Seasonal outlooks for hydrology and streamflow in the western U.S. Andy Wood, Alan Hamlet and Dennis P. Lettenmaier Department of Civil and Environmental

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 GroupWater Workshop

Portland, ORSeptember 22, 2004

Topics Background on forecasting system

Selected Results for winter 2003-04

Current forecasts

Final Comments

Background

Background: Forecast System Schematic

NCDC met. station obs.

up to 2-4 months from

current

local scale (1/8 degree) weather inputs

soil moisturesnowpack

Hydrologic model spin up

SNOTEL

Update

streamflow, soil moisture, snow water equivalent, runoff

25th Day, Month 01-2 years back

LDAS/other real-time

met. forcings for spin-up

gap

Hydrologic forecast simulation

Month 6 - 12

INITIAL STATE

SNOTEL/ MODIS*Update

ensemble forecasts ESP traces (40) CPC-based outlook (13) NCEP GSM ensemble (20) NSIPP-1 ensemble (9)

* experimental, not yet in real-time product

Seasonal Climate Forecast Data Sources

ESP

ENSO/PDO

ENSO

CPC Official Outlooks

Seasonal Forecast

Model (SFM)

CAS

OCN

SMLR

CCA

CA

NSIPP-1 dynamical

model

VIC Hydrology Model

NOAA

NASA

UW

CPC outlooks: probabilities => anomalies => ensembles

precipitation

CPC outlooks: probabilities => anomalies => ensembles

temperature

Background: Hydrology Model

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

Background: Estimating Initial Conditions estimating spin-up period inputs

dense station network for model calibration

sparse station network in real-time

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

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

spatial weighting function

elevationweightingfunction

SNOTEL/ASP

VIC cell

• 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 Conditions SWE state adjustment (using SNOTEL/ASP obs)

April 25, 2004

Background: Streamflow Forecast Locations

in development: Colorado R., Upper Rio Grande

Columbia R. basin

California

Snake R. basin



Current forecasts

Final Comments

Initial Conditions for Winter 2003-04

Soil Moisture and Snow Water Equivalent (SWE)



Initial Conditions for Winter 2003-04CPC estimates of seasonal precipitation and temperature

normal to wetnear normal

temperatures

Dec-Jan-Feb








normal to dry generally warm

Mar-Apr-May


very dry hot

March Only








normal to wet slightly warm

Jun-Jul-Aug







Winter 2003-04: PNW streamflow

By Fall, slightly low flows were anticipated

By winter, moderate deficits were forecasted

Aside: volume forecast format

UPPER HUMBOLDT RIVER BASIN

Streamflow Forecasts - May 1, 2003

<==== Drier === Future Conditions === Wetter ====>

Forecast Pt ============ Chance of Exceeding * ===========

Forecast 90% 70% 50% (Most Prob) 30% 10% 30 Yr Avg

Period (1000AF) (1000AF) (1000AF) (% AVG.) (1000AF) (1000AF) (1000AF)

MARY'S R nr Deeth, Nv

APR-JUL 12.3 18.7 23 59 27 34 39

MAY-JUL 4.5 11.3 16.0 55 21 28 29

LAMOILLE CK nr Lamoille, Nv

APR-JUL 13.7 17.4 20 67 23 26 30

MAY-JUL 11.6 15.4 18.0 64 21 24 28

N F HUMBOLDT R at Devils Gate

APR-JUL 5.1 11.0 15.0 44 19.0 25 34

MAY-JUL 1.7 7.2 11.0 50 14.8 20 22

Aside: what is a “normal” year in “most-probable % of average” terms?

PNW CALI

median mean median mean

runoff histogram runoff histogram

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 madeseveral times monthly:1st, mid-month, late

(UW’sESP unconditional and CPC forecasts shown)

UWRFC

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 madeon 1st of month

(UW’sESP unconditional forecasts shown)

UW

RFC

Last winter: volume forecastsfor a sample of PNW locations

OCT 1, 2003 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


NOV 1, 2003 Summer Runoff Volume Forecasts compared to OBS

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OBS %avgRFCUW ESP


DEC 1, 2003 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


JAN 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


FEB 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


MAR 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


APR 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


MAY 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


JUN 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP


JUL 1, 2004 Summer Runoff Volume Forecasts compared to OBS

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110M

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OBS %avgRFCUW ESP



Current forecasts

Final Comments

December

Winter ClimateForecasts Dominate

Hydrologic State Variables Dominate

June March

April 1 SWE (mm)

CPC-based climate forecasts

CPC-based SWE (% average) forecasts

JJASON DJF MAM

CPC-based soil moisture (anomaly) forecasts

JJASON DJF MAM

CPC-based runoff (anomaly) forecasts

JJASON DJF MAM

PNW monthly flow forecasts



Current volume forecast summary

September 1 Forecasts compared to Climatology

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100

110M

ICA

AR

EV

EL

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AC

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AR

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WW

AN

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LIB

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CO

LF

AH

HO

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RR

RC

HIE

FP

RIE

SD

WO

RS

ICE

HA

DA

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ET

RIN

IS

HA

ST

CL

EA

RS

AC

_B

CO

TT

OO

RO

VI

SM

AR

TB

EA

RC

FO

L_

IC

ON

SU

PR

D-C

N_

HO

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_M

EL

DP

R_

IL

K_

MC

MIL

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ESPfcst-med ENSO-med

CPCfcst-med OBS med%avg

Canada WA-OR-ID California



Current forecasts

Final Comments

For more information:

www.hydro.washington.edu/Lettenmaier/Projects/fcst/

Final Comments 2005 will resemble 2004, but flow deficits will be smaller

forecast system is for research, not operations

strengths of western U.S. forecast system may lie at boundaries of current forecasting approach strengths longer lead times use of non-traditional inputs visualization disaggregation diagnosis

apparent skill cannot be determined from one season alone

misc slides

Framework: Estimating Initial Conditions snow cover (MODIS) assimilation (Snake R. trial)

Snowcover BEFORE update

Snowcover AFTER update

MODIS update for April 1, 2004 Forecast

snowadded

removed

Framework: Downscaling Climate Model output

NCEP GSM and NSIPP-1

Framework: Bias-correcting Climate Model output

numerous methods of downscaling and/or bias correction exist

the relatively simple one we’ve settled on requires a sufficient retrospective climate model climatology, e.g., NCEP: hindcast ensemble climatology, 21 years X 10 member NSIPP-1: AMIP run climatology, > 50 years, 9 member

specific to calendar monthand climate model grid cell

Framework: Downscaling CPC outlooks spatial unit for raw forecasts is the Climate Division (102 for U.S.)

13 percentile values (from 0.025 to 0.975) for P and T are given

Framework: Downscaling CPC outlooks

downscaling uses Shaake Shuffle (Clark et al., J. of Hydrometeorology, Feb. 2004) to assemble monthly forecast timeseries from CPC percentile values

Results: Initial Conditions for Current Winter


Documents

Seasonal outlooks for hydrology and streamflow in the western U.S. Andy Wood, Alan Hamlet and Dennis P. Lettenmaier Department of Civil and Environmental