Water evaporating from the plant surface or soil surface, is called evaporation .
Water vaporizing inside the plant leaves and diffusing through leaf pores to the ambient air is called transpiration. In addition to weather, irrigation frequency, plant height, plant roughness, canopy ground shading, plant-water relations, and crop age all affect ETc rates. Table 1 gives ETo historical averages for the areas shown on the California evaporative demand map. Together, the ETo rate and the presence of water on or near the soil surface are the principal factors determining ETc rates during the initial growth period. When the soil surface is wet, the ETc rate is typically at or slightly above the ETo rate.