Abstract
Abstract In this study, new estimates of monthly freshwater discharge from continents, drainage regions, and global land for the period of 2003–05 are presented. The method uses observed terrestrial water storage change estimates from the Gravity Recovery and Climate Experiment (GRACE) and reanalysis-based atmospheric moisture divergence and precipitable water tendency in a coupled land–atmosphere water mass balance. The estimates of freshwater discharge are analyzed within the context of global climate and compared with previously published estimates. Annual cycles of observed streamflow exhibit stronger correlations with the computed discharge compared to those with precipitation minus evapotranspiration (P − E) in several of the world’s largest river basins. The estimate presented herein of the mean monthly discharge from South America (∼846 km3 month−1) is the highest among the continents and that flowing into the Atlantic Ocean (∼1382 km3 month−1) is the highest among the drainage regions. The volume of global freshwater discharge estimated here is 30 354 ± 1212 km3 yr−1. Monthly variations of global freshwater discharge peak between August and September and reach a minimum in February. Global freshwater discharge is also computed using a global ocean–atmosphere mass balance in order to validate the land–atmosphere water balance estimates and as a measure of global water budget closure. Results show close proximity between the two estimates of global discharge at monthly (RMSE = 329 km3 month−1) and annual time scales (358 km3 yr−1). Results and comparisons to observations indicate that the method shows important potential for global-scale monitoring of combined surface water and submarine groundwater discharge at near-real time, as well as for contributing to contemporary global water balance studies and for constraining global hydrologic model simulations.
Highlights
As a key component of the water cycle, freshwater discharge integrates a host of physical and biogeochemical processes crucial for sustaining ecosystems, influencing climate and related global change
We present large-scale, monthly estimates of freshwater discharge using Gravity Recovery and Climate Experiment (GRACE) derived monthly terrestrial water storage changes in a combined land– atmosphere water mass balance, for the 3-yr period from 2003 to 2005
In this study we have presented new estimates of terrestrial freshwater discharge from basin to continental scales
Summary
As a key component of the water cycle, freshwater discharge integrates a host of physical and biogeochemical processes crucial for sustaining ecosystems, influencing climate and related global change. We present large-scale, monthly estimates of freshwater discharge using Gravity Recovery and Climate Experiment (GRACE) derived monthly terrestrial water storage changes in a combined land– atmosphere water mass balance, for the 3-yr period from 2003 to 2005. While often used as a surrogate for net outflow in basin-scale water balance studies (Gutowski et al 1997; Seneviratne et al 2004; Betts et al 2005), in-channel streamflow measured at the gauging stations may in reality represent only a part of the net freshwater flux and is incomplete for comprehensive budget analyses (Oki et al 1995; Alsdorf and Lettenmaier 2003; Syed et al 2005). Large-scale flow diversification in deltaic regions, floodplain inundation, direct groundwater flows [estimated to be ;10% of global runoff; Zektser and Loaiciga (1993)], and drainage into wetlands [composed of ;4% of the global land area; Prigent et al (2001)] are some of the many pathways of basin water outflow that are not registered by conventional stream gauges
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