Evaporative Power in the Tropical Forest of the Panama Canal Zone

Abstract

The physical processes that affect evapotranspiration and the technique of measuring the drying influence of the air in the tropical forest of the Panama Canal Zone during the rainy season were examined. The time rate of evaporation from the surfaces of white spherical porcelain atmometers was the measurement used to determine the evapotranspiration power of this environment. Evaporation, temperature, vapor pressure deficit and relative humidity profiles are shown from the forest floor through 28.5 m to the top of the canopy. The mean hourly rate of evaporation at the top of the canopy is 4 times that near the forest floor but may vary from as little as 2.8 times to as much as 12 times the rate of evaporation near the forest floor. Mean hourly rates of evaporation at the top of the canopy and near the forest floor change dramatically from day to night. The average daytime rate near the surface was about 2.5 times the nighttime rate, while at the top of the canopy it was about 4 times the nighttime rate. The saturation vapor pressure deficit is closely related to the evapotranspiration power, but the slight air movements in the moist stagnant spaces below the forest canopy and the active convection of relatively dry air from above the canopy into the trunk spaces below appear to be the dominant factors.