1. A West-wide Seasonal to Interannual Hydrologic Forecast System
Andy Wood, Alan Hamlet, Seethu Babu, Marketa McGuire and Dennis P. Lettenmaier
Univ. of Washington 1
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. 2 3
Climate Forecasts
Downscaling of Climate Model Forecasts
Hydrologic Model (Liang et al., 1994)
Hydrologic Forecasting Simulations
Forecast Products
streamflow
soil moisture
runoff
snowpack
derived products
model spin-up
forecast ensemble(s)
climate forecast information
climatology ensemble
1-2 years back
start of month 0
end of mon 6-12
NCDC met. station obs. up to 2-4 months from current
LDAS/other real-time met. forcings for remaining spin-up
data sources
snow state information
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
NCEP Official Outlooks (pending)
Review of Pilot Implementation: Columbia River Basin in Winter 2003
Major Improvements for Current Forecasting System (starting Sept. ‘03) 4 5
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
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
Trinity
Whiskeytown
Shasta
Oroville (SWP)
Folsom
Clear Creek
American River
Feather River
Trinity River
Sacramento River
Dam
Power Plant
River
Transfer
Delta
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
Apr 1, 2003
Dec 28, 2002
Jan 15, 2003
Feb 1, 2003
ESP /
ENSO /
PDO
FCSTS
Colorado River
San Joaquin River
Dam
Power Plant
River/Canal
Transfer
Eastman, Hensley, & Millerton
New Don Pedro & McClure
Delta
New Hogan
Pardee & Camanche
Stanislaus River
Tuolumne & Merced Rivers
Delta Outflow
Mokelumne River
Calaveras River
San Joaquin River
New Melones
San Luis
Streamflow hydrograph forecasts (example from February 1)
Feb 1
observed
observed
NSIPP-1 / NCEP GSM
FCSTS
Streamflow volume forecast comparison with NWS / NRCS official forecasts
computer failure halted UW 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)
Simulated System Storage (acre-ft)
Simulated System Storage (acre-ft)
min
max
fcst. ens. mean
historical mean
References / Acknowledgements
Wood, A.W., E.P. Maurer, A. Kumar and D.P. Lettenmaier, 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).
The authors acknowledge the support of NOAA/OGP, the IRI/ARCS Regional Applications Project, and the NASA Seasonal-to-Interannual Prediction Project (NSIPP).