Determination of reference evapotranspiration by the Bowen Ratio Method using psychrometers installed at various heights

Abstract

Meaning to contribute for an adequate use of water resources, this study aimed to compare daily forecasts of reference evapotranspirations (ETo), obtained through the Bowen ratio method with psychrometers set at various heights, to those obtained by the FAO-56 Penman-Monteith method. The trials were carried out in Fortaleza, Ceará State, Brazil at the Meteorology Experimental Station of Universidade Federal do Ceará, covered with American grass (Bouteloua americana (L) Scribin). The area was irrigated with the help of dripping hoses and the amount of water to be replaced was estimated according to soil moisture characteristics. Soil moisture conditions were evaluated with tensiometers. The values of the variables related to the weather were collected in an automated weather station plus the Bowen Ration System (BRS), which was set up with psychrometers made up with thermocouples of copper-constantan. Aiming to analyze the likely effects of the various heights/lengths ratios of the psychrometers installed, the BRS arms positions were altered, in four different periods of data collection. The values of ETo were calculated with an MS-Excel spreadsheet, and the reliability of the methodology was evaluated by the correlation coefficients between the Wilmott index and the c-index. The experimental studies have lead to the conclusion that the arms position of the Bowen ratio system greatly affects the reliability of the method used to estimate the reference evapotranspiration.