Quantification of Drainable Water Storage Volumes in Catchments and in River Networks on Global Scales using the GRACE and/or River Runoff
Authors: Riegger Johannes 
Published: March 2018

Abstract. The knowledge of storage volumes in catchments and in river networks is essential for the management of water resources and for a comprehensive description of the environment water in the context of climate change. Measurements of water storage variations by the GRACE gravity satellite or by ground based observations of river or groundwater level variation do not allow to determine the respective total storage volumes, which could be considerably larger than the mass variations themselves. In addition mass variations measured by GRACE comprise all storage compartments whether they are hydraulically coupled, contributing to river runoff, or uncoupled like soil moisture, isolated surface water or snow and ice. The possibility to determine the hydraulic time scale from observed runoff and GRACE for the first time allows to quantify the total Drainable Storage i.e. the volume freely draining with gravity comprising all coupled storages in the catchment and in the river network on global scales. As investigations of the runoff–storage (R–S) relationship using GRACE have shown, the R–S relationship can be characterized as a Linear Time Invariant (LTI) System for hydraulically coupled storage compartments (Riegger and Tourian, 2014). Thus, the respective hydraulic time constant of the total system and the corresponding Drainable Storage can be determined once the observed phase shift is adapted either directly or by a model. However, even though the observed phase shift is already considered in modelling approaches its physical reason and the information it carries is not understood in detail so far. A possible reason for the observed phase shift might be found in the river network storage, which so far has not been addressed separately in the R–S relationships. Opposite to storages draining in parallel (as for overland and groundwater flow) a sequence of storages leads to a temporal delay or a phase shift. This might explain the different phasing of the catchment, the river network storage and the total water storage. In order to investigate such a phasing effect a system of cascaded storages for the catchment and river network is set up with different hydraulic time constants and is mathematically solved by piecewise analytical solutions. Tests of the scheme with synthetic recharge time series show that the parameter estimation either versus deviations in total mass or runoff reproduces the time constants for both, the catchment τC and the river network τR in a unique way and allows to quantify the respective storage volumes individually. The application of the Cascaded Storage approach to the Amazon catchment leads to very good agreements of calculated and measured total mass and river runoff (Nash–Sutcliffe for signals > 0.96, for residuals > 0.72). The signal amplitudes and the phase shift between GRACE and river runoff are reproduced very well. The calculated river network mass highly (0.96) correlates with the observed Flood area from the Global Inundation Extent from Multi-Satellites data set (GIEMS) and corresponds to the determined Flood volumes. The implementation of a river network storage in sequence to catchment storages thus describes and explains the observations w.r.t. phasing and signal amplitudes and allows a discrimination of the storage volumes in the catchment and the river network. As the parameter optimization either versus river runoff or GRACE mass deviations leads to comparable results, river runoff and storage volumes can be determined from recharge and GRACE even for ungauged catchments. It is mainly the quality of the recharge data used that limits the quality of the results. Thus further developments in hydrometeorological data products are expected to improve the quantification of river runoff and drainable water storage volume from space.