Alfalfa Canopy Resistance From Lysimetric and Radiometric Measurements.

The widely used Penman-Monteith (PM) equation to find the value of evapotranspiration (ET) contains a value for the canopy resistance (rc, s m-1) that, unlike the other equation parameters, cannot be measured directly. Different values for rc have been suggested but none have been directly nor experimentally proven. We proposed an alternative for the P-M equation that is not subject to a weakness in its derivation, by solving the surface energy balance for ET by iteration (Lascano and Van Bavel, 2007; Agron. J. 99:585–590). Further, it can be used to find the value of rc, also by iteration, provided the value of ET is measured accurately by lysimetry. Using data obtained in Bushland, TX we calculated hourly rc values for an irrigated alfalfa crop that ranged from 28 to 73 s m-1 on a midsummer day during daylight hours. We then used these values to calculate the hourly values on three other days from the energy balance and compared these with the actual values, measured by lysimeters. As lysimeters are not generally available, we repeated obtaining hourly values for rc by measuring the crop surface temperature with long-wave radiometers (Conaway and Van Bavel, 1976; J. Appl. Meteorol. 6:650–655) also using the energy balance by iterating. The hourly values of rc from radiometric measurements were more variable and when used to calculate ET on the same three days, the results were more scattered. We conclude that the proposed calculation of rc from lysimetric measurements is preferable to a so-called "reference" value in an inaccurate equation.