Two- and One-layer Implicit Energy Balance Solutions Compared with the One-layer Explicit Penman-Monteith Solution for Evapotranspiration of Alfalfa.

Alfalfa (Medicago sativa) is one of two crops commonly used to establish reference evapotranspiration (ET) for use in water use estimation for other crops. The other is grass. In the modern paradigm for estimating crop water use, both alfalfa (~0.5-m tall) and grass (cool-season variety, mowed, ~0.12-m tall) reference ET have been supplanted by standardized sets of equations that establish reference ET values using a Penman-Monteith (PM) single layer, explicit energy balance formulation applied using standard weather data. The formulation is made explicit using the Bowen ratio; and operationally it is assumed that only air temperature need be known, not both air and canopy temperature. Although the standardized equations include those for estimating the contribution of evaporation from the soil surface, the PM formulation is fundamentally a single-layer (or “big-leaf”) concept. In practice, the reference ET value is multiplied by a crop coefficient, which changes according to plant growth, to render an estimate of crop ET. One problem of modern crop ET estimation is that the crop coefficients do not transfer well across different environments. In large part, this may be due to the assumption that the canopy is at air temperature, an assumption that becomes less true in hot, advective, arid environments. We compared the PM approach to a one-layer implicit energy balance solution and to a two-layer (canopy and soil) solution, neither of which assumes equality of air and canopy temperatures. Weather and water use data for alfalfa grown under reference ET conditions on large weighing lysimeters were used in the analysis.