Land Use and Water Quality
When the National Forest System was established in 1905, one of its first mandates was to restore the watershed function of forests. Today an estimated 80 percent of U.S. freshwater resources originate in forests, with much of the nation's drinking water coming from the estimated 192 million acres of our national forests, which actually make up only 30 percent of U.S. forested land. According to the U.S. Environmental Protection Agency, more than 60 million people in 3,400 communities rely directly on national forests for their drinking water.
What is a Watershed?
A watershed is a basin of sloping land surrounded by ridges and drained by a stream. The Coweeta basin, which contains dozens of separate watersheds, is ideal for hydrologic research. The site was strategically selected due to its topography and aspect and the unusually high rainfall (70 to 90 inches per year). The solid bedrock underlying the soils permits the hydrologist to account for most of the rainfall that enters the basin. Many of the watersheds in the basin are very similar in terms of size, climate, soils, and vegetation. The relationship between rainfall and streamflow before disturbance has been charted for many of the watersheds since 1934. To test a theory or evaluate a management practice, scientists can manipulate conditions on a watershed and then compare results with those from a similar undisturbed watershed that serves as a reference.
Since the 1930s, 32 weirs, or stream gauging stations, have been installed on streams in the Coweeta basin; 16 of these weirs are currently operational. Streamflow data has been collected from the weirs since the 1930s using automatic recorders that continuously monitor the height of the water in the weirs which is later translated into streamflow using a mathematical formula based on the dimensions of the weir blade. Because the weirs were precisely constructed, streamflow can be calculated day and night, through storm and sunshine, throughout the year. Sediment that accumulates in the ponding basin constructed behind each weir can also be measured and streamwater chemistry data has been monitored since 1972.
Water quality in the South has been shaped by three centuries of intensive land use, with clearing for agriculture starting in the 1700s, and unregulated logging beginning shortly after the Civil War and lasting through the 1920s. The period between 1860 and 1920 was the most destructive known, with widespread clearing of southern forests without any few erosion control measures. Logging peaked in 1909 and stayed high until 1920, when only a few stands of virgin forest remained. Rivers were filled with sediment from mountain slopes; many still run muddy from those times.
Like most of the Southern Appalachians, the Coweeta basin was heavily harvested in the 1920s. At the time, very little scientific information was available about the impacts of unregulated logging on water quality, but it was clear to the naked eye that large amounts of sediment had reached the streams when mountain watersheds were logged.
Land Use Change and Water Quality
How important are forests to maintaining high quality water in the southern Appalachians? Studies at Coweeta have shown that forests produce the cleanest water among other land uses – forested watersheds have consistently been shown to have lower sediment and nutrients in streamwater. Healthy forests soak up nutrients from the soils as they grow. For example, the lowest levels of nitrogen are usually found in waters draining forested watersheds, while the highest levels are found in water from agricultural and urban uses. One study by Coweeta scientists showed that as the stream flows from a forest water quality is high, but as the stream flows through a more urban setting water quality then declines from the inputs from non-forest landscapes. However, when intermixed with other land uses, undisturbed forests can improve streamwater quality draining agricultural and urban areas.
In a recent study, Coweeta scientists showed that the combination of dilution from clean water draining forested watersheds and in-stream processes, improved water quality in a stream draining an urbanized headwater watershed.