Global hydrobelts: A new method to define the world’s river basins

November 11th, 2013

Prof. Michel Meybeck, Dr. Matti Kummu & Dr. Hans Dürr

This article is the first part of a two part series. The second part can be read here.

Global water assessments are generally performed on the basis of countries, political regions, or continents. However, these units cannot accurately perform this task because they:

(i) mask or minimize the great disparity of water resources within continents and large countries (e.g. USA, Russia, Brazil, China, Australia),

(ii) do not support integrated analysis of water-related issues at the river basin level, which is generally considered to be the optimum scale1,2, and,

(iii) are ill-suited for research into the response of river basins to current Global Change (including climate change and other human-induced environmental changes)3.

On the other hand, considering individual river basins at the global scale is of limited practical use because there are approximately 8,000 basins in total4,5. Selecting the fifty greatest basins exceeding 500,000 km2 would only account for half of the continental area. An approach to assessments is therefore needed that combines the level of detail found at the basin scale with a global level of coverage. The answer is river basin aggregation.

In a recent study6 we have proposed a novel reporting scale for water issues that generates cross-continent ‘hydrobelts’. These regions are generated through: (1) natural hydrological basin boundaries, and (2) an aggregation based on the characteristic hydroclimatic features of types of river basins: annual water runoff (q, mm/year) and average annual air temperature (T, °C) (see Table 1). The result is eight hydrobelts (see also Figure 1):

Boreal belt (BOR) , only developed in the Northern Hemisphere and centered around 62°N,

, only developed in the Northern Hemisphere and centered around 62°N, North and South Mid-Latitude belts (NML and SML) centered around 43°N and 34°S respectively,

centered around 43°N and 34°S respectively, North and South Dry belts (NDR and SDR) centered around 29°N and 27°S respectively,

centered around 29°N and 27°S respectively, North and South Sub-tropical belts (NST and SST) centered around 17°N and 17°S respectively, and

centered around 17°N and 17°S respectively, and Equatorial belt (EQT) centered at 3°S.

This delineation was facilitated by previous global aggregations of the 8,000 river basins into 156 coastal catchments that allowed reporting of river outflows into oceans.7,8 The final aggregation arose from multiple attempts in which the main criteria were:

(i) hydrobelts are delineated by natural hydrological basins that cannot be divided (e.g. headwaters are not separated from lowlands),

(ii) belts are defined on the basis of their hydroclimate, and

(iii) the belts follow continental boundaries [with one minor exception].

Hydroclimatic characteristics of hydrobelts

This new approach defines eight hydrobelts with similar hydrological and thermal river regimes, glacial and postglacial history of basins, internal drainage distribution, and sensitivity to climate variations. A general symmetry is observed for temperature and annual water runoff between corresponding North and South belts (Table 2). The runoff types, varying from 31 to 960 mm/y, are particularly well characterised by the hydrobelt segmentation.

Hydrological similarities between rivers within a given belt and in analogous North and South belts are also very clear. Boreal belt rivers are characterized by a long frozen low stage, and late-spring, early-summer peak flows generated by snowmelt (e.g. the Yukon, Mackenzie, Nelson, Churchill, Pechora, Ob, Yenissei, Lena, Kolyma, and Amour rivers). Mid-latitude rivers have mixed regimes, often combining snowmelt and rain fed regimes (e.g. the Columbia, Mississippi, Saint Lawrence, Danube, Volga, Indus, Ganges, Brahmaputra, Yangtze, and Yellow rivers). Rivers located in the Subtropical belts are characterized by highly seasonal runoff regimes with a marked low stage ,e.g. the Magdalena, Senegal, Niger, Godavari, Irrawaddy, Mekong rivers (Northern Subtropical belt), and the Sao Francisco, Parana, and Zambezi rivers (Southern Subtropical belt). Equatorial belt rivers are characterized by high runoff throughout the hydrological cycle (e.g. the Orinoco, Amazon, Congo rivers and some much smaller basins in SE Asia).

The North and South Dry belts are characterized by their numerous endorheic basins, i.e. those not drained to the ocean (e.g. the Great Basin, Aral Sea, Tarim, and Kerulen basins in the Northern Dry belt, and Altiplano, Mar Chiquita, Okavango, and Lake Eyre basins in the Southern Dry belt), and by their allogenic rivers, i.e. those fed by run-off mostly in their upper basins, including many of the world’s ‘water towers’9, e.g. the Colorado, Rio Grande, Nile, Chari/Logone, Shatt el Arab, Amu Darya and Syr Darya, Kerulen, and Tarim basins in the Northern Dry belt, and the Orange, Okavango and Murray basins in the Southern Dry belt. Dry belts also correspond to 94% of the arheic or desert land, that is conventionally considered to be land with runoff of less than 3 mm/y.4,5

The average hydrobelt temperature is also very distinct, ranging from -6.6°C to +23.9°C. The greatest difference between neighbouring belts occurs between the Boreal belt (-6.6°C) and the Northern Mid Latitude belt (+9.1°C). The Boreal belt is essentially frozen, with 74.8% of permafrost land, and 56.7% of its area was covered by glaciers during the last ice age.

