What it means
The Groundwater Management Act is the 1980 state law that decides whether a new neighborhood in metropolitan Phoenix may exist. In everyday Scottsdale talk it has shrunk to a phrase, “the 100-year rule”: a developer who wants to plat six or more lots inside the Phoenix Active Management Area must prove to the state that water will be there, physically, legally and continuously, for a century. Scottsdale clears that bar through a designation of assured water supply that covers its whole service area, which is why the city can keep approving homes while edge towns scramble.
You hear the act invoked at the Kiva whenever a rezoning is argued, in every story about Rio Verde Foothills since its water was cut off, and in the state’s running fight over how much growth the desert can carry. The full map of Arizona water law, surface rights, groundwater and the Colorado River, is in the Arizona water law guide; this page is about the one law locals name.
Where you’ll meet it
In the fine print of a subdivision’s public report, which must cite a certificate or a designation. In Scottsdale Water planning documents. On the county line, where unincorporated land runs on wells and the rule often never applied. And in June each year since 2023, when the Department of Water Resources updates its Phoenix-area groundwater model and the development industry holds its breath.
Where the word came from
For most of Arizona’s history groundwater belonged to whoever owned the land above it, and by the 1970s the Valley was pumping far more than rain and rivers put back. The federal government, which was building the Central Arizona Project to bring Colorado River water, lost patience: it would not finish the canal for a state that would keep draining its aquifers anyway. Governor Bruce Babbitt used that threat to force farmers, mines and cities, who agreed on nothing, into a study commission and then into a deal. The result was signed on June 12, 1980, and the Ford Foundation later named the code one of the ten most innovative programs in state and local government.
The act did four things that still structure the Valley. It created the Arizona Department of Water Resources (ADWR). It drew Active Management Areas around Phoenix, Tucson, Pinal and Prescott (Santa Cruz was split off later), each with a management goal; for Phoenix the goal was safe yield by 2025, a long-term balance between what is pumped and what is recharged. It grandfathered existing pumping rights and put new wells under permit. And it invented the assured water supply program: no new subdivision in an AMA without a hundred years of water, demonstrated to the state. Rules written in 1995 sharpened the test, and the Central Arizona Groundwater Replenishment District, created in 1993, gave developers without their own renewable supply a way to buy in. The “100-year rule” is the part of all this that entered the language.
What it is not
The act is not a statewide law in practice. Outside the AMAs most of rural Arizona still pumps with little regulation, which is why the fights over Willcox and the Kingman area are about extending the 1980 model, not replacing it. It is also not a water allocation: it says nothing about who gets Colorado River or Salt River water. That is the job of the CAP contracts and the Water Users’ Association seniority system.
The fine print
The rule has a hole, and Scottsdale’s neighbors fell through it. Dividing land into five or fewer lots is not a “subdivision,” so the 100-year test is never triggered. Do that over and over across unincorporated desert and you get Rio Verde Foothills: hundreds of homes on wildcat splits, on wells or hauled water, with no provider obliged to serve them. When Scottsdale ended standpipe sales to non-residents on January 1, 2023, citing its drought plan, the gap became national news, and the Legislature’s fix, a standpipe district signed into law on June 19, 2023, was a patch on the lot-split loophole rather than a repeal of it.
The second limit is the goal itself. On June 1, 2023 ADWR published a new Phoenix AMA model showing about 4.86 million acre-feet of demand over the next hundred years that groundwater could not meet, and it stopped issuing new assured-supply determinations that rely on groundwater alone. Existing designations, Scottsdale’s included, were untouched; the pressure fell on fast-growing edge cities and on Pinal County. The 2025 safe-yield deadline arrived, in other words, with the goal unmet by the state’s own numbers. Governor Katie Hobbs signed the Ag-to-Urban law on June 30, 2025, letting developers earn assured-supply credit by retiring farm pumping, the latest in a series of adjustments rather than a rewrite.
Scottsdale’s position inside all this is comfortable but not free. Its designation rests on Colorado River water through the CAP, Salt River Project water for the old farmland south of the Arizona Canal, recycled water and a modest groundwater share, and on the drought plan that cut off Rio Verde. The Colorado River shortage is the variable: a designation is only as good as the renewable supply behind it.