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Tracking Freshwater from Space


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Title: Tracking Freshwater from Space

Tracking Freshwater from Space
Funded by the Terrestrial Hydrology Program at
NASA Jared Entin, Program Manager
Eric Wood, Doug Alsdorf
  • Science Societal Questions
  • Spaceborne Measurement Methods
  • WatER Water Elevation Recovery Mission
  • Future Directions for the SWWG

Everyone is welcome to join us!
Water Energy Fluxes in Global Water Cycle
From Land Cover Land Use Change Missions (e.g.,
LandSat, MODIS, etc.)
From Precipitation (GPM, TRMM), Clouds
(CloudSat), and Soil Moisture Missions (HYDROS,
  • Global Needs
  • Surface water area for evaporation direct
  • DS and Q

From Soil Moisture Mission (e.g., SMOS, HYDROS)
DS Qout Qin (P-E)
The Science Agenda
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.
The ability to measure, monitor, and forecast
the U.S. and global supplies of fresh water is
another high-priority concern. Agencies, through
the NSTC (National Science and Technology
Council), should develop a coordinated,
multi-year plan to improve research to understand
the processes that control water availability and
quality, and to collect and make available the
data needed to ensure an adequate water supply
for the Nation's future.
The Difficulty of In-Situ Measurements
Gauges are designed for in-channel hydraulics yet
are incapable of measuring the diffusive flow
conditions and related storage changes in these
photos of the Amazon floodplain and Arctic.
Instead of cross-sectional methods, the ideal
solution is a spatial measurement of water
heights from a remote platform.
Non-Channelized Flow
100 Inundated!
  • 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. Stokstad, E., Science, 285, 1199, 1999
  • 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. Shiklomanov et al., EOS, 83, 13-16,

Resulting Science Societal Questions
How does this 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.
What is the role of wetland, lake, and river
water storage as a regulator of biogeochemical
cycles, such as carbon and nutrients? E.g.,
Rivers outgas as well as transport C. Ignoring
water borne C fluxes, favoring land-atmosphere
only, yields overestimates of terrestrial C
What are the implications for global water
management and assessment? The ability to
globally forecast freshwater availability is
critical for population sustainability. Water
use changes due to population are more
significant than climate change impacts.
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?
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 require
    intensive expensive in-situ efforts (cover at
    least 4 Earths land 1gauge/500 km2 X 50,000
    gtgt 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 ?S concomitant
    with other hydrologic missions (e.g., soil
    moisture, precipitation). Q ?S verify global
    hydrologic models.

Non-Channelized Flow
Matthews, E. and I. Fung, GBC, 1, 61-86, 1987.
Typical Problems With Q From 2D Imagery
Iskut River, Alaska
Extreme Flood
Effective width determined from SAR imagery and
discharge for three braided rivers in the Arctic.
Discharge was determined from a gauge at a
downstream coalescing of channels. The three
curves represent possible rating curves to
predict discharge in the absence of gauge data.
Normal Flood
Critical Problems 1. Relies on in-situ
measurements to derive Q and DS, 2. Does not
provide h, dh/dt, dh/dx no hydraulics
Smith, L.C., Isacks, B.L., Bloom, A.L., and A.B.
Murray, Water Resources Research, 32(7),
2021-2034, 1996. Smith, L.C., Isacks, B.L.,
Forster, R.R., Bloom, A.L., and I. Preuss, Water
Resources Research, 31(5), 1325-1329, 1995.
Storage Change Discharge from Radar Altimetry
Presently, altimeters are configured for
oceanographic applications, thus lacking the
spatial resolution that may be possible for
rivers and wetlands.
Water Slope from Altimetry
Classified SAR Imagery

Note loss of gauge data after 1997
Birkett, C.M., Water Resources Res.,1223-1239,
1998. Birkett, C.M., L.A.K. Mertes, T. Dunne,
M.H. Costa, and M.J. Jasinski,Journal of
Geophysical Research, 107, 2003.
?S and Floodplain Hydraulics from Repeat Pass
Interferometric SAR
Perspective views of dh/dt. Surface water
mission should be capable of measuring these
12 Jul 96 15 Apr 96
29 Jun 97 2 Apr 97
Flow hydraulics vary across these images.
Floodplains are not bathtubs. Arrows indicate
that dh/dt changes across floodplain channels.
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.
Channel Slope and Amazon Q from SRTM
Water Slope from SRTM
Observed Manacapuru Gauge 96300 m3/s Estimated
from SRTM 93500 m3/s
Channel Geometry from SAR
Bathymetry from In-Situ
Avoid using in-situ bathymetry, instead
repeatedly measure dh/dx for dQ/dt
Hendricks, Alsdorf, Pavelsky, Sheng, AGU
Abstract, 2003
Problems 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 200,000 km2 and
    does not isolate surface water from its
  • Between track spacing of radar and lidar
    altimeters is much greater than 100 km, thus
    easily missing many important lakes and
  • Low Temporal Resolution
  • Repeat pass interferometric SAR requires two
    data-takes, thus typical ?t is one month or much
  • SRTM operated for just 11 days in February of
  • 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).

The International WatER Mission
Water Elevation Recovery Mission 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
Courtesy of Ernesto Rodriguez, NASA JPL
Everyone is welcome to be a participant in WatER! Participants
from 14 countries on 4 continents are already
Future Directions for the SWWG
  • Water Quality Temperature
  • Transport of Constituents (sediments, nutrients,
  • Hydrologic models capable of petabytes of
    satellite based data
  • Field programs to test methods
  • Global health issues

About 3,000,000 people die each year from Malaria.
Please join us! undergrads, grads, researchers,
faculty, all!
  • There is a great need for global, spatially
    distributed measurements of h, dh/dt, and dh/dx
    for improving global water, climate, and
    biogeochemical models as well as flooding
    dynamics and water management practices.
  • The ideal solution is a satellite mission with
    temporal and spatial resolutions compatible with
    planned hydrologic missions and modeling efforts,
    i.e., WatER!
  • Future directions for the SWWG include developing
    spaceborne methods of quantifying water quality,
    sediment nutrient transport, and temperature.
    Need hydrologic models capable of petabytes of
    data. Concerns also include impact of water
    bourne diseases.