The latent heat transfer associated with tree transpiration is one of the influences of trees on the surrounding physical environment and it is important to predict the transpiration rate of trees correctly in order to predict and evaluate the urban heat and water budgets. When trees are planted as a countermeasure against urban warming in street canyon design, it is necessary to predict the influence of commonly planted trees on typical summer days accurately rather than that of a particular tree on a certain day. Various prediction models for transpiration rate and stomatal conductance have been proposed. In this study, in order to predict the transpiration rate of trees widely planted as road side trees on a typical summer day at a certain time with high accuracy, transpiration rate is predicted with the Penman Monteith method incorporating the Jarvis type stomatal conductance model. Penman Monteith method is one of the transpiration models tha t can predict the time variation of transpiration rate on a single tree scale and Jarvis type stomatal conductance model is one of the most widely used stomatal conductance models today. In this study, the time variation of transpiration rate of Zelkova serrata, one of the species widely planted as roadside trees in Japan, on typical summer days was clarified using long term measurement result. Based on the result, model parameters in the Jarvis type stomatal conductance model proposed by Ward et al 23,24) were identified for the prediction of transpiration rate on typical summer days.
The influences of solar radiation (condition of cloud cover), soil water content and vapor pressure deficit on the transpiration rate were analyzed. As for the solar radia tion (condition of cloud cover), the peak of the transpiration rate tended to appear earlier in sunny mornings than cloudy mornings. As for the soil water content, decrease in the transpiration rate was observed due to the continuous decrease of the soil w ater content for three days when the time variation of the vapor pressure deficit and the solar radiation was almost equal. As for the vapor pressure deficit, the increase in the daily total transpiration rate associated with the increase in the daily mean vapor pressure deficit was hardly observed, suggesting that the stomat a of the water stressed tree were closed to prevent excessive drying. As a result of these analyses, the time variation of transpiration rate of Zelkova serrata on typical summer days was clarified, and model parameters in the stomatal conductance model were identified based on the data.
The accuracy of transpiration rate and leaf surface temperature prediction using the model which incorporates the stomatal conductance model with the identified parameters into the Penman Monteith method was validated . The model reproduced the tendency that the transpiration rate of the whole single tree peaked in the morning and then decreased . The tendency that the transpiration rate in the tree crown is the highest at the top of the crown, where it receives the most solar radiation, followed by the bottom part, where it receives less solar radiation, and the lowest at the middle part, where it receives little solar radiation was also reproduced. As for the leaf surface temperature, the tendency that the leaf surface temperature varies little at each point in the crown was also reproduced.