EVALUATI NG SURFACE RESISTANCE FOR ESTIMATING CORN AND POTATO EVAPOTRANSPIRATION WITH THE PENMAN–MONTEITH MODEL
2001
The Penman–Monteith (P–M) evapotranspiration (ET) model has been shown to be adequate for estimating daily
reference crop ET (ETo). However, the proper evaluation of surface resistance to vapor exchange (rs) has been a limiting factor
for using the model to directly estimate daily ET for other crops. A popular approach to quantify rs, referred to as canopy
resistance (rc), is based on single–leaf resistance (rL) and leaf area index (LAI) but does not account for non–stomatal
contributions (“excess resistance”) to rs such as leaf boundary layer resistance and aerodynamic resistance within the canopy.
Field measurements of ET, rL, and LAI for full–canopy conditions were used to develop an empirical crop height and
LAI–dependent relation to estimate excess resistance (ro). An alternative rs term was defined as ro + rc and used in conjunction
with an aerodynamic resistance term (ra) 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 rs definition including excess resistance (rs = ro + rc), ra calculated from the top of the canopy, and
vapor pressure deficit (VPD) and solar irradiance (Rs) functions to adjust rL.
PM–2, which was essentially identical to PM–1 with no VPD or Rs adjustment to rL.
PM–3, which used ‘standard’ rs (rS = rc) and ra (based on a zero–plane displacement height approximately 2/3 of crop
height) calculations with VPD and Rs adjustments to rL.
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 rc 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).
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