The Colorado Surface Water Supply Index (SWSI) Performance Since 1981 Nolan Doesken Colorado State...
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Transcript of The Colorado Surface Water Supply Index (SWSI) Performance Since 1981 Nolan Doesken Colorado State...
The Colorado The Colorado Surface Water Surface Water
Supply Index (SWSI) Supply Index (SWSI) Performance Since Performance Since
19811981Nolan DoeskenNolan Doesken
Colorado State ClimatologistColorado State ClimatologistColorado Climate CenterColorado Climate CenterColorado State UniversityColorado State University
So whose idea was this So whose idea was this anyway???anyway???
BackgroundBackground
► WATF (Water Availability Task Force) was WATF (Water Availability Task Force) was created within the Drought Response Plan to created within the Drought Response Plan to objectively assess water supply informationobjectively assess water supply information Initially used the Palmer Drought Severity Index Initially used the Palmer Drought Severity Index
to trigger drought response actions.to trigger drought response actions. Soon discovered that the Palmer Index was too Soon discovered that the Palmer Index was too
slow to respond to changes and seemed to slow to respond to changes and seemed to perform poorly in seasonal snowpack perform poorly in seasonal snowpack environmentsenvironments
SWSI was later developed to aid in water SWSI was later developed to aid in water availability monitoring in mountainous areas.availability monitoring in mountainous areas.
SWSI UsageSWSI Usage
SWSI IntroductionSWSI Introduction► Purpose: to be an indicator of basin-wide water Purpose: to be an indicator of basin-wide water
availability for the mountain water dependant availability for the mountain water dependant sector, be predictive and permit comparison of sector, be predictive and permit comparison of water supply conditions between basins to assess water supply conditions between basins to assess relative drought severity (Shafer and Dezman, relative drought severity (Shafer and Dezman, 1982)1982)
► SWSI seemed to be more robust and responsive SWSI seemed to be more robust and responsive to changing conditions than the Palmer index for to changing conditions than the Palmer index for the mountainous Western U.S. It accounts for the mountainous Western U.S. It accounts for known components of water supply -- reservoir known components of water supply -- reservoir storage, streamflow, precipitation and snowpack. storage, streamflow, precipitation and snowpack. SWSI seemed more understandable than the SWSI seemed more understandable than the “black box” PDSI model“black box” PDSI model
SWSI CalculationSWSI Calculation► SWSI = (aPSWSI = (aPsnowsnow + bP + bPprecprec + c P + c Pstrmstrm + dP + dPresvresv -50) / 12 -50) / 12
a = snowpack weighting factora = snowpack weighting factor b = precipitation weighting factorb = precipitation weighting factor c = streamflow weighting factorc = streamflow weighting factor d = reservoir weighting factord = reservoir weighting factor
► a+b+c+d = 1a+b+c+d = 1 P = probability of non-exceedance (percent) for P = probability of non-exceedance (percent) for
component component ii snow, prec, strm and resv are the snowpack, snow, prec, strm and resv are the snowpack,
precipitation, streamflow and reservoir precipitation, streamflow and reservoir components, respectively.components, respectively.
Subtraction of 50 centers the distribution around Subtraction of 50 centers the distribution around zerozero
Division by 12 bounds the values between -4.17 Division by 12 bounds the values between -4.17 and +4.17 to mimic range of Palmer Drought and +4.17 to mimic range of Palmer Drought Index.Index.
(Shafer and Dezman, 1982)
SWSI CalculationSWSI Calculation
► Normalization was accomplished by using Normalization was accomplished by using non-exceedance probabilities -- allowing non-exceedance probabilities -- allowing for comparison across varying climates.for comparison across varying climates.
► Requires representative long-term data or Requires representative long-term data or accurate estimates – appropriately accurate estimates – appropriately representing both wet and dry extremes.representing both wet and dry extremes.
► Each of the components is weighted by Each of the components is weighted by overall impact in the basin. overall impact in the basin. Weighting factors change with season (winter Weighting factors change with season (winter
versus summer -- snowpack or streamflow)versus summer -- snowpack or streamflow)
(Shafer and Dezman, 1982)
Weighting CoefficientsWeighting Coefficients
►May seem arbitrary, but were May seem arbitrary, but were thoughtfully developed based on known thoughtfully developed based on known reservoir capacity as percent of reservoir capacity as percent of average annual streamflow. average annual streamflow.
►Specific process was followed for each Specific process was followed for each basinbasin
►Authors knew that coefficients might Authors knew that coefficients might change monthly, but stuck with two-change monthly, but stuck with two-season approach for simplificationseason approach for simplification
Winter SWSI Weighting Factors
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
South Platte Arkansas Rio Grande Gunnison Colorado Yampa/White SanJuan/Dolores/Animas
Basin
Wei
gh
tin
g F
acto
r
Snowpack/Streamflow
Precipitation
Reservoir Storage
Summer SWSI Weighting Factors
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
South Platte Arkansas Rio Grande Gunnison Colorado Yampa/White SanJuan/Dolores/Animas
Basin
Wei
gh
tin
g F
acto
r
Snowpack/Streamflow
Precipitation
Reservoir Storage
Input DataInput Data► Snowpack (SWE)Snowpack (SWE)
6-7 Snow course/SNOTEL stations per basin.6-7 Snow course/SNOTEL stations per basin.► Streamflow – monthly total volume flowsStreamflow – monthly total volume flows
Gages on major streams above diversions were preferable.Gages on major streams above diversions were preferable. 1-4 gages per basin chosen by location, regulation and data 1-4 gages per basin chosen by location, regulation and data
availability.availability.► Reservoir StorageReservoir Storage
Only managed non-random variableOnly managed non-random variable Both irrigation and municipal reservoirs used to reduce Both irrigation and municipal reservoirs used to reduce
dependence of previous values.dependence of previous values. South Platte uses 18 reservoirs, others 4-6, none on South Platte uses 18 reservoirs, others 4-6, none on
Yampa/White/N.PlatteYampa/White/N.Platte► PrecipitationPrecipitation
4-6 National Weather Service stations per basin.4-6 National Weather Service stations per basin. Cumulative water-year values used Dec – May (mimic Cumulative water-year values used Dec – May (mimic
snowpack), individual monthly amounts for remainder of snowpack), individual monthly amounts for remainder of year.year.
