March 21, 2011

Kevin WernerNWS Colorado Basin River Forecast Center

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Current CBRFC Water Supply Forecast Methodology

Colorado Basin River Forecast Center

One of 13 River Forecast CentersEstablished in the 1940s for water supply forecastingThree primary missions:1. Seasonal Water supply forecasts for water management2. Daily forecasts for flood, recreation, water management3. Flash flood warning support

www.cbrfc.noaa.gov

Water Supply Forecasts

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Generated seasonally• Typically January through

June• Updated monthly or as

needed

Forecast runoff volume (usually April – July)

Probabilistic

Web Reference: www.cbrfc.noaa.gov/gmap/gmapm.php?wcon=checked

Water Supply Forecast Methods

Statistical Forecasting Statistical Regression Equations Primary NOAA/RFC forecast method from 1940’s to mid 1990’s. Primary NRCS/NWCC forecast method Historical Relationships between flow, snow, & precipitation (1971-2000+) Tied to a fixed runoff period (inflexible)

Ensemble Simulation Model Forecasting A component of a continuous conceptual model (NWSRFS) Continuous real time inputs (temperature, precipitation, forecasts) Accounts for soil moisture states (SAC-SMA) - drives runoff efficiency Builds and melts snowpack (Snow-17) – output feeds SAC-SMA Flexible run date, forecast period, forecast parameters. Evolving toward ESP as primary forecast tool at NOAA/RFCs

Equations built on relationships between the inputs and the output

Statistical Water Supply (SWS)

?

Output Variable: April-July streamflow volume

at Provo-Woodland

Equations built on relationships between the inputs and the output

Statistical Water Supply (SWS)

Input Variable: Trial Lake Snow

Source: NRCS

Water Supply Forecast Methods

Statistical Forecasting Statistical Regression Equations Primary NOAA/RFC forecast method from 1940’s to mid 1990’s. Primary NRCS/NWCC forecast method Historical Relationships between flow, snow, & precipitation (1971-2000+) Tied to a fixed runoff period (inflexible)

Ensemble Simulation Model Forecasting A component of a continuous conceptual model (NWSRFS) Continuous real time inputs (temperature, precipitation, forecasts) Accounts for soil moisture states (SAC-SMA) - drives runoff efficiency Builds and melts snowpack (Snow-17) – output feeds SAC-SMA Flexible run date, forecast period, forecast parameters. Evolving toward ESP as primary forecast tool at NOAA/RFCs

Forecastprecip / temp

General RFC ModelW

eath

er a

nd C

limat

e Fo

reca

sts

RiverForecastSystem

parameters

Observed Data

Analysis &Quality Control

Calibration

modelguidance

Hydrologic Model Analysis

hydrologicexpertise &judgment

OutputsGraphics

River Forecasts

RFC Models

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Snow Model: SNOW-17 Temperature Index Snow model

RFC forecast uses a snow model and a rainfall-runoff model:SNOW-17: Temperature index model for simulating snowpack accumulation and meltSacramento Soil Moisture Accounting Model: Conceptual hydrologic model used to generate runoff

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Calibration

• Process to assign parameter values to the runoff and snow modules within the model. Unique set for each basin (and sub-basin)

• Quality of calibration can vary greatly from basin to basin depending on data availability, period or record, quality of data, hydrology of the basin, etc.

San Juan Basin

San Juan-Pagosa Springs(PSPC2)

Upper (11000-12644)

Middle (8500-11000)

Lower (7198-8500)

San Juan-Pagosa Springs(PSPC2)

Weather and Climate Forecasts

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RFC forecast system incorporates both weather and climate forecasts:Weather forecasts integrated into daily operations with forecaster control over point and basin average values

Water supply forecasts typically only use QPF during late season or in lower basinWhen QPF is used, it is used in a deterministic manner

Climate forecasts integrated into seasonal water supply forecasts through probability shifts of forcing ensembleClimate forecasts are typically only considered in lower basin and only in ENSO years

Forecastprecip / temp

Wea

ther

and

Clim

ate

Fore

cast

s

RiverForecastSystem

parameters

Observed Data

Analysis &Quality Control

Calibration

modelguidance

Hydrologic Model Analysis

hydrologicexpertise &judgment

OutputsGraphics

River Forecasts

Decisions

Rules, values, other factors, politics

Forecast Process

CBRFC Research Needs

1. Improve precipitation analysis

2. Improve use of weather and climate forecasts

3. Develop reliable ensemble forecast system 4. Improve physical

process understanding and modeling

5. Decision Support: Work with stakeholders to use forecasts

Water Supply Forecast Overview

SWS (Statistical Prediction)

ESP (Hydrologic Model Prediction)(River

Forecast Centers)

VIPER (Statistical Prediction)

(Water and Climate Center)

Forecast Coordination

Official Coordinated

Forecast

Water Managers and Users

Other Inputs....

Decisions

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Statistical 50% exceedance Forecast:222 kac-ft

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Ensemble 50% exceedance Forecast:230 kac-ft

ESP applications

CBRFC currently provides “raw” ensemble time series forecasts to several groups:

• Denver Water• Pacificorps (Bear River)• USBR (Gunnison, Utah,

and MTOM)Forecasts updated daily in

winter/springAvailable via CBRFC

webpage23

Denver Water:• Long history of using ensemble forecasts

for risk management• Download CBRFC ensemble forecasts into

reservoir operations spreadsheet (right)• Optimize reservoir operations by

minimizing negative impacts

LAKE POWELLFORECAST PROBABILITY DISTRIBUTION

OBSERVED INFLOW VOLUMEWATER YEAR 2010

6789

1011121314

AUG-71%

SEP-65%

OCT-55%

NOV-50%

DEC-36%

JAN-21%

FEB-25%

MAR-19%

CHANCE OF EQUALIZATION

MA

F

6.007.008.009.0010.0011.0012.0013.0014.00

required volume

30% exc volume

70% exc volume

50% exc volume

Southern CA MWD:• Requested forecast for probability of

equalization releases from Lake Powell

• USBR determines inflow volume required to trigger equalization from 24 month study

• CBRFC uses regulated ESP forecast to determine probability of reaching the required volume

Applications of Probabilistic Flow Forecasts

Forecast Coordination

Forecasts are coordinated with NRCS on a monthly basis. Forecasters at each agency compare forecasts, analyze differences, and come up with a official, coordinated forecast.

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NRCS Preferred Forecast:235 kac-ft

NOAA Preferred Forecast:225 kac-ft

Coordinated Forecast:230 kac-ft

Water Supply Forecast Overview

SWS (Statistical Prediction): 222 KAFESP (Hydrologic Model Prediction) : 230 KAF(River

Forecast Centers)

VIPER (Statistical Prediction) : 236 KAF(Water and

Climate Center)

Forecast CoordinationNOAA: 225 KAFNRCS: 236 KAF

Official Coordinated

Forecast:230 KAF

Water Managers and Users

Other Inputs....

Decisions

Forecast Verification

www.cbrfc.noaa.gov -> Water Supply -> Verification

Beginning in 2008, CBRFC began verifying all water supply forecasts both for current year and systematically over previous years

Current forecast system has skill

SWS and ESP have somewhat different error characteristics

Skill is typically near zero in Jan/Feb and increases substantially into spring

Discussion

• Unmet stakeholder requirements:• Assessment and incorporation of weather and climate forecasts

into water supply forecasts• Forecast horizon out to two years• Objective (and therefore repeatable) forecast system

• CBRFC is committed to working with partners (you all) to meet these and other requirements

• Need to move forward in a responsible way that builds on or bridges from current forecast system

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