Whitespar Watersheds

Treatments

The Whitespar watersheds were treated in 3 phases reflecting the changing emphasis on chaparral management over time. Each treatment phase required a pretreatment stream discharge calibration period lasting several years to establish the annual runoff relationship between the control and the treated watershed. The 3 treatment phases on Whitespar A and B were experiments designed to address emerging questions.

The Phase 1 experiments were applied to Whitespar B in the 1960s to determine whether annual streamflow could be increased by killing the chaparral brush and trees in and along the main channel (essentially a riparian treatment). Previous studies in California indicated that clearing trees along channels would substantially decrease evapotranspiration and increase streamflow. The main emphasis in the 1950s and early 1960s was managing chaparral vegetation to produce water yield increases. The riparian areas throughout the Central Arizona Highlands were viewed as major consumers of available water, causing reduced streamflow. Conversion of these woody riparian species to grass or other species that use less water was promoted. A decade later, these riparian systems were recognized as important recreation sites and wildlife habitats that required preservation and enhancement to maintain their sustainability.

Within the context of earlier research emphasis on riparian treatment, 37 acres (about 15% of the watershed) of the channel area were treated with soil-applied herbicides that was hand applied underneath shrubs and small junipers in March 1967. Intershrub spaces were not treated to avoid killing grasses and forbs. Larger junipers were either cut or girdled. The single application of the soil-applied herbicide gave 80% to 90% control of the shrubs and made a follow-up treatment unnecessary. The channel treatment effect was evaluated for 7 yr, after which the Phase 2 experiment was initiated.

The Phase 2 experiment began in 1973 with a second treatment on Whitespar B. This experiment was started after the effect of the channel treatment on streamflow had stabilized and been evaluated. The objective of the Phase 2 experiment was to determine whether ridgeline brush-control would affect annual streamflow volume. The treatment consisted of treating the boundary ridges and a main centrally-located ridge with soil-applied herbicides broadcast aerially. A follow-up treatment was necessary in 1976 because of uneven chemical distribution and poor shrub control after the aerial application. The overall shrub reduction on the areas treated was about 85%. The combined area was about 20% of the watershed (49 acres). The evaluation was for 7 yr.

The Phase 3 experiment was the last applied on the Whitespar watersheds. In February 1981, soil-applied herbicides were applied by helicopter in a mosaic pattern on Whitespar A. The treatment was applied to about 55% of the watershed (168 acres). This treatment pattern was designed to incorporate what had been learned from previous experiments into a design that would increase water yield without degrading wildlife habitat or other watershed resource values. Hydrology, soil-plant water relations, and wildlife habitat improvement were considered in designing a mosaic conversion pattern on Whitespar A that was aesthetically pleasing and technically feasible to implement by applying herbicide with a helicopter. Because of esthetics and wildlife values, all ponderosa pine and Gambel oak stands were excluded from treatment. The ponderosa pine sites were used extensively for roosting doves and pigeons and contained big game trails leading into the watershed. Gambel oak sites along stream channels are valuable javelina (Tayassu tajacu) and deer habitat.