Although hydrobelts were designed to present the hydroclimate of continents in a symmetrical way, some important differences remain between corresponding North and South belts (Figure 2 and Table 2) due to:

(i) the uneven distribution of land masses (i.e. a larger land mass in the Northern Hemisphere) and lack of continental land mass south of 55°S (i.e. absence of a Boreal belt in the Southern hemisphere),

(ii) the continental climate that is only found in the Northern belts, and

(iii) the Central Asian mountains and high plateaux with no equivalent in the Southern hemisphere.

For example, these three factors cause the Northern Mid Latitude belt (NML) (9.1°C) to be much colder than its Southern equivalent (the SML) (14.5°C).

Application of hydrobelts

The hydrobelts concept facilitates the reporting of water resources and the analysis of world rivers basins, particularly within the earth system sciences. As they are derived from the geographic limits of river basins, hydrobelts are well-positioned to assess impacts of Global Change on rivers, aquatic biodiversity issues, and river geochemistry and biogeochemistry (e.g. carbon and silica cycles).

In addition to earth sciences, the hydrobelts concept can also be applied to the social sciences and used to inform water governance. In the second part of this two part series we will demonstrate this broader utility by using the hydrobelts framework to create a population-focused water security indicator at the sub-continental level.

This article is the first part of a two part series. The second part can be read here.

References:

1. Millennium Ecosystem Assessment (MEA) (2005), Ecosystems and Human Well-being: Synthesis. Island Press: Washington, DC.

2. World Water Assessment Programme (WWAP) (2009), Water in a Changing World. UNESCO. Earthscan: London.

3. Steffen W. (ed.) (2004), Global Change and the Earth System: A Planet under Pressure. Springer.

4. Vörösmarty, C.J., Fekete, B.M., Meybeck, M. and R.B. Lammers (2000a), ‘Geomorphometric attributes of the global system of rivers at 30-minute spatial resolution’, Journal of Hydrology 237: 17-39.

5. Vörösmarty, C.J., Fekete, B.M., Meybeck, M. and R.B. Lammers (2000b), ‘The global system of rivers: its role in organizing continental land mass and defining land-to-ocean linkages’, Global Biogeochemical Cycles 14: 599-621.

6. Meybeck, M., Kummu,M. and H.H. Dürr (2013), ‘Global hydrobelts and hydroregions: improved reporting scale for water-related issues’, Hydrolical and Earth System Sciences 17: 1093-1111.

7. Meybeck, M., Dürr, H.H. and C.J. Vörösmarty (2006), ‘Global coastal segmentation and its river catchment contributors: a new look at land-ocean linkage’, Global Biogeochemical Cycles 20:GB1S90, doi:10.1029/2005GB002540.

8. Dürr, H.H., Laruelle, G., van Kempen, C., Slomp, C., Meybeck, M., and H. Middelkoop (2011), ‘Worldwide Typology of Nearshore Coastal Systems: Defining the Estuarine Filter of River Inputs to the Oceans’, Estuaries and Coasts 34: 441-58.

9. Viviroli, D., Dürr, H.H., Messerli, B., Meybeck, M. and R. Weingartner (2007), ‘Mountains of the world – water towers for humanity: typology, mapping and global significance’, Water Resources Research 43: W07447.

This article summarises material from a full-length, open-access journal article in Hydrology and Earth System Sciences 17: 1093-1111, 2013, entitled ‘Global hydrobelts and hydroregions: improved reporting scale for water-related issues?’. Dr. Michel Meybeck is Director Emeritus of Research at the National Centre for Scientific Research, University Paris 6, France. He has been working on rivers hydrology and geochemistry at the global scale since 1976. He was scientific adviser in water-related international programmes as the GEMS -Water programme (1978-1998), and was part of scientific committees of IGBP, BAHC-IGBP and LOICZ- IGBP. Dr. Meybeck can be contacted at michel.meybeck@upmc.fr. Dr. Matti Kummu is Assistant Professor at the Water and Development Research Group, Aalto University, Finland. His research focuses on the linkages between human actions, food security, and water resources at different spatial scales, from local to global. Dr. Hans Dürr is Assistant Professor in the Ecohydrology Group at the University of Waterloo, Canada. He works on continental waters, rivers, groundwaters and estuaries, and their typologies and geochemistry at the global scale, at the University of Paris 6, at the Utrecht University, the Netherlands, and at Waterloo University.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.