Characterization of runoff- storage relationships by satellite gravimetry and remote sensing

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. Our spectral analysis supports the fact that the R-S relationships can be characterized as a Linear Time Invariant (LTI) System. As a consequence in time domain an adjustment of time lag leads to correlation of 0.98 between runoff and storage. 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 an LTI system. This system behavior thus allows for a direct determination of river runoff from GRACE mass and vice versa for unmanaged catchments, provided that the coupled/uncoupled storage components can be quantified remote sensing.