Canopy Resistance Studies of Cattails

The one-step application of the Penman-Monteith evapotranspiration (ET) computation method from weather data requires canopy resistance (rc) and aerodynamic resistance (ra) factors specific to the vegetation. A physically based estimation of these factors would help achieve reproducible and accurate results. Concurrent measurements of stomatal conductance of cattails (Typha domingensis) with a porometer, ET with a lysimeter, and weather data were made to develop physically based seasonal canopy and aerodynamic resistance factors for a cattail wetland in south Florida. The conductance and transpiration variability on different sections and leaves of a single cattail plant were accounted for with a sampling procedure. The mean leaf conductance occurs at the apical section of the outer green leaf and the basal section of the second leaf. Wind profiles over the site were modeled from average wind speed measurements at three heights at the site to estimate aerodynamic resistance (ra). A seasonal cattail canopy resistance factor developed in this study (50 s m–1) was evaluated by comparing computed and measured daily ET data. The relationship between canopy resistance and errors in ET estimation showed that more error is encountered with underestimation of canopy resistance than overestimation.