Tracking Fresh Water from Space

1
Tracking Fresh Waterfrom Space
Funded by the Terrestrial Hydrology Program at
NASA Jared Entin, Program Manager
Doug Alsdorf NASA SWWG Chair alsdorf.1_at_osu.edu
www.geology.ohio-state.edu/swwg
2
Outline
www.geology.ohio-state.edu/swwg

• Important Hydrologic Science Questions
• Why Satellite Based Observations Are Part of the
  Answer
• The Present and the Future
• What Needs to be Done
• Opportunities for USGS

3
Fundamental Goal

• Our goal is not to launch a satellite, rather our
  goal is to answer important hydrologic science
  questions.
• Answers to these questions will only come from a
  collective effort of all hydrologists, of which a
  spaceborne measuring system is an essential
  piece.
• Spaceborne measurements of surface waters do not
  replace in-situ stream gauges.

4
Our 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.
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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.
http//www.whitehouse.gov/omb/memoranda/fy04/m04-2
3.pdf
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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
  precipitation
• DS and Q

From Soil Moisture Mission (e.g., SMOS, HYDROS)
DS Qout Qin (P-E)
7
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,
  2002

8
Wetlands RequireSpatial View

• Only 1 of 8000 Amazon floodplain lakes has been
  measured for annual water balances!
• 7 Gauges on channels, how do they define flow
  across floodway?
• Annually inundated area in Amazon is 750,000
  km2!
• Gauge data is only sporadically available, if at
  all.
• Worlds largest river, yet Q and DS are poorly
  known.
• Situation is much worse for Congo and other
  remote basins.

634 USGS gauges but only 7 Amazon gauges
Potomac R. at DC 400 m3/s but Negro R. 40000
m3/s
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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.

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.
10
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

0N 72W
(L. Hess photo)
8S 54W

• 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 Distribution of Wetlands and Lakes
Requires Satellite Perspective

• 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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Example Africas Lakes, wetlands and reservoirs
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?
?S is 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.
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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 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.
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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.
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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 its
  measurement.
• 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 ?t 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).

The following slides review these technologies
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ICESat Targeting of Lower Mississippi River
targeted path mode track 2.5 off-nadir
targeted path coincident w/ river reach
8-day reference track
22 km
Slide Courtesy David Harding, NASA GSFC
18
Lower Mississippi River Extent, Stage Slope
2.5 Off- Nadir
Slide Courtesy David Harding, NASA GSFC
19
Predicted GRACE Detectability of Modeled Monthly
Changes in Terrestrial Water Storage
Orange bars are changes in total soil and snow
water storage modeled by the Global Soil Wetness
Project. Error bars represent the total
uncertainty in GRACE-derived estimates, including
uncertainty due to the atmosphere, post glacial
rebound, and the instrument itself. Modified
from Rodell and Famiglietti 1999.
20
Improved estimation of continental water storage
changes from GRACE satellite-to satellite
tracking data Han et al., in review w/ GRL
GRACE measurements are a summation of P, E, ?S,
and groundwater changes. In the Amazon,
floodplain ?S is the dominant portion of the
anomaly. Note the northward movement of the ITCZ
from January to June.
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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

DS
Note loss of gauge data post 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, 2002.
22
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
Abstracts, 2003 2004
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?S and Floodplain Hydraulics from Repeat Pass
Interferometric SAR
Perspective views of dh/dt. Surface water
mission should be capable of measuring these
hydraulics.
12 Jul 96 15 Apr 96
29 Jun 97 2 Apr 97
Views are 70x70km
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
DEM
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.
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Floodplain Flow Directions from Interferometric
SAR
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Measurement Goals

• Hydraulics Required h, dh/dx, dh/dt
• Spatial Sampling Images with pixels of 100 m
• Need between track sampling, not just
  conventional altimeter profiles. (see Ernestos
  work regarding lakes and networks)
• Image pixel sizes should be small enough to
  measure 100 m wide channels. (see dh/dt maps
  from interferometric SAR)
• Height accuracy needs to be capable of deriving
  slope from lowland rivers (e.g., Amazon 1
  cm/km see SRTM and Charon Birketts T/P work)
• Geographic coverage to 75 degrees North.
• Temporal Sampling Repeats weekly
• Need to capture the majority of discharge from
  any basin.
• Amazon floodwave is regular and lasts almost a
  year
• Arctic floods occur during annual spring melt and
  last for less than a month.

