Soil-Plant-Atmosphere System

This chapter describes the soil-plant-atmosphere (SPA) system to support the development and calculation of evapotranspiration (ET). The SPA system involves absorption and storage of energy in the soil; storage of water in the soil; evaporation from soil and plant surfaces; transpiration from leaves of plants; and transfer of water vapor, sensible heat, and radiation from the soil and plants to the atmosphere. The chapter focuses on items of concern to engineers in solving problems involving evaporation (E) and ET from plant and soil communities, conducting research in E and ET, and managing water supplies and delivery systems. The physical basis of the evaporation process comprises physical properties of liquid water, vapor, and air. Most soils literature uses the terms capillary potential, matric potential, and soil-water suction or tension when referring to the pressure potential of water in soils.