Long-Term Hydroclimatic Variability and Land Use Change in the Curuá-Una Watershed: Implications for the Hydrological Regime of the Amazon's First Hydropower Plant

The Amazon rainforest is often imagined as a vast, unchanging sea of green, but in reality, it is a dynamic engine where water, climate, and life are locked in a constant, delicate dance. At the heart of this system is the hydrological cycle, the process by which rain falls, soaks into the soil, feeds rivers, and eventually returns to the atmosphere. In the Amazon, this cycle is the lifeblood of the region, supporting immense biodiversity and providing the water power needed to generate electricity for millions. However, this system is under pressure from two distinct forces: the shifting patterns of global climate, which alter how much rain falls and when, and the transformation of the land itself, as forests are cleared for farming and grazing. Understanding how these two pressures interact is critical, especially for the communities and industries that rely on the steady flow of the region's great rivers. When the forest disappears or the rain patterns shift, the rivers change, and those changes can ripple out to affect everything from local wildlife to the lights in our cities.

In the eastern Amazon, a specific stretch of the Curuá-Una River has become a focal point for scientists trying to untangle these complex threads. This river is home to the first large-scale hydropower plant built in the Brazilian Amazon, a facility that has been generating electricity since the late 1970s. For decades, researchers have known that the region is changing, but they have lacked a long-term, integrated view of how rainfall, river flow, and land use have evolved together over time. A new study by researchers at the Federal University of West Pará has filled this gap by looking at more than seventy years of rainfall and river flow records, combined with forty years of satellite images showing how the land has been used. Their work reveals a story of a landscape in transition, where the rain is slowly becoming less abundant, yet the river is behaving in ways that might seem surprising at first glance.

The researchers began by examining the history of the rain. They found that the total amount of rain falling on the watershed has been declining significantly over the decades. This drop is not happening evenly throughout the year; it is driven primarily by a reduction in rainfall during the wet season, which typically runs from March to April. A major shift in the climate pattern occurred around 1975, marking a turning point where the region began to receive less water than it did in previous decades. This change was abrupt and has persisted, suggesting that the climate in this part of the Amazon has entered a new, drier phase.

At the same time, the landscape above the river was undergoing a dramatic transformation. Using satellite data from 1985 to 2024, the team mapped the loss of forest cover with precision. They discovered that the forest, which once covered nearly the entire watershed, has been reduced to about two-thirds of its original size. The primary driver of this loss was not the immediate conversion of forest to large-scale soybean farms, as is often assumed, but rather the clearing of land for cattle pasture. Between 1985 and 2024, the area dedicated to pasture grew from a tiny fraction of the land to nearly a quarter of the watershed. While agricultural expansion did pick up speed after the year 2000, the initial and most significant wave of deforestation was for grazing land. The pattern of this loss followed the roads, creating a "fishbone" shape of cleared land radiating out from the highways, a classic signature of human settlement in the Amazon.

One might expect that with less rain and fewer trees to hold the soil, the river would simply run lower and more erratically. However, the data told a more nuanced story. While the total amount of water flowing in the river each year remained relatively stable, the nature of that flow changed. The highest floods did not get significantly bigger, but the lowest flows, which occur during the dry season, actually increased. This means that the river is becoming more reliable during its driest times, a counterintuitive result given the loss of forest and the decline in rainfall. The researchers suggest that this shift is likely due to the replacement of deep-rooted trees with shallow-rooted pasture grass. Trees drink deeply and release large amounts of water into the air, while pasture grass uses less water, leaving more of it to run off into the river, even as the total rain decreases.

Despite these massive changes in the climate and the landscape, the river's response time to the rain has remained remarkably steady. When it rains, the river responds within about one month, a rhythm that has not changed over the decades. This consistency suggests that the fundamental way the watershed processes water has not been broken, even as the volume of water and the type of land covering it have shifted. The study highlights that the river is not just a passive recipient of rain but a system that adapts to the new conditions created by both climate change and human land use.

The implications of these findings extend far beyond the Curuá-Una River. As the Amazon continues to face the dual threats of a drying climate and expanding agriculture, understanding how these forces combine is essential for managing water resources. For the hydropower plant that has powered the region for nearly fifty years, the increase in low-flow conditions offers a glimmer of resilience, ensuring that electricity generation can continue even during dry spells. However, the study also serves as a warning. The stability of the river's flow is a result of a specific balance between the remaining forest, the new pasture, and the changing rain. If the balance tips too far, or if the climate shifts further, the system could reach a breaking point. The research underscores that the future of water security in the Amazon depends not just on protecting the remaining forests, but on understanding the complex, often unexpected ways in which the land and the sky interact to shape the rivers that sustain them.