Watershed Research and Education
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Watershed 3

Area: 362 ac (147 ha)
Elevation: 4,960 to 5,509 ft (1,513 to 1,680 m)
Vegetation: Utah Juniper
Parent Material: Basalt
Years: 1958 - 1983
Treatment: Herbicide application
Year Treated: 1968

Aerial herbicide treatment
Aerial herbicide treatment

Untreated condition, 1969
Untreated condition, 1969
Objective: To determine how killing pinyon-juniper overstory with an herbicide affects streamflow, erosion and sedimentation, vegetation, and wildlife. Prior to treatment, streamflow from WS 3 was calibrated against streamflow from the designated control WS 2.
Treatment: All but 8 ha (20 ac) of this watershed were sprayed with an herbicide mixture of picloram and 2,4-D in 1968. Most of the watershed was sprayed by helicopter, while the edges were treated from the ground to prevent herbicide drift onto adjacent areas. This treatment killed 83 % of the trees, but left them standing. Growth of some weeds and grasses was slowed temporarily but soon recovered.
Response: This was the only pinyon-juniper treatment to produce a meaningful increase in water runoff. Annual water yields rose 11.4 mm (0.45 inch) or 65 % above pretreatment levels. Researchers concluded that standing dead trees, which use no water, provide a partial shield to the soil from drying wind and sun. With ample moisture remaining in the soil, less rain and snow melt will soak in, and more will become runoff.

The average annual livestock forage increase was minor, only about 73 kg per ha (65 pounds per ac).

The standing dead trees were cut and sold for firewood in 1975
References: Baker, M.B., Jr. 1984. Changes in streamflow in an herbicide-treated pinyon-juniper watershed in Arizona. Water Resources Research 20:1639-1642.

Clary, W.P., M.B. Baker, Jr., P.F. O'Connell, T.N. Johnsen, Jr., and R.E. Campbell. 1974. Effects of pinyon-juniper removal on natural resource products and uses in Arizona. USDA Forest Service, Research Paper RM-128.

Johnsen, T.N., Jr. 1980. Picloram in water and soil from a semiarid pinyon-juniper watershed. Journal of Environmental Quality 9:601-605.
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