Accuracy of evapotranspiration rates determined by the water-budget method, Gila River flood plain, southeastern Arizona

Abstract

Evapotranspiration by phreatophytes (primarily saltcedar) was determined by the water-budget method for 5,500 acres (2,230 ha) of the Gila River flood plain in southeastern Arizona. The water budget consists of 12 components including surface and subsurface flow through the study area, precipitation on the area, and soil-moisture changes in the unsaturated soil profile. Nine years (1963-71) of hydrologic data were collected on four reaches within the area. These data provided over 400 measurements of evapotranspiration for two-or three-week periods. Midway through the study the vegetation was removed from the flood plain. The evapotranspiration measurements are therefore defined for both natural vegetative cover and essentially bare-ground conditions. This report shows how each component of the water budget was evaluated, demonstrates the significance of each component in relation to the total evapotranspiration, and describes the methods used to evaluate the relative accuracy of each component. The two most significant components of the water budget are, generally, the Gila River inflow and outflow. One of the least significant is tributary inflow, which occurred only 4 percent of the time during the 9-year study. Soil-moisture change is highly significant during periods of low streamflow and is one of the more difficult components to measure. The groundwater flow components are the least variable in the water budget, fluctuating only in response to seasonal changes in the downvalley groundwater slope. The total measurement error of each component consists primarily of a sampling error which is dependent on the number of observation points used to measure the component. This error is time variant, reflecting both the variability in repetitive measurements and the error due to missing data. Included in the total measurement error is a bias error which gives a constant overestimate or underestimate of the component. Only the groundwater flow components introduce a measurable bias error, but the direction of this error is unknown and its magnitude in relation to evapotranspiration is relatively insignificant. The total measurement error in evapotranspiration is not related to the magnitude of evapotranspiration but rather to the total volume of water moving through the area. Thus, the minimum errors occur during the midsummer months of maximum evapotranspiration when streamflow is low and precipitation is negligible. Evapotranspiration rates computed for reach 1 indicate that phreatophyte clearing reduced summer rates by nearly 45 percent. The average computed measurement errors in summer evapotranspiration rates, before and after clearing, are ±59 percent and ±113 percent, respectively, and the average measurement error in the change in summer evapotranspiration as a result of clearing is nea^y ±200 percent. These large computed measurement errors are shown to overestimate substantially the true measurement variable in evapotranspiration. The computed errors do give, however, a good indication of the relative sign...

Evapotranspiration before and after clearing phreatophytes, Gila River flood plain, Graham County, Arizona

“…The collection and conversion of data for these components are described in the following pages. The errors involved in computing the individual components and in evaluating ET as the residual in the budget equation were analyzed and described by Burkham and Dawdy (1970) and Hanson and Dawdy (1976).…”

“…The total accumulated precipitation for a budget period was computed as an average weighted value from 2where P = the average weighted precipitation for the budget period, in inches, Pj = the accumulated precipitation at gage j for the budget period, in inches, Aj = the area assigned to gage;, in acres, and n = total number of gages in a reach. Hanson and Dawdy (1976) analyzed the measurement errors associated with the precipitation data. Mean annual precipitation for the 8-year period 1964-71 was 11.15 in.…”

“…Evapotranspiration was related, by use of empirical equations, to potential evapotranspiration and to the type of vegetation. Error analyses of the components of the water budget, prepared by Burkham and Dawdy (1970) and Hanson and Dawdy (1976), were used to evaluate the reliability of the results.…”

Introduction to Phytoremediation of Contaminated Groundwater

“…The removal of the phreatophytes permitted more groundwater to discharge to the surface-water body. In a different study, the water-budget method was used to determine evapotranspiration for saltcedar that grew along the Gila River flood plain in Arizona (Hanson and Dawdy 1976). Along a 4-mi (6.4 km) reach, it was estimated that phreatophytes consumed 80 acre-ft/year of groundwater that, therefore, was not available to supply the adjacent surface-water body.…”

Measurement of evapotranspiration in phreatophyte areas, Smith Creek Valley and Carson Desert, west-central Nevada, 1983

“…In the Great Basin, ground water is lost primarily through transpiration by phreatophytes and evaporation from bare soils. Early estimates of ET were based on water-budget methods, empirical techniques, and meteorological techniques (Penman, 1948;Robinson, 1970;Hanson and Dawdy, 1976;Pennington, 1980). Technical advancements have facilitated such theoretical approaches for estimating ET as mass-transfer and energy-balance methods.…”