ABSTRACT
Water can be found in three different states, solid (ice), liquid, and gas (water vapor). Evaporation is the process of change of the state of water from liquid to vapor. Water in the liquid state is known to everyone because it is a necessity on a daily basis. We drink water, use it to wash utensils and clothes, and water our lawns. However, the vapor form of water is less familiar because seeing something makes it easier to believe in it, and water vapor cannot be seen. Although water vapor is also an inseparable part of our daily life, especially if we are living in humid, semihumid, or coastal areas, we commonly refer to it as humidity. In the presence of vegetation, evaporation and transpiration are together known as evapotranspiration. Evaporation and evapotranspiration are important components of the water cycle in the hydrosphere. For evaporation to occur, energy is required. Meteorological conditions strongly influence evaporation rates and amounts from an area. Surface soil conditions also influence evaporation, for example, presence of mulch on the soil surface could decrease evaporation. Other factors that can influence evaporation from soil are soil depth, soil structure, and degree of homogeneity of soil. Important aspects of energy transport, other than those discussed in Chapter 11, through soil are evaporation, transpiration, and root water uptake. Actual evaporation from soil and transpiration from plants are affected by root zone soil water dynamics, and their determination requires simultaneous knowledge of water flow and root water uptake in soils. In this chapter, we discuss soil water dynamics due to evaporation, water uptake by roots, and evapotranspiration. Some of the lecture notes provided by Dr. Pete Wierenga, a soil physicist at NMSU, to students are incorporated in this chapter.
