EFFECTS OF NEGATIVE WATER PRESSURES ON EVAPORATION FROM SMALL EVAPORIMETERS

Evaporation tests were conducted using black Bellani plate and carborundum block evaporimeters operated at water pressures of 0 to − 64 cm. In both cases, the reduction in recorded evaporation was rapid as the pressure changed from 0 to − 2 cm. A large part of this change was attributed to loss of water by dripping from the evaporating surfaces. Below − 4 cm, a decrease in pressure was accompanied by a slight, nonsignificant reduction in evaporation from Bellani plates, and a larger and highly significant reduction in evaporation from carborundum blocks. In studies of routine daily records in which water pressure at each evaporimeter was allowed to lessen by daily increments from a maximum of − 4 cm to a minimum of − 40 cm, this range in pressure had no significant effect on the rate of evaporation. It was concluded that within this pressure range, any effects of pressure on evaporation were not great enough to be of concern under field conditions.