Understanding Arizona’s groundwater “transportation basins”
Sarah Porter is a well-known Arizona water policy expert and director of the Kyl Center for Water Policy, a pillar of the Arizona Water Innovation Initiative, a multi-year partnership with the state led by Arizona State University’s Julie Ann Wrigley Global Futures Laboratory in collaboration with the Ira A. Fulton Schools of Engineering.
Here, Porter discusses the little-known but critical groundwater issue of Arizona’s transportation basins with Faith Kearns, AWII’s director of research communication.
Faith Kearns (FK): Can you talk about how Arizona’s transportation basins came to be where they are?
Sarah Porter (SP): “Transportation basins” are four Arizona groundwater basins from which raw groundwater may be legally transported. Groundwater is water filling the spaces within and between soil particles and rock formations underground. A groundwater basin is a relatively hydrologically distinct body of groundwater. Arizona has 51 groundwater basins as designated by the Arizona Department of Water Resources (ADWR).
In the mid-1980s, Phoenix, Mesa and Scottsdale acquired farmlands in rural areas of the state with the intention of someday transporting groundwater pumped from under those farmlands into their service areas. In 1991, the Arizona legislature enacted the Groundwater Transportation Act, which generally prohibits the transportation of groundwater from one basin to another. But the Act carves out four sub-basins from which groundwater transportation is permitted: Butler Valley, McMullen Valley and the Harquahala Irrigation Non-Expansion Area in western Arizona, and Big Chino in Yavapai County.
Eventually, the cities sold their “water farms,” but since then the City of Buckeye and the Town of Queen Creek have made investments in transporting groundwater from the Harquahala Basin. Scottsdale has also acquired new farmland in the Harquahala Basin to ensure a water supply for two golf courses.
FK: What is the state’s process for weighing the needs of urban and rural communities?
SP: It’s worth noting that apart from the prohibition on groundwater transportation, outside of Arizona’s Active Management Areas (AMAs) and Irrigation Non-Expansion Areas, there are no state laws in place to protect the groundwater supplies for the people, communities and industries that already rely on them. There is no process for weighing the needs of one rural groundwater user against those of another, and a current groundwater user has no protection against a new user arriving and using up the supply.
The Groundwater Transportation Act imposes limitations specific to each transportation basin. Groundwater may be transported only from lands that were previously irrigated: between January 1, 1975 and January 1, 1980 in the Harquahala, before January 1, 1988 in McMullen and between January 1, 1975 and January 1, 1990 in the Big Chino. The volume of water that can be transported from each basin is limited, only certain entities are eligible to withdraw groundwater for transportation and only certain entities are permitted to use transported groundwater.
Almost all of the land in Butler Valley is state or federally owned. The state lands were set aside as a groundwater reserve for the Initial Active Management Areas (Phoenix, Pinal, Prescott, Tucson and Santa Cruz). The Act permits transportation of Butler Valley groundwater only from land owned by the state or a political subdivision of the state and limits the transfer of title to land from which groundwater is transported to the state or its political subdivisions.
The Act requires an entity that transports groundwater to pay a groundwater transportation fee to the county from which the water is transported, and the Act gives landowners a right to sue for damages caused by the transportation of groundwater.
In determining the extent of the damages, the court must consider the steps the transporting entity has taken to mitigate injury, for example, retiring lands from irrigation, reducing other uses of water, implementing water conservation measures or acquiring alternative water supplies for the landowner. Owners of land from which groundwater is being transported are also required to control weeds and dust.
In July 2025, ADWR approved proposals by the City of Buckeye and the Town of Queen Creek for transporting groundwater from the Harquahala Basin. Because Buckeye and Queen Creek’s proposals include commitments by local landowners to reduce groundwater withdrawals for local crop irrigation, ADWR concluded that “the projected storage depletion in the transportation scenario is nearly identical and slightly less than the projected storage depletion in the ‘business as usual’ scenario.”
FK: Do you foresee the state revisiting or redefining transportation basins in the future?
SP: Never say never. But the four transportation basins were designated almost 35 years ago and some Phoenix-area communities have made significant investments in reliance on the designations. Others have looked to those basins as a future water supply or a bridge supply if there are deep cuts to Colorado River supplies. Eliminating or redefining transportation basins would potentially be disruptive to those plans.
FK: What does having three of Arizona’s four designated transportation basins mean for La Paz County specifically and their efforts to address groundwater overdraft? What are their options?
SP: To be sure, transportation basins are not just a La Paz County thing. Over half of the Harquahala Basin is in Maricopa County, and about half of the McMullen Valley Basin lies in Maricopa and Yavapai Counties.
Also, the groundwater overdraft currently seen in La Paz County is due to withdrawals for local agricultural use, not for groundwater transportation. Both the Harquahala and McMullen Valley basins are experiencing aquifer depletion of over 12 inches per year as measured by the 10-year median rate of change of ADWR’s index wells; this can be seen on the Kyl Center’s Arizona Water Blueprint groundwater level change map.
According to ADWR estimates, agriculture accounts for over 96 percent of demand in the Harquahala Basin and over 99 percent in the McMullen Valley Basin. In the Harquahala, demand for groundwater for agricultural irrigation exceeds supply by around 300 percent; in McMullen Valley demand for groundwater for agricultural irrigation exceeds supply by over 500 percent.
To ensure that groundwater transportation doesn’t add to aquifer depletion, the recently approved Harquahala transfers may be a helpful model. If local landowners who benefit from groundwater transportation agree to reduce their local use of groundwater, the net depletion can be the same as “business as usual.”
One potential significant customer for transported groundwater is the Central Arizona Groundwater Replenishment District (CAGRD), operated by the Central Arizona Project (CAP). The CAGRD is responsible for replenishing groundwater used by member lands (subdivisions) and member providers in the Phoenix, Pinal and Tucson AMAs. In recent years, about half of new housing in these areas has relied on the CAGRD. In 2021, the CAP adopted a policy for acquiring new replenishment supplies requiring CAP to “consider potential third-party impacts to the local community associated with any proposed water acquisition/transfer.”
FK: So the CAGRD might transport groundwater from one basin in order to recharge depleted aquifers in an AMA?
SP: Yes, because within the AMAs, groundwater used by new subdivisions must be replenished. Within the AMAs, there is a high degree of groundwater regulation. That is not the case in most parts of the state.
In the bigger picture, one thing Arizona could do better is consumer notification related to water and land ownership. The state could take steps to make sure that when people buy a parcel of land, information about the water supply, or lack of supply, for that parcel is clearly disclosed. I think most Arizonans would agree that people buying land in transportation basins should be notified that local groundwater may not be a reliable long-term water supply.
FK: What are the infrastructure needs for transporting water, how would they be paid for and how long would it take to build?
SP: For Buckeye and Queen Creek, there are two options for moving transportation basin groundwater to their communities: the CAP canal or a pipeline. The CAP canal runs right through La Paz County, but to meet water quality standards local groundwater might require some amount of treatment before it can be put in the canal. That means pumping the water, treating it and piping it to the canal.
The other option is to pipe the water directly to its place of use. This might involve moving the water to a local aquifer where it is stored and pumped for use when needed.
Either option is expensive and potentially challenging to finance. In particular, the financing for water infrastructure projects often require the approval of existing water users, who may be resistant to having their rates raised to ensure water supplies for growth.
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