1. HyMet Runoff Volume
Forecast Model
October 2010 Presentation | by Wendell Tangborn
HyMet, Inc. | 19001 Vashon Hwy SW Suite # 201 | Vashon, WA 98070 | www.hymet.com

2. Outline
 Introduction
 Structure of HyMet Model
 Characteristics of HyMet Model
 Forecast Examples
 Comparisons
 Conclusions
 Accuracy
 Advantages Over other Models
 Differences
1

3. Introduction

4. Forecasts of
Columbia River inflow
at Grand Coulee,
Lower Granite and
The Dalles are
distributed on a
weekly basis
beginning late
November through
mid-September
for the upcoming
2010-2011 water
year.
3
Runoff Volume Forecasts

5. 4
Importance of Runoff Volume
 Importance of Columbia
River runoff volume in
the Pacific Northwest
region:
 Hydropower Electricity
 Irrigation
 Fisheries
 Recreation

6. 6
Structure of Model

7. 6
Runoff Model Flow Chart

8. 7
Area Altitude Profile for The Dalles Basin

9. 9
Characteristics of Model

10.  57 Precipitation stations are selected from a list
of 2197 stations located in Montana, Idaho,
Oregon, Wyoming and Washington.
 Calibration period of a selected precipitation station
regressed with basin runoff: 1969-85
 Verification period of a selected precipitation station
regressed with basin runoff: 1986-2010
 Forecast coefficients are first determined in
calibration, then applied in verification for each
precipitation station
9
Selection of Precipitation Stations

11. Temperature Station10
Weather Stations Used in Model

12. 1. Precipitation Multiplier
2. Precipitation Intercept
3. Fraction of Snowmelt or Rain to Soil
4. Maximum Precipitation Multiplier
5. Ablation from Temperature without Precipitation
6. Ablation from Temperature with Precipitation
7. Ablation from Temperature Range
8. Snowmelt Factor
9. Factor for Soil Moisture ET
10. Factor for Snowpack Sublimation
11. Fraction of Snowmelt or Rain to Groundwater Storage
12. Groundwater Outflow Multiplier
13. Groundwater Outflow Exponent
14. Lower Lapse Rate Threshold
15. Upper Lapse Rate Threshold
11
Model Calibration: 15 Coefficients

13. R2
= 0.9549
0
20
40
60
80
100
0 5 10 15 20 25
Basin Water Storage (Inches)
RunoffVolume(MAF)
12
Basin Water Storage vs. Forecast Volume
Grand Coulee Dam April 1, 2009

14. 14
Forecast Examples

15.  Weekly forecasts contain 7
pages
Forecast seasons:
 Forecast day- Sep30
 January- July
 April - July
 April - September
 January - September
 Forecast in MAF
 Forecast as % of (1971-
2000)Mean
 R-square and Mean Error %
 Confidence levels for forecast
Season
14
Page 1 – Forecast Overview

16.  Grand Coulee, Lower
Granite and The Dalles
Basins
 Daily basin water storage from
beginning of water year to
September 30
15
Page 2 - Water Basin Storage

17. 16
Page 3 - Water Basin Storage
 Grand Coulee, Lower Granite
and The Dalles Basins
 Distribution of mean basin water
storage with elevation

18. 17
Page 4 - Hydrograph
 Grand Coulee, Lower Granite
and The Dalles Basins
 Mean (1969-2009)
 Actual natural flow condition up
to the forecast day
 Actual regulated flow condition
up to forecast day
 Forecast hydrograph

19. 18
Page 5 - Hydrograph
 Grand Coulee, Lower Granite
and The Dalles Basins
 Cumulated runoff volume charts
for each basin

20. 19
Page 6 – Precipitation & Temperature Summary
 Average, observed and
deviation of each parameter
for the entire season, and for
previous week
 Precipitation (inches)
 Temperature (degrees)
 Reconstructed Natural Inflows
(MAF)

21. 20
Page 7 - Confidence Levels
 Calculated from Forecast and
Error
 Example of 90% lower
confidence level for Jan-Jul
forecast:
 CL90=Forecast-1.645 *Error
 Forecast = 87% (Equal chance
observed runoff will be above or
below 87%)
 CL=68% (9 in 10 chance runoff
will be at least 68%
 1 in 10 chance observed will be
less than 68%)

22. 22
Comparisons

23. 22
Comparisons of HyMet Forecasts
 Comparison Charts of HyMet, RFC and ESP
methods for Jan-Jul and Apr-Sep forecast
seasons
 Grand Coulee Basin
 The Dalles Basin
 Lower Granite Basin

24. 23
Comparisons – Grand Coulee Basin

25. 24
Comparisons – The Dalles Basin

26. 25
Comparisons – Lower Granite Basin

27. 27
Conclusions

28. 27
 HyMet forecasts have proven equally, if not more
accurate compared to other forecast methods
Accuracy
HyMet RFC ESP
WY/BASIN GCL LWG TDA GCL LWG TDA GCL LWG TDA
2005 2.8 11.0 4.5 5.8 10.1 7.4
2006 5.8 7.9 5.0 7.0 6.6 6.4
2007 3.9 21.8 5.3 2.0 13.2 4.8 5.4 15.8 8.4
2008 5.6 11.3 7.1 2.9 2.5 2.6 3.2 5.1 3.9
2009 7.3 13.9 3.9 8.1 11.2 2.5 7.8 9.0 4.3
2010 5.0 6.8 4.4 5.9 13.8 9.9 8.3 13.7 10.9
MEAN 5.1 12.1 5.0 5.3 9.6 5.6 6.2 10.9 6.9

29. 28
Advantages
 Advantages of HyMet forecasts over others:
 Equally, if not more accurate
 More informative
 Weekly delivery; directly to your email inbox
 Longer forecast season
 Excel worksheet available for any date in water
year
 Distribution to multiple users (5 per subscription)
 Differences:
 We do not incorporate the future varying weather
conditions in the model and assume normal future
precipitation for the season.

30. 30
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