- Bryan Swistock
The Importance of Forests to Water
Forested watersheds provide many benefits to humans. One of the most important is to provide clean, clear drinking water but forested watersheds also support fish and wildlife and provide endless recreational opportunities. An often overlooked benefit of forested watersheds is pollution dilution. Many of our towns and cities rely on clean water coming from upstream, forested areas to allow us to dispose of wastewater and other pollutants into the stream while still maintaining a relatively clean environment. While irrigation is a relatively small use of water from forested watersheds, water used for electricity generation and other industrial processes is a major use of water that is supported by forested watersheds.
The importance of forested watersheds to drinking water for humans was studied by the American Water Works Association which is the professional organization of drinking water suppliers in the United States. They conducted a study that related the cost of treatment for drinking water supplies to the amount of forested land in the water supply watershed. Watersheds that had 60 percent or more of the land area covered in forests provided drinking water that required about half of the water treatment costs in comparison to water coming from watersheds with 30 percent or less forested land.
So where does all of the water originate from in forested watersheds? The “Hydrologic Cycle” or the movement of water through our environment starts with precipitation falling to the watershed land surface as either rain or snow (Figure 1).
When we add up all of the rain and melted snow, we can look at the depth of water that would cover the surface over an entire year. The depth of annual precipitation in Pennsylvania ranges from about 32 inches in many northern counties to over 48 inches in the western mountains and the Poconos. This precipitation forms the beginning of the Hydrologic Cycle. While approximately 40 inches of precipitation falls on our watersheds, about half of that water quickly evaporates back to the sky. This “evapotranspiration” is caused by direct evaporation from the sun or, more importantly, from the use of water by plants during their growth – called transpiration. Of the 20 inches of water that does not evaporate or transpire, about 13 inches slowly moves through soil and rock to recharge the groundwater while about 7 inches directly runs off the land surface into streams and lakes. As the term Hydrologic Cycle implies, water is constantly moving through this cycle with evapotranspiration from one watershed becoming precipitation, recharge and runoff for another. While our annual hydrologic cycle results in about half of incoming precipitation evaporating and the other half reaching water sources, the hydrologic cycle in any given month can be dramatically different.
A further complicating factor on our simple Hydrologic Cycle is the role of differing types of vegetation. Watersheds dominated by coniferous forests will have higher percentages of water lost to evapotranspiration since these trees are actively growing all year long. Watersheds with deciduous forests, on the other hand, are still dominated by evapotranspiration but these watersheds will lose less water to tree growth resulting in more water reaching groundwater and streams.
Water reaching a stream may do so during low flows (called baseflow) or during rain storms (called stormflow). During baseflow, the water in the stream is dominated by groundwater which is water that is stored in rock and flows underground to the stream. The quality of the water in the stream during baseflow is greatly influenced by the type of rock underneath the watershed surface. For example, watersheds with lots of limestone will have streams that are less acidic than watersheds dominated by shale or sandstone.
Groundwater entering the stream during baseflow conditions has often traveled a considerable distance and time underground before reaching the stream. Some of the water may have only traveled for days but other flow paths can take months or even years. Hydrologists can use various methods to age water in a stream to determine approximately how long ago it fell as rain or snow. Penn State studies of several small, forested watersheds have found that the water flowing during baseflow in these small streams was, on average, from rain or snow that fell one to five years ago! (DeWalle et al. 1997)
But what happens when stream are at a high flow after a rain storm or melting snow? On a forested watershed, you’ll note that the stream water is still clear even during high flows while other nearby streams may be muddy. That’s because the roots associated with the dense forest and understory vegetation hold the soil tightly in place and prevent erosion from causing sediment to enter the stream. The dense roots and especially old, decayed root cavities also provide many holes in the soil, called macropores, where water can be easily transmitted through the soil to the stream. Thus soils in forested watersheds act much like a sponge – absorbing large amount of water, filtering out sediment and other pollutants, and allowing the movement of large volumes of water.
The ability of forest soils to absorb and move water is most easily seen through measures of infiltration. Infiltration is the rate at which water is absorbed into and through soil. It is usually measured in inches of water movement per hour. Infiltration measurement can be done by pouring a measured amount of water into a water-tight ring installed on top of the soil and measuring how long it takes for the water to infiltrate completely into the soil. If this experiment is done on an undisturbed soil on a forested watershed, infiltration rates in excess of 15 inches per hour are commonly measured for dry soil (Figure 2).
As soils are disturbed or compacted, the loose structure and macropores are lost causing a reduction in the infiltration capacity. For example, a typical residential lawn might only be able to infiltrate one inch of water per hour. It is most important to compare this infiltration capacity with the typical rate of rainfall for a given area. For example, if we have a lawn that infiltrates just one inch of water per hour and we get a heavy thunderstorm that dumps two inches of rainfall in one hour, we can expect about half of that water to run off from the lawn rather than infiltrate into the soil. But, the same storm in a forested watershed would not produce runoff since the rainfall rate is still far below the ability of the soil to infiltrate water. Since infiltration rates in forested areas are usually much higher than any natural rainfall rate, we would expect all water to infiltrate in the forest and none to run-off the surface.
