The Need for Satellite Based Observations of Global Surface Waters: Perspective of the NASA Surface Water Working Group Workshop on Hydrology from Space Toulouse September 29, D. Lettenmaier, D. Alsdorf, C. Vörösmarty, C. Birkett

Outline The Lack of Global Discharge and Water Storage Change Measurements Resulting Science Questions Why Satellite Based Observations Are Required to Answer These Questions Potential Spaceborne Solutions Some ideas for moving the agenda forward The Lack of Global Discharge and Water Storage Change Measurements Resulting Science Questions Why Satellite Based Observations Are Required to Answer These Questions Potential Spaceborne Solutions Some ideas for moving the agenda forward Amazon Floodplain (L. Hess photo)

Lack of Q? Keep these measuring approaches in mind

Lack of Q and ΔS Measurements: An example from Inundated Amazon Floodplain 100% Inundated! 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. How does water flow through these environments? (L. Mertes, L. Hess photos)

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.

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, 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.

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.

Model Predicted Discharge vs. 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 Lenters, J.D., M.T. Coe, and J.A. Foley, Surface water balance of the continental United States, : Regional evaluation of a terrestrial biosphere model and the NCEP/NCAR reanalysis, J. Geophysical Research, 105, , Coe, M.T., Modeling terrestrial hydrological systems at the continental scale: Testing the accuracy of an atmospheric GCM, J. of Climate, 13, , REAN2: NCEP/DOE AMIP Reanalysis II GSM, RSM: NCEP Global and Regional Spectral Models ETA: NCEP Operational forecast model OBS: Observed Runoff (mm/day) OBS REAN2RSMETAGSM J F M A M J J A S O N D

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, , For 2025, Relative to 1985

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 CO 2 Evasion = Basin Wide CO 2 Evasion 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 CO 2 Evasion = Basin Wide CO 2 Evasion 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, , (L. Hess photos)

CO 2 Evasion in the Amazon Over 300,000 km 2 inundated area, samples of CO 2 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? (8S,72W) (0,72W) (0,54W) (8S,54W) 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, , 2002.

Global Wetlands Wetlands are distributed globally, ~4% of Earth’s land surface Current knowledge of wetlands extent is inadequate Matthews, E. and I. Fung, Methane emission from natural wetlands: global distribution, area, and environmental characteristics of sources, Global Biochemical Cycles, v. 1, pp , Prigent, C., E. Matthews, F. Aires, and W. Rossow, Remote sensing of global wetland dynamics with multiple satellite data sets, Geophysical Research Letters, 28, , 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].

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

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 km 2 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.

Solutions from Radar Altimetry Birkett, C.M., Contribution of the TOPEX NASA radar altimeter to the global monitoring of large rivers and wetlands, Water Resources Res., , 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, 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. 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.

0 km 20 Solutions from Interferometric SAR for Water Level Changes 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, , 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, , JERS-1 Interferogram spanning February 14 – March 30, “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. These water level changes, 12 +/- 2 cm, agree with T/P, 21 +/ cm.

What is needed? Stage measurement (e.g. from altimetry) is highly useful (especially for lakes, reservoirs, and wetlands) but is not enough For rivers, discharge and inundation extent are the key variables of interest, for lakes, wetlands, and reservoirs surface area and stage A strategy is needed to obtain plausible estimates of river discharge and surface water storage change without surface observations, while maximizing the utility of existing in situ networks (e.g. for algorithm calibration and verification) There are particular challenges for ice-covered rivers (relevant to most high latitude discharge) Remote sensing offers the potential for obtaining a different kind of data (e.g. dynamics of surface water spatial variations) and should not be viewed as simply a gage replacement strategy

The Technology Challenge Technology development needs to recognize the science requirements: Ability to estimate what we need to know Q (i.e., velocity, slope)? Need to know  S? Both? Are additional variables required: i.e., channel-widths, inundation areas, depths? An Example (next slide): The interferometric altimeter concept of Ernesto Rodriguez of JPL includes two Ka- Band antennae on a 10 m boom providing 50 km wide swath with multi-looking providing cm-scale heights of the water surface and surface velocity.

What is needed for a U.S. – Europe (and perhaps Japan) partnership Recent EOS and Science articles, and this workshop, have created momentum and visibility for an eventual mission Much needs to be done though Development an aircraft instrument? Virtual mission concept? Possible creation of a European working group to further the science, technology, and building an international community?

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

Conclusions: Lack of Q and ΔS measurements cannot be alleviated with more gauges (e.g., wetlands = diffusive flow). This lack leads to a poor basis for evaluation of global hydrologic and climate model predictions (and perhaps eventually assimilation of direct measurements of a key flux and state variable in the water balance). Ideal solution is a satellite mission capable of measureing river discharge and surface extent, and lake, reservoir, and wetland storage change. International partnerships are highly desirable, and perhaps essential, to move a community agenda forward