Evaluating surface resistance for estimating corn and potato evapotranspiration with the Penman-Monteith model

The Penman-Monteith (P-M) evapotranspiration (ET) model has been shown to be adequate for estimating daily reference crop ET (ET(o)). However, the proper evaluation of surface resistance to vapor exchange (r(s)) has been a limiting factor for using the model to directly estimate daily ET for other crops. A popular approach to quantify r(s), referred to as canopy resistance (r(c)), is based on single-leaf resistance (r(L)) and leaf area index (LAI) but does not account for non-stomatal contributions ("excess resistance") to r(s) such as leaf boundary layer resistance and aerodynamic resistance within the canopy. Field measurements of ET, r(L), and LAI for full-canopy conditions were used to develop an empirical crop height and LAI-dependent relation to estimate excess resistance (r(o)). An alternative r(s) term was defined as r(o) + r(c) and used in conjunction with an aerodynamic resistance term (r(a)) calculated from the top of the canopy. Using climatic data collected over corn and potato canopies, three variations of daily P-M ET were compared: PM-1, which used the r(s) definition including excess resistance (r(s) = r(o) + r(c)), r(a) calculated from the top of the canopy, and vapor pressure deficit (VPD) and solar irradiance (Rs) functions to adjust r(L). PM-2, which was essentially identical to PM-1 with no VPD or Rs adjustment to r(L). PM-3, which used 'standard' r(s) (r(S) = r(c)) and r(a) (based on a zero-plane displacement height approximately 2/3 of crop height) calculations with VPD and Rs adjustments to r(L). All three methods of P-M ET were compared with daily crop ET measurements made with a Bowen ratio energy balance (BREB) system. For corn, the results of this study suggested significant resistance to vapor transfer in excess of the r(c) used in standard calculations of daily ET with the P-M equation. This behavior was not seen with the calculation of daily ET for potatoes. The direct application of the standard P-M equation to estimate daily corn ET (i.e., PM-3) is not recommended because excess resistance not accounted for by this model will often lead to substantial ET overestimation. However, estimation of excess resistance based on canopy features was not found satisfactory (i.e., PM-1 and PM-2).