Soil water management implications during the constant rate and the falling rate stages of soil evaporation

Abstract

Desorptivity is a soil physical term used to describe the ability of soil to lose water by evaporation. Theoretically, it has been established that evaporation of water from soil during the falling rate stage, when water becomes limiting, is proportional to the square root of time. The proportionality constant is called desorptivity. Using ten soils with different textures from different locations in Ghana, evaporation was effected in direct sunshine during the day and with a fan in the laboratory during the night. The study was carried out in dry harmattan winds during the day. From the cumulative evaporation ( E) and time ( t) curves, it was deduced that evaporation in dry harmattan winds was similar to that in an environment with constant evaporativity. The falling rate stage of evaporation began after 5 days of continuous evaporation. During the falling rate stage, E was observed to be proportional to t 1 2 , as predicted by theory. The desorptivity ( Sd) calculated during the falling rate stage of evaporation was observed to increase exponentially with water content of the soil during the falling rate stage. The desorptivity gave a negative correlation with per cent sand and a positive correlation with per cent clay. However, neither correlation was statistically significant. The loss of water during the constant rate stage far exceeded that during the falling rate stage. Therefore, the study suggests that, in the tropics, the best time to control soil water loss owing to evaporation would be the constant rate stage