Based on the high infiltration capacity in the forest soils, most water reaching the stream moves through underground flow paths instead of over the surface of the ground. Penn State studies have found that the water in a stream in a forested watershed is typically comprised of about 75% groundwater and 20% soil water with just 5% coming from runoff or precipitation directly falling on the stream.
Now let’s address some water quality issues in forested watersheds. Since the vast majority of water reaching a stream in a forested watershed is infiltrating and moving through soil and rock, the bedrock and resulting soils in the watershed are the most important variable to explain the natural water quality in the stream. Streams in areas dominated by sandstone and shale bedrock tend to be slightly more acidic with lower levels of alkalinity, calcium and magnesium. Conversely, water coming from the carbonate, limestone bedrock areas would be less acidic but much harder, with much higher levels of calcium, magnesium and total dissolved solids.
While rock and soil types largely control the natural water quality in a stream, pond or water well, there are a variety of land use changes that can dramatically change this water quality and, in some cases, impair the water for drinking or other uses. Activities common to Pennsylvania include agriculture, development, mining, gas drilling, timber harvesting, or industrial sites. Each of these activities can have unique pollutants that enter nearby water. For example, mining may cause increased levels of iron, sulfate and acidity due to the rock that is exposed. While each of these activities usually occur on a local scale, acid rain can affect all forested watersheds, even those in very remote and natural areas of the state. Years of research by Penn State and others have shown that acid rain had dramatically changed the water quality, fish and aquatic insects in many streams in Pennsylvania – especially those in northern and western parts of the state underlain by naturally more acidic sandstone and shale bedrock.
Now let’s look at how various changes to the forest might affect the water cycle overall. In a watershed comprised of undisturbed forests, about two thirds of the incoming precipitation will be evaporated or transpired and one third becomes stream flow. As we change the forest by clearcutting and then creating permanent vegetation changes to meadows, crops or bare soil, the percent of precipitation reaching the stream increases dramatically. In an agricultural watershed, about 50% of precipitation reaches the stream while streams in urban areas may see more than 70% of precipitation reaching the stream and only 30% being evaporated or transpired. These land uses changes have implications for regional climate since less water would be evaporated and available for precipitation in other watersheds.
These same land use changes can also have a dramatic influence on stream water quality. The disturbance to the soil can result in greater runoff and erosion which can increase sediment in nearby streams and groundwater. Removal of trees from near the stream can result in greater sunlight reaching the water surface causing increased stream temperatures. Finally, with little vegetation remaining to use nutrients like nitrogen, we would expect to see more nutrients in the stream. This can create downstream problems with excessive nutrients that are a concern to the Chesapeake Bay and other receiving waters. It is important to realize that all of these impacts to water can be minimized or completely eliminated through the use of careful Best Management Practices.
One of the most important Best Management Practices to protect water quality from timber harvesting and many other land use activities is the use of riparian buffer strips. This involves either protecting or planting trees in strips adjacent to streams, lakes or other water bodies. These strips can be as narrow as 20 feet to provide roots to protect the stream bank but wider buffer strips provide many other important benefits including the uptake of nutrients, filtering sediment, providing shade and falling leaves for stream aquatic life and important habitat for many species of fish and wildlife near and in the stream. A study at Penn State in the 1970’s found that streams on forested watersheds with an intact buffer strip had summer water temperatures that never exceeded 70 degrees while streams where buffer trees were removed had elevated stream temperatures exceeding 80 degrees in July (Figure 3). This stream temperature increase would be lethal to many coldwater fish that are native to most small, headwater streams in Pennsylvania such as brook trout.
So let’s review how forested watersheds function hydrologically. The combination of a thick litter layer of old leaves along with a dense network of macropores in the soil provide high infiltration rates that readily transmit water to the stream. Forest vegetation growth results in high percentages of evapotranspiration which reduces the amount of water available to the stream. The dense network of tree and plant roots holds the soil in place and prevents significant erosion – even during heavy storms. By reducing surface runoff and promoting soil water and groundwater flow, forested watershed provide streams with moderated flows that never get too low or too high and have relatively cool and constant water temperatures.
DeWalle, D.R., P.J. Edwards, B.R. Swistock, R. Aravena and R.J. Drimmie, 1997. Seasonal Isotope Hydrology of Three Appalachian Forest Catchments, Hydrological Processes11:1895-1906.
Rishel, G.B., J.A. Lynch, and E. S. Corbett, 1981. Seasonal stream temperature changes following forest harvesting, Journal of Environmental Quality, 11(1):112-116.