The Need for Satellite Based Observations of Global Surface Waters: Perspective of the NASA Surface

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The Need for Satellite Based Observations of
Global Surface Waters Perspective of the NASA
Surface Water Working Group
D. Lettenmaier, D. Alsdorf, C. Vörösmarty, C.
Birkett
Workshop on Hydrology from Space Toulouse Septembe
r 29, 2003
www.swa.com/hydrawg/
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Outline
Amazon Floodplain (L. Hess photo)

• 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

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Lack of Q?
Keep these measuring approaches in mind
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Lack of Q and ?S 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)
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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.
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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.
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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.

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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.
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Resulting Science Questions
For 2025, Relative to 1985

• 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.
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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.
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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.
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Global Wetlands

• Wetlands are distributed globally, 4 of Earths
  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.
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Saturated extent from RADARSAT - Putuligayuk
River, Alaska
a.
b.
c.
d.
e.
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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 ?S.
• Wetlands are globally distributed (cover 4
  Earths 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 ?S concomitant
  with other NASA hydrologic missions (e.g., soil
  moisture, precipitation). Q ?S verify global
  hydrologic models.

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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.
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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.
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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

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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 DS? 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.

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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?

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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
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Conclusions

• Lack of Q and ?S 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

www.swa.com/hydrawg/