A) Le complément du Colloque sur l'évaporation à Berkeley

Abstract

Abstract Due to the intensive development of hydroelectric power projects and the utilization of water resources, waterbalance calculations of the projected ponds, storage works and natural reservoirs assume an ever-increasing importance. The majority of reservoirs to a varying extent become overgrown with quatic plants. In estimating total evaporation from a reservoir it is necessary to determine evaporation losses from its overgrown part. Evaporation from reed (Phragmites communis) grown in special evaporation pans was studied during several years to determine the possible amount of evaporation from aquatic growth at reservoirs in different zones of insufficient moisture supply. According to the results of the observations, total evaporation from reed whose transpiration amounts to 80 per cent of total evaporation losses, depends upon the meteorological characteristics of the season and the amount of the transpirating vegetal mass. Depending on the meteorological conditions of the season, total evaporation from the reed growth at the reservoirs under study varied, during the observational period, within 440 mm to 1 700 mm for the vegetation season. The following formula is recommended for calculating seasonal evaporation values: E = E max −ae −kD where: E—mean total evaporation from aquatic growth in mm/month for the vegetative season; D—mean vapour pressure deficit over the reservoir at a height of 200 cm, in mb, for the calculation period; E max—for the zones under consideration is 290 mm per month; a and k—1000 and 0.31 respectively; e—base of natural logarithms. An important question for extensively used water-balance calculations is that of the ratio between evaporation from open water surface of reservoirs (Ew ) and from the quatic growth in the latter (Ep ). The investigations carried out show that, depending on the air saturation deficit values in the zone and the velocity of wind over the reservoir, the ration between the above-indicated values vary within 0.8 to 2.5.