The SWWG Hydrologic Science Agenda

1
Outline

• The SWWG Hydrologic Science Agenda
• Technologies for Measuring ?S and Q (i.e., why
  altimeters are ideal)
• The Virtual Mission

2
Our Science Agenda
3
Water Energy Fluxes in Global Water Cycle
DSf Qout Qgw (P-E) Qin
From Land Cover Land Use Change Missions
From Precipitation, Clouds, and Soil Moisture
Missions

• Global Needs
• Surface water area for evaporation direct
  precipitation
• DS and Q

From Soil Moisture Mission
4
Lack of Q?
Keep these measuring approaches in mind
5
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)
6
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.
7
Globally Declining Gauge NetworkWorking Group
is not a gauge replacement strategy, but..

• 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.
8
Resulting Science Questions

• How does the lack of stream flow and storage
  change 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 a
  key element of the water cycle
• Unfortunately, climate model runoff predictions
  are not in agreement with observed stream flow
• Stream flow provides control verification,
  currently not available over much of the globe

9
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.
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.
11
Resulting Science Questions
China

• What is the hydrology of flooding in urban and
  agricultural areas?
• Flooding imposes clear dangers, but the lack of
  water heights during the passage of the flood
  wave and the lack of concomitant inundation
  mapping limit important hydraulic modeling that
  would otherwise predict the zones of impact.
• Modeling and prediction of flood hazards can be
  used to understand the consequences of land use,
  land cover, and climatic changes for a number of
  globally-significant, inhabited floodplains.
• Inundation hydraulics must account for varied
  water sources as well as the interaction of the
  flow with floodplain topography, vegetation, and
  standing water. Unfortunately, verification and
  calibration of the hydraulic models suffer
  greatly from a lack of floodplain water height
  measurements during flood events and the extent
  of inundation.

U.S.
Europe
India
Europe
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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?

Hess, L.L, J.M. Melack, E.M.L.M. Novo, C.C.F.
Barbosa, and M. Gastil, Dual-season mapping of
wetland inundation and vegetation for the central
Amazon basin, Remote Sensing of Environment, 87,
404-428, 2003.
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Wetlands RequireSpatial View

• Lake Calado The only floodplain lake, of 8000,
  where the water balance has been measured
  throughout annual hydrograph.
• 7 Gauges on channels, how do they define flow
  across floodplain? Ita to Man 12,000 inundated
  km2 1/2 Maryland, 634 USGS gauges, Potomac at
  D.C. 400 m3/s Negro40000 m3/s
• Worlds largest river, yet Q and DS are poorly
  known.

Lesack, L.F.W. and J.M. Melack, Flooding
hydrology and mixture dynamics of lake water
derived from multiple sources in an Amazon
floodplain lake, Water Resources Research, 31,
329-345, 1995.
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Global Distribution 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 Hess et al, RSE 2003 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 2003.
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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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.
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Science Questions from the ESE Research Strategy
Variability
Forcing
Response
Consequence
Prediction
x
x
Precipitation, evaporation cycling of water
changing?
Atmospheric constituents solar radiation on
climate?
Clouds surface hydrological processes on
climate?
Weather variation related to climate variation
(floods)?
Weather forecasting improvement?
Ecosystem responses affects on global carbon
cycle?
Global ocean circulation varying?
Changes in land cover land use?
Consequences in land cover land use?
Transient climate variations?
x
x
x
Surface transformation?
Changes in global ocean circulation?
Coastal region change?
Trends in long-term climate?
Global ecosystems changing?
Stratospheric ozone changing?
Stratospheric trace constituent responses?
Future atmospheric chemical impacts?
x
x
Ice cover mass changing?
Sea level affected by climate change?
Future concentrations of carbon dioxide and
methane?
Motions of Earth interior processes?
Pollution effects?
YellowPrimary BlueSecondary X
Interdisciplinary NRA
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Outline

• The SWWG Hydrologic Science Agenda
• Technologies for Measuring ?S and Q (i.e., why
  altimeters are ideal)
• The Virtual Mission

20
Spaceborne Measurements of Surface Water Mass
Flux
Qtot DSmc DSfp Qtr Qatm Qsoil Qgw

• 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 (gt 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)

