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Watershed 2 was used as a hydrologic reference or control watershed. A paired watershed methodology was used to evaluate treatment response. Two watersheds with similar characteristics (e.g., size, vegetation, precipitation, and soil type) were selected and before any watershed manipulation was done, runoff from each watershed was measured for several years to determine streamflow variations under pretreatment conditions.
The number of years required depends on year to year variability normally experienced. In the semi-arid southwestern United States, it usually take about 7 years of pretreatment calibration to define a pretreatment relationship. During this time, the quantity and quality of other natural resources were also inventoried (e.g., soil loss, forage production, animal types and populations).
With pretreatment measurements completed, one of the watersheds was designated to be the untreated or "control" watershed. It was shown to respond to environmental influences in a particular manner to the watershed where the experimental manipulation or treatment was to be applied. Measurements continued on both the experimental and control watersheds for several years after a treatment was applied.
Streamflow, sediment production, and water quality were monitored regularly, and other resources were reinventoried periodically. Changes caused by the management practice applied to the experimental unit were evaluated by comparing posttreatment values to the pretreatment data relationships. |
