TRANSPIRATION OF AN OAK FOREST AS PREDICTED FROM POROMETER AND WEATHER DATA

Transpiration of an oak forest located on one of the Castricum lysimeters is predicted from the Penman-Monteith equation and a model of canopy conductance based on porometer measurements of stomatal conductance. An upper limit to daily transpiration of 2. 1 mm is introduced to regulate daily water loss. The model predicts transpiration to within 10% of observed losses on a seasonal basis. From measurements of soil moisture content and groundwater depth it appeared that the trees first depleted the unsaturated zone and only when this moisture reservoir was emptied, started to use water from the saturated zone. A possible physiological basis of the upper limit to daily transpiration is discussed.