The Yahara Watershed reaches around the city of Madison and its defining lakes. It’s a large stretch of land, spanning farms and forests and dotted by the occasional construction site that slowly reshapes and urbanizes its traditional farms and prairies.
In the center are five lakes, each one fed by the rain that flows down the periphery of the watershed and into the Yahara River, ultimately leading through the rivers and streams that join the Mississippi and drain into the Gulf of Mexico almost a thousand miles away.
In a Birge Hall lab seemingly isolated from that network of water that flows around it, Jiangxiao Qiu studied models of the region, observing data sets created from Department of Natural Resources mapping, UW-Madison’s Center for Limnology and other organizations. They were maps of phosphorous and nitrates, spread across the Yahara Watershed’s surface waters and the groundwater below.
Qiu was looking at the layout of the land, how much of that land may have been farmland or wetland, how it was orientated and how all of those factors came together to influence the quality of the water that drained through the Yahara Watershed.
According to Qiu, whose research was published recently in the journal “Ecosphere,” the arrangement of the land does influence that water quality.
Qiu, working with UW-Madison Zoology professor Monica Turner, defined that arrangement of land according to the composition and configuration of the land. According to Qiu, the composition of the land, meaning the type and features of the land, outweighed the configuration in regards to impacting water.
“If you want to improve freshwater benefits, just changing the configuration is not enough to actually see large freshwater benefits,” Qiu said. “You actually need to change the relative abundance, as well as the kinds of land cover, to enhance the hydrologic services.”
In the context of the Yahara Watershed, that would primarily mean working with farmland. According to UW-Madison’s Water Sustainability and Climate Project, the Yahara Watershed covers almost 170,000 acres of farmland.
“The biggest impact is from the agricultural lands, as well as the nutrient inputs,” Qiu said. “It’s those croplands, with fertilizer application and manure application.”
According to a study conducted by the city of Madison in 2010, fertilizers spread on farmlands contributed greatly to the amount of phosphorous in the Yahara Watershed. That phosphorous, in turn, fed the blue-green algae blooms that make swimming dangerous and shut down beaches.
While the composition of a landscape may contribute the most to water quality in the Yahara Watershed, changes to the configuration of the land can also have an impact, though small.
Qiu’s study pinpointed several areas where those small changes could contribute greatly, areas where the amount of cropland and wetland approached a certain threshold.
“If you target certain areas in this watershed, you can get a large benefit with the least amount of land changes,” Qiu said. “Those are areas where the amount of croplands are close to the threshold.”
According to Qiu, rearranging these lands so there are more prominent edges of forests, like the UW Arboretum or Lakeshore Nature Preserve, could improve surface water quality.
“If you increase the edge of agricultural landscapes share with natural habitats,” Qiu said. “If you imagine where the water flows with more nutrients, those forests and grasslands can grab nutrients as the water passes through.”
Smaller changes in urban greenspace can also have an impact. The addition of greenspace or rain gardens could improve the flow of water into the ground, improving the supply and quality of the groundwater aquifer that supplies Madison with 10 billion gallons of water a year, according to Madison Water Utility.
Though Qiu’s study suggests that minor changes in the arrangement and composition of land could largely improve the water quality, especially in the targeted areas the study highlights, major improvements would take some time.
“It’s going to be a long process that involves a lot of actors to participate,” Qiu said. “[But] the landscape can provide a lot of benefits to humanity. There’s a lot of ways you can protect or conserve the ability of the landscape to sustain those benefits.”
The study was supported by the Water Sustainability and Climate Project, a program dedicated to studying the impacts of water quality in the Yahara Watershed. The WSCP is funded by the National Science Foundation.