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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
• A detailed analysis of the science impact of
  these results will follow from the Virtual
  Mission (more later in the presentation)

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Summary of Future Technologies

• Radar Altimetry
• Through delay-doppler and SAR-like processing,
  radar altimetry is capable of along-track 100 m
  samplings (Johns Hopkins APL).
• Waveform histories and tomographic modeling
  should allow height resolutions approaching 1 cm
  (JPL).
• An interferometric altimeter is capable of
  providing an image of elevation values SRTM and
  Wide-swath Ocean Altimeter heritage (JPL).
• Lidar Altimetry
• Along-track spatial resolution is already 70 m
  and should be capable of 20 m samplings (GSFC).
• Height resolutions are already 3 cm and better
  (U.Texas CSR).
• Images of heights might be achieved through
  multiple lidar beams because off-nadir returns of
  2.5º are demonstrated from space (GSFC). But,
  swaths 10s of km wide are not possible.
• Some cloud and vegetation penetration is
  possible, but density limits are not known
  (GSFC).
• Spaceborne performance is erratic (Mars MOLA
  OK, Earth GLAS Poor)

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Why Radar Altimetry?

• Only method capable of producing images of high
  resolution water surface elevation measurements
• can provide h, dh/dx, and dh/dt
• Is technology evolution, not revolution
• Radar altimetry has already been successfully
  used in space on a number of missions
• Does not require double-bounce like repeat pass
  interferometric SAR
• The water surface is highly reflective, thus is
  easily measured at near nadir

29
SRTM C-Band vs. X-Band
SRTM C-Band DEM
SRTM X-Band DEM
SRTM used incidence angles of 30 to 60 degrees.
Although C-band yields some returns from the
water surface, X-band produces returns
everywhere. Changing to Ka-band and 1 to 3
degrees incidence, should provide returns over
all water surfaces.
Compare
SRTM C-Band DEM
SRTM X-Band DEM
No Returns
Compare
Hendricks and Alsdorf, AGU Abstract, 2004
30
Surface Water Interferometric Altimeter Concept

• 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
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Science Goals

• Primary
• To determine the spatial and temporal variability
  in freshwater stored in the worlds terrestrial
  water bodies.
• Secondary (potentially)
• Secondary science has not been determined,
  possibilities include
• 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, glacial ice, etc.

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What Needs to be Done

• Determine spatial and temporal sampling
  resolutions required to answer hydrologic
  questions.
• For example, the regular Amazon floodwave may
  need a ?t of just a few weeks, but the sudden
  Arctic Spring melt requires a much more frequent
  observation.
• What are the cost vs. science trade-offs
  represented by varying spatial and temporal
  resolutions? Is there a cut-off below which no
  valuable science can be gained?
• Are both discharge and storage change required?
• Surface water velocities measured from space will
  be flawed by wind-induced waves, instead use
  water slope and Mannings equation. But, still
  requires some knowledge of water depths (i.e.,
  channel cross sectional area). Can get dQ/dt
  without in-situ?
• ?S is a simple spaceborne measurement, but is ?S
  sufficient to constrain water and energy cycle
  models? Q will not be measured from space, it
  will be a derived model-assimilation product, so
  what is the Q accuracy from this approach?
• Technology Demonstrations
• What is the capability to penetrate clouds and
  vegetation?
• Does the instrument provide reliable off-nadir
  measurements of h?
• Is surface water science sufficient to support an
  entire satellite mission?
• Which national and international groups would
  participate?
• If other science is joined with a surface water
  mission, what technology and orbital compromises
  are required to ensure a healthy mission for all
  participating science groups?

