Introduction Page 8A |
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The picture on the left shows six of the HBEF watersheds. Researchers have conducted forest cutting experiments on three of them, as you can see here (notice the three large, narrow patches of land that are different than the surrounding forest). You will learn more about these watersheds on Page 9 of the Introduction. By studying what moves into and out of watersheds, scientists can learn about how the watershed works. HBEF scientists are especially interested in how water enters and leaves watersheds, and more importantly, what is in that water. To understand how water (including the elements and nutrients in it) moves through watersheds, we first need to know how much water is entering (input) and how much is leaving the watershed (output). As the graphic to the right shows, measuring precipitation (input) is straightforward. Three precipitation collectors (click here to see one of these "rain gauges") are located in each of the experimental watersheds at the HBEF, and collect all the precipitation that falls into them. Forest service technicians measure how much precipitation has collected in the gauges every week, and can then calculate how much precipitation has fallen on the entire watershed during that week - and after enough time, over the entire year.
Measuring water output is a bit more tricky. Water can flow out of a watershed in 3 ways:
Researchers use weirs to measure how much water leaves a watershed in the drainage stream. However, measuring water movement through soil is extremely difficult. Can you think of why it would be hard to measure how much water is flowing through soil? How would you do it? Fortunately, in the HBEF, the bedrock layer is almost completely impermeable. Therefore, not much water can seep through soil and out of the watershed - in fact, scientists believe that only 1% of the water coming into the watersheds at the HBEF leaves via cracks in bedrock. Instead, some water may seep down through the "soil mantle" (see graphic above) until it hits bedrock, and will then flow downhill on the bedrock. All the weirs in the HBEF were built directly on bedrock, so any water moving through soil will eventually enter the weir basin. Thus, all water leaving a watershed via streams OR soil can be measured at these weirs. This can all be referred to as "Streamflow." Water may also leave a watershed via evapotranspiration. Because the only way water can leave a watershed in the HBEF is via streamflow or evapotranspiration, the equation below is true: Precipitation = Streamflow + Evapotranspiration Therefore,
since we can measure precipitation and streamflow, it is not necessary to actually
measure evapotranspiration. For example, if 100 water units of precipitation fell
on a watershed in one year, and 60 units were measured flowing out of the watershed
at the weir, than 40 units of water must have left the watershed via evapotranspiration. Scientists not only measure the volume of water entering and leaving HBEF watersheds, they also measure the amount of many elements and nutrients in the water. As an example, consider nitrate (NO3-), a form of nitrogen that is available to plants, and at high levels is a water pollutant. If, over the course of an entire year, scientists measured 500 units of nitrate entering a watershed in precipitation, and 650 units of nitrate leaving a watershed in streamflow, they might be able to conclude that the watershed was losing nitrate and was a source of pollution to the stream. Why do you think it could it be important to know if watersheds are losing or accumulating plant nutrients over long periods of time? By measuring the input and output of any element or nutrient in a specific watershed, it is possible to determine net gains or losses for that watershed. HBEF watersheds are small and often very similar to one another. Understanding the gains and losses of water and nutrients in one watershed, and then comparing this to other watersheds (that may have been experimentally altered in some way) comprises the Small Watershed Concept. Of course, as with many things there are exceptions to the rule, and the story can be more complicated (and interesting). Some elements and nutrients can leave watersheds in a gaseous phase (for example, carbon dioxide), and other nutrients can enter watersheds in a dry form (not in precipitation). Many HBEF scientists conduct research on these exceptions, so we are always getting closer to fully understanding the inputs and outputs. This in-depth research helps scientists to understand watershed budgets. Later in this tour you will learn that research in the HBEF is not conducted only on the watershed level. However, as you continue through the next several pages, keep in mind the Small Watershed Concept and how it can be used to study forest ecology over long periods of time. If you are interested in learning more about weirs, click here. This will take you to part of the Watershed 6 tour, separate from the Introduction you're currently viewing. The text in the WS6 tour is slightly more difficult to read (it's written for a college or college-educated audience). Use your back button to return here when you're finished. |
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