HYDRASAT in the context of the Global Water Cycle

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HYDRASAT in the context of the Global Water Cycle

• Dennis P. Lettenmaier
• Department of Civil and Environmental Engineering
• University of Washington
• NASA River and Wetlands Working Group Meeting
• Chicago OHare Hyatt
• Nov 4-5, 2002

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Hornberger Report Science Questions

• What are the underlying causes of variation in
  the water cycle on both global and regional
  scales, and to what extent is this variation
  induced by human activity?
• To what extent are variations in the global and
  regional water cycle predictable?
• How will variability and changes in the cycling
  of water though terrestrial and freshwater
  ecosystems be linked to variability and changes
  in the cycling of carbon, nitrogen and other
  nutrients at regional and global scales?

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Global observation issues associated with
Hornberger Report SQ 1 2

• Storage
• Snow AMSR
• Glaciers and ice sheets ICESAT
• Atmosphere various
• Soil moisture HYDROS (?)
• G/W GRACE and follow-on (?)
• Lakes and reservoirs none
• Fluxes
• ET none
• Precipitation GPM
• Streamflow none
• Subsurface none
• Others (glaciers etc) none

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Where are the uncertainties?

• Precipitation is reasonably well characterized in
  developed countries by in situ observations, and
  although less than ideal, by real-time reporting
  (over GTS). Hole in land observations is in
  Third World, and this will be the primary
  (surface observation) benefit of GPM
• At present, no global capability to characterize
  runoff dynamics, and their alteration by land
  use, water management, etc. at any spatial scale,
  save perhaps continental
• Direct observation of ET is beyond our grasp at
  this point, aside for flux towers (small number
  of essentially point observations) and indirect
  methods
• Conclusion Without streamflow observations,
  there is no way of tracking the dynamics of the
  land hyrology system it is the primary forced
  variable (and also via assimilation, tells us
  something about the total basin storage (even if
  P E)

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Prediction

• P Coupled model output, evolving potential for
  assimilation
• E Derived variable, essentially no observations
• SM R/S appears to be only way of getting at
  spatial variability, strong candidate for
  assimilation (but not without many issues)
• Snow similar to SM
• Streamflow Essentially not used in any real
  time coupled prediction. Why not?
• No observations in real-time (not really true in
  U.S.)
• Management effects (resolvable, but real-time
  data sets dont exist)
• Most coupled models dont simulate runoff or
  streamflow well

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Technical implications and issues

• Size of drainage area (arguably needs to be order
  of n coupled model grid cells)
• Repeat (how frequent for useful info, and how
  does that depend on spatial scale)
• Spatial coverage (where is the payoff greatest,
  e.g., remote areas)

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• Conclusion
• If we want to develop a capability for global
  hydrologic modeling and prediction, we must have
  an observation system that includes streamflow
  and surface water storage, and a strategy for
  incorporation of these observations into earth
  system modeling