Estimating evapotranspiration from wheat using weather measurements and carborundum or Piché evaporimeter

Abstract

Hourly evaporation rates from a Piché evaporimeter installed inside a Stevenson screen were compared with rates measured from a Piché exposed to the atmosphere, but screened from direct short- and long-wave radiation (shaded) and with rates registered on a carborundum evaporimeter similarly exposed. Strict linear relationships exist between evaporation from the shaded Piché evaporimeter, the Piché in a screen and the shaded carborundum evaporimeter. The latter was 1.6 times more sensitive than the shaded Piché. The shaded Piché was 1.4 times more sensitive than the screened Piché. Regression equations for the aerodynamic term (AT) in the Penman-Monteith equation were developed. AT was expressed in terms of hourly evaporation measured by a Piché in a Stevenson screen ( Epsc), an exposed, but shaded Piché ( Eps), and a carborundum evaporimeter ( Ecs). The following relationships were found: 1. (i) AT = 0.018 ( Epsc) + 0.09 (mm h −1) r = 0.95 2. (ii) AT = 0.014 ( Eps) + 0.05 (mm h −1) r = 0.96 3. (iii) AT = 0.008 ( Ecs) + 0.06 (mm h −1) r = 0.95 Estimates of maximum evapotranspiration rates, ETm, obtained by substituting these relationships for AT in the Penman-Monteith equation were then tested against lysimeter measurements of ETm. In 1985, values of the index of agreement varying between 0.94 and 0.95, and mean absolute difference varying from 0.10 to 0.09 mm h −1, for the three types of evaporimeters were obtained from these comparisons. In 1986, values of the corresponding test parameters were 0.92 and 0.09 mm h −1 when the carborundum evaporimeter was used. These values demonstrate the reliability of the proposed model. While the sensitivity of the evaporimeters varied greatly, no one proved more accurate than any other. The carborundum evaporimeter proved to be the most sensitive.