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New Scenario:

"Current Policy" or "Continued Changes in

Practice"

ESHMC Meeting

13 November 2007

B. Contor

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Purpose of Scenario

• Rationale– Changes in practice could occur even w/o

change in policy– Hydrologic changes might also occur

• The Question:– Where might we be headed if any of these

things were to actually happen?

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Proposed Approach

• Steady-state– we don't know how fast changes might occur

• Superposition– so we can tease out the effects of individual

hypothetical components

• Not fine tuned– this is all highly speculative anyway; no

utility in extreme refinement of numbers

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• Assessment of probability:– No assessment?– Limited qualitative assessment?

• Reporting format:– Single report summarizing all analyses

Proposed Approach

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Potential changes to evaluate

• Irrigated agriculture ET

• Continued conversion to sprinklers

• Canal lining

• Urbanization

• Managed recharge

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Irrigated agriculture ET

• Increase:– Possibly caused by

• climate change (hotter, drier)• crop mix (more alfalfa & corn)• changes in crop varieties• more intense management

– Model 10% of 2006 ET as discharge, on GW & Mixed-source lands, plus SW lands where net recharge > 0.5 feet/year

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Irrigated agriculture ET

• Decrease:– Possibly caused by

• climate change (wetter; also, more C02 = stomatal control)

• crop mix (more beans, grain & potatoes)

– Model 10% of 2006 ET as recharge on all ag lands

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Conversion to sprinklers

• Assume all remaining lands will be converted

• Two mechanisms on all lands– increased net acreage (bumps, field roads,

high spots)– increased vigor due to better timing of

irrigation

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Conversion to sprinklers

• Additional mechanism on water-short lands: – Improved CU reduces percolation loss– recovered water is available for ET on

former dry spots

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Conversion to sprinklers

• Modeled stress based on current sprinkler percent & ET adjustment factors– (1.05 - 1.00) * (1 - current Spr. %) * (2006

ET), modeled as discharge– If net SW recharge < 0.5 ft/year, model

additional 0.25 ft/year discharge

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Canal Lining

• Assumptions:– Only changes in diversions, returns, and

CU will affect the water budget– All other impacts of lining are only changes

in spatial distribution– Lining will occur only if financial incentive

exists

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Canal Lining

– Limited financial incentives exist• No incentive for reduced diversions

– natural flow: goes to next junior– storage: rental pool price is too low to justify cost of

lining

• No incentive to increase returns or spills– goes downstream to next user

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Canal Lining

– There are only two meaningful financial incentives

• Reduce GW pumping on mixed-source lands (no change in water budget)

• Increase CU (better crops) on SW-only lands (changes water budget)

– Increased CU on SW-only lands will occur only where crops are currently water stressed

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Canal Lining

– There are only two meaningful financial incentives

• Reduce GW pumping on mixed-source lands (no change in water budget)

• Increase CU (better crops) on SW-only lands (changes water budget)

– Increased CU on SW-only lands will occur only where crops are currently water stressed

DANGER, WIL ROBISON!

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Canal Lining

• Proposal– If net SW recharge > 0.5 ft/year, no change

due to canal lining– If net SW recharge < 0.5 ft/year, change =

15% of SW diversion volume, represented as extraction from aquifer

– Spatially apply to main canals• Calibration data• Hyd2mil shapefile

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Urbanization

• Use 2004 & 2006 NAIP aerial images to assess annual rate of change in size & shape of urban areas– Rexburg– Rigby– Idaho Falls– Pocatello– Jerome

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Urbanization

• Use rates of change to construct year-2011 polygons (2006 + 5)

• Intersect w/current irrigated-lands polgyons– GW & Mixed: No change– Not irrigated: No change– SW-irrigated: 30% of current net SW

recharge, applied as negative stress (discharge) to the model

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Urbanization Rationale

• GW & Mixed-source irrigated lands, non-irrigated lands:– all in-home & landscape irrigation will be supplied

by transfer of GW rights or other fully-mitigated GW pumping

• SW-irrigated lands– in-home use supplied by transfer or other fully-

mitigated GW pumping– landscape irrigation supplied by existing SW rights

but at reduced diversion & recharge rates

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Managed Recharge

• Key assumptions– managed recharge is "current policy"– the future may bring some limited success

in achieving this policy

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Managed Recharge

• Conceptual approach– absolute lower limit = zero– absolute upper limit

= old recharge scenario– lower limit for this scenario

= old recharge scenario * "X"– upper limit for this scenario

= old recharge scenario * "Y"

(0 <= X <= Y <= 1.0)

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• We can set "X" and "Y" by combining preferences of ESHMC members

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(End)