Whitespar Watersheds - Watersheds

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.

Whitespar Watershed B

AREA: 246 ac ( ha)
SLOPE:
ASPECT: Southeast facing
ELEVATION: 5,900 to 7,200 ft (1,800 to 2,195 m)
VEGETATION: Chaparral-dominant shrubs are shrub live oak and true mountainmahogany (crown cover considered medium density, 50 to 60 %).
PARENT MATERIAL: Fine-grained granite
GAGE: 120° V-notch
PERIOD OF RECORD: 1958 through 1983

OBJECTIVE: To determine how converting chaparral vegetation to grass affects streamflow, erosion and sedimentation, vegetation, and wildlife. Prior to treatment, streamflow from WSB was calibrated against streamflow from the designated control WS A. Streamflow was ephemeral on both watersheds prior to treatment.

TREATMENT: Conversion of channel-side brush in 1967 to grass was the first of a series of treatments to eventually convert the entire watershed (Hibbert et al. 1974). The intent of this chemical treatment was to eliminate all shrubs and juniper trees using water from the moist channel environment. By eliminating these plants, water concentrating along the stream channel would be subject to less transpiration loss before contributing to streamflow. Bounds of the treated zone were set at 30 vertical feet above and no more than 75 feet horizontally from the channel. Due to the lack of perennial stream flow, plant cover in the channel zone was not what is considered typically "riparian". There were no sycamore, alder, cottonwood, or other species often found along streams.

The treatment covered 38 acres, 15 % of the watershed. Pelleted fenuron (25 % active ingredient-ai) was placed by hand underneath shrubs and small junipers in March 1967 (Hibbert et al. 1974). The average rate of application on the 38 acres was 23.2 pounds (ai) per acre. Intershrub spaces were not treated to avoid killing grasses and forbs.

The second treatement was made in 1973 to convert an area of similar acreage on the upper slopes in a strip surrounding the watershed just inside the boundary to simulate a fuel break.

RESPONSES: The single application of fenuron gave 80 to 90 % control of the shrubs eliminating need for further treatment of the area (Hibbert et al. 1974). The native grasses and forbs increased substantially on the treated area, providing good cover except in the patches of Gambel oak where establishment of grasses and forbs was slow. Cattle graze both watersheds, although B was fenced after treatment to control grazing.

During the second and particularly the third summer after treatment, flannel mullein (Verbascum thapsus) a disturbance species flourished along the moist channel bottoms. By the fifth growing season these plants were no longer numerous. Since flow increased throughout these years, it is apparent that the vigorously growing weeds did not consume as much water as the brush.

More details of treatments and treatment response can be found in Hibbert and Ingebo 1971, Hibbert et al. 1974, and Ingebo 1971 and 1972.

SELECTED REFERENCES

Hibbert, Alden R. and Paul A. Ingebo. 1971. Chaparral treatment effects on streamflow. p. 25-34. In 15^th Annual Arizona Watershed Symposium, Phoenix, AZ, September 1971, 15:?????.

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.

Ingebo, P.A. 1972. Converting chaparral to grass to increase streamflow. Hydrology and Water Resources in Arizona and the Southwest. 2:181-192.

Whitespar Watersheds: Description | Treatments & Practices | Results | Watersheds