Description:
From 1955 to 1988 peak discharge data were collected from two watersheds (one logged, one un-logged) at the HJ Andrews Experimental Forest in Blue River, Oregon. Both watersheds have similar elevation and lie on the western slopes of the Oregon Cascades in the Willamette National Forest. The logged (i.e. treated) watershed in the study was 100% clearcut -- presumably at the beginning of the research. It is not clear whether additional clear cutting was performed during the period of study. The un-logged watershed (i.e. the "control") had vegetation consisting mainly of "100- to 500-year-old Douglas fir, western hemlock, and western red cedar in closed canopy stands, with increasing amounts of Pacific silver fir above 800 m." One goal of the research was to "quantify long-term changes in storm hydrograph behavior associated with clear-cutting and road construction".
Peak discharge from both watersheds was collected for 352 storm events. Discharge (sometimes called flow) is measured in units of volume per time. Examples of units of measure for discharge include "gallons per second" and "cubic feet per minute"; in this data set, discharge is recorded in "cubic meters per second", or m3/s. The logged watershed in this study has an area of about 45 square kilometers (km2) while the un-logged watershed has an area of 21 km2. To eliminate the effects of area on discharge, the data must be adjusted or "normalized". This is done by dividing discharge by basin area to give what is referred to as "Unit Discharge", with measurement units in "cubic meters per second per square kilometer" (m3/s/km2). Unit discharge tells us how much flow comes off a standard sized piece of land in the watershed, on average.
Vegetation in forests, especially large trees, acts like a sponge, soaking up water and slowly releasing it. On the flip side, a lack of vegetation allows precipitation to flow more quickly through the soil and into the groundwater, where it may exit the watershed in a stream or river. This data set is in agreement with these facts: we observe that for most storm events, peak unit discharge is always higher in the logged watershed. On the graph, we observe that most data points lie above the imaginary 45 degree or y = x line, indicating that the x coordinate (un-logged) is smaller that the y coordinate (logged). Students may want to examine the distribution of differences of unit discharge between the two watersheds. How significant are the differences in peak unit discharge? Is the distribution nearly normal? Students may also conjecture as to why the data "spreads out" as unit discharge increases.
Data presented at this website is modified from that found at the HJA site, and the graph displayed above was created by QELP. Data sets [available at the HJA website] were provided by the Forest Science Data Bank, a partnership between the Department of Forest Science, Oregon State University, and the U.S. Forest Service Pacific Northwest Research Station, Corvallis, Oregon. Significant funding for these data was provided by the National Science Foundation Long-Term Ecological Research program (NSF Grant numbers BSR-90-11663 andDEB-96-32921).
For more information on the US Long Term Ecological Research Network (LTER), visit http://lternet.edu/sites/
Information about the HJ Andrews Experimental Forest can be found at http://www.fsl.orst.edu/lter/
