Abstract
The PenmanMonteith (PM) evapotranspiration (ET) model has been shown to be adequate for estimating dailyreference crop ET (ETo). However, the proper evaluation of surface resistance to vapor exchange (rs) has been a limiting factorfor using the model to directly estimate daily ET for other crops. A popular approach to quantify rs, referred to as canopyresistance (rc), is based on singleleaf resistance (rL) and leaf area index (LAI) but does not account for nonstomatalcontributions (excess resistance) to rs such as leaf boundary layer resistance and aerodynamic resistance within the canopy.Field measurements of ET, rL, and LAI for fullcanopy conditions were used to develop an empirical crop height andLAIdependent relation to estimate excess resistance (ro). An alternative rs term was defined as ro + rc and used in conjunctionwith an aerodynamic resistance term (ra) calculated from the top of the canopy. Using climatic data collected over corn andpotato canopies, three variations of daily PM ET were compared:PM1, which used the rs definition including excess resistance (rs = ro + rc), ra calculated from the top of the canopy, andvapor pressure deficit (VPD) and solar irradiance (Rs) functions to adjust rL.PM2, which was essentially identical to PM1 with no VPD or Rs adjustment to rL.PM3, which used standard rs (rS = rc) and ra (based on a zeroplane displacement height approximately 2/3 of cropheight) calculations with VPD and Rs adjustments to rL.All three methods of PM 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 usedin standard calculations of daily ET with the PM equation. This behavior was not seen with the calculation of daily ET forpotatoes. The direct application of the standard PM equation to estimate daily corn ET (i.e., PM3) is not recommendedbecause 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., PM1 and PM2).
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