Experimental Long-Lead Streamflow Forecasts for the Columbia River Basin

on the Web

Alan F. Hamlet**

Dennis P. Lettenmaier

JISAO Climate Impacts Group

Department of Civil and Environmental Engineering

University of Washington

** corresponding author:

hamleaf@u.washington.edu, Department of Civil Engineering, Box 352700, University of Washington, Seattle, WA 98195, 206.543.5454

Abstract

Recent advances in long-lead climate forecasts have made it possible to produce streamflow forecasts roughly six months earlier than current operational forecasts that rely on snow pack measurements. The JISAO Climate Impacts Group (CIG) and the Department of Civil Engineering at the University of Washington are currently making experimental streamflow forecasts for the Columbia River which are initially issued in early June, and predict the spring and summer streamflow for the subsequent year. These forecasts provide apparent opportunities to make the Columbia Basin reservoir operating system more directly responsive to interannual climate variability in the early fall and winter months. In the current operating system, the so called "critical" and "assured refill" rule curves that restrict releases for hydropower in the period from August to December are based on the critical and third lowest flow sequence respectively. These rule curves provide appropriate protection of energy capacity and reservoir refill in low flow conditions, but appear to be overly restrictive in normal and high flow years. A new rule curve called the "refill to least flood" curve has been proposed by the authors (http://www.ce.washington.edu/~hamleaf/use_of_forecasts.html) for the major storage dams to make effective use of extended streamflow forecasts and provide more management flexibility in the fall and early winter while protecting against potential low flow conditions when appropriate.

Development of these kinds of streamflow forecasts in an operational setting, and implementation of the related changes in the Columbia Basin reservoir operating system to make use of them is a complex, costly, multi-step process requiring the participation of a large group of institutions and water managers. The first step is to disseminate basic information to the water management community about the new forecasting techniques that are available and their potential usefulness. While not replacing publication in refereed journals, the world wide web has been an effective tool in providing this outreach for the CIG's research in streamflow forecasting to a wide audience. Each June, a experimental long-lead ensemble streamflow forecast for the Columbia River at The Dalles is published on the web (http://www.ce.washington.edu/~hamleaf/DallesForecast.html) along with a description of how these forecasts are produced and their potential usefulness in Columbia Basin operations. By providing these forecasts in real time, the lead time, performance, and usefulness of these forecasts, as well as supporting documentation, can be readily explored by the user community.

 

Overview of Recent Improvements in Forecasting Streamflow in the PNW

Variability of streamflow in the Columbia River has been shown to be strongly correlated with two recurrent climate phemonena, the El Niño/Southern Oscillation (ENSO)(see e.g. Battisti and Sarachik 1995), and the Pacific Decadal Oscillation (PDO)(Mantua et al 1997) ( Piechota et al 1996, Mantua 1997, Hamlet and Lettenmaier 1999). With advancement in the ability to forecast winter ENSO conditions with lead times of about six months, streamflow forecasts for summer with lead times of about 12 months are made possible. This effective doubling of the climate forecast lead time is made possibile by the physical characteristics of the Columbia River and the dependence of summer streamflow on average winter climate conditions and snowpack, which are in turn most strongly correlated with winter ENSO and PDO.

Figure 1 and Table 1 show an ensemble forecast of natural streamflow and corresponding summer runoff volumes for the Columbia River at The Dalles, Oregon for water year 1999. The forecast was made available on an experimental basis June 1, 1998, based on the 1998-1999 ENSO forecast for La Niña conditions, and heuristic methods for determining the phase of the PDO in real time (Hamlet and Lettenmaier, 1999). This type of long-range ensemble forecast shows promise for improving reservoir operations in the Columbia River Basin.

Figure 1 Long-range ensemble streamflow forecast for water year 1999 (PDO cold/ENSO cold) [Light gray lines are forecast ensemble members associated with the forecast climate category, heavy black lines are the simulated minimum and maximum streamflow for each month for 1948-1988, and the red dashed line is the observed naturalized streamflow]

 

 

Percentile

Forecast 1999 Average Streamflow at the Dalles Apr-Aug

(kcfs)

Forecast 1999 Volume Runoff at The Dalles Apr-Aug.

(million acre-ft)

90th

439

133

80th

430

130

70th

406

123

60th

393

119

50th

380

115

40th

379

115

30th

341

103

20th

318

97

10th

299

91

Table 1 Percentile Flows and Runoff Volumes at The Dalles 1999 Water Year Ensemble Forecast

The forecasting scheme is implemented using a macro-scale hydrology model for the Columbia Basin (Figure 2) and observed meteorological driving data selected from the historic record. A schematic of the forecasting method is shown in Figure 3. On about June 1 an ENSO forecast is typically available for the coming winter. The phase of the PDO generally persists from the preceding year, and heuristic methods based on the occurrence of extreme high or low flow events in preceding years are used to identify regime shifts in the PDO in an objective manner (Hamlet and Lettenmaier, 1999). Given the assumed phase of the PDO and the ENSO forecast for the coming winter, precipitation and temperature data from water years associated with these climate conditions are selected from the historic record and used to drive the hydrology model.

