The Transpiration of Emersed Water Plants: Its Measurement and Its Relationships

Abstract

1. Emersed water plants transpire large amounts of water. 2. With one exception (water lily), the evaporation taking place from a water surface occupied by emersed water plants is much greater than that which takes place from a free water surface of the same area and subjected to the same external conditions. 3. The amount of evaporation from a water surface on which water lilies are growing is less than that which takes place from a free water surface of the same area and subjected to the same external conditions. 4. The amount of evaporation from a water surface occupied by emersed water plants depends upon the following factors: (a) the species of plant; (b) the density of plant stand; (c) the amount of plant surface exposed to the evaporating power of the air; (d) the height of the plant growth above the level of the water; (e) external factors (physical factors, like wind, temperature, relative humidity, etc.); (f) internal factors (chemical and physiological phenomena within the plant). 5. Transpiration from emersed water plant surfaces occurs both by day and by night, but transpiration by day is greatly in excess of that by night. 6. For different species there is no constant ratio between rate of transpiration and the area of surface exposed. 7. The rate of evaporation from the transpiring surface of an emersed water plant during the day may equal, and in some cases may exceed, that from a free water surface of the same area; but at night the evaporation from a free water surface is usually greatly in excess of that from a transpiring plant surface. 8. Wind, temperature, and relative humidity undoubtedly have a great influence on the rate of transpiration of emersed water plants, but these factors alone do not explain the great difference existing between transpiration of emersed water plants and the evaporation from a free water surface. 9. These data are of economic importance in indicating what plants should be grown in and what plants should be excluded from storage reservoirs in regions of small rainfall and scant water supply.