Answers from Virtual Mission
Answers from Virtual Mission
Answers from SRTM, WSOA
Penetration for 20 opening
SRTMYes
CNES, USGS (hopefully!), CSA?
Depends on ESSP and EE announcements
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The Virtual Mission

• Overall VM Goal To provide information over the
  short term (by mid 2005) that would make viable a
  proposal for a surface water mission in the
  upcoming ESSP (Earth System Science Pathfinder)
  competition, the first stage of which is expected
  to be announced in early 2005.
• What is the VM The VM is a synthetic hydrologic
  model of a continental-scale basin with an
  embedded floodplain and channel hydraulics model.
  By controlling the various hydrologic parameters
  (precipitation, evaporation, infiltration, energy
  balances, etc.), the runoff related boundary
  conditions of the channel and wetlands hydraulics
  models are known which thus allows a known
  relationship between samplings of various channel
  and wetland morphologies to water cycle science.
• Trade-Offs Science, technology, and cost
  trade-offs will be determined by sampling the
  modeled water surface at various resolutions
  related to alternate configurations of existing
  and space-ready technologies.
• Which Questions The VM will identify exact water
  cycle, carbon cycle, and natural hazards
  questions that can be answered from hydraulic
  measurements collected by a spaceborne platform.
  What can we expect to learn from an actual
  mission with such sampling? i.e., we need to
  demonstrate more than simply matching of
  in-situ measured Q, instead, need to demonstrate
  the value added science from an actual mission.
• ?S and Q? The VM will establish trade-offs
  between measuring storage changes versus
  measuring discharge.

34
VM1 Ohio R.

• First model runs of Virtual Mission Stage-I on
  the Ohio River. VIC supplies runoff from
  eighth-degree cells at each of the tributary
  input points (black dots) whereas LISFLOOD-FP
  derives floodplain and channel flows with their
  attendant water surface elevations, depths, and
  inundated areas. Although stream gauges are
  relatively common in the Ohio River Basin, many
  of the smaller tributaries are either ungauged or
  gauged too far upstream from the main channel.
  However, because VIC simulates runoff across a
  mesh grid, it is able to produce inflow values
  for all tributaries and along the main reaches.
  The record produced by VIC is also free of data
  gaps in time. These hydraulics will be sampled
  by the JPL instrument simulator to determine the
  capabilities of various technologies for
  capturing discharge and storage changes.

35
Building the Partnership

• Funding for satellite missions from upcoming
  announcements by ESAs Earth Explorer NASAs
  ESSP
• CNES-JPL are firmly committed to 50/50 science,
  technology, and funding partnership
• This mission is a community building effort
• The core team will be responsible for the
  mission, but everyone is invited to be a part of
  our goals

36
Opportunities for the USGS

• Measurements Calibration and Validation are
  absolutely critical for a successful mission.
• Science USGS has a rich history of hydrologic
  and hydraulic modeling the mission team needs
  this expertise!

37
Conclusions

• 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.
• A critical part of the solution is a satellite
  mission with temporal and spatial resolutions
  compatible with planned hydrologic missions and
  modeling efforts.
• Altimeter instruments are capable of measuring
  profiles of hydraulics, but only the
  interferometric altimeter provides high spatial
  and temporal resolutions that are required to
  answer science questions.
• We have formed an international partnership
  (CNES-JPL), but need additional national (USGS)
  and international partners (CSA, etc.)

www.geology.ohio-state.edu/swwg
38
Altimeter Instrument Concept

• Use Ka-band frequency (8 mm wavelength)
• 1.5 m reflector antenna gt 4.3 km beam limited
  footprint
• 500 MHz bandwidth (30 cm range resolution) gt 650
  m pulse limited footprint
• Use preset tracker based on known topography
• Reduce number of onboard averaging to minimize
  distortion
• Use full-deramp processing to reduce data rate
• For SAR mode, use bursts to reduce PRF, and
  onboard SAR compression (e.g., K. Raneys
  delay-doppler)
• Required transmit power (10W) available from
  solid-state technology
• SAR mode requires onboard processor, higher
  complexity and digital subsystem power

Courtesy of Ernesto Rodriguez, NASA JPL
39
Courtesy of Keith Raney, Johns Hopkins APL
40
Delay-Doppler Advantages

