Untitled DocumentEstimating Arizona's water reserves from space-borne gravity observations
Macro Theme Area:
Basin Scale Water Balance [Project ID: B24]
PI:
Peter Troch
CO-PI(s):
Matej Durcik
Basin focus:
Colorado
Specific area in
basin /
field sites:
N/A
Summary/Goals: How much surface and groundwater is stored across the state of Arizona? How much water storage change occurs in Arizona seasonally and from year to year? Today, no one can answer these important basic questions related to sustainable water. But addressing the latter question, while still difficult, is now possible thanks to technological progress in observing Earth's natural resources from space. In March 2002, NASA and DLR launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites to observe, with unprecedented accuracy, monthly changes in the Earth's gravity field. Over land, one can adjust the signal for all known processes that affect gravity, producing a signal closely related to changes in total water storage. To make GRACE derived water storage variations useful to water managers we have to resolve two outstanding/open issues:
(i) how to disaggregate vertically-integrated water storage information into its constituent components (viz. surface, soil and aquifer stores) and related hydrologic fluxes across these stores (precipitation, evaporation, recharge, discharge) and
(ii) how to merge this information with large-scale hydrologic models to improve seasonal predictions of water availability.
Solving the first problem (the focus of this project) will allow us to quantify large-scale monthly hydrologic fluxes (precipitation, P; evapotranspiration, ET; recharge, R, and discharge, Q) across the (horizontal) boundaries of the different catchment stores (land surface, vadose zone, and aquifers), thereby providing better closure of the monthly water balance at the river basin scale. The main goals are to:
i) map Arizona's total water storage change for the period of GRACE gravity observations (March 2002 to date);
ii) develop a strategy to disaggregate GRACE derived total water storage change into monthly hydrologic fluxes, such as precipitation, evaporation, recharge and runoff; this strategy will involve the combination of GRACE derived and modeled water storage changes by means of a monthly water balance model applied to the Lower Colorado River basin;
iii) develop, in collaboration with SAHRA, a methodology to continue monitoring and reporting of Arizona's total water storage changes for the entire duration of the GRACE mission (expected life time 8-10 years); and
iv) make the information available online through WSP's ArizonaWater.org site.
Activities and outcomes during past year:
During the first 6 months of the project (July to December 2006) we focused on the first 2 goals of this project. We have derived estimates of total water storage changes for the Upper Colorado (upstream from Lees Ferry) and the Colorado (upstream Imperial Dam) based on GRACE and have compared those with estimates derived from ECMWF operational re-analysis data applied to the River Basin Water Balance (RBWB)method developed by Seneviratne et al. (2004). Both estimates yield comparable results (in terms of amplitude and periodicity) but also indicate a phase shift between them, possibly related to aliasing effects within GRACE. These results will be presented at the AGU 2006 Fall Meeting in San Francisco.
Plans for the upcoming year:
During the second phase of the project (January to June 2007) we will continue working with GRACE and RBWB in an attempt to disaggregate verticall-integrated total water storage changes into its constituent components, and to compare those estimates with in-situ data.
