Assessment of Groundwater Depletion at Local Scale Using Satellite Data: A Case Study of Lahore District, Pakistan

Water is the invisible thread that holds a city together, flowing through pipes, filling taps, and sustaining the crops that feed its people. Beneath the surface of many cities lies a vast, hidden reservoir called an aquifer, a sponge-like layer of rock and soil that stores groundwater. This water is a vital resource, but it is not infinite. When people pump more water out of the ground than nature can replace through rain and rivers, the water table—the top level of that underground water—begins to sink. This phenomenon, known as groundwater depletion, poses a serious threat to the health of cities and the ecosystems that support them. For decades, scientists have struggled to track these changes accurately, especially in rapidly growing urban areas where data is often sparse or scattered. The challenge has been to find a way to see the invisible, to measure the slow, steady draining of these underground reserves across entire regions without needing to drill a hole in every single neighborhood.

To solve this puzzle, a researcher named Sonia Anwar from the University of the Punjab turned her attention to Lahore, the bustling capital of Pakistan's Punjab province. Lahore is a city of millions that relies heavily on groundwater for its daily needs, drawing water from deep underground through thousands of wells. The question Anwar set out to answer was simple yet critical: how fast is the water level dropping in Lahore, and where is the problem most severe? To find the answer, she did not rely on a single method. Instead, she combined two very different ways of looking at the world. First, she gathered hard numbers from the ground itself, collecting water level records from 220 different wells across the district, spanning nearly two decades from the year 2000 to 2019. These records provided a detailed, point-by-point history of how deep the water had to be pumped to reach the surface.

But looking at individual wells is like trying to understand the weather by only checking the thermometer in your own backyard. To see the bigger picture, Anwar looked up. She used data from a pair of satellites known as GRACE, which orbit the Earth and measure tiny changes in gravity. Because water has weight, when a large amount of groundwater disappears from an area, the mass of the Earth in that spot changes slightly, and the satellites can detect this shift. By analyzing these gravity measurements, she could estimate how much water was being lost across the entire region, effectively weighing the underground water from space. She then used powerful computer software to blend these two sources of information, creating a map that showed not just the numbers, but the story of the water's decline across the landscape.

The story the data told was one of steady, significant loss. Over the twenty-year period, the water table in Lahore dropped dramatically. In the year 2000, the average depth to the water was about 24 meters. By 2019, that depth had increased to nearly 42 meters. This means that, on average, the water level fell by almost one meter every single year. The statistical analysis confirmed that this was not a random fluctuation or a temporary dip; it was a consistent, long-term trend of depletion. The researchers found that the decline was happening across the board, but it was not happening evenly. The most severe drops were concentrated in the central and southern parts of the city, the very areas where the population is densest and where urbanization has transformed the land.

These findings point to a clear cause-and-effect relationship driven by human activity. The rapid growth of the city, with its expanding neighborhoods and industries, has created a demand for water that far exceeds what the ground can naturally replenish. As concrete and asphalt cover the soil, rainwater can no longer soak into the ground to refill the aquifer. Instead, it runs off into drains, while millions of liters are pumped out every day for drinking, washing, and industry. The study also looked at the role of the Ravi River, which flows near the city and historically helped recharge the groundwater. The data suggests that the river, now diminished by upstream diversions and reduced flow, contributes very little to replenishing the underground supply. While weather patterns and rainfall do cause the water levels to rise and fall slightly from year to year, the overwhelming driver of the decline is the imbalance between how much water is taken out and how much is put back in.

The implications of this discovery are profound for the future of Lahore. As the water table sinks deeper, it becomes more expensive and energy-intensive to pump water, and the quality of the water can deteriorate as deeper, saltier layers are drawn up. The study highlights that the current way of managing water is unsustainable. The researchers conclude that without immediate intervention, the city risks running out of this critical resource. They suggest that the solution lies in a combination of stricter rules on how much water can be pumped, better management of surface water, and the construction of systems to artificially recharge the aquifer, allowing rainwater to seep back into the ground. By combining the precision of ground-level measurements with the broad view from space, this research provides a clear, undeniable picture of a city losing its underground lifeline, offering a roadmap for how to stop the drain before it is too late.