Whitespar Watershed A

AREA: 303 ac (122 ha)
SLOPE:
ASPECT: Southest facing
ELEVATION: 5,900 to 7,200 ft (1,800 to 2,195 m)
VEGETATION: Chaparral dominant shrubs are shrub live oak and true mountain mahogany (crown cover considered medium density, 51 %).
PARENT MATERIAL: Bradshaw granite, a course granitic aggregate consisting of about equal parts quartz, orthoclase, and microcline.
GAGE: 120° V-notch
PERIOD OF RECORD: 1958 through 1983

TREATMENT:

No treatment from 1958 through 1981. During this period Watershed A was used as a control to evaluate changes occurring on watershed B.

In 1981, 168 acres (55 %) of watershed A was chemically treated in a mosaic pattern (Davis 1993. Treated areas were mainly chaparral, although a few gambel oak patches were included. Tebuthiuron pellets (20 % active ingredient-ai) were applied to the treatment areas from a helicopter equipped with a seeder calibrated to deliver 3 lb ai/acre. The mosaic pattern was designed to incorporate information learned from previous chaparral watershed studies (Hibbert et al. 1974, Ingebo 1971, and Davis 1984, 1987) to increase water yield, improve wildlife habitat, and maintain water quality and other watershed resource values. An important consideration in the design of this treatment was to mitigate nitrate releases to streamflow that followed previous conversion studies (Davis 1982, 1984, 1987, Davis and DeBano 1986). Hopefully, the use of the mosaic treatment design, in conjunction with a low application of herbicide2, would combine to reduce stream water contamination.

OBJECTIVE

Watershed A was originally used as a hydrologic reference 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.

SELECTED REFERENCES

Arizona State Land Department. 1962 The Arizona watershed program. Phoenix, AZ: Arizona State Land Department.

Barr G. W. 1956. Recovering rainfall. Part 1. Arizona Watershed Program. Cooperating: Arizona State Land Department, Water Division, Salt River Valley Water User's Association, University of Arizona, Tucson, Arizona. 33 p.

Davis, Edwin A. 1993. Chaparral control in mosaic pattern increased streamflow and mitigated nitrate loss in Arizona. Water Resources Bulletin 29:391-399.

Davis, Edwin A. 1987. Chaparral conversion to increase streamflow in Arizona: Sequential treatment extend duration of nitrate loss to stream water. Forest Science 33:89-103.

Davis, Edwin A. 1984. Conversions of Arizona chaparral to grass increases water yield and nitrate loss. Water Resources Research 20:1643-1649.

Davis, Edwin A. 1982. Stream water nutrient changes associated with the conversion of Arizona chaparral. In Proceedings of Symposium on dynamics and management of Mediterranean-type ecosystems, June 22-26, 1981, San Diego, California. USDA Forest Service General Technical Report PSW-58. p. 333-338.

Davis, E. A. and L. F. DeBano. 1986. Nitrate increases in soil water following conversion of chaparral to grass. Biogeochemistry 2:53-65.

Hibbert, A.R.; Davis, E.A.; Scholl, D.G. 1974. Chaparral conversion. Part I: Water yield response and effects on other resources. USDA Forest Service Research Paper RM-17, 36 p. Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Ingebo, Paul A. 1971. Suppression of channel-side chaparral cover increases streamflow. Journal of Soil and Water Conservation 26:79-81.