Andy%20Wood,%20Alan%20Hamlet%20and%20Dennis%20P.%20Lettenmaier

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A west-wide seasonal to interannual hydrologic
forecast system
Andy Wood, Alan Hamlet and Dennis P.
Lettenmaier University of Washington

• A retrospective forecast skill analysis for the
  NCEP seasonal forecasts over the entire western
  U.S. domain was undertaken to ascertain the value
  of the climate model forecasts, relative to the
  ESP forecast and climatological forecasts
  baselines. In general, the GSM retrospective
  forecasts did not improve upon the skill of the
  ESP streamflow forecasts however, in years when
  strong ENSO anomalies were present in the
  forecast initiation month, the GSM-based
  forecasts yielded skill increases in California
  and the Columbia River basin, but lower forecast
  skill (relative to ESP) in the Colorado and the
  upper Rio Grande River basins.

• 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 up to six months.
• We have implemented the Variable Infiltration
  Capacity (VIC) macroscale Climate forecast
  ensembles are presently taken from the NCEP
  Global Spectral Model (GSM) and the NASA NSIPP
  model, and will eventually be expanded to
  incorporate more models in a multi-model
  ensemble.
• As a benchmark, we also use the VIC model to
  produce parallel forecasts via the well-known
  Extended Streamflow Prediction (ESP) method. The
  ESP forecasts are further composited to provide
  ENSO and PDO conditioned ensembles, which past
  work has shown can considerably reduce seasonal
  forecast error variance.

Retrospective skill assessment for forecasts of
basin averages of hydrologic and climate variables
NOTE Skill Score 1 - RMSE(GSM) /
RMSE(baseline) where baseline is either
CLIM unconditional climatology ESP
ESP-derived forecast
Shown below are the skill scores for GSM-based
forecasts over 1979-1999, relative to two
forecast baselines (CLIM and ESP), for all years
(top 2 sets) and for a strong ENSO composite
(abs(Nino 3.4) gt 1).
GSM wrt CLIM
GSM wrt ESP
Strong ENSO Composite GSM wrt ESP
January Forecast
Models
NCEP GSM forecasts
VIC Hydrologic Model (Liang et al., 1994)

• T62 (1.9 degree) resolution
• 6 month forecast duration
• each month, ensemble product has
• 20 forecast members
• 210 rolling climatology members (derived from 10
  initial atmos. condition perturbations for each
  year of a 21 year climatology period)
• we use monthly total precip average temperature

NSIPP forecasts
October Forecast

• 2 x 2.5 degree (lat x lon) resolution
• 7 month forecast duration
• 9-member forecast ensembles
• fixed 50 year climatology based on 9 continous
  AMIP runs

numbered locations were used for retrospective
streamflow forecasting analysis (results not
shown)
(April and July forecasts not shown also,
streamflow forecasts not shown)

• Our initial model domain is the Pacific
  Northwest. Initial testing in real-time began
  with bi-monthly updates starting at the end of
  December, 2002, and ran through April 2003.
• Upgrades to the modeling system during the test
  period included a) the development of a simple
  method for assimilating snow water equivalent
  observations at the start of the forecast, and b)
  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 also describe the development of a set of
  reservoir system models for the western U.S., and
  their implementation within the system to produce
  ensemble forecasts of reservoir system storages,
  operations and releases.

Real-time Hydrologic Forecasting for Columbia
River Basin in Winter 2003
Forecast Approach upgrades
Reservoir system forecasts
Use of real-time SWE observations (right) (from
the 600 station USDA/NRCS SNOTEL network and
several ASP stations in BC, Canada, run by
Environment Canada) to adjust snow state at the
forecast start date (left) (spin-up met. data
improvements method not shown)
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)
Initial hydrologic condition estimates
Streamflow hydrograph forecasts (example from
February 1)
Ongoing Work
blue/red are storage boundaries green is ensemble
mean thick red is historical average black init.
cond. with normal climate

• implementing remainder of western U.S. domain
  with real-time forecasts to recommence in Sept.
• working on alternative spin-up meteorology
  approaches
• expanding products to include spatial fields
  (snow, soil moisture), wider reservoir system
  coverage
• improving web site (http//www.ce.washington.edu/p
  ub/HYDRO/aww/w_fcst/w_fcst.htm)
• developing a downscaling approach for official
  forecasts from NCEP and other centers
• pursuing linkages to NRCS and NWS streamflow
  forecasting operations groups

Streamflow volume forecast comparison with NRCS
official forecasts
References
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, A
Simple hydrologically Based Model of Land Surface
Water and Energy Fluxes for GSMs, J. Geophys.
Res., 99(D7), 14,415-14,428, 1994.