When PET energy is available, free water on plant and soil surfaces (interception water) readily evaporates, with little surface interaction or vapor resistance. Therefore, the PET value is reduced by the amount of interception evaporation before plant and soil water evaporation is computed. This interception evaporation becomes the first of the three components for estimating actual ET (AET).

Interception is specified as a storage device with a constant maximum capacity which represents the potential interception. This storage is filled by precipitation and sprinkler irrigation, and depleted by PET. Both plant and soil surfaces will accumulate surface water films during rainfall which is then available for interception evaporation, ie. water which evaporates with no surface resistance. Defining the potential interception is not obvious. Few data are available and the concept is somewhat nebulous. Each plant canopy has some ability to intercept water and prevent that portion of the precipitation from becoming infiltration or runoff. Residues and the uppermost soil surface similarly hold water films.

Limited data suggests that 0.10 inch (2.5 mm) is a nominal interception amount for agricultural crops (Chow, 1964 p.6-10). Soil surfaces without the waxy surfaces of most leaves would likely have half this amount. Maximum values for plant and soil surfaces are input variables. Refinements not included could account for time variation as the canopy develops, different canopy architecture or various soil surfaces and depressions.

Interception can accumulate to become a significant proportion of the water budget. Thirty or forty precipitation events are common in many agricultural climates, which results in some 3.0 to 4.0 inches (75 to 100 mm) of interception evaporation as one component of perhaps 25 to 30 inches (635 to 760 mm) of annual actual ET.

Field Hydrologic Processes | Precipitation | Infiltration | Potential ET
Soil Water Evaporation | Plant Transpiration | Crop Water Stress
Root Water Uptake | ActualET | Soil Water Redistribution | Irrigation