The rate of water loss from the soil by evaporation depends on the drying capacity of the atmosphere just above the ground and the water content in the surface layers. The evaporation rate is directly related to the net radiation (see p26) from the sun which can be measured with a solarimeter (see p43). Evaporation rates increase with higher air temperatures and wind speed or lower humidity levels. As water evaporates from the surface, the water films on the soil particles become thinner. The surface tension forces in the film surface become proportionally greater as the water volume of the film decreases. This leaves water films on the particles at the surface with a high surface tension compared with those in the films on particles lower down in the soil. The increased suction gradient causes water to move slowly upwards to restore the equilibrium. Whilst the surface layers are kept moist by water moving slowly up from below, the losses by evaporation, in contrast, are quite rapid. Consequently the surface layers can become dry and the evaporation rate drops significantly after 5 to 10 'mm of water' is lost. Evaporation virtually ceases after the removal of 20 'mm of water' from the soil. Maintaining a dry layer on the soil surface helps conserve moisture in the soil below. Evaporation from the soil surface is almost eliminated by a leaf canopy that shades the surface, thus reducing air flow and maintaining a humid atmosphere over the soil. Mulches (see p335) can also reduce water loss from the soil surface.
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