Disproportionate Contributions of Baseflow to Nitrogen and Phosphorus Export in an Agricultural Watershed of Southwest China

Water in a river is rarely just rain running off the land. Even when the sky is clear, a steady stream often continues to flow, fed by water that has soaked deep into the ground and is slowly making its way back to the surface. This hidden flow, known as baseflow, acts like a slow, steady heartbeat for a watershed, carrying dissolved materials from the soil into the river long after a storm has passed. For decades, scientists and policymakers have focused their attention on the visible, fast-moving water that rushes down hillsides during heavy rains, assuming this surface runoff is the main culprit behind pollution in lakes and streams. They have spent years trying to stop fertilizers and animal waste from washing away during these storm events. However, this focus on the surface has left a critical question unanswered: how much pollution is quietly traveling underground, hidden from view, and slowly seeping into our waterways?

In the northern part of Yunnan Province, China, a large freshwater lake called Erhai Lake has been struggling with water quality problems for years. The lake is surrounded by intensive farming, where crops and livestock are the main source of income. While the surrounding land is a major source of nutrients like nitrogen and phosphorus, which can cause harmful algae blooms, researchers have not fully understood how these nutrients reach the lake. A team of scientists set out to investigate the Miju River, the largest river flowing into Erhai Lake, to see if this hidden underground flow was playing a bigger role in pollution than anyone realized. They wanted to know if the steady, slow-moving water was carrying a disproportionate amount of nutrients compared to the fast, storm-driven water, and what factors were driving this movement.

To answer these questions, the researchers gathered ten years of data, from 2015 to 2024, tracking the daily flow of the river and the monthly levels of nutrients in the water. They faced a challenge because they could not directly measure the water moving underground. Instead, they used a mathematical tool to separate the river's total flow into two parts: the fast, storm-driven water and the slow, steady baseflow. After testing several different methods to ensure accuracy, they selected the most reliable one to isolate the baseflow. They then used a statistical model to estimate how much nitrogen and phosphorus were moving with that baseflow, carefully subtracting the contribution from wastewater treatment plants to focus only on the agricultural runoff.

The results revealed a surprising imbalance. While the steady underground flow made up only about 41 percent of the total water moving through the river, it was responsible for carrying nearly 44 percent of the total nitrogen and a striking 56 percent of the total phosphorus. This means that the slow, hidden water is delivering more than half of the phosphorus pollution, even though it moves less water overall. This finding challenges the common belief that phosphorus mostly travels attached to soil particles during heavy rains. The researchers suggest that in this specific environment, the underground water is dissolving the phosphorus and carrying it along in a way that makes it behave very similarly to nitrogen, which is naturally dissolved.

To understand what drives these nutrient loads, the team used a sophisticated computer model to rank the importance of various factors, such as rainfall, temperature, humidity, and the amount of water flowing in the river. They found that the most powerful driver was simply the total amount of water flowing in the river, which acts as the physical carrier for the nutrients. The next most important factors were related to how dry or wet the soil had been over the preceding days. A measure of how many days had passed without rain, and a calculation of the total water deficit over the past week, were the key switches that determined how much nutrient was available to be washed into the groundwater. Surprisingly, the specific weather conditions of the day, like the exact temperature or the amount of rain falling at that moment, were far less important than these longer-term patterns of wet and dry.

Perhaps the most significant discovery was that the factors driving nitrogen and phosphorus were almost identical. In many other environments, these two nutrients behave very differently, with nitrogen moving easily in water and phosphorus sticking to soil. But in the baseflow of this agricultural watershed, their paths converged. The steady underground flow seemed to dissolve the phosphorus, making it travel in lockstep with the nitrogen. This suggests that the traditional view of treating these two pollutants as separate problems with different causes might be incorrect for this type of environment. Instead, the movement of both nutrients is controlled by the same hydrological rhythm: the total flow of water, the history of dry spells, and the accumulation of moisture in the soil.

The study concludes that managing water quality in such agricultural regions requires looking beyond the surface. If the goal is to protect lakes like Erhai, strategies must address the slow, steady seepage of nutrients from the ground, not just the flash floods that wash soil away. The researchers found that simple, measurable indicators, such as counting consecutive dry days and tracking the total water deficit, could serve as early warning signs for when the risk of nutrient pollution is high. By understanding that the hidden flow is a major contributor to pollution, and that nitrogen and phosphorus are moving together in this hidden pathway, water managers can develop more effective, unified strategies to protect these vital freshwater resources.