Estimating continental hydrology parameters from existing space missions: the need for a dedicated surface water mission N. M. Mognard(1), A. Cazenave(1), D.E. Alsdorf(2), E. Rodriguez(3), (1) LEGOS, Toulouse, France; (2) Ohio State University, USA; (3) JPL-CalTech, USA

OUTLINE 1. The estimate of continental hydrology parameters from existing space missions: Computation of surface water volumes by combining altimetry and imagery (Mekong delta) Computation of discharge rates from water levels (South America) Computation of underground water storage by combining GRACE with surface water volumes 2. The need for a dedicated surface hydrology mission

Radar Altimetry and Gravimetry from Space

Global coverage of satellite altimeters

230 river stations ~ 100 sites Over Wetlands ~ 100 lakes ~ 50 reservoirs Current status

Mekong Basin : Comparison between altimeter-derived water level and in situ data T/P and ERS2 coverage T/P-in situ (RMS 17 cm) T/P-ERS2-ENVISAT T/P-ERS2 (RMS 16 cm) From Frappart F. et al., Water volume change in the lower Mekong basin from satellite altimetry and imagery data, Geophy. J. Int., 167, , 2006.

Mekong Basin: (July to December 2003) Seasonal spatio-temporal change of water volume (by combining altimetry and imagery from SPOT/Vegetation) m From Frappart F. et al., Water volume change in the lower Mekong basin from satellite altimetry and imagery data, Geophy. J. Int., 167, , 2006.

Water storage change in the MEKONG Basin from GRACE: Comparison with hydrological models

April 2003 September 2003

AMAZON RIVER From Zakharova E., et al., Amazon river discharge estimated from the Topex/Poseidon altimetry, C.R. Géosc., Acad. Sciences, 338, ,

Amazon River Discharge (m 3 /s) Rating curve: (Observed discharge versus T/P water level) In situ discharge measurement (m 3 /s) T/P-based discharge Jatuarana station From Zakharova E., et al., Amazon river discharge estimated from the Topex/Poseidon altimetry, C.R. Géosc., Acad. Sciences, 338, ,

Water level and discharge

La « Grande Rivière » basin, N-E Canada: Prediction and Optimisation for hydro- electric production

Water And Terrestrial Elevation Requirements: WATER Science Surface Hydrology Goals Primary: –To determine the spatial and temporal variability in freshwater stored in the world’s terrestrial water bodies. Secondary (potentially): –Inundation area provides carbon fluxes at air-water boundary (e.g., CO 2 ) –High resolution h images allow plume and near shore studies –Calculation of ocean water slopes for bathymetry and ocean circulation –Differences between sea ice and water surface allow ice-freeboard calculations, thus thickness. –Repeated topographic measurements for floodplains, glacial ice, etc.

WATER Measurement Goals Hydraulics Required: h, dh/dx, dh/dt Spatial Sampling: Images with pixels of ~100 m –Need between track sampling, not just conventional altimeter profiles. –Image pixel sizes should be small enough to measure ~100 m wide channels. –Height accuracy needs to be capable of deriving slope from lowland rivers –Geographic coverage to 75 degrees North. Temporal Sampling: Repeats ~weekly –Need to capture the majority of discharge from any basin. Amazon floodwave is regular and lasts almost a year Arctic floods occur during annual spring melt and last for less than a month.

 Only method capable of producing images of high resolution water surface elevation measurements  can provide h, dh/dx, and dh/dt  Strong Heritage: Is technology evolution, not revolution  Radar altimetry has already been successfully used in space on a number of missions (Topex/POSEIDON, ERS1/2, ENVISAT, JASON,..)  SRTM was a radar interferometer  Extensive JPL technology investment in WSOA WATER Technology: KaRIN: Ka-Band Radar INterferometer Courtesy E. Rodriguez, JPL

