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 MethodA 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
Treated and Control WatershedsWith 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 DestroyedThe 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
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 OpportunitiesWater 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
Vegetation and Snow ManagementVegetation can be managed
in several ways to reduce evapotranspiration:
Decrease stand density by various practices to reduce transpiration
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 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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