Long-Range Streamflow Forecasting Products and Water Resources Management Applications in the Columbia River Basin Alan F. Hamlet, Andy Wood, Dennis P. Lettenmaier JISAO Center for Science in the Earth System University of Washington IRI Workshop on Applicable Methods for use of Seasonal Climate Forecasts in Water Management Palisades, NYNov, 2002

Hydrological Characteristics of the Columbia Basin Elevation (m) Avg Naturalized Flow The Dalles Snowmelt Dominant Winter Climate Determines Summer Peak Flows

A history of the PDO warm cool warm A history of ENSO Pacific Decadal OscillationEl Niño Southern Oscillation

In 4 out of 5 test years, accurate categorical ENSO forecasts (warm, neutral, cool) have been available in June preceding the water year  1999  2000  2001  2002 

Effects of the PDO and ENSO on Columbia River Summer Streamflows Cool Warm

Global Climate Models Regional Climate Models Hydrology Models Water Resources Models Overview of Modeling Linkages Used in UW Forecasting Systems Streamflow Bias correction Downscaling Observed Meteor. Data Optimization or Simulation Resampling

Variable Infiltration Capacity (VIC) Model

Method 1: A Resampling Approach

ENSO PDO Run Initialized Hydrologic Model Ensemble Streamflow Forecast Select Temperature and Precipitation Data from Historic Record Associated with Forecast Climate Category Climate Forecast Schematic for Forecasting Experiments Using Resampled Observed Data

Time Line for 12-month Lead Time Retrospective Forecasts Climate Forecast Estimated Initial Conditions Forecast Ensemble Lead time = 12 months Statistical Methods Based on Snowpack

Red trace is long-term simulated climatological mean Blue trace is ensemble forecast mean The Dalles Natural Streamflow (cfs)

The Dalles Natural Streamflow (cfs) Red trace is long-term simulated climatological mean Blue trace is ensemble forecast mean

Method 2: An Approach Using Linked Climate and Hydrologic Models

Domain of West-Wide Experimental Forecasting System

Simulations start of month 0end of month 6 Forecast Products streamflow soil moisture runoff, snowpack VIC model spin-up VIC forecast ensemble climate forecast information (from climate model output, or terciles) VIC climatology ensemble 1-2 years back NCDC meteorol. station obs. up to 2-4 months from current LDAS/other meteorol. forcings for remaining spin-up data sources NOTE: In using climate model output, BIAS is a major obstacle.

Recent results: streamflow Columbia R. Basin hindcast analysis GSM- and ESP-derived ensembles for , all years using RMSE-skill score wrt. climatology Results Both ensembles show skill (from initial conditions), but ESP outperforms GSM in most locations (in figure, larger circle = higher skill) Explanation Poor precipitation simulation in GSM JAN forecasts RMSE-Skill Score JAN forecast of FEB-JUL flow

Water Management Applications to the Columbia River Basin

Climate Forecast Streamflow Forecast Water Management Decisions Conceptual Overview of the Use of Climate Forecasts for Water Management Demand Forecast Hydropower Irrigation Urban Water Supply Flood Control Fish Flows

ColSim Monthly Time Step Water Management Model Flood Control Hydropower Irrigation Instream Flow Recreation Navigation Management Objectives:

Constructing the Current Operations Energy Content Curve (wet year) No Forecast Information Used Forecast Information Used

Least Amount of Flood Evacuation Expected Based on Forecast Refill Curve to Least Flood Target Based on Lowest Streamflow Forecast Constructing a New Reservoir Rule Curve called the “Refill to Least Flood” Curve

Expected Wet Year Cool PDO/La Nina Increased Available Water in Fall over Current Operations Adjustments to Energy Content Rule Curves

Expected Dry Year Warm PDO/El Nino Decreased Available Water in Fall over Current Operations

Non-firm marketing with long-range forecasts increases here Traditional non-firm marketing is focused here

Energy Production and Revenue from ColSim model simulations Increasing Emphasis on Fall Energy Marketing

Reliability of System Objectives

Conclusions Improvements in the ability to forecast ENSO with long lead times and greater understanding of the role of decadal scale variability have made possible experimental streamflow forecasts with lead times of about 12 months in the PNW. With needed changes to the reservoir operating system, retrospective resampling forecasts are sufficiently skillful to increase simulated hydropower revenues in the Columbia basin by about $150 million/yr on average. Strengths of the Retrospective Resampling Approach: Faithful reproduction of historic climate signals (ENSO/PDO) Straight-forward validation procedures Weaknesses: Susceptible to forecast errors associated with unusual or evolving climate variability (e.g. WY2001) PDO forecasts at an early state of development

Strengths of climate-model-based forecast system: Problems in transferring meaningful signals between climate and hydrologic models have largely been resolved Improved real-time data assimilation (better estimates of hydrologic initial conditions) Potential to forecast unusual or evolving climate variability better than resampling methods Weaknesses: Currently little or no increase in skill over ESP forecasts from the same initial hydrologic state due primarily to poor precipitation forecasts from climate models.