Calibration of the priestley-taylor evaporation model for Ethiopia

Abstract

In practice reservoir planning and operations, irrigation design and water balance studies require estimates of reliable evaporation. The universal standard models of the Penman E₀ and the Penman-Monteith ET₀ are used to estimate open water evaporation and evapotranspiration respectively. The models rely on accurate measurements of climatic elements such as temperature, humidity, wind speed, and solar energy with good spatial and temporal coverage. However, practicing hydrologists, irrigation engineers and planners face challenge of reliable estimate of evaporation when only temperature data are available as the case in many study areas of Ethiopia and elsewhere. To overcome this challenge, a number of simplified temperature-based evaporation models notably the Priestley-Taylor, Blaney-Criddle and Hargreaves have been developed. Their applicability is, nevertheless, subject to rigorous local calibrations and without calibration they have limited validity to tropical areas.

There is a need, thus, to precise estimate of E₀ and ET₀ based on only temperature data for Ethiopia. This paper presents locally calibrated coefficients α for the Priestley-Taylor model applicable for Ethiopia to estimate open water evaporation E_o and Reference Evapotranspiration ET₀ based on maximum and minimum temperature as well as readily derivable elevation and radiation data. In order to calibrate α, regression is done between the Priestley- Taylor model estimate (independent variable) and E_o Penman model estimate as dependent variable for each month of the 167 Class I stations. Similarly, regression is done between the Priestley- Taylor estimate (independent variable) and ET_o Penman-Monteith model estimate (dependent variable). It is found that the Priestley-Taylor coefficients α applicable over Ethiopia to estimate monthly E₀ is 1.11 and to estimate ET₀ is 0.96.

Keywords: Ethiopia, Irrigation-water - requirement, Open water Evaporation, Priestley-Taylor, Penman-Monteith