The years associated with each climate category are based on retrospective definitions of climate state, using the Niño3.4 (for ENSO) and PDO climate indices (winter averages more than 0.5 standard deviations from the mean in each case). Nino3.4 is a numerical measure of sea surface temperatures in the tropical Pacific which may be used to identify and categorize ENSO events (Trenberth, 1997). The PDO (as it is used here) is a decadal-scale climate phenomenon with a characteristic period of about 50 years and bimodal, epochal events typically lasting 20 to 25 years (Mantua et al, 1997). The two most recent epochs for the PDO were cold phase from 1948-1976, and warm phase from 1977-1996(?). There is evidence in the streamflow record that there may have been a PDO regime shift in 1996 or 1997.

Having established the climate forecast, the hydrology model is then initialized using the meteorological data from the previous water year, and each of the meteorological sequences associated with the climate state forecast for the coming water year are used to simulate the streamflow, producing an ensemble forecast for the coming year (Figure 1). Post processing then produces summaries like those shown in Table 1. The grid-based hydrologic model is capable of resolving a large number of river locations in the Columbia basin, and a similar streamflow forecast can be provided at each of these points.

The main advantage of the experimental methods discussed here over traditional methods based on simulated or observed accumulated snowpack is that the experimental methods have demonstrable forecast skill for predicting summer streamflow early in the water year (fall and early winter). Traditional methods are generally not sufficiently accurate prior to January 1 (Lettenmaier and Garen 1979).

 

Figure 2 Schematic of grid-based Variable Infiltration Capacity (VIC) macro-scale hydrology model

 

Figure 3 Schematic of resampling forecasting method

 

Importance of the World-Wide Web as an Outreach Tool

The Columbia reservoir system operating plan has evolved historically to make use of streamflow forecasts as their skill and reliability have been proven. The kinds of long-range streamflow forecasts described above show promise in providing useful information for water resources managers earlier in the yearly planning process. While the experimental performance is encouraging, the time, financial resources, and institutional coordination that would be required to implement these kinds of forecasts in an operational setting for the Columbia Basin, design alternative reservoir operating plans to make best use of the forecasts, and conduct planning studies to determine the effectiveness of these alternatives are considerable. The first step in such a process is to make general information about the forecasting procedures available to those who would make use of it and provide retrospective and real-time forecasts on an experimental basis.

Peer reviewed journal articles provide a well-recognized and permanent archive for the dissemination of technical information on the forecasting methods (Hamlet and Lettenmaier 1999), and provide confidence that the methods and analysis underlying the forecasts have been carefully reviewed by knowledgeable and objective third parties. Journal articles, however, are not particularly helpful in allowing interested water managers to readily assess the feasibility of the forecasting method in real time, nor is this forum appropriate for providing regular updates on the performance of recent forecasts by comparing to observed streamflow after the fact. For this purpose, the World-Wide Web (WWW) is nearly ideal. At essentially no additional cost to either researchers or those visiting the site, real-time streamflow forecasts, an overview of the forecasting methods, and a forecast archive can be made available to demonstrate the timing, accuracy, and usefulness of information contained in these kinds of forecasts. Changes or additions can also easily be made based on feed back from those visiting the site.

Providing access to the gateway URL also provides a convenient way of responding to requests for information. This information is easily conveyed via e-mail, by links from other web sites, or handouts in public meetings and workshops such as the Water Resources Workshops given each fall by the Climate Impacts Group.

Description of the The Dalles Streamflow Forecasting Web-Site and Links

 

Figure 4 Overview of gateway web site and links

 

A single gateway URL is available at:

http://www.ce.washington.edu/~hamleaf/DallesForecast.html

This site (Figure 4 A) provides an overview of the forecasting method, recent forecasts, links to the sites listed below, and selected references.

 

The web site for the JISAO Climate Impacts Group (Figure 4 B)

http://www-jisao.atmos.washington.edu/PNWimpacts/main.html

provides general information on PNW climate variability and recurrent climate patterns like ENSO and PDO.

 

The Pacific Northwest Marine Environmental Laboratory El Niño Theme Page (Figure 4 C)

http://www.pmel.noaa.gov/toga-tao/el-nino/nino-home.html

provides general information about ENSO and links to real-time monitoring and forecast information that is the basis of the streamflow forecasting methodology.

 

The web site on PDO regime shifts (Figure 4 D)

http://www.ce.washington.edu/~hamleaf/RegimeShifts.html

describes an empirical method for identifying PNW climate regime shifts in real time.

 

The forecast archive (Figure 4 E)

http://www.ce.washington.edu/~hamleaf/forecast_archive.html

contains a group of retrospective forecasts.

