|Managing Arid and Semi-Arid
Historical Perspective on the Central Arizona Water
WaterToo Much or Too Little
Water has been recognized as an important resource in central Arizona and the arid Southwest that has affected populations occupying the Salt River Valley for centuries (Baker 1999b). Water related activities have been documented since about 200 BC, when Hohokam Indians settled the Valley and constructed canals to irrigate their fields. Europeans began to settle in the Phoenix area in the late 1860s and depended on irrigation water from the Salt River for agriculture. However, water supplies fluctuated greatly because the river often flooded in winter and dried up in the summer. There were no impoundments to store water for the dry seasons.
In 1904, the Salt River Water Users Association signed an agreement with the United States government under the National Reclamation Act to build a dam on the Salt River below the confluence with Tonto Creek. The Roosevelt or Theodore Roosevelt Dam, the first of six dams on the Salt and Verde Rivers, was completed in 1911 and the Salt River Project was established.
Sediments in the Reservoirs
In the early 20th century, watershed managers became concerned that erosion on the adjacent and headwater watersheds of the Salt River would move sediment into the newly constructed Roosevelt Reservoir and decrease its capacity. Measurements indicated that 101,000 acre-ft of coarse granitic sediments had accumulated behind Roosevelt Dam between 1909 and 1925. The Summit Plots, located between Globe, Arizona and Lake Roosevelt, were established in 1925 by the USDA Forest Service 15 mi upstream from Roosevelt Dam to study the effects of vegetation recovery, mechanical stabilization, and cover changes on stormflow and sediment yields from the lower chaparral zone (Rich 1961).
Research Studies: Increase Water and Control Sediment
During the mid-1950s, the amount of water stored in project lakes was very low and, as a consequence, apprehension arose among some residents that a serious water shortage impended. Groundwater supplies in the Salt River valley were also being rapidly depleted, and pumping costs were rising steadily.
Long-term studies at Sierra Ancha Experimental Forest in central Arizona showed some potential for increasing runoff by converting chaparral shrublands to grass. Based on this, a belief existed that water yield from the Salt and Verde Watersheds could be significantly increased by treatment of various vegetation types. Suggestions for water-yield improvement included widespread burning of chaparral, eradication of pinyon-juniper by burning and mechanical methods, and prescribed burning in ponderosa pine.
In the summer of 1955, several ranchers met with a USDA Forest Service representative and an official with the Salt River Project on the Beaver Creek watershed near Flagstaff. These people were concerned that increasing densities of trees and shrubs on upland watersheds on the Salt and Verde River basins were reducing the flow of streams and the supply of livestock forage.
Barr ReportAs a result of this meeting, the University of Arizona was commissioned to investigate the potential for increasing water yield from the state's forests and ranges. The somewhat optimistic university findings, titled Recovering rainfall (Barr 1956) and better known as the Barr Report, suggested that surface-water runoff from mountain watersheds might be increased by replacing high water-using plants, such as trees and shrubs, with low water users such as grass. This 1956 report resulted in demand for an immediate action program.
Arizona Water Program
In responding to this demand, the Arizona Water Program of the USDA Forest Service was initiated in the late 1950s to evaluate the usefulness of selected vegetative management programs in increasing water yields and other multiple resource benefits in the Salt River Basin (Arizona State Land Department 1962). The Beaver Creek watershed became a significant component of this program of experimental studies in the pinyon-juniper and ponderosa pine types.
Experimental WatershedsTwenty pilot watersheds were established between 1957 and 1962 to test the effects of vegetation management practices on water yield and other resources (Brown, H.E. et al. 1974). Of the 20 watersheds, 18 were from 66 to 2,036 acres in size; 3 in the Utah juniper type, 3 in the alligator juniper type, and 12 in the ponderosa pine type. The other 2 catchments, encompassing 12,100 and 16,500 acres of ponderosa pine forests, were set aside to demonstrate the effects of management practices on areas that managers work with operationally.
Stream gauges were built at the outlets of all watersheds, while sediment-measuring devices, in which suspended sediments and bedloads could be collected, were constructed on some. A network of precipitation gauges was installed throughout the study area (Baker 1982b).
Timber, herbage, and wildlife resources were inventoried (the latter by the Arizona Game and Fish Department) on a system of permanently-located primary sampling units established on each pilot watershed (Brown, H.E. et al. 1974, Clary et al. 1974). Point sampling techniques were used to monitor stand structures, tree-stem form, and species composition over time. These sampling points were also center-points for plots of varying sizes on which regeneration success, herbaceous vegetation, wildlife populations and habitat preferences, and hydrologic conditions were sampled.
Resource SamplingYields of water, timber, forage, and other natural resource products from the pilot watersheds were determined before any treatments were applied to provide the needed pre-treatment calibration information. Using a paired-watershed approach, one watershed was then altered through vegetative manipulation and the other was held in its original condition as a control for evaluations of potential changes in these yields. If a resource change was detected after treatment, it was attributed to the treatment implemented.
To refine the findings from the studies on the pilot watersheds for use over a wide range of conditions, 24 smaller watersheds, 12 to 40 acres in size, with more uniform soil, plant life, and topography, were established in the early 1970s to sample the range of diverse ecological characteristics in the ponderosa pine forests (Brown, H.E. et al. 1974). To compare the findings from watersheds with soils developed on basalt and cinders (55% of the Salt-Verde River Basin) to watersheds on soils formed from sedimentary rocks (45% of the Salt-Verde River Basin), 3 of these smaller watersheds were established on limestone soils at Rattle Burn (Campbell et al. 1977) and 4 were established on sandstone and tertiary alluvium soils near Heber (2 per each soil parent material) (Ffolliott and Baker 1977).
Home | Highlands Page
7 May 2002 credits