Hydrological and Sediment Yield Response Impacts to Combined Changes of Land Use Land Cover and Climate in the Sub-basin of Ethiopia

Water and soil are the twin foundations of life in any landscape, but they are not static treasures; they are constantly shifting, shaped by the hands of people and the whims of the sky. When forests are cleared for farms or when rain patterns shift due to a warming world, the delicate balance of a watershed changes. A watershed is simply the area of land where all the rain that falls drains into a single river system, like a giant funnel. In these systems, water moves in different ways: some runs off the surface into streams, some seeps sideways through the soil, and some sinks deep down to recharge underground aquifers. At the same time, the soil itself can be washed away, a process known as erosion, which carries sediment into the rivers. Understanding how these two forces—human land use and changing climate—work together is critical for communities that depend on these resources for drinking water, irrigation, and food security. If we cannot predict how these systems will behave in the future, we cannot plan for the droughts or floods that may lie ahead.

In the highlands of Ethiopia, within the vast Omo-Gibe river basin, a researcher named Meseret Bekele Toma set out to understand exactly how these forces are interacting in the Ajora-Woybo sub-basin. This area is a vital source of water, feeding five perennial rivers that eventually join the Gibe III reservoir, a key asset for the region's energy and agriculture. The landscape here is a mix of high, cool plateaus and hot, dry lowlands, with soil that ranges from deep, fertile clay to coarse, nutrient-poor rock. To see what the future holds, the researcher built a digital model of this entire watershed. This model acts like a virtual laboratory, allowing scientists to run simulations of how the land and climate might change over the coming decades. The study looked at two specific scenarios for the future: one where global warming is moderate and another where it is severe. It also projected how the land use would likely change, assuming current trends of converting forests and shrublands into farmland continue. By combining these two sets of changes, the study aimed to see if the effects of a changing climate and a changing landscape would cancel each other out, or if they would make the problems worse.

The results of these simulations paint a clear, if concerning, picture for the Ajora-Woybo watershed. The model suggests that in the coming decades, the total amount of water available in the system will shrink. Specifically, the amount of water flowing over the land surface, the water moving sideways through the soil, and the water stored deep underground are all projected to decrease. Under the most severe warming scenario, the study projects that surface runoff could drop by nearly 5 percent, and groundwater flow could fall by more than 6 percent by the 2070s. This reduction is driven primarily by the changing climate, which is expected to bring less rain to the area. However, the story is different for evapotranspiration—the process where water moves from the soil and plants into the air. This is projected to rise, increasing by over 6 percent in the near future, as higher temperatures cause more water to evaporate.

Perhaps the most striking finding concerns the soil itself. While the water flowing through the system is expected to decrease, the amount of soil being washed away and carried by the rivers is projected to increase. The model indicates that sediment yield could rise by more than 8 percent in the near future. This happens because the land is changing; as forests and shrublands are replaced by crops, the ground becomes more vulnerable to erosion. The study found that while climate change is the main driver reducing the water supply, the changes in how people use the land are the primary reason for the increase in soil loss. Interestingly, the two forces sometimes work in opposite directions. For example, while climate change tends to reduce surface runoff, the conversion of land to agriculture tends to increase it. In the combined simulation, these opposing forces partially canceled each other out, leading to a "neutralizing" effect where the total change in water flow was less dramatic than if only the climate had changed. However, this balance did not protect the soil; erosion continued to climb.

The timing of these changes also matters. The study looked at the seasonal rhythms of the region, which are defined by three distinct seasons: the heavy rains of the kiremt, the lighter rains of the belg, and the dry bega season. The simulations suggest that the flow of water will become more erratic. During the main rainy season, the flow might increase slightly, but during the dry seasons, it is expected to drop significantly. This pattern suggests that the region could face more extreme events, with periods of intense flooding followed by severe droughts. The upstream areas of the watershed, where farmers rely on the land for their livelihoods, are particularly vulnerable. The simulations show that these areas will see a reduction in the water available for agriculture, a problem that could become increasingly difficult to manage as the population grows.

Ultimately, this research highlights that managing water and soil resources requires looking at both the sky and the ground at the same time. The study concludes that while the climate is the dominant force changing the water balance, the way people use the land is the critical factor determining how much soil is lost. The findings suggest that simply reacting to climate change is not enough; strategies must also address how the land is managed. Reducing deforestation and adopting farming methods that hold the soil in place are essential steps. The study emphasizes that the interaction between land use and climate is complex and varies from place to place, meaning that local solutions are necessary. For the farmers and planners in the Ajora-Woybo watershed, these insights provide a roadmap for preparing for a future where water is scarcer and the soil is more fragile, urging a shift toward sustainable practices that can protect these vital resources for generations to come.