Andy Wood, Alan Hamlet, Seethu Babu, Marketa McGuire and Dennis P' Lettenmaier

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A West-wide Seasonal to Interannual Hydrologic
Forecast System
Andy Wood, Alan Hamlet, Seethu Babu, Marketa
McGuire and Dennis P. Lettenmaier
Univ. of Washington
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Components of Overall Real-time Forecasting
Approach

• OVERVIEW
• We have implemented the Variable Infiltration
  Capacity (VIC) macroscale hydrology model over
  the western U.S. at 1/8 degree spatial resolution
  for experimental ensemble hydrologic prediction
  at lead times of six months to 1 year.
• Forecasts are made once monthly, using simulated
  initial conditions based on real-time
  observations of temperature and precipitation.
• Benchmark climate forecasts are constructed via
  the well-known Extended Streamflow Prediction
  (ESP) method of the U.S. National Weather
  Service. The ESP forecasts are further
  composited to provide ENSO and PDO-conditioned
  ensembles, which past work has shown can
  considerably reduce seasonal forecast error.
• Experimental hydrologic forecasts are also made,
  using climate forecast ensembles taken from the
  NCEP Global Spectral Model (GSM) and the NASA
  NSIPP-1 model. These will also eventually
  include the NCEP official seasonal outlooks.

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Hydrologic Model (Liang et al., 1994)
Climate Forecasts
Downscaling of Climate Model Forecasts
NCEP GSM forecasts

• T62 (1.9 degree) resolution
• 6 month forecast duration
• 20-member ensembles, monthly P, T

Statistical Approach

• Bias-correction of climate model ensembles of
  monthly PCP, TEMP, at climate model scale
• Spatial disaggregation to 1/8 degree hydrologic
  model scale
• Temporal disaggregation from monthly to daily
  time step
• detailed in Wood et al. (2002)

NSIPP-1 Tier 1 forecasts

• 2 x 2.5 degree (lat x lon) resolution
• 7 month forecast duration
• 9-member forecast ensembles, monthly P, T

ESP forecasts

• VIC model resolution (1/8 degree)
• historical 12 month daily sequences from 1960-99

NCEP Official Outlooks (pending)
Major Improvements for Current Forecasting System
(starting Sept. 03)
Review of Pilot Implementation Columbia River
Basin in Winter 2003
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Our initial forecast domain was the Pacific
Northwest. Real-time bi-monthly updates began at
the end of December, 2002, and ran through April
2003.

• NRCS SNOTEL / EC ASP observed SWE anomalies are
  interpolated in distance and elevation to
  hydrologic grid cell elevation bands, and
  linearly combined with simulated anomalies, to
  adjust the hydrologic model state at the start of
  the forecast.
• 2) spin-up met. data improvements method not
  illustrated

Forecast Web Page
Current conditions nowcasts for soil moisture /
SWE
http//www.hydro.washington.edu/Lettenmaier/Projec
ts/fcst/
Primary Upgrades to the forecasting system
included 1) the development of a simple method
for assimilating snow water equivalent
observations at the start of the forecast, 2) a
modification of the surface forcing estimation
immediately prior to the forecast start using a
set of real-time index stations in lieu of the
Land Data Assimilation System (LDAS) real-time
forcings. We began adapting a set of reservoir
system models for the western U.S to produce
ensemble forecasts of reservoir system storages,
operations and releases.
example obs SWE anomalies
corresponding SWE adjustment
Columbia River
Sacramento River
Selected Results
Expanded hydrologic nowcasting / forecasting
domain (ultimately to include US west of
Mississippi R. and update weekly rather than
monthly all at 1/8 degree lat/lon)
SYNOPSIS early winter snowpack deficits
recovered somewhat, but ultimately led to
moderate streamflow deficits in spring and summer.
Spatial Forecasts for P, T, SWE, Soil Moisture,
Runoff
Initial hydrologic condition estimates
Snow Water Equivalent
Monthly streamflow forecast distributions
Jan 15, 2003
ESP / ENSO / PDO FCSTS
Colorado River
San Joaquin River
Streamflow hydrograph forecasts (example from
February 1)
NSIPP-1 / NCEP GSM FCSTS
Streamflow volume forecast comparison with NWS /
NRCS official forecasts
Other Changes / Ongoing Work
Reservoir system forecast experiments

• improving late spin-up forcing procedure through
  addition of more index stations (currently 100,
  with change to 350 in next month)
• re-evaluating the NASA/NOAA NLDAS 1/8 degree
  forcing product as a potential real-time forcing
  in Western U.S.
• automating nowcast / initial condition simulation
  to occur on weekly basis
• expanding forecast products to include spatial
  fields (snow water equivalent, soil moisture,
  runoff)
• adopting selected experimental reservoir system
  forecasts as routine products
• developing a downscaling approach for official
  forecasts from NCEP and other centers
• pursuing linkages to NRCS and NWS streamflow
  forecasting operations groups
• comparing nowcasts with retrospective simulations
  now in progress extending back to 1915.

6-Month Ensemble Forecasts of System Storage for
the Columbia River Basin Using VIC Streamflow
Forecasts and the ColSim Reservoir Model
Initialized by Observed Reservoir Elevations (
Feb 1, 2001)
References / Acknowledgements
Wood, A.W., E.P. Maurer, A. Kumar and D.P.
Lettenmaier, 2002. Long Range Experimental
Hydrologic Forecasting for the Eastern U.S., J.
Geophys. Res., 107(D20). Liang, X., D. P.
Lettenmaier, E. F. Wood and S. J. Burges, 1994.
A Simple hydrologically Based Model of Land
Surface Water and Energy Fluxes for GSMs, J.
Geophys. Res., 99(D7). The authors acknowledge
the support of NOAA/OGP, the IRI/ARCS Regional
Applications Project, and the NASA
Seasonal-to-Interannual Prediction Project
(NSIPP).