 Profiling altimeters miss too many rivers and lakes whereas imaging methods sample all of the world’s water bodies.  Data base includes 3700 rivers and 6500 lakes, ranked by Q or Area  Profiles from an altimeter  10-day repeat: misses ~45% of rivers and 80% of lakes (misses 22 of 150 largest R and 21 L of 150)  16-day repeat: i.e., Terra, misses ~30% of rivers and ~70% of lakes (14 R and 9 L in top 150)  35-day repeat: i.e., ERS, misses ~20% of rivers and ~55% of lakes (5 R and 1 L in the top 150)  Swaths from an interferometric radar altimeter  10-day repeat: misses ~7% of the rivers and lakes (misses 49 th and 95 th of 150 largest R and 4 of 150 L)  16-day repeat: samples all (misses only ~1% of rivers and lakes; 90% of R & L sampled at least twice during repeat)  35-day repeat: samples all (90% R & L sampled 4 or more times during repeat)  A detailed analysis of the science impact of these results will follow from the Virtual Mission (more later in the presentation) Images are Required : Coverage Study Results for 3 Orbital Repeat Cycles Courtesy E. Rodriguez, JPL

From the standpoint of global water issues, what would be the impact of the proposed WATER mission? Freely available data on water storage for water bodies larger than ~1 km Capability to produce river discharge estimates for many rivers with width > ~ m Understanding how reservoirs are operated (presently there is no coherent data base for reservoir storage) Major implications for the ability to predict floods and droughts globally Major implications for water resources and human health (2 billion incidences of water borne diseases per year globally!)

Conclusions Scientific Objectives: WATER will measure terrestrial surface water storage changes and discharge, which are critical for understanding the land surface water balance. Societal Objectives: WATER will facilitate societal needs by (1) improving our understanding of flood hazards and the ability to forecast floods by measuring water surface elevations in large rivers and floodplains, which are critical for hydrodynamic models; (2) mapping space-time variations in water bodies that contribute to disease vectors (e.g., malaria); and (3) provide freely available data in near- real time on the storage of water available for potable and other human uses in lakes, rivers, and wetlands in support of water management decision making, particularly in trans-boundary river basins. Measurements Required: WATER will provide repeated (at time intervals of ~3 to ~16 days, depending on location) measurements of spatial fields of water surface elevations (h) for wetlands, rivers, lakes, and reservoirs. Each successive h measurement will allow computation of both spatial variations (water surface slope,  h/  x) and temporal changes in elevation  h/  t, hence allowing computation of both storage changes, and hydraulic gradients which are a primary determinant of river discharge. Technology Description: WATER is an interferometric altimeter which has a rich heritage based on (1) the many highly successful ocean observing radar altimeters, (2) the Shuttle Radar Topography Mission (SRTM), and (3) the development effort of the Wide Swath Ocean Altimeter (WSOA). It is a near-nadir viewing, 120 km wide, swath based instrument that will use two Ka-band synthetic aperture radar (SAR) antennae at opposite ends of a 10 m boom to measure the highly reflective water surface. Interferometric SAR processing of the returned pulses yields a 5m azimuth and 10m to 70m range resolution, with elevation accuracy of ± 50 cm. Polynomial based averaging increases the height accuracy to about ± 3 cm. The repeat cycle will be 16 days thus yielding a global h map every 8 days. Estimated cost, including launch vehicle, bus, interferometer, downlinking, and ground segments is about $300M. Criteria Met: WATER will meet high priority targets identified by President Bush’s Cabinet. The Offices of Science & Technology Policy (OSTP) and Management & Budget (OMB) have both called for a U.S. focus on our “ability to measure, monitor, and forecast U.S. and global supplies of fresh water.” It will contribute strongly to ESAS Panel Themes 5 (Water resources and the global hydrologic cycle), 3 (Weather), 4 (Climate), 2 (Ecosystems), 6 (Human Health), and 1 (Societal needs). The mission is an affordable ESSP class design; all components already being space tested. WATER is already an international effort with a large support community.

WATER: Water And Terrestrial Elevation Recovery satellite mission Thank You for your Attention