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Braided River Discharge From SAR
Extreme Flood Event on Iskut R., Alaska
Normal Flood Event on Iskut R., Alaska
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.
Need a method that does not rely on in-situ
measurements to derive Q and DS.
Smith, L.C., Isacks, B.L., Bloom, A.L., and A.B.
Murray, Estimation of discharge from three
braided rivers using synthetic aperture radar
(SAR) satellite imagery Potential application to
ungaged basins, Water Resources Research, 32(7),
2021-2034, 1996. Smith, L.C., Isacks, B.L.,
Forster, R.R., Bloom, A.L., and I. Preuss,
Estimation of discharge from braided glacial
rivers using ERS-1 SAR First results, Water
Resources Research, 31(5), 1325-1329, 1995.
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Storage and 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

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, 2003.
23
Channel Slope and Discharge from SRTM
SRTM
Water Slope from SRTM
Channel Geometry from SAR

Q
Mannings n
Observed96297 m3/s Estimated93498 m3/s
24
Storage Change from Interferometric SAR
Like interferometry, the ideal instrument will
provide high spatial resolution measurements of
water surface area combined with accurate water
surface elevations, such as those from altimetry.
25
?S and Floodplain Hydraulics from Interferometric
SAR
Interferometric phase showing dh/dt from April 15
to July 12, 1996. Flow hydraulics vary across
this image. Arrows indicate that dh/dt changes
across floodplain channels.
Perspective view of dh/dt
SRTM DEM
SAR
The ideal spaceborne technology would be capable
of measuring these hydraulics!
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GRACE Applications in Hydrology Terrestrial Water
Storage Variations

• Rodell and Famiglietti (1999) explored potential
  to detect water storage variations on land using
  GRACE
• Compared best available models of water storage
  variations (from Global Soil Wetness Project) to
  predicted errors in GRACE-derived estimates for
  20 large watersheds globally
• Found that GRACE will be able to detect water
  storage variations in basins gt 200,000 km2 on
  monthly, seasonal and annual time scales.
• Uncertainty dominated by errors in atmospheric
  mass estimates in large basins and instrument
  errors in small basins
• Significant potential for improved climate model
  initialization and data assimilation

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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.
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Why Altimetry?

• Only method capable of high resolution water
  surface elevation measurements
• can provide h, dh/dx, and dh/dt
• Is technology evolution, not revolution
• Both radar and lidar altimetry have already been
  used in space
• Does not require double-bounce like
  interferometric SAR
• The water surface is highly reflective, thus
  should be easily measured at nadir

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Outline

• The SWWG Hydrologic Science Agenda
• Technologies for Measuring ?S and Q (i.e., why
  altimeters are ideal)
• The Virtual Mission

30
The Virtual Mission

• Overall VM Goal To provide information over the
  short term (by mid-to-late 2004) 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 (?).
• VM Goals To demonstrate the feasibility of
  collecting surface water storage and extent
  variations from a spaceborne platform, and to
  evaluate their ability to improve predictions of
  the water and carbon cycles.
• What are the spatial and temporal sampling
  resolutions required to answer the previous
  hydrologic science questions?
• Are both discharge and storage change required?
  Q is probably much more difficult to remotely
  measure than ?S.
• 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.

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Parts of the VM

• A macroscale water and energy balance model
  implemented at the continental scale capable of
  simulating over large river basins
  evapotranspiration, soil moisture, snow
  accumulation and ablation, runoff and streamflow,
  and surface area and storage variations in lakes
  and wetlands
• A river hydraulics model that will route runoff
  generated by the macroscale hydrology model
  through various channel and floodplain
  morphologies
• A reservoir management model that will represent,
  for a large transboundary continental river
  basin, variations in reservoir levels,
  corresponding variations in reservoir inflows and
  water demands, and implications of direct
  measurements of water levels for international
  water negotiations.

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Expected Results of the VM

• 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.
• Identification of key water cycle, carbon cycle,
  and natural hazards questions that can be
  answered from hydraulic measurements collected by
  a spaceborne platform.
• Evaluate the feasibility of near real-time
  processing and classification of SAR and of
  optical imagery for surface water extents over
  large, continental scale areas.
• Trade-offs between measuring storage changes
  versus measuring discharge.

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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 poorly constrained global
  hydrologic models.
• Conceptually, the ideal solution is a satellite
  mission with temporal and spatial resolutions
  compatible with planned missions and modeling
  efforts.
• But, this point needs to be proven via modeling

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