• Better open-ocean performance
• SSH, SWH, WS precisions 2-times better
• More averaging (multi-looking)
• Full spatial control over Doppler cells being
  averaged
• Small along-track footprint
• Typically 250 meters at 30 Hz rate,
  independent of SWH
• Smaller spacecraft altimeter
• Less transmitter power required
• Specular scatterers identified in Doppler
  spectrum
• Enhanced response to small inland (calm) water
  surfaces
• Robust measurements in water/terrain areas
• Small footprint, Doppler-smart tracker
• Flight-proven
• NASA Incubator airborne D2P

Courtesy of Keith Raney, Johns Hopkins APL
41
Future Work with ICESat Surface Water Data

• Quantify signal return amplitude from surface
  water as a function of off-nadir angle,
  atmospheric cloud/aerosol optical depth, and
  surface water roughness
• Examine frequency-of-surface-return global
  climatology
• spatial and temporal variability of observing
  through clouds/aerosols
• Evaluate surface water retrieval capability for
  inundated forests
• Quantify accuracy of slope measurements for along
  channel profiles, multiple crossings of river
  meanders, and areas of inundation
• Establish absolute accuracy of stage retrievals
  by comparison to in-situ gauges
• Demonstrate stage change retrieval from repeat
  profiles

Courtesy of David Harding, NASA GSFC
42
Tomographic Height Estimation

• Use entire range/time power history instead of
  single waveforms
• Use imager to obtain water mask and geolocation
• Generate simulated waveform templates and
  optimize fit with data by varying river height
  and reflectivity
• The problem can be recast as a Maximum Likelihood
  or MAP estimation problem for a limited set of
  model parameters
• Formal estimates of measurement errors can be
  obtained by error propagation

Courtesy of Ernesto Rodriguez, NASA JPL
43
SWWG is Addressing these NASA Earth Science
Enterprise Questions

• KEY How are global precipitation, evaporation,
  and the cycling of water changing? How are
  global ecosystems changing? (variability)
• Global water cycle models require mass and flux
  balances from Q and DS
• Inundation area provides CO2, CH4 exchange with
  the atmosphere, and seasonal variations in C
• Global measurements of Q and DS provide for the
  management of fresh water resources
• What changes are occurring in global land cover
  and land use, and what are their causes? How is
  the earth's surface being transformed? (forcing)
• Floods significantly alter the land surface
  whereas their cause is linked, in part, to within
  catchment changes in land cover and land use
• KEY What are the effects of clouds and surface
  hydrologic processes on Earths climate? How do
  ecosystems and biogeochemical cycles respond to
  and affect global environmental change?
  (response)
• Wetlands, reservoirs, lakes all provide
  significant areas for evaporation and direct
  reception of precipitation these need to be
  fully incorporated in GCMs
• CO2, CH4 evasion from the water surface, and
  their fluvial transport are important components
  in the C-balance of wetland ecosystems
• How are variations in local weather,
  precipitation and water resources related to
  global climate variation? (consequences)
• Real time observations of Q and DS provide
  constraints on flood waves (e.g., flooded area,
  wave velocity) resulting from local to regional
  storms what is the global distribution of these
  in connection to climate oscillations (e.g.,
  ENSO)?
• How well can transient climate variations be
  understood and predicted? (prediction)
• Potential of assimilating Q and DS in global
  water cycle and climate models will allow past
  response to weather and climate for predicting
  future scenarios.

44
Confluence of Rio Tapajos with Amazon

• Closer examination of flat area shows
  differences between GLA06 and GLA14 products
  (different analysis of digitized echo waveform)
• GLA06 based on max peak
• GLA14 based on entire pulse
• Indicative of vegetation
• Track crosses Amazon and upstream Rio Tapajos
• Region from 2.4? to -3.2? appears to be along
  Rio Tapajos

45
GLAS Precision Estimate

• Residuals to low degree polynomial fit to
  elevation on preceding chart represent GLAS
  precision
• Both GLAS data products give similar result (echo
  waveform is Gaussian)
• 40 Hz points shown (no averaging)
• Over this water surface, the precision is lt 3 cm
• May be remaining decimeter level altitude bias,
  but elevation slope is very accurate