PhD Graduation

Promotor: Prof.dr.ir. P.A. Troch (University of Arizona, Tucson, USA)
Co-Promotor: Prof.dr.ir. R. Uijlenhoet

Recent progress in accurately monitoring temporal gravity variations by means of superconducting gravimeters and satellite geodesy provides unprecedented opportunities in closing the water balance. This thesis deals with the relation between temporal gravity variation and water storage change.

A superconducting gravimeter observes with high accuracy (few~nm/s$^2$) and high frequency (1~Hz) the temporal variations in the Earth's gravity field, in Moxa, Germany. Hourly gravity residuals are obtained by time-averaging and correcting for Earth tides, polar motion, barometric pressure variations, and instrumental drift. These gravity residuals are significantly affected by hydrological processes (interception, infiltration, surface runoff and subsurface redistribution) in the vicinity of the gravimeter.

First, time series analysis and distributed hydrological modeling techniques were applied to investigate the effect of hydrological processes on observed terrestrial gravity residuals. It is shown that the short-term response of gravity residuals to medium to high rainfall events can be efficiently modeled by means of a linear transfer function. This transfer function exhibits an oscillatory behavior that indicates fast redistribution of stored water in the upper layers (interception store, root zone) of the catchment surrounding the instrument.

The relation between groundwater storage and gravity residuals is less clear and varies according to the season. High positive correlation between groundwater and gravity exists during winter months when the freezing of the upper soil layers immobilizes water stored in the unsaturated zone of the catchment. Similar results are found in the application of a distributed hydrological model to detect gravity variation. Observed gravity change is then considered as an integrator of catchment-scale hydrological response (similar in nature as discharge measurements), and therefore used to constrain catchment-scale hydrologic models.

Results indicate that a lumped water balance model for unsaturated storage and fluxes, coupled with a semi-distributed hydraulic groundwater model for saturated storage and fluxes, successfully reproduces both gravity and discharge dynamics.

Since its launch, the Gravity Recovery and Climate Experiment (GRACE) mission has been providing estimates of surface mass anomalies for the entire globe. Despite the coarse spatial (a few hundred kilometers) and temporal (1~month) resolution, the mission has proven to deliver valuable data for continental scale river basin water balance studies. Recently released GRACE gravity field coefficients represent a significant improvement over previous releases. The potential of such distributed GRACE measurements is investigated in a smaller, partly mountainous, partly semi-arid basin, namely the Colorado River Basin (CRB).

For the period 2003-2005, monthly 1~degree GRACE data from different releases are correlated with different spatial distributions of hydrologic simulations (VIC model) and in-situ observations. High spatial correlations between VIC and GRACE are found for most of the CRB, where snow and groundwater dominate Upper and Lower CRB respectively, and soil moisture affects the entire CRB.

Results show the need to combine hydrological information from the surrounding basins to apply GRACE data in a basin like CRB. The differences in various GRACE products for the same basin also need to be addressed.