Seasonal outlooks for hydrology and streamflow in the western U.S.

Seasonal outlooks for hydrology and streamflow in the western U.S. Andy Wood, Alan Hamlet and Dennis P. Lettenmaier Department of Civil and Environmental Engineering for UW Climate Impacts Group Water Workshop Portland, OR September 22, 2004

Topics • Background on forecasting system • Selected Results for winter 2003-04 • Current forecasts • Final Comments

Background

soil moisture snowpack streamflow, soil moisture, snow water equivalent, runoff local scale (1/8 degree) weather inputs INITIAL STATE Hydrologic model spin up Hydrologic forecast simulation NCDC met. station obs. up to 2-4 months from current LDAS/other real-time met. forcings for spin-up gap ensemble forecasts ESP traces (40) CPC-based outlook (13) NCEP GSM ensemble (20) NSIPP-1 ensemble (9) SNOTEL / MODIS* Update SNOTELUpdate 25th Day, Month 0 1-2 years back Month 6 - 12 Background:Forecast System Schematic * experimental, not yet in real-time product

CCA NOAA CAS OCN CPC Official Outlooks SMLR CA Seasonal Forecast Model (SFM) VIC Hydrology Model NASA NSIPP-1 dynamical model ESP ENSO UW ENSO/PDO Seasonal Climate Forecast Data Sources

CPC outlooks: probabilities => anomalies => ensembles precipitation

CPC outlooks: probabilities => anomalies => ensembles temperature

Background: Hydrology Model

sparse station network in real-time dense station network for model calibration Background:Estimating Initial Conditions estimating spin-up period inputs Problem: met. data availability in 3 months prior to forecast has only a tenth of long term stations used to calibrate and run model in most of spin-up period Solution: use interpolated monthly index station precip. percentiles and temperature anomalies to extract values from higher quality retrospective forcing data -- then disaggregate using daily index station signal.

Background:Estimating Initial Conditions SNOTEL assimilation Problem sparse station spin-up period incurs some systematic errors, but snow state estimation is critical Solution use SWE anomaly observations (from the 600+ station USDA/NRCS SNOTEL network and a dozen ASP stations in BC, Canada) to adjust snow state at the forecast start date

spatial weighting function elevation weighting function SNOTEL/ASP VIC cell Background:Estimating Initial Conditions SNOTEL assimilation • Assimilation Method • weight station OBS’ influence over VIC cell based on distance and elevation difference • number of stations influencing a given cell depends on specified influence distances • distances “fit”: OBS weighting increased throughout season • OBS anomalies applied to VIC long term means, combined with VIC-simulated SWE • adjustment specific to each VIC snow band

Background:Estimating Initial ConditionsSWE state adjustment (using SNOTEL/ASP obs) April 25, 2004

in development: Colorado R., Upper Rio Grande Background:Streamflow Forecast Locations California Columbia R. basin Snake R. basin

Initial Conditions for Winter 2003-04 Soil Moisture and Snow Water Equivalent (SWE)

Initial Conditions for Winter 2003-04 CPC estimates of seasonal precipitation and temperature Dec-Jan-Feb near normal temperatures normal to wet

Initial Conditions for Winter 2003-04 CPC estimates of seasonal precipitation and temperature Mar-Apr-May normal to dry generally warm

Initial Conditions for Winter 2003-04 CPC estimates of seasonal precipitation and temperature March Only very dry hot

Initial Conditions for Winter 2003-04 CPC estimates of seasonal precipitation and temperature Jun-Jul-Aug normal to wet slightly warm

Winter 2003-04: PNW streamflow By Fall, slightly low flows were anticipated By winter, moderate deficits were forecasted

Aside: volume forecast format

median mean median mean runoff histogram runoff histogram CALI PNW Aside: what is a “normal” year in “most-probable % of average” terms?

Winter 2003-04:seasonal volume forecastsComparison with RFC forecast for Columbia River at the Dalles, OR UW forecasts made on 25th of each month RFC forecasts made several times monthly: 1st, mid-month, late (UW’s ESP unconditionaland CPC forecastsshown) UW RFC

Winter 2003-04:seasonal volume forecastsComparison with RFC forecast for Sacramento River near Redding, CA UW forecasts made on 25th of each month RFC forecasts made on 1st of month (UW’s ESP unconditional forecasts shown) RFC UW

Last winter: volume forecastsfor a sample of PNW locations

April 1 SWE (mm) Winter Climate Forecasts Dominate Hydrologic State Variables Dominate December June March Range =16.7% of ensemble summer mean

CPC-based climate forecasts

CPC-based SWE (% average) forecasts SON DJF MAM JJA

CPC-based soil moisture (anomaly) forecasts SON DJF MAM JJA

CPC-based runoff (anomaly) forecasts SON DJF MAM JJA

Seasonal outlooks for hydrology and streamflow in the western U.S.