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 1960-99 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 http://www.hydro.washington.edu/Lettenmaier/Projects/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 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, 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).