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Central and Southern Arizona are fortunate to have a diverse set of water supplies, including more than 1.5 million acre-feet of Colorado River water delivered by Central Arizona Project (CAP). 

CAP Canal in Southern Arizona

But the region also faces challenges and uncertainty about future water supply and demand conditions, including the impacts of shortages on the Colorado River. To help evaluate and plan for possible future conditions, CAP has developed the CAP Service Area Model (CAP:SAM).

CAP:SAM projects water supply and demand conditions for all major water users in CAP’s three-county service area, and is designed to easily generate “what-if” scenarios.  The model can simulate a wide range of future conditions, including variable rates and patterns of growth, shortage impacts, effluent reuse, aquifer recharge and recovery and complex supply portfolio management decisions. CAP:SAM relies on data that comes from CAP, the Arizona Department of Water Resources, the county associations of governments, and others.

The CAP:SAM model is organized into four basic steps:

Project Demands

To simulate municipal water demand, CAP:SAM produces individual projections for 80 public and private water utilities accounting for more than 99% of the demand in the municipal sector, 23 Agricultural Irrigation Districts and other Grandfathered Irrigation Rights, 12 Tribes and Tribal Districts, and over 20 other user categories including the Central Arizona Groundwater Replenishment District (CAGRD), Arizona Water Banking Authority, and industrial users such as mines and power plants.

Determine Supplies

The CAP:SAM model tracks the total legal and physical supply availability for 16 supply types in each projection year. Supplies are categorized into effluent, surface water (including Salt River Project) and CAP water. The supply of long term storage credits and groundwater allowances are tracked and debits and credits occur through time. To model the CAP supply from the Colorado River, CAP:SAM allows the user to input an annual diversion supply for Arizona, demands from on river users, total system losses, and the net storage to CAP’s storage reservoir.CAP:SAM also uses externally generated Colorado River supply scenarios from the Colorado River Simulation System.

Request Supplies

Requests represent the maximum preferred use of a particular supply that is legally available to an entity without consideration of the demands of that entity. The basic approach is to load the provider’s maximum entitlement for each supply type. Entitlement volumes can then be individually adjusted either by percentage, or by setting a limit to represent specific preferences or operational limitations.

Requests for CAP supplies are separated into treated water and groundwater delivered through annual storage and recovery, as well of preferences for earning underground long term storage credits with the remaining CAP entitlement. The request portion also includes ranked preference for 36 storage facilities that include deliveries to irrigation districts as in-lieu groundwater storage, or to spreading basins. Entitlements can also be modified through time based on transfers, leases, exchanges, reallocations, and priority conversions.

CAP Canal next to Southern Arizona homesFulfill Demands

In the final model step, information from each of the other steps is brought together and reconciled.The model steps through each supply type in a defined sequence, incrementally satisfying the demand of each entity based on their request and their volume of unsatisfied demand.

Use of CAP:SAM

CAP:SAM capabilities and results played a large role in the development of the technical component of the CAGRD’s 2015 Plan of Operation. More recently CAP has had the opportunity to contribute to two ongoing regional planning efforts: the-West Salt River Basin Study and the Lower Santa Cruz River Basin Study. These collaborative basin studies are funded by the Bureau of Reclamation, with extensive in-kind contributions from study participants. As part of each study’s larger scope, the CAP:SAM computer model is being used to explore a range of supply and demand scenarios. If funded by the Bureau, a similar planning effort may take place for the Pinal area; planning support by CAP and CAP:SAM is also anticipated.

Moving forward CAP:SAM will be used to: 1) Broaden and deepen understanding of CAP water supply and demand issues; 2) Explore the impacts of big picture phenomena on the service area; 3) Examine and test potential solutions to water challenges; 4) Provide a tool to help inform water management decisions.

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