Land-Atmosphere Interactions, Evapotranspiration

Evapotranspiration (ET) is the movement and transfer (i.e., flux) of water as a liquid at the Earth’s surface to the atmosphere as a gas. ET is a combination of open water evaporation and plant transpiration ( , which is the Sublimation transition of solid water (i.e., ice, snow) to vapor due to low atmospheric pressure (i.e., high altitude), dry air and high sunlight, is generally considered separate from ET.). Sources of open water evaporation could include oceans, seas, lakes, rivers, ponds, puddles, and water on objects such as plants, buildings, rocks, the soil surface (including movement of water vertically through the soil to the surface), or in the context of measuring devices such as a pan. Plants take up water from the soil through their roots, transferring that water through their stems via conduits called xylem to their leaves, where it is used in the process of photosynthesis. The photosynthetic machinery in leaves (e.g., chlorophyll) takes in CO2 from the atmosphere through stomatal pores, and combines it with water and energy (i.e., light) to create sugars used to maintain and grow plant tissue and functions. While stomata are open, plants may lose water from their leaves to the atmosphere – this water loss is called transpiration. Plants regulate the opening and closing of their stomata to minimize water loss (closed), yet maximize CO absorption (open). 2 Energy is required to break the strong bonds that hold water molecules together as a liquid – when those bonds break, the individual water molecules may enter the surrounding atmosphere as vapor. Energy may be in the form of heat, radiation, or pressure. Regardless of the availability of energy, water molecules may not be able to enter the atmosphere if the atmosphere is already saturated with moisture (humidity) or if there is no wind to facilitate the transfer of the molecules from the water source to the atmosphere. The wind itself may be differentially influenced by friction as it passes over smooth versus rough surfaces. Therefore, solar radiation (or, indirectly, air temperature), air humidity, and wind speed are the main climate influences on ET. The main vegetative controls include leaf and canopy structures, regulation of stomata, and rooting dynamics. Finally, soil characteristics control soil moisture retention of precipitation inputs. All of these potential controls vary in influence depending on the system in question, as well as the associated spatial and temporal scales of analysis (Fisher et al. ). 2011