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The evapotranspiration is indispensable for the planning and management of water resources in watersheds and for the design of important engineering projects. The accuracy of its estimation depends, among other things, on the proper determination of the date and frequency of irrigation. One of the most accurate equations for estimating the reference crop evapotranspiration (ETo) is the Penman-Monteith equation, which was derived from the Penman equation published in 1948, which in turn is based on the energy balance equation. In this paper we present the theoretical foundations of Penman’s equation from the perspective of the first law of thermodynamics and the theory of energy balance as a contribution for the students and engineers who use Penman’s or Penman-Monteith equations in a utilitarian way within the historical and theoretical framework of the origin of this equation.

Physical-Mathematical Foundations of the Penman Equation from the Perspective of the First Law of Thermodynamics and Energy Balance Theory