Tracking Fresh Water from Space

1. Tracking Fresh Water from Space WatER is an international satellite mission to meet the requests of the President’s OSTP & OMB directive to “measure, monitor, and forecast the U.S. and global supplies of fresh water” and to meet the U.N.’s resolution “that the goals of the Decade should be a greater focus on water related issues at all levels and on the implementation of water-related programmes and projects…” WatER initiated from NASA’s Surface Water Working Group and from a similar European community of scientists, engineers, and users. We welcome everyone to join and participate in WatER! Doug Alsdorf, U.S. WaTER PI alsdorf.1@osu.edu Nelly Mognard, EU WatER PI nelly.mognard@cnes.fr Seed funding from the Terrestrial Hydrology Program at NASA: Jared Entin, Program Manager www.geology.ohio-state.edu/water www.geology.ohio-state.edu/swwg

2. The WatER Mission Alsdorf, D. and D. Lettenmaier, Science, 1485-1488, 2003. Alsdorf, D., D. Lettenmaier, C. Vörösmarty, & the NASA Surface Water Working Group, EOS Transactions AGU, 269-276, 2003.

3. Why Use Satellite Based Observations Instead of More Stream Gauges? • Wetlands and floodplains have non-channelized flow, are geomorphically diverse; at a point cross-sectional gauge methods will not provide necessary Q and ΔS. • Wetlands are globally distributed; require intensive & expensive in-situ efforts (cover at least 4% Earth’s land; 1gauge/500 km2 X $50,000 >> half a billion dollars) • Decreasing gauge numbers makes the problem only worse. Political and Economic problems are real. • Need a global dataset of Q and ΔS concomitant with other hydrologic missions (e.g., soil moisture, precipitation). Q & ΔS verify global hydrologic models. Non-Channelized Flow Matthews, E. and I. Fung, GBC, 1, 61-86, 1987.

4. Ohio R. Floods are the number one hazard Two of Several Issues Motivating WatER Runoff (mm/day) Observed Water Cycle and Climate Modeling • How does the lack of measurements limit our ability to predict the land surface branch of the global hydrologic cycle? e.g., In locations where gauge data is available, GCM precipitation and subsequent runoff miss streamflow by 100%; the question is unanswered for ungauged wetlands, lakes, and reservoirs throughout the world. Models Flooding Hazards • Flooding imposes clear dangers, but the lack of water heights and inundation mapping during the passage of the flood wave limit important hydraulic modeling that would otherwise predict the zones of impact. • Essentially, can we predict flooding hazards which could be used to understand the consequences of land use, land cover, and climatic changes for a number of globally-significant, inhabited floodplains? Roads et al., GCIP Water and Energy Budget Synthesis (WEBS), J. Geophysical Research,2003.

5. Global Health, Water Resources, Management Issues that Motivate WatER • Ability to globally forecast freshwater availability is critical for population sustainability. • Water use changes due to population are more significant than climate change impacts. • Predictions also demonstrate the complications to simple runoff predictions that ignore human water usage (e.g., irrigation). • Major health issues are tied to fresh water. • Disease vectors such as malaria are a function of mosquito habitats, which in turn, are directly related to water surface areas. Yet, we do not have any archival or contemporary mapping of these highly dynamic and sometimes intermittent water bodies. • International security is tied to trans-boundary watersheds. • 98.5% water in Euphrates from Turkey; Syria totally dependent; Iraq heavily dependent. Turkey’s Southeastern Anatolia Project (GAP) includes 22 dams for hydroelectric and irrigation (=22% of total flow). Water is Turkish resource, like oil is to Iraq. • Egypt politically controls the Nile, including its uppermost reaches: discourages withdrawals by neighboring countries from Lake Victoria • Remote measurements of surface water volumes and fluxes creates free information for all, removing questions regarding who has how much. For 2025, Relative to 1985 About 3,000,000 people die each year from Malaria Turkey’s GAP Vörösmarty, C.J., P. Green, J. Salisbury, and R.B. Lammers, Global water resources: Vulnerability from climate change and population growth, Science, 289, 284-288, 2000. Frank Schwartz, Personal Communication

