Evaporation and transpiration are responsible for returning massive quantities of precipitation back to the atmosphere. While evaporation is present in a watershed, the dominant pathway for water to return to the atmosphere is via transpiration. Transpiration describes the process of plants extracting water from the soil through their roots and releasing it to the air through their leaves.

According to EM 1110-2-1417:

The fundamental water balance relationship that a continuous simulation model must satisfy to accurately represent the hydrologic cycle is: 

runoff = precipitation - evapotranspiration

Consequently, estimating evapotranspiration is of major importance.

Evaporation

Evaporation is the process of converting water from the liquid to the gaseous state. The process happens throughout a watershed. Water evaporates from the surface of lakes, reservoirs, and streams. Water also evaporates from small depressions on the ground surface that fill with precipitation during a storm. Water held in pore spaces near the surface of the soil may evaporate. Finally, precipitation that has landed on vegetation may evaporate before it can fall from the vegetation to the ground.

Thermodynamics of Evaporation

Evaporation is the conversion of water from the liquid to the gaseous state and involves large quantities of energy. The latent heat of vaporization is the amount of energy required to affect the state transition. The latent heat of vaporization is 539 calories per gram of water at 1 atmosphere of pressure. Additional energy is required to bring the water from its current temperature to the boiling point; water must be at the boiling point before it can vaporize. The energy required to raise the temperature of water is 1 calorie per Celsius degree. As energy is adsorbed in the water, some of the water molecules will begin moving faster. This increased movement is increased kinetic energy and is the result of applying energy to the water. The most common source of energy to drive evaporation is shortwave radiation from the sun. Eventually the fastest water molecules break free from the water and move into the air as water vapor. This process at the molecular level is repeated many times per second over the water surface. When taken in bulk, this is the process of evaporation. Because the fastest water molecules are the ones to break free to the air, the molecules left behind in the water are moving slower. A slower speed means a lower kinetic energy. Energy is equivalent to temperature so the temperature of the water will decrease during evaporation, even as some of the water is converted to vapor in the air.

Factors Affecting Evaporation

Many factors affect the amount of evaporation occurring in a watershed at any given time, and the factors are interrelated. However, some generalities exist. Evaporation occurs as long as the vapor pressure of the water is greater than the vapor pressure of the air. Therefore, evaporation will decrease as the relative humidity increases and will stop when the relative humidity reaches 100%. A number of environmental factors affect the vapor pressure of water and air through difference mechanisms. Evaporation will increase as the ambient temperature of the water increases. Several sequential days of high temperature could warm water and increase evaporation, but also shallow water is usually warmer than deep water. Evaporation decreases as the atmospheric pressure increases, or example when a high pressure system is present. Conversely, evaporation increases as atmospheric pressure decreases, such as with increasing elevation. Note however that increasing elevation typically is associated with decreases in temperature and the net result can be unpredictable. Wind also increases evaporation by removing the thin layer of vapor saturated air that forms over the water under calm conditions.

Measuring Evaporation

The most common way to measure evaporation is with special pans specifically designed for the purpose. Water is added to the pan and the depth of water is measured using either calibrated graduations marked on the side of the pan or measurement tools. The measurements are usually taken daily. Regardless of the method, the measurement relates directly to the equivalent depth of water that is evaporating at the location where the pan is sited. The pan must be periodically refilled with water.

Transpiration

Transpiration is the process of plants removing water from the soil and expelling it to the atmosphere. The water is extracted by the roots, travels through the plant vascular system, and exits through structures called stomata on the underside of the leaves. Some of the soil water is retained for the biological processes of the plant, while the process of evaporation that happens in the stomata cools the plant.

