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?