Studies on the water relations of Pinus sylvestris in plantation conditions. IV. Direct observations on the rates of transpiration, evaporation of intercepted water, and evaporation from the soil surface.
cf. F.A. 27 No. 106.] The total evaporation from a plantation of Pinus sylvestris at Crowthorne, Berkshire, was estimated on 44 randomly chosen sample days during the three years 1957-59, from measurements of the water lost from isolated blocks of soil within the plantation, and of the loss of weight of detached branches or leaves, suspended in the places from which they had been removed, during the 10 min. after they were detached. Experiments showed that the effect of detachment on the rate of water loss from branches and leaves was probably negligible in the conditions of these experiments. The branches or leaves were sampled so as to represent different layers of the canopy and different aspects. The rates of water loss from them were expressed per unit weight of leaf, and the amounts of leaf present per ha. in the plantation were determined by a sampling procedure, from which the pattern of change in the weight and area of the canopy could be determined. The dry weight of the foliage varies from 5000-6000 kg./ha. at the end of winter to 9000-10, 000 kg./ha. in summer, and the leaf area index (total surface) appears to vary seasonally between about 2.5 and 4.5. The total evaporation from the plantation exceeded E0 (potential evaporation from an open water surface) on rainy days (when the foliage was wet). It was less than E0 on rainless days from November to May but was not significantly different from E0 during June to October. The mean evaporation determined in this way was similar in amount to the mean evaporation estimated from the rainfall during the periods of soil water deficit in the three years. KEYWORDS: Evapotranspiration \ Foliage quantity and weight \ Hydrology \ Interception \ precipitation \ Pinus sylvestris water relations \ Transpiration measurement \ detached leaves