The Need for Satellite Based Observations of Global Surface Waters

1. The Need for Satellite Based Observations of Global Surface Waters Funded by the Terrestrial Hydrology Program at NASA D. Lettenmaier, D. Alsdorf, C. Vörösmarty, C. Birkett www.swa.com/hydrawg/

2. Amazon Floodplain (L. Hess photo) Outline • The Lack of Discharge and Water Storage Change Measurements • Resulting Science Questions • Why Satellite Based Observations Are Required to Answer These Questions • Potential Spaceborne Solutions • Your Participation is Welcomed (please see our web page for a list of participants) www.swa.com/hydrawg/

3. Lack of Q? Keep these measuring approaches in mind

4. Lack of Q and ΔS Measurements: An example from Inundated Amazon Floodplain Singular gauges are incapable of measuring the flow conditions and related storage changes in these photos whereas complete gauge networks are cost prohibitive. The ideal solution is a spatial measurement of water heights from a remote platform. 100% Inundated! How does water flow through these environments? (L. Mertes, L. Hess photos)

5. Example: Braided Rivers It is impossible to measure discharge along these Arctic braided rivers with a single gauging station. Like the Amazon floodplain, a network of gauges located throughout a braided river reach is impractical. Instead, a spatial measurement of flow from a remote platform is preferred.

6. Globally Declining Gauge Network • “Many of the countries whose hydrological networks are in the worst condition are those with the most pressing water needs. A 1991 United Nations survey of hydrological monitoring networks showed "serious shortcomings" in sub-Saharan Africa, says Rodda. "Many stations are still there on paper," says Arthur Askew, director of hydrology and water resources at the World Meteorological Organization (WMO) in Geneva, "but in reality they don't exist." Even when they do, countries lack resources for maintenance. Zimbabwe has two vehicles for maintaining hydrological stations throughout the entire country, and Zambia just has one, says Rodda.” • “Operational river discharge monitoring is declining in both North America and Eurasia. This problem is especially severe in the Far East of Siberia and the province of Ontario, where 73% and 67% of river gauges were closed between 1986 and 1999, respectively. These reductions will greatly affect our ability to study variations in and alterations to the pan-Arctic hydrological cycle.” Stokstad, E., Scarcity of Rain, Stream Gages Threatens Forecasts, Science, 285, 1199, 1999. Shiklomanov, A.I., R.B. Lammers, and C.J. Vörösmarty, Widespread decline in hydrological monitoring threatens Pan-Arctic research, EOS Transactions of AGU, 83, 13-16, 2002.

7. Resulting Science Questions • How does this lack of measurements limit our ability to predict the land surface branch of the global hydrologic cycle? • Stream flow is the spatial and temporal integrator of hydrological processes thus is used to verify GCM predicted surface water balances. • Unfortunately, model runoff predictions are not in agreement with observed stream flow.

8. Runoff (mm/day) 1.25 1.00 0.75 0.50 0.25 0.00 J F M A M J J A S O N D OBS REAN2 GSM RSM ETA Model Predicted Discharge vs. Observed REAN2: NCEP/DOE AMIP Reanalysis II GSM, RSM: NCEP Global and Regional Spectral Models ETA: NCEP Operational forecast model OBS: Observed • Mouth of Mississippi: both timing and magnitude errors (typical of many locations). • Within basin errors exceed 100%; thus gauge at mouth approach will not suffice. • Similar results found in global basins Roads et al., GCIP Water and Energy Budget Synthesis (WEBS), J. Geophysical Research,in press 2003. Lenters, J.D., M.T. Coe, and J.A. Foley, Surface water balance of the continental United States, 1963-1995: Regional evaluation of a terrestrial biosphere model and the NCEP/NCAR reanalysis, J. Geophysical Research, 105, 22393-22425, 2000. Coe, M.T., Modeling terrestrial hydrological systems at the continental scale: Testing the accuracy of an atmospheric GCM, J. of Climate, 13, 686-704, 2000.

9. For 2025, Relative to 1985 Resulting Science Questions • What are the implications for global water management and assessment? • 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). 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.

10. Resulting Science Questions • What is the role of wetland, lake, and river water storage as a regulator of biogeochemical cycles, such as carbon and nutrients? • Rivers outgas as well as transport C. Ignoring water borne C fluxes, favoring land-atmosphere only, yields overestimates of terrestrial C accumulation • Water Area x CO2 Evasion = Basin Wide CO2 Evasion (L. Hess photos) Richey, J.E., J.M. Melack, A.K. Aufdenkampe, V.M. Ballester, and L.L. Hess, Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2, Nature, 416, 617-620, 2002.

11. (0,54W) (0,72W) (8S,72W) (8S,54W) CO2 Evasion in the Amazon • Over 300,000 km2 inundated area, 1800+ samples of CO2 partial pressures, 10 year time series, and an evasion flux model • Results: 470 Tg C/yr all Basin; 13 x more C by outgassing than by discharge • But what are seasonal and global variations? If extrapolate Amazon case to global wetlands, = 0.9 Gt C/yr, 3x larger than previous global estimates; Tropics are in balance, not a C Sink? Richey, J.E., J.M. Melack, A.K. Aufdenkampe, V.M. Ballester, and L.L. Hess, Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2, Nature, 416, 617-620, 2002.

