Separating secular depletion from drought: a detrended storage-deficit characterization of groundwater across Arizona’s Active Management Areas from GRACE observations (2002–2023)

Water is the lifeblood of Arizona, a state where the sun beats down on vast deserts and the Colorado River is a distant, sometimes unreliable, promise. For decades, the people living there have relied heavily on the water hidden beneath their feet, pumping it from deep underground reservoirs to drink, grow food, and keep their cities running. But this hidden supply is not infinite, and it is changing in ways that are hard to see from the surface. To understand what is happening, scientists have turned to a unique tool: satellites that can weigh the Earth. These satellites, part of a mission called GRACE, do not take pictures of the ground; instead, they measure tiny changes in gravity caused by the movement of water. When a region loses a massive amount of water, the land becomes slightly lighter, and the satellites can detect that shift. This ability to see the invisible flow of water across entire landscapes has opened a new window into how our planet's water systems are behaving.

However, looking at this data presents a tricky puzzle. When scientists see a steady drop in groundwater levels over twenty years, it is difficult to tell if a dry spell is just a temporary drought—a short-term shortage compared to normal—or if it is part of a much deeper, long-term problem where the water is simply disappearing forever. Imagine trying to listen to a single drumbeat in a song that is getting quieter and quieter every second; without adjusting for the fading volume, you might think every beat is weak, when in reality, some beats are just as loud as they used to be, but the background noise has changed. In Arizona, this confusion matters because the solutions for a temporary drought are very different from the solutions for a permanent loss of water. If managers treat a long-term decline as a temporary drought, they might wait for rain that never comes. If they treat a temporary dry spell as a permanent loss, they might panic and make unnecessary cuts to water supplies.

A new study led by Md Rayhan at the Bangladesh University of Engineering and Technology tackles this exact problem by looking at six specific regions in Arizona known as Active Management Areas. These areas, which include major cities like Phoenix and Tucson as well as rural zones like Douglas and Prescott, are the places where the state strictly regulates water use. The researcher analyzed twenty-one years of data, from 2002 to 2023, to separate the slow, steady decline of groundwater from the sharp, temporary dips caused by drought. The study confirms what many suspected: every single one of these six regions is losing groundwater at a significant rate. The water levels are dropping by amounts ranging from about five to eight millimeters every year, a steady decline that is happening regardless of whether it is raining or not. This is the secular depletion, the long-term loss of storage that accumulates over time.

But the real breakthrough of the study comes from how the researcher looked at the data after removing that long-term decline. By mathematically smoothing out the steady drop, the scientist could see the true shape of the droughts that were hiding underneath. Without this adjustment, a standard way of measuring drought would have suggested that the entire period after 2019 was a single, unbroken disaster. Once the long-term trend was stripped away, a different picture emerged. The data revealed that while the water is indeed vanishing over the long haul, the droughts themselves are distinct events that come and go. Some regions experience droughts that are frequent but relatively short and mild, while others face fewer droughts that are much longer and more severe.

The study found that the six regions do not all suffer in the same way. The areas around Prescott and Phoenix, for example, experience fewer drought events, but when they do happen, they last longer and hit harder. The deepest droughts in these areas saw water levels drop to nearly four times the normal variation below the trend line. In contrast, the regions of Douglas, Tucson, and Santa Cruz face droughts that happen more often, but these events tend to be shorter and less intense. Despite these differences in how the droughts feel, the study shows that the regions are still connected; when one area goes dry, the others often follow, suggesting that a large-scale weather pattern is affecting the whole state. Yet, the local response varies, meaning that a one-size-fits-all approach to water management would fail to address the specific needs of each community.

Perhaps the most important finding is that the ranking of which areas are in the most trouble changes depending on how you look at the problem. If you only look at the speed of the long-term decline, Douglas is the most stressed area, losing water faster than any other. However, if you look at the severity of the temporary droughts, Phoenix moves up the list of concern. Phoenix has the slowest rate of long-term water loss, but it suffers from some of the most intense and prolonged drought events when the long-term decline is accounted for. This distinction is vital for water managers. It means that a city like Phoenix, which might seem safer because its water table is dropping slowly, actually faces a high risk of severe, temporary shortages that require immediate emergency planning. Conversely, Douglas faces a relentless, steady drain that requires long-term strategies to reduce water use permanently.

The research also introduced a way to measure the total amount of water that has been lost and never recovered. By tracking how far the current water levels have fallen below the highest levels ever recorded, the study showed that some areas, particularly Prescott and Douglas, have dropped more than two hundred millimeters below their historical highs. This is not just a temporary dip; it is a cumulative loss that represents a fundamental shift in the health of the aquifer. The study concludes that to manage water wisely in Arizona, officials need to stop treating depletion and drought as the same thing. They need to use tools that can tell the difference between a slow, steady leak and a sudden, sharp dry spell. By separating these two processes, the study provides a clearer map of the risks facing the state's water supply, offering a foundation for decisions that can protect both the immediate needs of the population and the long-term survival of the underground reservoirs.