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University of Arizona

Watershed Management Research

Experimental Manipulation

Experimental manipulation is a research approach that has been used extensively in watershed research. A number of whole watershed treatments have been conducted to test research hypotheses, to obtain quantitative information on pertinent environmental issues, and to validate process-related formulations used in simulation models.

Paired watershed Method—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). One of the watersheds was designated to be the untreated or "control" watershed and the other watershed has a particular treatment applied to it.

Treated and Control Watersheds—With pretreatment measurements completed, one of the watersheds was designated to be the untreated or "control" watershed. The control watershed was shown to respond to environmental influences (pretreatment water yield regression) 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.

Matter is Not Created or Destroyed—The physical law of continuity stated simply is that "matter is neither created or destroyed". A given environment or ecosystem over a period of time receives an average input of precipitation and produces an average output of streamflow. The difference between these two amounts is basically what is used by the vegetation growing on the site. Because matter or in this example precipitation is neither created or destroyed, if one can change the amount of water used by the vegetation there will be more water available for streamflow or percolation into the soil. Therefore, the key to increasing water yield is the replacement of deep-rooted or high water using plants with shallow-rooted or low water using plants.

Differences in water use by shallow and deep rooted plants

Water Budget

Of the average 23.4 million ha-m (190 million acre-ft, 14.2 area inches, or 360 mm) of rain and snow that fall each year on the Colorado River Basin, in which the Arizona experimental watersheds lies, more than 90 % of it evaporates or is used by plants (Hibbert 1979). With such large amounts of water being returned to the atmosphere, even a slight reduction in this loss would leave substantially more water available for streamflow. As an example, if evapotranspiration over the entire Colorado River Basin could be reduced by only 1 %, the surface water supply would increase on the average by 0.2 million ha-m (1.75 million acre-ft) annually. However, the opportunity to significantly reduce evapotranspiration by management of vegetation and snow is limited to certain vegetation types.

Annual water budget for Arizona

Water Yield Opportunities—Water yield improvement, as it pertains to forest and rangelands, is based on the premise that streamflow and/or ground water are increased by an amount equal to the net reduction in evapotranspiration (Hibbert 1979). According to Hibbert, little opportunity exists to reduce transpiration where precipitation is less than about 460 mm(18 inches) (Figure from Hibbert) and is exceeded by potential evapotranspiration (warm, dry portions in figure ), because precipitation does not penetrate far into the soil, and one cover type is about as efficient as another in using the available water. The greatest opportunity to increase runoff by reducing transpiration exists where precipitation exceeds 460 mm (18 inches) and potential evapotranspiration (determined by the Thornthwaite method) exceeds 380 mm (15 inches). This kind of climate promotes vigorous growth of vegetation capable of using large amounts of water. Therefore, modifying plant cover under these conditions can substantially increase water yield.

Vegetation and Snow Management—Vegetation can be managed in several ways to reduce evapotranspiration:

  • Decrease stand density by various practices to reduce transpiration and interception.
  • Convert plant cover from one cover type to another that uses less water (type conversion).
  • Create openings in forest cover to reduce transpiration and to redistribute snow, thus concentrating the snow to reduce evaporation, to increase snowmelt efficiency, and to enhance contributions to streamflow.
  • Establish trees, large shrubs, or fences in windswept, treeless areas to pile snow in large drifts, thereby reducing evaporation losses from the snow.

The best opportunity to increase water yield by snow management is in cold climates where blowing snow can be concentrated in forest openings or trapped in large drifts to reduce evapotranspiration and to reduce losses to sublimation.

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8 May 2002