 

Links to the UW hydrology home page (Figure 4 F)

http://www.hydro.washington.edu/Lettenmaier/Welcome.html

provides access to information about the hydrology models used in the experimental forecasts.

 

A draft paper describing an alternative operating system to make use of long-lead forecasts (Figure 4 G) is available on the web as a MS Word document.

http://www.ce.washington.edu/~hamleaf/use_of_forecasts.html

 

Conclusions

Recent advances in streamflow forecasting present apparent opportunities to improve water resources management in the Columbia Basin. A first step in moving towards use of these kinds of forecasts in an operational setting is to provide technical information and real-time forecasts to Columbia Basin water managers on an experimental basis. Peer-riewed journal articles provide a permanent archive and a well-established method of ensuring the quality and accuracy of findings, but do not provide an effective means for demonstrating the lead time, performance, or usefulness of streamflow forecasts in real time, nor the ability to update past forecasts with the observed streamflow for retrospective evaluation of their performance. For these purposes, the world wide web has proven to be a nearly ideal outreach tool, providing essentially free access to real-time forecasts and information with minimal effort from the CIG staff.

Selected References

Barnston, A.G., H.M. Van den Dool, S.E. Zebiak, T.P. Barnett, M. Ji, D.R. Rodenhuis, M.A. Cane, A. Leetmaa, N.E. Graham, C.R., Ropelewski, V. E. Kousky, E.A. O'Lenic, and R.E. Livezey: 1994, Long-lead seasonal forecasts---where do we stand?, Bull. Am. Met. Soc., 75, 2097-2114.

Battisti, D.S. and E. Sarachik, 1995: Understanding and Predicting ENSO. Review of Geophysics, 33, 1367-76

Garen, D.C., 1992, Improved Techniques in Regression-Based Streamflow Volume Forecasting, J. Water Resources, Planning and Mgmt. Div., ASCE, 118(6), pp. 654-670

Garen, D.C., 1998, ENSO Indicators and Long-Range Climate Forecasts: Usage in Seasonal Streamflow Volume Forecasting in the Western United States, Session H22E-03, AGU Conference, Fall

Hamlet, A. F., Lettenmaier, D. P., 1998: Effects to Columbia Basin Water Resources Associated with Climate Variability and Operating System Design, Summary of Hydrology Results, Year 3 Report, JISAO Climate Impacts Group, University of Washington.

Hamlet, A. F., Lettenmaier, D. P., 1999: Columbia River Streamflow Forecasting Based on ENSO and PDO Climate Signals, ASCE Journal of Water Res. Planning and Mngmt, (in press)

Latif, M., T.P. Barnett, M.A. Cane, M. Flugel, N.E. Graham, H. von Storch, J.-S. Xu, and S.E. Zebiak, 1994,A Review of ENSO Prediction Studies, Climate Dynamics, 9, 167-179

Lettenmaier, D.P., and D.C. Garen, 1979, Evaluation of streamflow forecasting methods, Proceedings, Western Snow Conference, Sparks, Nevada, 48-55.

Liang, X., D.P. Lettenmaier, E. F. Wood, and S. J. Burges, 1994, A Simple Hydrologically Based Model of Land Surface Water and Energy Fluxes for General Circulation Models, J. Geophys. Res., 99, D7, pp14,415-14,428

Koch, R. W., Buller, D., 1993, Forecasting Seasonal Streamflow: Columbia River at The Dalles, Report to Bonneville Power Administration, June

Mantua, N., S. Hare, Y. Zhang, J. M. Wallace, R. Francis, 1997, A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production, Bulletin of the American Meteorological Society, Vol 78, pp. 1069-1079, June

Mantua, N., 1997,Relationships Between a "Naturalized" Columbia River Flow Record and Large Scale Climate Variations Over the North Pacific, JISAO Climate Impacts Group, Year-2 Progress Report, University of Washington, Seattle Washington, July

Nijssen, B., D.P. Lettenmaier, X. Liang, S.W. Wetzel, E. F. Wood, 1997, Streamflow Simulation for Continental-Scale River Basins, Water Resources Research, 33,4, pp 711, Apr

Piechota, T.C., Dracup, J.A.,1996, Drought and Regional Hydrologic Variations in the United States: Associations with the El Niño/Southern Oscillation. Water Resources Research, 32(5), 1359-1373.

Piechota, T.C., J.A. Dracup, and R.G. Fovell, 1997. Western U.S. Streamflow and Atmospheric Circulation Patterns During El Niño-Southern Oscillation (ENSO). Journal of Hydrology, 201(1-4), 249-271.

Trenberth, K. E. 1997. The definition of El Niño. Bull. Amer. Meteor.Soc. 78: 2771-2777

Twedt, T. M., Schaake, J.C., Peck, E. L., 1977, National Weather Service Extended Streamflow Prediction, Proceedings, Western Snow Conference, Albuquerque, NM, April