Root-Water Uptake

Water uptake does not begin with the roots; it begins within the stomata which are usually found on the underside of leaves. The stomata are tiny chambers with an opening to the air that can be regulated by the plant. The stomata are opened or closed in response to many different environmental and physiological factors. When the stomata are open, water vapor leaves through the opening as long as the relative humidity is less than 100%. The source of the vapor is water that evaporates inside the stomata, where it is found in the space between the cells that form the walls of the stomata. The evaporated water causes a meniscus to form in the space between the cells and a consequent capillary force is transferred to the vascular system of the plant. The capillary force is transmitted through the water in the vascular system from the leaves down to the roots. Microscopic hairs on the roots keep them in contact with the moist soil. Water is thus drawn into the roots due to the transmitted capillary force. The water moves throughout the vascular system of the plant performing functions such as transporting suspended nutrients. Water evaporating in the stomata performs the critical function of cooling the plant. If the stomata close for any reason, almost all water uptake by the plant will stop. Most plants transpire during daylight hours and cease at night.

Factors Affecting Transpiration

The factors affecting the rate of transpiration are related both to the amount of water in the soil and the plant. The factors are interrelated in very complex ways, but there are some generalities. One of the functions of transpiration is the cooling of the plant. The plant will attempt to increase transpiration during periods of high temperature to avoid heat damage, and may reduce transpiration in cool temperatures. The plant may open the stomata to initiate transpiration, but it can only occur if there is sufficient soil water. Plants cannot extract water in the soil below the permanent wilting point. This is the water content equivalent to the maximum capillary force which the plant can exert to extract water from the soil matrix. Different species of plants have differing abilities to generate capillary force in the vascular system. Further, the grain size distribution of the soil can also affect the permanent wilting point and changing the amount of water in pore spaces susceptible to the capillary force exerted by the plant. It is also the case that plants have a greater need for water during a rapid growth phase, and use less water after reaching maturity. Transpiration is affected by the density of plants in the landscape and by overall health. Finally, because transpiration is driven by evaporation of water in the stomata, all of the factors that effect evaporation also effect transpiration in a secondary manner.

Measuring Transpiration

The most accurate and complete method to measure transpiration is with a lysimeter. The lysimeter is essentially a large steel container on the order of 5 meters in diameter and 1 meter deep that sits on a scale. The container is installed in an agricultural field so that the top of the lysimeter is even with the ground surface. The container is filled with soil and crops are planted in the lysimeter and the surrounding field. Irrigation may be applied. The scale installed under the lysimeter measures the weight of the steel container, soil, water in the soil, and the crops. The reduction in weight on two subsequent days is equal to the amount of water evaporated plus the amount transpired. Given the density of water and the weight of water, the volume of water can be calculated. The combined evaporation and transpiration can then be calculated from the volume of water and the area of the lysimeter.
Transpiration can also be measured using the eddy covariance technique. Eddies are turbulent vorticies of air carrying varying concentrations of water vapor. If the amount of moving air and the concentration of water vapor in the air can be measured, then the transpiration can be calculated. This requires the installation of a tower at the site where transpiration will be measured. It is important that areas upwind of the measurement site be characteristic of vegetation at the site. Measurements may not be possible if the ground in the vicinity of the site is not level. Instruments for measuring vertical windspeed and air moisture content are installed on the tower. Separate instruments are used to monitor upward and downward eddies. Complex mathematics is used to take the instrument measurements and determine the net amount of water moving up from the ground surface. The upward moving water vapor is the result of transpiration.

Combined Evapotranspiration

It can be relatively straightforward to measure evaporation from an open water body such as a lake. However, measuring transpiration separately from evaporation over vegetation is very difficult. Consider that the trees, grass, or crops will be transpiring during daylight hours. Also, any water on the ground surface between the plants will be evaporating. Any measurement techniques will record the sum of evaporation and transpiration. In most cases it is not important in the context of hydrologic simulation to be able to separate the two distinct processes. The important component of the hydrologic cycle is the water that is removed from the soil and returned to the atmosphere. Therefore, an inability to measure the evaporation and transpiration separately is not a limitation in hydrologic simulation. It is almost always the case that evaporation and transpiration are combined and termed evapotranspiration.