Global hydrobelts and hydroregions: improved reporting scale for water-related issues?

Global-scale water issues such as its availability, water needs or stress, or management, are mapped at various resolutions and reported at many scales, mostly along polit- ical or continental boundaries. As such, they ignore the fun- damental heterogeneity of hydroclimates and natural bound- aries of river basins. Here we describe the continental land- masses at two levels: eight hydrobelts strictly limited by river basins, defined at a 30 0 (0.5 ) resolution, which are decom- posed on continents as 26 hydroregions. The belts were de- fined and delineated, based primarily on the annual average temperature (T ) and run-off (q), to maximise inter-belt dif- ferences and minimise intra-belt variability. This new global puzzle defines homogeneous and near- contiguous entities with similar hydrological and thermal regimes, glacial and postglacial basin histories, endorheism distribution and sensitivity to climate variations. The mid- latitude, dry and subtropical belts have northern and southern analogues and a general symmetry can be observed forT and q between them. The boreal and equatorial belts are unique. Population density between belts and between the continents varies greatly, resulting in pronounced differences between the belts with analogues in both hemispheres. Hydroregions (median size 4.7 M km 2 ) are highly con- trasted, with the average q ranging between 6 and 1393 mm yr 1 and the average T between 9.7 and +26.3 C, and a population density ranging from 0.7 to 0.8 p km 2 for the North American boreal region and some Australian hydroregions to 280 p km 2 for some Asian hy- droregions. The population/run-off ratio, normalised to a reference pristine region, is used to map and quantify the global population at risk of severe water quality degradation. Our initial tests suggest that hydrobelt and hydroregion divi- sions are often more appropriate than conventional continen- tal or political divisions for the global analysis of river basins within the Earth system and of water resources. The GIS files of the hydrobelts and hydroregions are available at the supplement of this article and at doi:10.1594/PANGAEA.806957 as well as geotypes.net.