6. Problems & Opportunities with Currently Operating Technologies • Low Spatial Resolution: • The spatial resolution of currently operating radar altimeters is low and not capable of accurately measuring water surface elevations across water bodies smaller than ~1 km. • GRACE spatial resolution is ~500,000 km2 and does not isolate surface water from total water column. • Between track spacing of radar and lidar altimeters is much greater than 100 km, thus easily missing many important lakes and reservoirs. • Low Temporal Resolution: • Repeat pass interferometric SAR requires two data-takes, thus typical Δt is one month or much greater. • SRTM operated for just 11 days in February of 2000. • Special Requirements: • Repeat pass interferometric SAR measurements of dh/dt only work with “double-bounce” travel path which results from inundated vegetation. Repeat pass interferometric SAR does not work over open water (i.e., dh/dt measurements are not possible). • No Hydraulics Measured • Existing image based methods rely on in-situ measurements to derive Q and ΔS. Q vs Image curves are unique to each location and do not provide h, dh/dt, dh/dx – no hydraulics

7. Coverage Study Results Courtesy Ernesto Rodriguez, JPL • Coverage from a pulse limited altimeter severely under samples rivers and especially lakes • 16-day repeat (i.e., Terra) coverage misses ~30% of rivers and ~70% of lakes in the data bases (CIA-2; UNH; UH) • If one restricts the study to the largest rivers and lakes, coverage is much better, but still misses some major rivers and lakes • 16-day repeat coverage misses 14 rivers and 9 lakes in the top 150 as ranked by discharge and area, respectively • The rivers which are covered can have only a few visits per cycle, leading to problems with slope calculations • 120 km swath instrument misses very few lakes or rivers • ~1% for 16-day repeat and ~7% for 10-day repeat 6500 lakes and 3700 rivers

8. Channel Slope and Amazon Q from SRTM + =Q dh/dx Manning’s n Water Slope from SRTM Observed Manacapuru Gauge: 90,500 m3/s Estimated from SRTM: 84,800 m3/s Channel Geometry from SAR Bathymetry from In-Situ Need to avoid using in-situ bathymetry, instead use data assimilation. LeFavour and Alsdorf, in review with GRL

9. ΔS and Floodplain Hydraulics from Repeat Pass Interferometric SAR Perspective views of dh/dt. Surface water mission should be capable of measuring these hydraulics. 29 Jun 97 – 2 Apr 97 12 Jul 96 – 15 Apr 96 Views are ~70x70km Flow hydraulics vary across these images. Floodplains are not bathtubs. Arrows indicate that dh/dt changes across floodplain channels. DEM 11 Apr 93 – 26 Feb 93 Alsdorf et al., Nature, 404, 174-177, 2000; Alsdorf et al., Geophysical Research Ltrs., 28, 2671-2674, 2001; Alsdorf et al., IEEE TGRS, 39, 423-431, 2001.

10. KaRIN: Ka-band Radar INterferometer • 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 (e.g., Topex/POSEIDON) • SRTM was an interferometric radar • Extensive JPL technology investment in WSOA • Does not require “double-bounce” like repeat pass interferometric SAR • Water surface is highly reflective, thus is easily measured at near nadir • Compared to SRTM • Order of magnitude better vertical resolution over water • Near nadir look angle, max 4.5º, not SRTM’s ~30º to ~60º

11. KaRIN: Ka-band Radar INterferometer • Ka-band SAR interferometric system with 2 swaths, 50 km each • WSOA and SRTM heritage • Produces heights and co-registered all-weather imagery • 200 MHz bandwidth (0.75 cm range resolution) • Use near-nadir returns for SAR altimeter/angle of arrival mode (e.g. Cryosat SIRAL mode) to fill swath • No data compression onboard: data downlinked to NOAA Ka-band ground stations These surface water elevation measurements are entirely new, especially on a global basis, and thus represent an incredible step forward in hydrology. Courtesy of Ernesto Rodriguez, NASA JPL

12. WatER is Broadly Applicable Science • WatER: Water Elevation Recovery • To determine the spatial and temporal variability in freshwater stored in the world’s terrestrial water bodies. • WATER: Water And Terrestrial Elevation Recovery • A global DEM every 8 days • Additional Science: • Inundation area provides carbon fluxes at air-water boundary (e.g., CO2) • 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 • Complete remaining DEM north of 60N (low relief terrains) • Glacial ice elevations • Change detection: vertical component only from multi-temporal DEMS (doubtful that Ka-band provides temporal coherence)

13. Building the Partnership: • Funding for satellite missions from upcoming announcements by ESA’s Earth Explorer & NASA’s ESSP • Everyone involved is firmly committed to a full sharing of science, technology, and funding • This mission is a community building effort • The core team will be responsible for the tedious day-to-day aspects of the mission, but everyone is invited to be a participant in WatER!

14. 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 $270M. 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. www.geology.ohio-state.edu/water