SWAT-based evaluation of multi-decadal land use dynamics and hydrological response in the data-scarce Nambul River Watershed, Manipur

Water does not simply fall from the sky and vanish; it travels. When rain hits the ground, its journey depends entirely on what lies beneath it. If it lands on a forest floor, the soil acts like a sponge, soaking up the water and releasing it slowly into streams over time. If it lands on a concrete road or a packed dirt field, the water cannot sink in. Instead, it rushes over the surface, gathering speed and volume until it becomes a sudden, powerful surge. This basic principle of how land shapes water flow is the foundation of watershed science. For cities built along rivers, understanding this relationship is a matter of safety. If a city expands by paving over its natural floodplains, the river may swell faster and higher than before, turning a routine rainstorm into a disaster. The question for planners is not just whether the water level will rise, but how much the changing landscape accelerates that rise.

In the heart of Imphal, a city in the northeastern Indian state of Manipur, the Nambul River cuts through a densely populated landscape. This river is the lifeblood of the region, eventually feeding into the vast Loktak Lake, a protected wetland of international importance. Yet, for the people living along its banks, the river is also a source of recurring anxiety. Between 2008 and 2021, officials recorded the river's water level every day. The records show a troubling pattern: on 361 days, the water crossed the warning line, and on 75 days, it surged past the flood level. The city has grown rapidly, transforming agricultural fields and low-lying areas into neighborhoods of concrete and brick. It seems logical to assume that this hardening of the landscape is the primary driver of the flooding, forcing more water into the river channel with every storm. To test this assumption, a researcher at Dhanamanjuri University turned to a powerful computer simulation tool known as SWAT, which stands for Soil and Water Assessment Tool. This software acts as a digital laboratory, allowing scientists to recreate the river's behavior under different conditions without waiting for the next monsoon.

The researcher set up a detailed digital map of the Nambul River watershed, an area covering roughly 183 square kilometers. The model was fed with real-world data: the shape of the land, the type of soil, and daily rainfall records stretching back decades. Crucially, the study focused on three specific moments in time: 2001, 2011, and 2021. For each of these years, the researcher mapped exactly how the land was used, distinguishing between forests, farms, water bodies, and built-up urban areas. The computer model was first calibrated against actual water flow measurements from the late 1990s and early 2000s to ensure it was accurate. Once the model proved it could reliably mimic the river's past behavior, the researcher ran a series of experiments. They kept the weather exactly the same for all three scenarios but swapped the land-use maps. This allowed them to isolate the effect of the changing landscape, asking the computer: "If the rain is the same, but the ground has changed from a farm to a city, how does the river respond?"

The results of this digital experiment were surprising. The study found that while the landscape of the Nambul River watershed changed dramatically over two decades, the total amount of water flowing out of the watershed remained almost exactly the same. Between 2001 and 2021, the area covered by buildings grew by about 15 percent, while dense forest cover shrank by more than 30 percent. Despite these massive shifts in the physical environment, the computer simulations showed that the average annual flow of the river changed by less than one percent. In fact, the total volume of water moving through the river was so consistent across the three decades that the tiny differences fell within the margin of error for the model itself. This suggests that the sheer force of the monsoon rains and the steep slopes of the surrounding hills are the dominant forces driving the river's total volume, overpowering the subtle effects of land conversion.

However, the story does not end with the total amount of water. The study revealed that the nature of the water's movement has likely become more dangerous, even if the total volume has not changed. The computer simulations indicated that while the forest and scrubland used to hold water and release it slowly, the new urban landscape allows water to rush over the surface much faster. This acceleration means that during heavy storms, the water arrives at the city all at once rather than trickling in over time. This explains why Imphal experiences such frequent and severe flash floods, with the river crossing flood levels dozens of times a year, even though the total yearly water supply has not increased. The flooding is not caused by a massive increase in the river's overall water, but by the speed at which that water is delivered to the city streets.

The findings challenge the simple idea that more concrete always equals more water. In this specific, hilly, monsoon-driven environment, the relationship between land use and river flow is far more complex. The study concludes that managing the Nambul River requires a holistic approach that goes beyond just counting the amount of water. It demands a focus on slowing down the rush of water through the city, restoring floodplains, and protecting the remaining forests that act as natural buffers. The research highlights that in data-scarce regions where detailed measurements are hard to come by, computer models can provide vital insights, but their results must be interpreted with care. The river's behavior is a complex dance of climate, geology, and human activity, and solving the flood problem in Imphal will require understanding all three, not just the changing skyline.