ABSTRACT
Soil water is lost from the soil surface by evaporation and from the rooting zone by plant transpiration. 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 from the soil surface is almost eliminated by a full leaf canopy. This shades the surface, thus reducing air flow and maintaining a humid atmosphere over the soil, but at this point soil water is removed via the plant. Where a leaf canopy covers the soil, the rate of water loss becomes more closely related to transpiration rates. The potential transpiration (PT) rate represents the estimated loss of water from plants grown in moist soil with a full leaf canopy. Water loss for each month does not vary much over the years and so potential transpiration rates based on past records can be used in the calculation.
