Surface-Energy and Air-Mass Interactions and the Concept of Reference ET

The conservation of energy at the evaporating surface controls the close linkages among solar or net radiation, the primary energy source; sensible heat flux to and from the air and to and from the soil; and the resulting conversion of energy into latent heat flux. This chapter describes the general interactions of the available surface energy and air-mass along with common approaches used to obtain evapotranspiration (ET) estimates. The combination energy balance–aerodynamic equations, such as those proposed by Penman or Slatyer and McIlroy, model how various weather parameters interact to affect energy partitioning at evaporating surfaces. The chapter explains why the term reference ET has replaced the term potential evaporation in irrigation design, planning, and management. Reference ET is a visual and practical means to establish a near-maximum index of climatic demand for water and is straightforward to measure.