GRDC
Freshwater Fluxes into the World's Oceans

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.

Mean freshwater fluxes 1901–2016 from continents into World’s oceans
 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.

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The structure of these tables is based on Baumgartner & Reichel (1975), table XXXV. All numbers are rounded to integer values. The totals are based on the non-rounded values and therefore differ from the summed rounded values.
Abbreviations:
AFR Africa
ARC Arctic Ocean
ASI Asia
ATL Atlantic Ocean
ANT Antarctica
AUS Australia and Oceania
CAS Caspian Sea
EaC Eastern Coast
EaS Eastern Shore
EUR Europe
IND Indian Ocean
NAM North America
NoC Northern Coast
PAC Pacific Ocean
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 (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 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 (Weedon et al. 2018) with monthly precipitation scaled to GPCC (WFDEI-GPCC, see 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), rgdal (Bivand et al. 2015), rgeos (Bivand et al. 2017), sp (Pebesma and Bivand 2005), 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.