Abstract
Continental water storage is a key variable in the Earth system that has never been adequately monitored globally. Since variations in water storage on land affect the time dependent component of Earth's gravity field, the NASA Gravity Recovery and Climate Experiment (GRACE) satellite mission, which will accurately map the gravity field at 2–4 week intervals, may soon provide global data on temporal changes in continental water storage. This study characterizes water storage changes in 20 drainage basins ranging in size from 130,000 to 5,782,000 km2 and uses estimates of uncertainty in the GRACE technique to determine in which basins water storage changes may be detectable by GRACE and how this detectability may vary in space and time. Results indicate that GRACE will likely detect changes in water storage in most of the basins on monthly or longer time steps and that instrument errors, atmospheric modeling errors, and the magnitude of the variations themselves will be the primary controls on the relative accuracy of the GRACE‐derived estimates.
Highlights
The benefitsthat would resultfrom improvedmonitoring and understandingof variationsin continentalwater storageSoil moisture,groundwater,snow and ice, lake and river are evident
Example,soilwater linksthe water, energy,andbiogeochemi- satellitemission.The goal of GRACE is to measureEarth's cal cyclesi;ts highheat capacityprovidesthermalinertia over gravityfieldwith unprecedentedaccuracyat 2-4 weekintervals multiple timescales;and it fuels evapotranspiration,which [Tapley,1997].Becausemassmovementsof water at andbelow helpsto sustainstormsthroughthe processof precipitation Earth'ssurfaceare a major contributorto the time dependent recycling[Brubakeret al., 1993;Eltahir and Bras,1996]
Frozenwater andliquidwater storedbelowsoilsin aquifers gravitymeasurementsobtainedby GRACE couldbe inverted play importantroles in the Earth systemand hold practical to produceestimatesof changesin water storagefor given significancefor society.Seasonalmelts replenish soils and terrestrialregions[Dickeyet al., 1997;Wahret al., 1998].Estistreamsw, hile groundwaterprovidesbaseflow to streamsand matesof the absolutemagnitudeofwaterstoragewouldnotbe sustainsdeep-rootedplantsthroughperiodsof drought.Predictingthe magnitudesof springmeltsand the availabilityof groundwateriscriticalfor naturalhazardpreparednesasndfor obtainablea, ndthe techniquewouldintegratechangesin soil moisture,snow,groundwater,and the other componentsof continentalwater storageto producean estimateof the net agriculturaland domesticwater resourcesmanagement
Summary
Soil moisture,groundwater,snow and ice, lake and river are evident. a systemfor routinely monitoring water, and vegetativewater are the principalcomponentsof changesin terrestrialwaterstorageisnotyet in place.Groundcontinental(or terrestrial,total) water storage. it basedtechniquesare labor intensiveand provideonly point constitutesonly about 3.5% of the water in the hydrologic estimatesof water storage[e.g.,Famigliettei tal., 1998].Microcycle,continentalwater storagehasa tremendousinfluenceon waveremotesensingof soilmoistureby satelliteshowspromclimate and weather aswell as being fundamentalto life on ise [e.g.,Jacksonand LeVine, 1996], but it will not provide land. Massredistributionassociatedwith changesin water storage on land has additionaleffectson the Earth systembeyond those described above. Note that in thispaper,the terms"variationsin waterstorage"and"changesin water storage"are usedinterchangeablyto denoteAS. This paperinvestigatesthe potentialof GRACE to provide estimatesof continentawl ater storagevariationsby comparing simulatedfieldsof water storagechangesto the expectedaccuracyof GRACE-derived AS measurementsT. Spatialvariationsin techniquewill enable GRACE to map the gravity field at the gravityfield were observedprimarilywith the aid of pen- intervalsof 2 weeksor longer,with an accuracyequivalentto a dulums. Gutierrezand Wilson[1987] attemptedto computeperturba- dependentgravitysignalover land, sothat modelsof surface tions in the orbits of the LAGEOS and Starlette satellites pressurechangesand PGR mustsupplythe data necessaryto causedby seasonalredistributionsof air massand terrestrial removetheir effects.On monthlyto annualtimescales,other water storage.To characterizethe changesin water storage, factorsinfluencingthe time variablegravitysignaloverinland they dividedthe land surface,neglectingAntarctica,into ap- regions(e.g.,the solidEarthtide) areassumedto benegligible.
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