Estimating the effects of understory removal from a Douglas Fir forest using a two‐layer canopy evapotranspiration model

Abstract

W. J. Shuttleworth's (1979) development of the Penman‐Monteith evaporation equation for multilayer, partially wet forest canopies was modified for application to the hypostomatous canopies of Douglas fir and salal. This theory was combined with standard hourly micrometeorological measurements, eddy diffusive, boundary layer and stomatal resistance functions, and canopy and root zone water balance equations to calculate evapotranspiration rates (E) from a Douglas fir forest with salal understory over extended periods during two growing seasons. Calculated values of E agreed to within 0.2 mm d−1 of values determined using Bowen ratio‐energy balance measurements. The courses of average root zone volumetric water content (θ) calculated for two extended periods agreed well with neutron probe measurements. Salal understory removal resulted in measured values of θ being only 0.01–0.03 m3 m−3 higher over the two periods, in close agreement with calculations. This corresponded to calculated tree transpiration rates being 0.4 mm d−1 higher on average, during the second half of both periods. These higher rates were confirmed by stomatal resistance measurements.