Global Freshwater Fluxes into the World’s Oceans (GRDC, 2021)
Freshwater input to the World’s oceans is part of the global hydrological cycle. During antiquity, the origin of water feeding the rivers was subject of speculations. With the Renaissance, however, the idea of the renewing water cycle with the parts precipitation, runoff and evaporation gained more and more acceptance. The first quantitative estimate of the compartments of the hydrological cycle is considered to be made by E. Brückner in 1905; the freshwater input into the World’s oceans was given there with 25000 km³/a (without polar glaciers). Since then, based on better observational data and methodologies, new estimates have been published frequently. An overview of selected publications can be found in the section Other Estimates.
The Global Runoff Data Centre (GRDC) has published the data product “Freshwater Fluxes into the World’s Oceans” since 2004. Since 2009 the calculations are based on the model results of the continuously developed water balance model WaterGAP (Hydrology Group, University of Frankfurt). The most recent version of the data product includes annual runoff values for the period 1901-2016, broken down in map and table displays by runoff from continents as well as into oceans, 5° and 10° latitude bands, and UNEP GIWA regions. The total freshwater input to the World’s oceans between 90°N and 60°S is computed to 40181 km³/a. Excluding Greenland, the total is 39693 km³/a. Excluding Caspian Sea, the total is 39828 km³/a.
| Arctic Ocean | Atlantic Ocean | Indian Ocean | Pacific Ocean | Sum Sea1 | |
|---|---|---|---|---|---|
| Europe | 645 | 1989 | - | - | 2634 |
| Asia | 2094 | 139 | 3932 | 6146 | 12311 |
| Africa | - | 3196 | 1001 | - | 4197 |
| Australia | - | - | 203 | 2450 | 2653 |
| North America | 1704 | 2974 | - | 2052 | 6730 |
| South America | - | 10418 | - | 883 | 11301 |
| Sum Land1 | 4442 | 18716 | 5139 | 11530 | 39828 |
| 1 slight deviations due to rounding |
This service is published as an interactive web application by the German Federal Institute of Hydrology (BfG). Disclaimer, Terms and Conditions of the BfG apply. Please cite in your publication the GRDC as the source of the data: Global Freshwater Fluxes into the World’s Oceans. Online provided by Global Runoff Data Centre. Koblenz: Federal Institute of Hydrology (BfG). URL, date of retrieval.
ARC Arctic Ocean
ASI Asia
ATL Atlantic Ocean
ANT Antarctica
AUS Australia and Oceania
CAS Caspian Sea
EaS Eastern Shore
EUR Europe
IND Indian Ocean
NAM North America
NoC Northern Coast
SAM South America
SEA Oceans
SoS Southern Shore
WeC Western Coast
WeS Western Shore
Background
This computation is based on the latest model output from the global hydrological model WaterGAP 2.2d (Hannes Müller Schmied et al. 2020). The WaterGAP model operates on the 0.5° x 0.5° CRU grid between 90°N and 60°S. A comprehensive overview on model structure and applications is given by H. Müller Schmied et al. (2021). The used model version considers human impacts and is calibrated and forced with a homogenized combination of WATCH Forcing Data (Boucher and Best 2010) and WFDEI meteorological forcing dataset (G. P. Weedon et al. 2018) with monthly precipitation scaled to GPCC (WFDEI-GPCC, see G. Weedon et al. 2014). The raw model output data (discharge) can be downloaded here: https://doi.pangaea.de/10.1594/PANGAEA.918447
For the accumulation of freshwater fluxes over different spatial aggregation levels, a reproducible workflow in the R programming language has been created. The process uses i.a. the R packages raster (Hijmans et al. 2015), rgeos (Bivand et al. 2017), sp (Pebesma and Bivand 2005), sf (Pebesma 2018), igraph (Csardi and Nepusz 2006) and hydts. The spatial areas considered are oceans (based on International Hydrographic Organization 1953), continents (Esri 2019), the division into regions according to the Global International Waters Assessment (GIWA, see Hempel and Daler 2004), 5° and 10° latitudinal bands and 5° coastline cells. In conformance with the previous version, for evaluation the outlet cell of the Amazon river has been assigned to the southern hemisphere, although in the model it is at the 0.25°N, 50°W cell (in the real world, the estuary is situated exactly on the equator). That means, that the flux from Amazon river is in its entirety assigned to the 10° latitude band between 0°N and 10°S, to the 5° latitude band between 0°N and 5°S, and to the 5° x 5° cell at 0°N-5°S/50°W-55°W.
For comparison, we contrasted our results with the GRUN dataset which was released in 2019 (https://doi.org/10.6084/m9.figshare.9228176, Ghiggi et al. 2019) and covers a similar time span. GRUN is also based on a 0.5° grid. It should be noted that GRUN is not linked to a routing module, so that the runoff values of the grid cells belonging to a catchment are merely summed up. This leads to the fact that possible evaporation losses due to reservoirs or lakes and water abstractions are not taken into account (e.g. Nile catchment) and in case of large catchments (e.g. Amazon) a significant lag of the runoff amplitudes occurs.
Details on input data, processing and results can be found in the report BfG-Mitteilung Nr. 36: Freshwater Fluxes into the World’s Oceans.
References
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* water balance of oceans: Pocean - Eocean + Q, Q incl groundwater non-drained by rivers
** values in the original source are given in 1015 kg/a
a including Southern Ocean
b southern parts of Atlantic, Indian and Pacific ocean
c Lake Aral & Caspian Sea only
d including endorheic basins
e Mediterranean and Black Sea
f northern portion of the ocean
g southern portion of the ocean
1 including Hudson Bay, Greenland Sea and Norwegian Sea
2 including Mediterranean Sea and Black Sea
3 separated from Southern Ocean along the Antarctic Convergence line
4 separated from Atlantic, Indian and Pacific Oceans along the Antarctic Convergence line
5 excluding inland sinks
* water balance of continents: Pcontinent - Econtinent, Q incl groundwater non-drained by rivers
** values in the original source are given in 1015 kg/a
a excl Greenland (700 km³/a)
b incl Greenland (700 km³/a)
c excl Antarctica
d islands within Antarctic Convergence, north of 60 latitude
e including endorheic basins (internal drainage)
f refers to Australasia
1 separated from Europe by Ural Mountains, Ural River, Caspian Sea, Caucasus Mountains, Black Sea, Bosporus and Dardanelles
2 includes Northern America, Central America and the Caribbean
3 separated from Asia by Ural Mountains, Ural River, Caspian Sea, Caucasus Mountains, Black Sea, Bosporus and Dardanelles
4 includes Australia, New Zealand, Melanesia, Micronesia and Polynesia (aka Oceania)
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