consistent flood risk assessment and management in data‐scarce regions, facilitating the reduction and mitigation of flood impacts in vulnerable communities across the globe. This article is categorized under: Science of Water > Water Extremes Engineering Water > Planning Water

January 1 is an arbitrary point to start a year for hydrological analysis. Water years offer a solution by shifting the start to a hydrologically meaningful point, but local and global definitions diverge considerably. Here we show that local water years reflect the seasonal cycles of snow accumulation, midlatitude storm tracks, and the intertropical convergence zone, producing a coherent global pattern that no existing global definition reproduces. We introduce the first observation-based global definition of the water year, derived from upstream snow water equivalent and discharge, that recovers this pattern and is applicable at a subcatchment scale. Applying it reveals that the point when a year starts systematically alters estimates of interannual variability, the spatial extent of significant changes, precipitation–flux correlations, and water-balance closure across the global land surface. The start of the water year is thus a first-order yet rarely scrutinized decision in Earth-system and water-cycle analysis.

In their seminal study, Buckley et al. (2010) identified two megadrought events in Mainland Southeast Asia during the mid-14th and early 15th centuries CE. These droughts, driven by monsoonal shift...

Water years have been used by water resource managers for more than a century to align hydrological phenomena with the annual precipitation cycle. The central idea is to start a new water year when specific hydrological conditions are met, so that precipitation carry-over from the preceding 12-months period is minimised. With the increase in global hydrological data and analysis, it is vital to understand first how to best determine the start of water years across the globe and to understand their effect on evaluating interannual changes. The only global analysis of the water year to date defines its start by the month of lowest stream discharge. While this definition works well in seasonal climates, it neglects snow dynamics, as the snowfall does not immediately contribute to discharge. In snow-dominated catchments, the lowest discharge often occurs just before the spring melt; consequently, precipitation from the previous water year significantly influences the discharge in the following water year. We present a new definition to be used in the global water year estimations. It utilizes the areal mean of snow water equivalent collected in upstream catchments and discharge of the target catchment to determine the starting month. This definition mirrors the dynamics of terrestrial water storage and aligns more closely with various national definitions. Applying this methodology across ERA5 Land variables and a combination of MSWEP, GLEAM, GRADES, and SWEML datasets, reveals significant differences in trends and coefficients of variation for annual hydrological fluxes when compared to the standard calendar year. Additionally, our snow-and-discharge-based definition minimizes water balance closure errors. Given these findings, we suggest that global interannual hydrological analysis should, at minimum, consider water years for a physically sound assessment.

Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.

Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.

Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.

Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.

Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.