Abstract
Freshwater discharge along continental margins is a key Earth system variable that is not well monitored globally. Here we propose a method for estimating monthly river basin outflows based on the use of new GRACE satellite estimates of terrestrial water storage changes in a coupled land‐atmosphere water balance. Using GRACE land water storage changes (which include changes in groundwater storage) in the water balance method results in more holistic estimates of basin discharge, which we call total basin discharge, that include not only streamflow, but the net of surface, groundwater and tidal inflows and outflows. The method was tested on the Amazon and Mississippi river basins, and could ultimately be applied to the major drainage regions and river basins of the globe. Estimated Amazon total basin discharge was well correlated with observed streamflow, but with a phase lag and underestimation of low flows. Estimated total basin discharge in the Mississippi river basin had a greater annual amplitude than observed streamflow, but showed good temporal covariance. Results for both basins highlight important differences between estimated total basin discharge and observed streamflow, at least part of which can be attributed to groundwater storage changes. Atmospheric moisture data and methods of GRACE data processing also contributed to the differences.
Highlights
Introduction andBackground[2] Freshwater discharge from the continents is central to understanding a wide range of climatic, geomorphic, hydrologic and ecologic processes in the Earth system [Famiglietti, 2004]
Estimated Amazon total basin discharge was well correlated with observed streamflow, but with a phase lag and underestimation of low flows
[19] Figure 1 shows that estimated total basin discharge and observed streamflow are in general agreement for the Amazon
Summary
[2] Freshwater discharge from the continents is central to understanding a wide range of climatic, geomorphic, hydrologic and ecologic processes in the Earth system [Famiglietti, 2004]. Alsdorf and Lettenmaier [2003] outline a plan for a hydrologyspecific interferometric altimetry mission [Rodriguez and Moller, 2004] to measure surface water elevations, their derivatives in space and time, and its lateral extent While this mission would primarily monitor terrestrial freshwater storage changes every 8 days, global river discharge (including both in-channel and overbank flow) would be a key derived product. While the water balance method using GRACE observations of terrestrial water storage changes cannot approach the temporal (near weekly) and spatial (tens of meters) resolution of a future interferometric altimetry mission for surface water discharge, it has the potential to complement such data by immediately providing an integrated measure of surface and groundwater outflows on monthly and longer-term time scales for the world’s major drainage regions [Graham et al, 1999] and river basins. The work described here demonstrates the potential for future, higher-resolution gravity missions for providing critical hydrologic information (snow, surface and groundwater storage changes; evapotranspiration and total basin discharge fluxes) at smaller spatial scales than are possible with the current GRACE mission
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