Evaluation of transpiration models with observations over a Douglas-fir forest

Hourly observations of eddy-covariance water vapour fluxes obtained over a Douglas-fir forest are used to evaluate three transpiration models. The models are (1) the Penman–Monteith model with a Jarvis type of formulation for the canopy resistance, including an explicit function for specific humidity deficit, (2) the Penman–Monteith model in which the specific humidity deficit response is replaced by a response to transpiration rate itself as described by Monteith (1995), and (3) a modified Priestley–Taylor formula. Model parameters are optimised against the observations of 43 dry days during the growing season. Systematic differences between model and observed transpiration could be related to wind direction. These deviations correspond with deviations found in the observed energy balance for the same wind directions. The mean square of the residuals is approximately two times larger than the value found for the uncertainty in the eddy-covariance measurements due to atmospheric statistics. Distinct responses to the specific humidity deficit, solar radiation, soil matrix potential and shoot growth are found. No temperature response is found. The response of transpiration to an increase of leaf area index during shoot growth suggests that the transpiration from new shoots is higher as that from older shoots. However, other physiological changes at the start of the growing season may play a role as well. An analysis of residuals shows that the Jarvis model gives good results for all conditions encountered. It is shown that the Monteith model can be formulated such that it is almost equal to the Jarvis model for this aerodynamically rough forest. Despite its simple formulation a modified Priestley–Taylor formula (including LAI and soil matrix potential response) gives reasonable results, although at moderate irradiation and high specific humidity deficit deviations are significant. A comparison with results from another coniferous forest (Thetford forest) shows that transpiration rates are only slightly larger despite the much larger leaf area index.