Series: Colorado Water Law Basics #14.3 - Case Study Addressing the Impact of the Drought Contingency Plan
In last week’s LWS blog, we discussed the water limitations on the Lower Basin States under different Tiers of the Drought Contingency Plan (DCP). Under a Tier 1 restriction, a total reduction of over 18 percent of Arizona’s allocation would be implemented. What does this mean for Arizona water users?
Arizona receives its allocation under the Colorado River Compact through the Central Arizona Project (CAP). The CAP is a system of canals that runs from Lake Havasu to Pima Mine Road in Pima County, AZ. The majority of the CAP was completed in 1994 and aimed to provide Arizona its share of Colorado River water.
Unfortunately, the Ag Pool portion of the CAP water is the primary water that will be reduced under the DCP, particularly if the Bureau of Reclamation declares a Tier 1 restriction in 2022 (a reduction of 512 KAF in its CAP allocation). [Update: A Tier 1 restriction was put in place as of January 1, 2022.] This means that Arizona agriculture will have to rely much more heavily on already over-drafted groundwater supplies to replace the lost CAP water.
Arizona has been working toward solutions to this problem for quite some time and has implemented large aquifer storage projects to help alleviate the water shortage problem. Other potential solutions include changing the types of crops grown to less water intense crops and other water saving measures.
LWS has been working with Arizona Public Service (APS) on a modeling study at one of its power plants to evaluate the effects of ongoing, and potentially increasing, DCP restrictions related to what that could mean to the sustainability of an already extensively used groundwater aquifer system.
Wells in the Pinal Active Management Area (AMA)
The Sundance power plant is an important resource on the APS power grid, serving the southern sector of Arizona. The power plant uses senior CAP water rights leased from the Gila River Indian Community, which is not expected to be reduced based on the DCP. While APS groundwater pumping is minor compared to overall pumping in the Pinal Active Management Area (AMA), on-site groundwater wells serve a vital role in maintaining power production in that they allow the plant’s blowdown water to not only be disposed, but also be put to beneficial use through irrigation mixing. In this way, APS is minimizing groundwater use while providing a vital power resource. For these reasons, it is very important that the groundwater resources being used by APS remain a sustainable resource.
To evaluate the sustainability of the APS wells, LWS developed a numerical MODFLOW model which assumes that, due to ongoing drought conditions in the Colorado River Basin, groundwater production will increase in the future if CAP Ag Pool water supplies are reduced due to the DCP. A worst-case scenario has been evaluated to assess sustainability of the APS groundwater wells, i.e., full replacement of CAP irrigation water with increased groundwater pumping, because these groundwater wells are so important to Sundance’s power generation potential.
Well response
The MODFLOW model developed by LWS was adapted from the regional Pinal Active Management Area model to focus on the area surrounding the Sundance plant. The cell size was refined from 2,640 by 2,640 feet to 330 by 330-foot cells to obtain more representative water level responses at the Sundance wells. The modeled pumping was changed from the MODFLOW .WEL package to the .MNW2 package which includes inputs like the screen interval and well radius and can also add well loss function parameters to the wells. In this way, not only are we modeling the regional aquifer response due to increased pumping, but we also see the predicted response at the APS wells.
Water levels with - and without - subsidence modeled.
By adding the MNW2 package to all the wells in the refined model, pumping amounts were also reduced if water levels in the aquifer became too low. The effect of continued subsidence on the drawdown was also evaluated, but we realized that adding subsidence provides a less conservative future outlook. This is because if subsidence occurs, water that is locked in clays becomes available to the aquifer, while if subsidence does not occur, the water never becomes available to the aquifer. Since the MODFLOW model calculates water levels and water budgets based on the aquifer, not including the subsidence in the future yields a more conservative estimate for the worst-case scenario was modeled.
A transient model was ultimately developed with a simulation period of 50 years to assess cumulative impacts related to the potential loss of CAP water related to DCP restrictions. The results of this modeling show that while groundwater levels will decline substantially over the next 50 years, the APS wells are completed deep enough below the groundwater table to not be affected by declines in the water table in the near future.
LWS has an experienced team of modelers that routinely use numerical models to evaluate complex surface water and groundwater hydrology issues, as well as address fate and transport of contaminants in groundwater. In addition, LWS often teams with other professionals to provide additional services related to modeling (click here for more information on collaborative modeling projects). For more information, please contact LWS at 303-350-4090 or contact our modeling team.
Maura Metheny, Ph.D., Senior Project Manager, maura@lytlewater.com
Anna Elgqvist, Project Engineer, anna@lytlewater.com
Marlena McConville, Staff Hydrogeologist, marlena@lytlewater.com
