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. The knowledge of water storage volumes in catchments and in river networksleading to river discharge is essential for the description of riverecology, the prediction of floods and specifically for a sustainablemanagement of water resources in the context of climate change. Measurementsof mass variations by the GRACE gravity satellite or by ground-basedobservations of river or groundwater level variations do not permit thedetermination of the respective storage volumes, which could be considerablybigger than the mass variations themselves. For fully humid tropical conditions like the Amazon the relationship betweenGRACE and river discharge is linear with a phase shift. This permits the hydraulic time constant to be determined and thus the totaldrainable storage directly from observed runoff can be quantified, if the phase shift can beinterpreted as the river time lag. As a time lag can be described by astorage cascade, a lumped conceptual model with cascaded storages for thecatchment and river network is set up here with individual hydraulic timeconstants and mathematically solved by piecewise analytical solutions. Tests of the scheme with synthetic recharge time series show that aparameter optimization either versus mass anomalies or runoff reproduces thetime constants for both the catchment and the river network τC and τR in a unique way, and this then permits an individualquantification of the respective storage volumes. The application to thefull Amazon basin leads to a very good fitting performance for total mass,river runoff and their phasing (Nash–Sutcliffe for signals 0.96, for monthlyresiduals 0.72). The calculated river network mass highly correlates (0.96for signals, 0.76 for monthly residuals) with the observed flood area fromGIEMS and corresponds to observed flood volumes. The fitting performance versus GRACE permits river runoff anddrainable storage volumes to be determined from recharge and GRACE exclusively, i.e. even forungauged catchments. An adjustment of the hydraulic time constants ( τC , τR ) on a training period facilitates a simpledetermination of drainable storage volumes for other times directly frommeasured river discharge and/or GRACE and thus a closure of data gapswithout the necessity of further model runs.

hydrology and earth system sciences

Quantification of drainable water storage volumes on landmasses and in river networks based on GRACE and river runoff using a cascaded storage approach – first application on the Amazon