12. Global Wetlands • Wetlands are distributed globally, ~4% of Earth’s land surface • Current knowledge of wetlands extent is inadequate • Amazon wetlands are much larger than thought in this view [Melack et al, in review ] • Putuligayuk River watershed on the Alaskan north slope: studies with increasing resolution demonstrate a greater open water area (2% vs. 20%; 1km vs. 50m) and as much as 2/3 of the watershed is seasonally flooded tundra [Bowling et al., WRR in press]. Matthews, E. and I. Fung, Methane emission from natural wetlands: global distribution, area, and environmental characteristics of sources, Global Biochemical Cycles, v. 1, pp. 61-86, 1987. Prigent, C., E. Matthews, F. Aires, and W. Rossow, Remote sensing of global wetland dynamics with multiple satellite data sets, Geophysical Research Letters, 28, 4631-4634, 2001.

13. 2000 = wet = dry Saturated extent from RADARSAT - Putuligayuk River, Alaska a. b. c. d. e.

14. 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 (cover ~4% Earth’s land; 1gauge/1000 km2 X $40,000 = $ 230M) • Declining gauge numbers makes the problem only worse. Political and Economic problems are real. • Need a global dataset of Q and ΔS concomitant with other NASA hydrologic missions (e.g., soil moisture, precipitation). Q & ΔS verify global hydrologic models.

15. Solutions from Radar Altimetry Topex/POSEIDON tracks crossing the Amazon Basin. Circles indicate locations of water level changes measured by T/P radar altimetry over rivers and wetlands. Presently, altimeters are configured for oceanographic applications, thus lacking the spatial resolution that may be possible for rivers and wetlands. Water surface heights, relative to a common datum, derived from Topex/POSEIDON radar altimetry. Accuracy of each height is about the size of the symbol. Birkett, C.M., Contribution of the TOPEX NASA radar altimeter to the global monitoring of large rivers and wetlands, Water Resources Res.,1223-1239, 1998. Birkett, C.M., L.A.K. Mertes, T. Dunne, M.H. Costa, and M.J. Jasinski, Surface water dynamics in the Amazon Basin: Application of satellite radar altimetry, accepted to Journal of Geophysical Research, 2002.

16. Lakes, wetlands and reservoirs in Africa Lakes & Wetlands from UMd land cover classification based on AVHRR (~1 km): JERS-1 Mosaics may show greater area, like the Amazon Total lake area = 844145 km2 (2.3 % of total land area) Topex/POSEIDON heights x area = storage changes Mean interannual variability for 5 lakes is ~200 mm; averaged over all of Africa is 5 mm, about 1/10th the equivalent value for soil moisture. What is the effect of all smaller water bodies? Not negligible and maybe 1/2 that of soil moisture. Sridhar, V., J.Adam, D.P. Lettenmaier and C.M. Birkett, Evaluating the variability and budgets of global water cycle components, 14th Symposium on Global Change and Climate Variations, American Meteo. Soc., Long Beach, CA, February, 2003.

17. 0 km 20 Solutions from Interferometric SAR for Water Level Changes These water level changes, 12 +/- 2 cm, agree with T/P, 21 +/- 10++ cm. JERS-1 Interferogram spanning February 14 – March 30, 1997. “A” marks locations of T/P altimetry profile. Water level changes across an entire lake have been measured (i.e., the yellow marks the lake surface, blue indicates land). BUT, method requires inundated vegetation for “double-bounce” travel path of radar pulse. Alsdorf, D.E., J. M. Melack, T. Dunne, L.A.K. Mertes, L.L. Hess, and L.C. Smith, Interferometric radar measurements of water level changes on the Amazon floodplain, Nature, 404, 174-177, 2000. Alsdorf, D., C. Birkett, T. Dunne, J. Melack, and L. Hess, Water level changes in a large Amazon lake measured with spaceborne radar interferometry and altimetry, Geophysical Research Letters, 28, 2671-2674, 2001.

18. ExistingInstruments • Water Surface Area: • Low Spatial/High Temporal: Passive Microwave (SSM/I, SMMR), MODIS • High Spatial/Low Temporal: JERS-1, ERS 1/2 & EnviSat, RadarSat, LandSat • Water Surface Heights: • Low Vertical & Spatial, High Temporal (> 10 cm accuracy, 200+ km track spacing): Topex/POSEIDON • High Vertical & Spatial, Low Temporal (180-day repeat): ICESat • Water Volumes: • Very Low Spatial, Low Temporal: GRACE • High Spatial, Low Temporal: Interferometric SAR (JERS-1, ALOS, SIR-C) • Topography: • SRTM (also provides some information on water slopes)

19. River Velocity & Width & Slope Measurements Concept by Ernesto Rodriguez of JPL Measure -Doppler Velocity Measure Topography Example of measurement of the radial component of surface velocity using along-track interferometry Measure +Doppler Velocity Basic configuration of the satellite

20. Conclusions: • Lack of Q and ΔS measurements cannot be alleviated with more gauges (e.g., wetlands = diffusive flow). • This lack leads to poorly constrained global hydrologic models. • Potential exists for a satellite-based solutions to these problems. www.swa.com/hydrawg/

21. Also of interest at this meeting: • Session HS15: “Satellite observations of rivers and wetlands ..” (Gallieni 3, 14:15-16:45 today) • NASA surface water working group meeting (immediately following session HS15; check with Doug Alsdorf for location)