MEMORANDUM

To:ESHMC

Fr:Bryce Contor

Date:31 October 2007

Re:"No Change in Policy" scenario

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This is a first cut at a possible description for the "No Changes in Policy" scenario.

  1. Modeling approach: Because we don't know how fast changes might take place, model all changes in steady-state superposition mode. Because these are hypothetical changes, we should not spend a lot of time fine-tuning the quantification of changes.
  2. Format of scenario: Several independent analyses.
  3. Probability of changes actually occurring: Either make no assessment, or make limited, qualitative assessments.
  4. Reporting format: Summarize results in a single report.

Potential changes to evaluate

  1. Change in ET for irrigated agriculture.
    This component will consider both a possible 10% increase in ET and a possible 10% decrease. Potential factors that could cause an increase include:

a)Climate change (longer growing season, warmer temperatures, reduced precipitation)

b)Changes in crop mix (more corn and alfalfa)

c)Changes in crop varieties

d)More intense management of farm crops

Potential factors that could cause a decrease are:

a)Climate change (increased precipitation, lower ET due to enriched CO2 allowing more stomatal control of transpiration)

b)Crop mix (more beans, small grains and potatoes)

The decrease in ET will be represented by modeling 10% of irrigated agriculture ET from year-2006 extended data. It will be applied as recharge.

The increase in ET will be represented by modeling 10% of ET as an extraction on all ground-water-only and mixed-source lands, and on all surface-water irrigated lands where current recharge from surface-water irrigation exceeds 0.5 feet per year.
On surface-water-only irrigated lands with less than 0.5 feet per year of recharge, the assumption is that there will not be adequate supply to support additional ET and crop stress will result, rather than a reduction in net recharge. No change will be represented on these lands.

  1. Continued conversion to sprinklers.
    Assume that all remaining lands will be converted to sprinklers. There are three mechanisms whereby this will increase consumptive use; a) increase in actual cultivated acreage; b) increased vigor of crops due to more timely irrigation; and, c) reduction in percolation on deficit-irrigated lands through improved uniformity of application.
    The first two mechanisms apply to all lands. Their effect will be represented by applying a negative stress of five percent of 2006 ET (since the gravity ET adjustment factor is 1.0 and the sprinkler factor is 1.05) times (1 - current sprinkler fraction).

The third mechanism applies to lands that are currently deficit irrigated. Due to non-uniformity of gravity irrigation methods, some percolation still occurs even though crops at times may be stressed in parts of fields. Sprinklers would allow more uniform application, reducing both crop stress and percolation. This mechanism will be represented by applying an additional 0.25 feet/year of negative stress on surface-water-only lands where current recharge from SW irrigation is less than or equal to 0.5 feet per year.

  1. Canal lining.
    The effect upon the aquifer of canal lining can only be understood in the context of change in diversions, change in consumptive use, and change in return flows. Unless lining changes these things, it simply works a change in the spatial distribution of recharge. This could occur, for instance, if the 'saved' water were used to reduce pumping of supplemental wells. In that case there would be a one-to-one offset (reduced supplemental pumping offsetting reduced recharge from canal leakage) and no net change in aquifer recharge.
    There is no financial incentive to reduce diversions of natural flow by lining canals because there is no ability to market "saved" natural flow; it just goes downstream to the next unsatisfied junior. Current rental-pool prices are likely low enough relative to lining costs that lining the canal to be able to place more storage in the rental pool, or to use less from the rental pool, does not make economic sense. Therefore, I propose that no reduction in diversions be represented as a result of canal lining.
    Because there is no financial benefit to canal companies from return flows or canal spills, I propose that no increase in return flows be represented.
    On canal companies that have large diversion depths, or that have large amounts of supplemental pumping, users probably already have adequate supplies to support full crop production. If so, it is unlikely that lining canals would result in increased consumptive use. However, on companies with short water supply and few supplemental wells, the saved water would allow improved crop production and therefore increased consumptive use.
    For entities where current recharge from surface-water irrigation is less than 0.5 feet per year, I propose representing the impact of canal lining as 15% of diversion volume as a negative impact, distributed uniformly along the length of the canal. It is acknowledged that canal leakage is probably greater than 15%, but it is likely that neither 100% of the canals would be lined nor that lining would be 100% effective in eliminating canal leakage. For other entities I propose no change.
    For canals already represented in the data set, the existing canal shapefiles will be used. For all other canals, the representation from existing dataset "hyd2mil.shp" will be used.
  2. Urbanization.
    Use 2004 and 2006 NAIP aerial photos to map the change in area for the cities of Rexburg, Rigby, Idaho Falls, Pocatello and Jerome. Find the annual percentage change in area and calculate an expected five-year change in area. Construct hypothetical growth polygons of the next five years of change in city footprints, honoring the calculated change in area and the patterns of change between 2004 and 2006.
    Intersect the growth polygons with current irrigated-lands polygons, and calculate expected stress based upon current irrigation status:

a)Ground-water only and mixed-source irrigation: Assume that landscape irrigation plus in-home usage (delivered to waste-water treatment plant and therefore not recharging the aquifer) is roughly equivalent to current net extraction for irrigation. No change.

b)Not irrigated: Assume that all needs must be satisfied by transfers of existing water rights. No change.

c)Surface-water irrigated. Assume that in-home uses will be supplied by transfer of existing GW rights. Assume that irrigation of lawns and landscaping continues with surface water, but at reduced diversion volume due to the increase in non-irrigated areas such as buildings and paving. Apply a negative impact to the aquifer equal to 30% of the current recharge from surface-water irrigation.

  1. Managed recharge:
    Assume that current policy is to pursue managed recharge, but that in the next five years success will be limited. Use the steady-state stress from the old recharge scenario, discounted heavily (actual discount yet to be determined). I propose running one simulation at a lower discount and another at a higher discount. "Zero" and the old recharge scenario will provide absolute upper and lower limits, within which the "no change in policy" range will lie.

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