GRACE in the Murray Darling Basin: Integrating Remote Sensing with Field Monitoring to Improve Hydrological Model Prediction
Kevin M. Ellett, Jeffrey P. Walker, Rodger B. Grayson, Adam B. Smith, and Matthew Rodell
Hydrological processes occurring throughout the earth's surface lead to temporal changes in the distribution of mass, which subsequently cause subtle changes in the earth's gravity field. The GRACE mission (Gravity Recovery And Climate Experiment) of NASA and the German Aerospace Centre will provide global data sets of changes in earth's gravity field at unprecedented accuracy over the next several years. This mission has the potential to provide the first-ever global measurements of changes in terrestrial water storage for large regions at monthly to annual time scales. In this paper we present a methodology designed to address two fundamental questions regarding the applicability of GRACE: (1) is the soil moisture component of terrestrial water storage change detectable in the vertically integrated gravity signal, and (2) can such large-scale measurements of gravity changes be used to improve our understanding and simulation of catchment-scale hydrological processes? The methodology involves three key components: (1) ground-based monitoring of gravity and terrestrial water storage changes at 40 sites throughout the Murrumbidgee catchment in Australia; (2) development of a modelling framework which includes the downscaling and disaggregation of GRACE data; and (3) using AMSR (Advanced Microwave Scanning Radiometer) remotely-sensed surface soil moisture observations to further constrain the downscaling and disaggregation. The GRACE data will be processed through assimilation into a hydrological model of the entire Murray-Darling Basin, and the results verified against the monitoring network. Preliminary results from 18 monitoring sites installed in 2001 suggest that changes in root-zone soil moisture represent the dominant fraction of terrestrial water storage changes occurring in the Murrumbidgee and the magnitude of such changes (monthly changes as high as 130 mm at the point-scale and 38 mm at the mean catchment-scale) should produce a statistically significant signal in both GRACE and ground-based observations of gravity.