James M. Blonquist Jr., Apogee Instruments, Inc., Logan, UT and Bruce Bugbee, Dept. of Plants, Soils, and Climate, Utah State University, Logan, UT
The Penman-Monteith (P-M) equation is widely applied to estimate evapotranspiration (ET) of crop fields.Currently, typical application of the P-M equation does not include stability effects in the calculation of the boundary layer conductance term. Additionally, the surface conductance term is usually assumed to be constant. The purpose of this study was to evaluate the sensitivity of ET calculations with the P-M equation to the two conductance terms and to develop simple equations to estimate these terms. Sensitivity analysis showed boundary layer conductance and surface conductance can impact ET estimates by up to 25 %. Based on simulations from a two-source (vegetation and soil) surface energy balance model for sensible and latent heat transport, a simple empirical modification was made to the equation used to calculate boundary layer conductance. A simple empirical equation was also derived to calculate potential surface conductance. Separate conductance equations were derived for the short crop and tall crop reference versions of the P-M equation. Measurements over alfalfa (tall crop) showed ET calculated using the new boundary layer and potential surface conductance equations ranged from 43 % lower to 7 % higher relative to ET calculated with standard application of the P-M equation (no stability effects, constant surface conductance). Additionally, daytime ET calculated with the new conductance equations more closely matched ET measured with a weighing lysimeter. We recommend that the new equations for calculation of boundary layer conductance and potential surface conductance be used in routine applications of the P-M equation, or in applications of recursive methods for ET calculation, recently suggested as an alternative to the P-M equation. While the proposed conductance equations can be applied in existing ET weatherstations, addition of an infrared (IR) sensor for surface temperature measurement facilitates use of the boundary layer conductance equation via more direct determination of surface stability and direct estimation of actual (rather than potential) surface conductance. An IR sensor also improves net radiation estimation (on weatherstations without a net radiometer) because outgoing net longwave radiation can be calculated from the Stefan-Boltzmann equation.