Utah Water Right Analysis used to develop the “Allocated” category in the proposed Utah/Nevada Snake Valley Agreement
This document is not part of the information package developed during the negotiations with Nevada but was assembled in August 2009 to document the history and nature of the accompanying documents which were developed as part of the negotiation process.
The two states agreed in late May 2006 to do a water right inventory which would be used in the negotiation process. Utah emphasized it could not create a fully definitive list of water rights without completing an adjudication because of the potential for diligence claims and other unsettled rights. In many cases there had been no evaluation of diligence claims filed, and many applications had elections to file water user claims which had not been completed. In light of that information the states agreed that rather than tie an agreement to specific water rights and their attributes each state would complete an inventory to estimate the magnitude of the depletion associated with existing rights which would be used in the agreement negotiations. Utah initially inventoried surface and groundwater uses recognizing there is no surface outflow from Snake Valley. The thinking was that whether the use was from surface or groundwater it had a direct impact on the groundwater system since any unused surface water finds its way to the groundwater system. Additionally, many of the surface water rights in Utah have supplemental groundwater sources making it difficult to separate where one ends and the other begins. After the initial inventory was completed and negotiations progressed Nevada asked that Utah further refine its inventory so the agreement could focus only on groundwater. Utah agreed to do so with the understanding that all surface waters which have springs as their source would be treated as groundwater and that lacking definitive information it would generally spilt sole supply on a 50/50 basis between surface and groundwater where a use had shared sources. There was no historical information to make a scientific determination of the split.
The process followed by Utah was as follows:
1) The Division created mapping from water right records which was overlain on aerial photography. Field visits were initiated to locations where irrigated acreage was identified in the water right record but didn’t match the imagery or irrigation was evident from the aerial imagery and no water right appeared. Effort was made to contact water users in the field to go over what the water right mapping indicates and resolve conflicts and duplication and verify what acreage was served from groundwater sources. The water right query output in spreadsheet form is included as: utah_waterrights_snake_summary.xls The spreadsheet summary output product from this mapping activity is attached as: final_acreage_breakdown.xls This work activity was focused in Snake Valley and did not include area 19 where the water right record was presumed to be more accurate.
2) Water right records and actual irrigation mapping were compared particularly where the water rights were based on diligence claims to eliminate gross overstatements of fact.
3) Water rights were grouped by service area and sole supplies assigned based on supplemental characteristics in the water right record and where necessary when surface and groundwater uses were mixed by arbitrarily assigning half of the sole supply to irrigation and half to surface (not on the permanent record but just for purposes of the agreement estimate). The intent was to provide adequate consideration in situations where there is likely a dependence on groundwater and which could increase if additional stress on the groundwater system tended to increase surface water lost to seepage into the groundwater system. The output product from this activity is under tab “Utah Rights” in the spreadsheet identified in item 4.
4) Acreage was totaled by geographic groups and compared to the gross irrigation occurring in the geographic area as a reality check. The spreadsheet product from this activity is included as: snake_surface_underground_analysis.xls under the Region Summary tab.
5) Acreage was added to the Snake Valley Water Right Summary spreadsheet. The output product from this activity is included as the Sources and Uses Tabulation tab on the spreadsheet listed under item 4.
6) Consumptive use (depletion) for irrigation was computed by assuming a uniform consumptive use duty for irrigation of 2.5 acre-feet/acre. Stockwatering consumptive use was computed using 0.028af/ELU and domestic use was computed using 20 percent of 0.45af/EDU.
7) A summary of the calculation is as follows:
20368 (Total Utah Acreage in Agreement Boundary)
- 4764 (acreage from surface sources)
- 1801 (post 1989 acreage)
- 732 (Nevada diversions used on Utah Acres)
= 13071 (total pre-89 acreage)
x 2.5 (consumptive use factor)
= 32677 (total consumption from irrigation)
+ 2068 (ELU from groundwater)
+ 156 (EDU from groundwater)
= 34901 (total pre-1989 consumptive use)
8) The allocation for pre-1989 rights is not dedicated to specific water rights but is an allocation of sufficient size believed by the state engineer sufficient to serve the rights inventoried. There may be unfiled diligence claims but to the best of the state engineer’s knowledge from the observations made there are not large uses occurring which have not been accounted for in Utah’s tabulation of rights.