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Forecasting Streamflow with the UW Hydrometeorological Forecast System Dennis P. Lettenmaier Photos from: www.metrokc.gov Department of Civil and Environmental Engineering, University of Washington GAPP Mountain Studies Workshop July 24, 2003 presentation prepared by Ed Maurer, Dept of Atmospheric Sciences, UW MM5-DHSVM Streamflow Forecast System UW Real-time MM5 DHSVM Distributed-Hydrology-Soil-Vegetation Model Completely automated In use since WY 1998 Streamflow and other forecasts For details: Westrick, K.J., P. Storck, and C.F. Mass, Description and Evaluation of a Hydrometeorological Forecast System for Mountainous Watersheds, Weather and Forecasting 17: 250-262, 2002. Penn State/NCAR Mesoscale Model MM5 Used throughout the world for both research and operational forecasting 48-hour (and some 72-hour and longer) forecasts run twice daily at the University of Washington High-resolution model (4-km) capable of capturing the complex orography of the region, including lee shading and windward precipitation enhancement FOR MORE INFO... http://www.atmos.washington.edu/mm5rt/ DHSVM land surface hydrology model •Physically-based, distributed model •Solves a water balance at each grid cell at each time step •Horizontal scales typically 30m to 150m •Designed for and extensively tested in complex terrain Details on DHSVM at: http://www.hydro.washington.edu/ DHSVM Calibration Calibration at 2 sites in Snohomish River Basin • Used all available meteorological observations (50sites), 1987-1991 • Used flow observations at two USGS gauges: – – Snoqualmie R. at Carnation Skykomish R. near Gold Bar Snoqualmie R. at Carnation Peaks flows and average water balance are well simulated by DHSVM when forced by observed meteorology UW Hydromet Domain - 2003 26 basins ~60 USGS Gauge Locations 48,896 km2 2,173,155 pixels DHSVM @ 150 m resolution MM5 @ 4 & 12 km Performance of Hydromet System Sauk Observed MM5-DHSVM NWRFC Snoqualmie Using the Hydromet system for MM5 diagnosis Onecourse, exceptionally bad forecast for Of, not all forecasts were thebad… Cedar R., events from January so 25 to Feb 4, 2003 Second peak: •Forecast:1200 cfs •Observed: 3700 cfs •Flood stages above bankfull occurred, and were not forecast Representative Meteorological Station – Mt. Gardner Precip Avg. Precipitation from 1/24 - 2/7: Observed: 1.0 mm/h Simulated: 0.7 mm/h Total difference: ~100 mm Average Temperature: Observed: +2.1C Predicted*: -0.1C SWE: Observed: -50 mm Predicted*: +100 mm MM5 biases in P and T combine to produce large underestimation in runoff Temp SWE Opportunity for Improving UW Hydromet Forecasts 1 – Precipitation/Temperature Bias Correction Remove systematic biases in P, T, at land surface 2 – IMPROVE-2 Take advantage of the IMPROVE-2 experiment to examine the interplay between observation density and bias correction performance 3 – Initial State Updating Assimilation of snow and soil moisture information from an observationally constrained data set. Snow State Updating with Observations Use ground observations (SNOTEL sites) to adjust the basin snow state Challenge: 45-50 snow water observations for 48,000 km2 domain – low density places high dependence on interpolation assumptions Summary • UW Hydrometeorological Forecast System provides accurate streamflow and snowpack predictions when forced with accurate meteorology and when properly initialized • Improvements in both initialization and meteorological forecasts are ongoing, by analyzing current flood events and retrospective analysis • The capabilities of the system are being expanded to include both probabilistic forecasts using ensembles, and to include landslide hazard evaluation Mountain hydrology issues for GAPP • Hydrologic prediction is essentially a one-way forced problem in these environments. Do the (atmospheric) models get the processes right, especially the interaction of topography and model physics? What can GAPP do to help improve the models? • How important are weather scale variations in precipitation and temperature to S/I predictability, and at what (spatial) scales? • Are models able to capture observed longterm changes in snow/rain partitioning within the transient snow zone of the western mountains? • Is there a rationale for a field campaign associated with cold season orographic processes in the west? If so, what needs to be done, particularly in light of results from PACJET, IMPROVE, etc.?