Estimating Evapotranspiration Using the SEBAL Model and Remote Sensing: Spatiotemporal Dynamics in Quara District, Northwest Ethiopia

Water is the invisible currency of the land, constantly moving between the soil, the plants, and the air. This movement, known as evapotranspiration, is a combination of two processes: water evaporating directly from the ground and water vapor released by plants as they breathe. In dry and semi-arid regions, where water is scarce, understanding exactly how much water is being lost to the atmosphere is critical. Farmers need to know how much to irrigate, and water managers need to plan for the future. For decades, measuring this loss has been difficult, often relying on weather stations that only tell a story for a single spot, leaving vast landscapes in the dark.

A team of researchers from universities in Ethiopia has now mapped this hidden water loss across the Quara District in the northwest of the country. Using a sophisticated computer model called SEBAL, which stands for the Surface Energy Balance Algorithm for Land, they turned satellite images into a detailed picture of water consumption. The model works by looking at the energy hitting the Earth's surface. Just as a person feels hot when standing in the sun, the land absorbs solar energy. This energy has to go somewhere; it either heats the air, warms the soil, or is used to turn liquid water into vapor. By calculating how much energy is left over after heating the air and soil, the researchers could determine exactly how much water was turning into vapor and rising into the sky.

The study focused on the dry season of 2020, analyzing satellite images taken on four specific dates between January and April. The researchers used images from the Landsat-8 satellite, which captures the Earth in high detail, allowing them to see individual fields, forests, and bare ground. They combined these images with data from weather stations and maps of the land's elevation to build a complete picture of the district's water cycle. The result was a series of maps showing exactly where water was being used and how that usage changed over time.

The findings revealed a landscape of stark contrasts. In areas covered by dense vegetation or irrigated crops, the land was actively releasing large amounts of water vapor. In these green zones, the daily water loss reached as high as 9.63 millimeters per day. In sharp contrast, bare soil and rocky areas, where no plants were growing and the ground was dry, lost very little water, with rates dropping to as low as 0.68 millimeters per day. The maps showed that the location of the water loss was not random; it followed the land itself. Valleys and areas near rivers, where the soil remained moist and vegetation was thick, consistently showed high water loss. Higher, drier ground with sparse grass or exposed rock showed very little.

As the dry season progressed from January to March, the amount of water lost to the atmosphere generally increased. This rise tracked with the increasing heat and the drying of the soil in areas that relied on rain. By March, the water loss hit its peak, driven by the intense sun and the lack of moisture in the rain-fed fields. However, by April, the pattern shifted slightly. While the air remained hot, the daily water loss decreased a bit. The researchers suggest this was likely due to the very first signs of the coming rainy season, which may have added a little moisture back into the soil, or perhaps farmers reduced their irrigation in preparation for the rains.

The study also highlighted the power of using satellites to solve local problems. In a region where weather stations are few and far between, the satellite-based method provided a way to see the whole picture at once. The researchers found that the model worked well, producing results that matched up with the few ground measurements they had. This approach offers a practical tool for managing water in Ethiopia and other dry regions. By knowing exactly which fields are drinking the most water and which areas are drying out, officials can make better decisions about where to send water, how to protect crops, and how to prepare for droughts. The work proves that looking down from space can provide a clear, detailed view of the water that sustains life on the ground.