Characterisation of Runoff – Storage Relationships By Satellite-Gravimetry and Remote Sensing

Abstract:

GRACE observations of the time dependent gravity field provide a direct measurement of the monthly state of mass and thus monthly total water storage in a catchment. This for the first time allows for a direct comparison of monthly runoff and water storage.

Investigations of global scale Runoff-Storage (R-S) relationships for different climatic conditions show distinct periodic characteristics with hysteresis for total water storage.

For fully humid tropical catchments hysteresis reveals a time invariant temporal delay from storage to runoff. An adjustment of time lag leads to a linear (R-S) relationship with correlation 0.98. Confirmed by the coherence of Fourier spectra, this allows to characterize this R-S relationship as a Linear Time invariant System (LTI).

Based hereon, the hypothesis of a R-S relationship characterized by the superposition of linear contributions from coupled/liquid storage and nonlinear contributions from uncoupled storages is investigated by means of remote sensing.

For boreal catchments MODIS snow coverage is used to separate total storage into coupled/liquid and uncoupled/solid components either directly by assigning frozen solid storage to the snow covered areas or indirectly by a model based aggregation of snow and liquid according to snow coverage. Both methods show, that the nonlinear part of the R-S relationship can be fully assigned to the uncoupled/solid storage while the relationship of runoff and liquid storage can also be characterized as LTI.

This system behavior thus allows for a direct determination of river runoff from GRACE mass or vice versa of water storage from measured runoff for unmanaged catchments, provided that the coupled/uncoupled storage components can be quantified by remote sensing.

Examples for river runoff time series determined from GRACE mass are presented for Amazon sub-catchments and with the additional integration of MODIS snow coverage also for the boreal catchments of Sibiria and North America.