PREFACEThis report is the first of a series of professional papers describing the hydrologic studies associated with the Gila Eiver Phreatophyte Project. The project plan is described in the report, including the details of observation. A sequence of fact finding, estimation, and decision is necessary in planning a project of this scope.First a study was made of the available information. Specific objectives were then denned, and the hydrologic problem was identified. Methods of measurement were investigated and a primary method was selected. The data requirements and statistical problems involved in the application of this method were then considered. A site was selected and the hydrologic characteristics investigated. Methods of data collection were determined and the project area was instrumented.The organization of this report follows the general sequence used in planning the project. A brief description of the facts considered and analyzed is presented to explain the reasons for making the planning decisions. The advice and assistance of many scientists were used in planning the project. The contributors to this report are the persons having the primary responsibility for making decisions concerning the planning and for subsequent reporting of results on the various phases of the project.
Basic hydrologic data for the Gila River Phreatophyte Project, Graham County, Arizona: U.S. Geological Survey Basic-data compilation, 13 p. and 400 p. of tables.
This report is one of several reports describing the hydrologic studies and associated activities related to the Gila River Phreatophyte Project. The objectives of this project were to study the hydrologic changes resulting from the removal of phreatophytes and substitution of grasses. Numerous methods have been used for removing the phreatophytes, preventing reinfestation, and establishing the replacement vegetation. Varying degrees of success in controlling the phreatophytes have been achieved and varying effects on the hydrology of the site have been observed. This report describes the management of flood-plain vegetation during the period of the hydrologic study.
Evapotranspiration from three reaches of the Gila River floodplain in Arizona was measured by the water budget during 1963-1971. Initially, the vegetation consisted of salt cedar and mesquite with densities of canopy ranging from 10 to 100%. The phreatophytes were removed from the study reaches in stages during 1967-1971. Perennial grass seed was applied but did not become established, and the postclearing vegetation was primarily annuals. Comparison of the evapotranspiration data from various reaches and comparison of data from before and after clearing required the application of an empirical equation. A consumptive use coefficient related to the description of vegetation was applied to an existing potential evapotranspiration equation based on macroclimatic observations. Initially, the vegetation description consisted of plant identification and canopy dimensions obtained by use of black and white aerial photography and ground measurements. In 1967, remote sensing in the form of color infrared aerial photography became available and densitometric interpretation was used to develop a spectral signature as the vegetation descriptor. DETERMINATION OF EVAPoTRANSPIRATIONThe evapotranspiration (ET) data used in this paper were obtained from the Gila River Phreatophyte Project, a 9-year study performed by the U.S. Geological Survey on a 15-mi (24 km) reach of the Gila River floodplain in southeastern Ari-Copyright ¸ 1976 by the American Geophysical Union. 40 zona. The objectives of this project as described by Culler et al. [1970] were (1) to evaluate water salvage by phreatophyte control on a floodplain typical of the areas of existing and proposed application, (2) to describe the hydrologic and ecologic variables of the study area sufficiently for extrapolation of water salvage data to other sites, and (3) to test and to develop methods for evaluating hydrologic variables in a large area. The first objective was met by using the water budget method to evaluate the change in ET as a result of clearing the phreatophytes from 5000 acres (2000 ha) of the Gila River floodplain. Thornthwaite [1948, Map A] classified the climate of the study area as semiarid. The reade• is referred to the works of Burkharn [1970], Culler et al. [1970], Weist [1971], and Hanson [1972] for detailed descriptions of the physical and climatic characteristics of the project site.The study began in October 1962 and continued through June 1971. The project area was divided into three contiguous reaches designated 1, 2 (which was further subdivided into 2a and 2b), and 3. The total volume of water lost by ET from a reach during a 14-or 21-day budget period was determined as the residual in a water budget equation involving 10 components, which defined the inflow, the outflow, and the change in storage for the reach. The ET data produced by the application of this equation are described by Hanson [1972].
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