Using GRACE as a tool to monitor continental hydrology

Using GRACE as a tool to monitor continental hydrology Dr. Brian Gunter Delft Institute of Earth Observation and Space Systems (DEOS) Delft University of Technology Beijing, 26-27 May 2008 Conference on Advanced Tools in Water Resources Management

Overview GRACE How does it work and why is it useful? Limitations Spatial and temporal resolution (orbits) Aliasing Impact of spatial filter Examples Outlook for the future June 4, 2008 2

Overview GRACE How does it work and why is it useful? Limitations Spatial and temporal resolution (orbits) Aliasing Impact of spatial filter Examples Outlook for the future June 4, 2008 3

Gravity Recovery And Climate Experiment (GRACE) Science Goals High resolution, mean & time variable gravity field mapping for Earth System Science applications. Mission Systems Instruments KBR (JPL/SSL) Accelerometer (ONERA) Star Cameras (DTU) GPS Receiver (JPL) Satellite (JPL/DSS) Launcher (DLR/Eurockot) Operations (DLR/GSOC) Science (CSR/JPL/GFZ) Orbit Launched: March 17, 2002 Initial Altitude: 500 km Inclination: 89 deg Eccentricity: ~0.001 Separation Distance: ~220 km Lifetime: 5 years Non-Repeat Ground Track, Earth Pointed, 3-Axis Stable

How does GRACE work? Courtesy: NASA June 4, 2008 5

High Frequency Content of KBR Dual One-Way Range Measurement June 4, 2008 6

Gravity & Earth System Science Measurements of the Earth s gravity field from space provides one part of the puzzle about global mass variations Oceanography: (GRACE + Radar Altimetry) Absolute Surface Currents Upper Ocean Heat Content and Heat Flux Deep Ocean Currents and Mass Transport Long Term Sea-Level Change Hydrology: (GRACE + in-situ data) Evapo-transpiration and Soil Moisture Change Aquifer depletion Glaciology: (GRACE + Ice-Sheet Altimetry + in-situ data) Polar Ice Sheet Mass Variations (Global Sea Level Change) Post-Glacial Rebound Solid Earth Sciences: (GRACE + in-situ data) Mantle & Lithospheric Density Variations June 4, 2008 7

Challenges GRACE observes the cumulative gravity change from all sources of mass transport. How can we separate the different components? Answer: by combining with models and in-situ measurements! June 4, 2008 8

Overview GRACE How does it work and why is it useful? Limitations Spatial and temporal resolution (orbits) Aliasing Impact of spatial filter Examples Outlook for the future June 4, 2008 9

GRACE temporal resolution From: Schmidt et al. submitted to Surv. in Geophys. 1 day 1 week 1 month increase temporal resolution decrease decrease spatial resolution increase June 4, 2008 10

GRACE temporal resolution Jan 2004 Sep 2004 Near 9-day repeat orbit From: Klokoćnik et al, J. Geodesy (2007) 305: 503 505 June 4, 2008 11

GRACE temporal resolution Different months have different accuracies From: Klokoćnik et al, J. Geodesy (2007) 305: 503 505 June 4, 2008 12

Aliasing Caused by high frequency (less than one month) mass variations Tides, atmosphere, etc. Are currently modeled out in data preprocessing (AOD) Models not perfect Any errors in models go directly into gravity solutions June 4, 2008 13

Spatial filtering Why do we need to filter GRACE data? Noise increases for higher degrees North-South striping pattern June 4, 2008 14

Spatial filtering Before filtering and after June 4, 2008 15

Choice of spatial filter important! From: Klees et al, GJI (2008) in review

Overview GRACE How does it work and why is it useful? Limitations Spatial and temporal resolution (orbits) Aliasing Impact of spatial filter Examples Outlook for the future June 4, 2008 17

Amazon Basin > 5,000,000 km 2 GRACE observed geoid height variations in 2003, due to changes in (sub-) surface water storage From: Tapley et al, Science (2004) 305: 503 505 June 4, 2008 18

Mississippi Basin 3,247,804 km 2 Black GRACE Blue snow Dark brown soil moisture models Tan σ in soil moisture From: Rodell et al, Hydrogeology Journal (2007) 15: 159 166 June 4, 2008 19

Illinois 140,998 km 2 Comparison between observed and GRACE total water storage from Aug 2002 to Nov 2005 (correlation coefficient 0.83) From: Yeh et al, Water Resour. Res. (2006) 42 June 4, 2008 20

GRACE results over China 795,000 km 2 Shiyang and Yellow River basins See talk later this afternoon by Ellen Hermans (15:00-15:30) for more details! 41,600 km 2 June 4, 2008 21

Overview GRACE How does it work and why is it useful? Limitations Spatial and temporal resolution (orbits) Aliasing Impact of spatial filter Examples Illustrates strengths and weaknesses Outlook for the future June 4, 2008 22

The future GRACE currently projected to last until 2012-2016 Extended time series are valuable when examining multi-year trends GRACE follow-on mission expected Laser ranging system (10-9 m), Improved accelerometers Accuracy of results will improve as background models are refined (ocean, atmosphere, etc.) June 4, 2008 23

Conclusions GRACE is a valuable tool for hydrological studies Accuracies and resolution finite GRACE observes total water storage only Combination with other data sources essential to maximize the benefit of GRACE data Satellite, airborne, in-situ, etc. Especially important for separating hydrology components (SM, ET, etc.) Choice of spatial filtering important June 4, 2008 24