(Shafer and Dezman, 1982)
Example: Snowpack non-exceedance Example: Snowpack non-exceedance probability for the Rio Grande basinprobability for the Rio Grande basin
(Shafer and Dezman, 1982)
Things to considerThings to consider►Discontinuance of stations used in the Discontinuance of stations used in the
calculation.calculation.►When one or more extreme events occur When one or more extreme events occur
and go beyond the frequency distribution.and go beyond the frequency distribution. The time series must be reanalyzed.The time series must be reanalyzed.
►Changes in water management activities.Changes in water management activities. New or expansion of existing reservoirs. New or expansion of existing reservoirs. Changing reservoir operation “rules”Changing reservoir operation “rules”
(Shafer and Dezman, 1982)
Additional ConsiderationAdditional Consideration
►Have you looked at reservoir data Have you looked at reservoir data lately? lately? Probability distributions can be quite oddProbability distributions can be quite odd
What has 28 years of What has 28 years of SWSI computation SWSI computation
shown us?shown us?
Colorado River Basin SWSI
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
Date
SW
SI
South Platte River Basin SWSI
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
Date
SW
SI
Rio Grande River Basin SWSI
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
Date
SW
SI
Colorado SWSI Frequency
0
0.05
0.1
0.15
0.2
0.25
5.0 - 4.1 4.0 - 3.1 3.0 - 2.1 2.0 - 1.1 1.0 - 0.1 -0.0 - 0.9 -1.0 - -1.9 -2.0 - -2.9 -3.0 - -3.9 -4.0 - -4.9 -5.0 - -5.9
Bin
Fre
qu
ency
(<
= B
in)
South Platte SWSI Frequency
0
0.05
0.1
0.15
0.2
0.25
5.0 - 4.1 4.0 - 3.1 3.0 - 2.1 2.0 - 1.1 1.0 - 0.1 -0.0 - 0.9 -1.0 - -1.9 -2.0 - -2.9 -3.0 - -3.9 -4.0 - -4.9 -5.0 - -5.9
Bin
Fre
qu
ency
(<
= B
in)
Rio Grande SWSI Frequency
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
5.0 - 4.1 4.0 - 3.1 3.0 - 2.1 2.0 - 1.1 1.0 - 0.1 -0.0 - 0.9 -1.0 - -1.9 -2.0 - -2.9 -3.0 - -3.9 -4.0 - -4.9 -5.0 - -5.9
Bin
Fre
qu
ency
(<
= B
in)
Average Absolute Value of Monthly SWSI Change by River Basin
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
10 11 12 1 2 3 4 5 6 7 8 9
Month
Ab
so
lute
Mo
nth
ly C
han
ge
in S
WS
I
South Plate Monthly Change
Colorado Monthly Change
Rio Grande Monthly Change
Cumulative SWSI Values by Basin
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
450.0
500.0
Date
Cu
mu
lati
ve S
WS
I
S. Platte Cumulative SWSI
Colorado Cumulative SWSI
Rio Grande Cumulative SWSI
ReferencesReferences
► Shafer, B.A.; and L.E. Dezman. 1982. Development Shafer, B.A.; and L.E. Dezman. 1982. Development of a Surface Water Supply Index (SWSI) to assess of a Surface Water Supply Index (SWSI) to assess the severity of drought conditions in snowpack the severity of drought conditions in snowpack runoff areas. In runoff areas. In Proceedings of the Western Snow Proceedings of the Western Snow ConferenceConference, pp. 164–175. Colorado State , pp. 164–175. Colorado State University, Fort Collins, Colorado. University, Fort Collins, Colorado.
My personal concluding My personal concluding thoughtsthoughts
► SWSI was developed to be a practical toolSWSI was developed to be a practical tool► SWSI must be worth something or we SWSI must be worth something or we
would have quit using it long agowould have quit using it long ago► SWSI values below -3 for consecutive SWSI values below -3 for consecutive
months do correspond well to known months do correspond well to known drought conditionsdrought conditions
► WATF intended to test SWSI by WATF intended to test SWSI by comparing it to observed drought impacts comparing it to observed drought impacts – don’t think we ever did that, but always – don’t think we ever did that, but always perceived it to be practical and usefulperceived it to be practical and useful
More ThoughtsMore Thoughts
►WATF participants always felt that SWSI WATF participants always felt that SWSI could be improved -- just weren’t sure could be improved -- just weren’t sure it was worth the effort just to satisfy it was worth the effort just to satisfy WATFWATF
►Fundamental questionsFundamental questions How many basins? How localized?How many basins? How localized? Monthly coefficients?Monthly coefficients? Realities of inhomogeneous dataRealities of inhomogeneous data Predictive or Diagnostic?Predictive or Diagnostic?