​Mapping and assessing evapotranspiration using MODIS-based SEBAL for groundwater potential in the Sita–Swarna river basins, coastal Karnataka, India

Water is the lifeblood of the land, but it is also a ghost that vanishes into the air every day. In the humid, tropical regions of coastal India, where the monsoon rains are heavy and the forests are thick, this invisible loss is called evapotranspiration. It is the combined process of water evaporating from the soil and transpiring from plants, a massive daily exchange that returns water from the ground back to the sky. Understanding exactly how much water is lost, and where, is critical for managing the hidden reservoirs of groundwater that communities rely on when the rains stop. If engineers and planners cannot see where the water is going, they cannot effectively plan for the dry months or protect the aquifers from overuse. The challenge has always been that this process is difficult to measure across a whole landscape; traditional tools can only take a reading at a single spot, leaving vast areas of the countryside a mystery.

A team of researchers set out to solve this puzzle for two river basins in the Udupi district of Karnataka, India: the Sita and Swarna rivers. These basins are a dramatic slice of the Western Ghats, stretching from the high, forested mountain ridges down to the flat, built-up coastal plains where the rivers meet the sea. The team used a sophisticated method that turns satellite images into a map of water loss. By looking at the heat radiating from the Earth's surface and the greenness of the vegetation, they calculated the energy balance of the land. This allowed them to estimate how much water was being pulled from the ground and released into the atmosphere across the entire region, day by day, over the course of two years. They did not just guess; they cross-checked their satellite calculations against weather station data and compared the results across different seasons, from the wet monsoon to the dry winter.

The results painted a clear, if surprising, picture of how water moves in this landscape. The researchers found that the amount of water lost to the air varied significantly depending on the type of land and the time of year. In the Sita basin, daily water loss ranged from about 5.6 to nearly 20 millimeters, while in the Swarna basin, it was more modest, ranging from roughly 4 to 6 millimeters. The highest rates of water loss were not where one might expect the driest conditions, but rather over the dense, evergreen forests on the mountain ridges and over the open water of the river estuaries. These areas, rich in moisture and vegetation, were constantly breathing out water into the atmosphere. Conversely, the lowest rates of water loss were found over the built-up areas of the coastal plain, where concrete and buildings cover the ground.

This finding upended a common assumption used in groundwater planning. For a long time, many experts believed that areas with high water loss were poor candidates for recharging groundwater, because they seemed to be "using up" the water. The study showed that this logic was backwards for this region. The high water loss in the forests was actually a sign of health; it meant there was plenty of water available to be lost. These forested uplands, with their deep roots and permeable soil, are the true engines of groundwater recharge, soaking up the heavy monsoon rains and feeding the aquifers below. In contrast, the areas with the lowest water loss—the cities and towns—were the least likely to recharge the groundwater. Because the ground there is sealed by pavement and buildings, rain cannot soak in at all. The low water loss there was not a sign of a water surplus, but a sign that the water was being blocked from entering the earth entirely.

The study also revealed how the seasons change the story. During the winter, when the air is cooler but the soil is still holding moisture from the previous rains, the forests and water bodies continued to lose water at high rates. However, as the dry summer approached and the soil dried out, the water loss dropped sharply in the agricultural fields and settlements, even though the air was getting hotter and thirstier. This gap between what the air wanted (high demand) and what the ground could give (low supply) is exactly when the pressure on groundwater wells becomes most intense. The researchers found that the difference between the atmospheric demand and the actual water loss grew wider as the dry season progressed, signaling the precise moments when communities are most likely to over-pump the aquifers.

One of the most striking aspects of the research was the discovery of a computational artifact near the river mouth. The models showed an extremely high spike in water loss at the estuary, where the river meets the ocean. While this was partly due to the unique physics of open water and dry air mixing, the researchers determined that the numbers were likely exaggerated by the way the satellite data was processed. They treated this extreme value not as a precise measurement, but as an upper limit, a way to identify where the demand for water is most concentrated. This distinction is crucial; it shows that while the satellite method is powerful, it must be interpreted with care, especially in complex coastal environments where land, water, and air interact in tricky ways.

Ultimately, the work provides a new, spatially detailed tool for water managers. By mapping where water is actually being lost, the researchers have identified the specific zones where the dry-season demand is highest. This information allows engineers to place recharge structures, like check dams and percolation tanks, in the right places to catch runoff before it is lost. It also highlights where restrictions on water pumping are most needed to prevent saltwater from the ocean from creeping into the fresh groundwater. The study concludes that while evapotranspiration is a vital piece of the water puzzle, it cannot be used alone to judge the potential of an area to hold groundwater. Instead, it must be viewed alongside land use, soil type, and geology. In these coastal basins, the forests are the guardians of the water, and the cities are the barriers; understanding this dynamic is the key to keeping the water table stable in a changing climate.