Determining Average Irrigation Need

Determining Average Irrigation Need

Review Hearing Officer conclusions regarding supply needed to prevent material injury

Selecting year to use as baseline supply

Overview of adjustment technique to account for differing climatic conditions from baseline

IDWR – Spring 2009

Hearing Officer Conclusion

The minimum full supply established in the May 2, 2005, Order is inadequate to predict the water needs of SWC on an annual basis. Recommended Order at 50 It is based on a decade old year that does not

reflect current efficiencies such as the increased use of sprinkler irrigation and computer monitoring or changes in the amount of land irrigated. Recommended Order at 49

...it is time for the Department to move to further analysis to meet the goal of the minimum full supply but with the benefit of the extended information and analysis offered by the parties and available to its own staff. Recommended Order at 51

• The parties have attempted to establish water budgets that reflect the needs of SWC members using sophisticated analytical techniques, but the parties' analyses are too far apart to reconcile.

Recommended Order at 49 • (…conclusions in SWC's expert testimony are closer to being

acceptable…) Recommended Order at 50 TWIN FALLS CANALS COMPANY

IGWA AND SWC ANALYSIS CONCLUSIONS

0

100,000

200,000

300,000

400,000

500,000

600,000

SWC IGWA

VO

LU

ME

(A

CR

E-F

EE

T)

SEEPAGE LOSS

ET

Hearing Officer Conclusion

Water Budget Schematic

ETCanal seepageCanal seepage

Deep PercolationDeep Percolation

Return Return flowsflows

Canal Head DiversionCanal Head Diversion

DeliveryDeliveryLossesLosses

Ignore for now other components: precipitation and soil moisture

Water budget summary as percent of average diversion for Twin Falls Canal:

SWCOTHER5%DELIVERY

LOSS53%

CROP WATER NEED42%

IGWA CROP WATER NEED30%DELIVERY

LOSS61%

OTHER9%

ETETCanal seepage

Deep Percolation

Return flows

Canal Head DiversionCanal Head Diversion

Known Values

Water Budget Schematic

Water budget approach using satellite imagery based ET can be used to establishes apparent project efficiencies:

Apparent project efficiency = 41%

Apparent project efficiency can be used in adjustment process of average annual irrigation need

OTHER7%DELIVERY

LOSS52%

CROP WATER NEED41%

Development of an average annual irrigation need:

“Predictions of need should be based on an average year of need, subject to adjustment up or down depending upon the particular water conditions for the irrigation season” Recommended Order at 49 Adjustment can be made using the measured in-

season ET from satellite imagery and project efficiency

Propose using 2006 irrigation diversions as the average annual irrigation need, or baseline demand: Adjust for above normal winter/spring rains in 2006 Normal Heise gage runoff and adequate storage

supply ET values generated with Landsat data available

Correct 2006 diversion for soil moisture excess using ETIdaho data (in progress)

HEISE GAGE APRIL THROUGH JULY RUNOFF

0

2,000

4,000

6,000

8,000

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

RU

NO

FF

(1

00

0 A

CR

E-F

EE

T) AVERAGE 1971-2000

Heise gage runoff volume

Regional weather data

WEATHER DATA - REGIONAL CROP EVAPOTRANSPIRATION GROWING AND DEGREE DAYS

20

22

24

26

28

30

32

34

36

38

40

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

YEAR

RE

GIO

NA

L P

OT

EN

TIA

L C

RO

P E

T

0

500

1000

1500

2000

2500

3000

GR

OW

ING

DE

GR

EE

DA

YS

REGIONAL POTNTL CROP ET GROWING DEGREE DAYS AVG ET AVG GDD

APRIL TO SEPTEMBER PRECIPITATION, TWIN FALLS WSO

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

YEAR

PR

EC

IPIT

AT

ION

(IN

CH

ES

)

AVERAGE

Twin Falls growing season precipitation

TOTAL IRRIGATION YEAR DIVERSION

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

YEAR

DIV

ER

SIO

N (

AC

RE

-FE

ET

)

AVERAGE TOTAL DIVERSION

TFCC

Twin Falls Canal Company annual diversions

97% of average

Summary of Baseline Demands for Surface Water Coalition members based on 2006 irrigation diversion with upward

correction for average soil moisture

NOTE: PRELIMINARY, UNCHECKED DATA

2006 TOTAL DIVERSION

(AF)

SOIL MOISTURE

CORRECTION TO AVERAGE

(AF)

2006 ADJUSTED DIVERSION (BASELINE DEMAND)

(AF)

MINIDOKA NS 352,300 16,000 368,300 MINIDOKA SS (BID) 247,800 10,000 257,800 A & B ID 57,500 3,000 60,500 MILNER ID 55,500 3,000 58,500 RES. DIST #2 404,200 13,000 417,200 NS TWIN FALLS 968,600 32,000 1,000,600 TWIN FALLS SS 995,800 41,000 1,036,800

TOTAL 3,199,700

Summary of Baseline Demands for Surface Water Coalition members, comparison to historic average

diversions (cont.)

NOTE: PRELIMINARY, UNCHECKED DATA

BASELINE DEMAND

(AF)

1990-2008 AVERAGE

(AF)

BASELINE DEMAND AS % OF

AVERAGE

MINIDOKA NS 368,300 358,800 103%MINIDOKA SS (BID) 257,800 246,900 104%A & B ID 60,500 56,100 108%MILNER ID 58,500 57,800 101%RES. DIST #2 417,200 428,500 97%NS TWIN FALLS 1,000,600 1,030,400 97%TWIN FALLS SS 1,036,800 1,072,700 97%

TOTAL 3,199,700 3,251,200 98%

Hearing Officer Recommendation:

The concept of a baseline is that it is adjustable as weather conditions or practices change, and that those adjustments will occur in an orderly, understood protocolRecommended Order at 51

IDWR Proposed Protocol: Each SWC canal begins season with reasonable

in-season demand equal to adjusted 2006 diversions, and called baseline demand (BD).

IDWR Proposed Protocol

Determine crop water needs (CWN) during season using Landsat generated ET: CWN = (ET – PEFF) * Area

where PEFF = effective precipitationArea = canal company total irrigated

area

Calculate revised reasonable in-season demand (RISD) as season progresses: RISD = CWN÷ Ep

where Ep = project efficiency

SUMMARY

Replace minimum full supply with the reasonable in-season demand, with baseline equal to 2006 SWC diversions, adjusted upward for beginning season soil moisture In-season adjustments made relative to 2006 crop water needs using Landsat generated ET and effective precipitationBaseline year for reasonable in-season demand will be amended in the future to reflect current average conditions