Evapotranspiration
Evapotranspiration (ET) is a term used to refer to the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpiration (movement of water from the soil, through roots and bodies of vegetation, and then into the air). Evapotranspiration is an important part of the water cycle, and measurement of it plays a key role in agricultural irrigation and water resource management. Definition. Evapotranspiration is a combination of evaporation and transpiration, measured in order to better  understand crop water requirements, irrigation scheduling, and watershed management. The two key components of evapotranspiration are: Evapotranspiration is typically measured in millimeters of water per unit of time. Globally, it is estimated that between three-fifths and three-quarters of land precipitation is returned to the atmosphere via evapotranspiration. Factors that impact evapotranspiration levels. Levels of evapotranspiration in a given area are primarily controlled by the amount of water present, the amount of energy present (in the air and soil), and the ability of the atmosphere to take up water. Other factors. Vegetation type. Because water transpired through leaves comes  from the roots, plants with deep reaching roots can more constantly transpire water. Herbaceous plants generally transpire less than woody plants because they usually have less extensive foliage. Conifer forests tend to have higher rates of evapotranspiration than deciduous forests, particularly in the dormant and early spring seasons. This is primarily due to the enhanced amount of precipitation intercepted and evaporated by conifer foliage during these periods. Factors that affect evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, solar radiation, humidity, temperature, and wind. Isotope measurements indicate transpiration is the larger component of  evapotranspiration. Through evapotranspiration, forests may reduce water yield, except in unique ecosystems called cloud forests, and rainforests. Trees in cloud forests collect the liquid water in fog or low clouds onto their surface, which drips down to the ground. These trees still contribute to evapotranspiration, but often collect more water than they evaporate or transpire. In rainforests, water yield is increased (compared to cleared land in the same climatic zone) as evapotranspiration increases humidity within the forest (a portion of which returns quickly as precipitation experienced at ground level as rain). The density of the vegetation reduces temperatures at ground  level (thereby reducing losses due to surface evaporation), and reduces wind speed (thereby reducing the loss of airborne moisture). The combined effect results in increased surface stream flows and a higher ground water table whilst the rainforest is preserved. Clearing of rainforests frequently leads to desertification as ground level temperatures increase, vegetation cover is lost or intentionally destroyed by clearing and burning, soil moisture is reduced by wind, and soils are easily eroded by high wind and rainfall events. Soil and irrigation. In areas that are not irrigated, actual evapotranspiration is usually no greater than precipitation, with some buffer in  time depending on the soil's ability to hold water. It will usually be less because some water will be lost due to percolation or surface runoff. An exception is areas with high water tables, where capillary action can cause water from the groundwater to rise through the soil matrix to the surface. If potential evapotranspiration is greater than the actual precipitation, then soil will dry out, unless irrigation is used. Estimating evapotranspiration. Because atmospheric vapor flux is difficult to measure directly, evapotranspiration is typically estimated by one of several different methods. Indirect methods for estimating evapotranspiration. There are three  general approaches to estimate evapotranspiration indirectly. Catchment water balance. Evapotranspiration may be estimated by evaluating the water balance of a drainage basin. The water balance equation relates the change in water stored within the basin ("S") to its input and outputs: formula_1 The input is precipitation ("P") and the outputs are evapotranspiration (which is to be estimated), streamflow ("Q"), and groundwater recharge ("D"). If the change in storage, precipitation, streamflow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows: formula_2 Energy balance. A second methodology to estimate the actual  evapotranspiration is the use of the energy balance. formula_3 where λE is the energy needed to change the phase of water from liquid to gas, R is the net radiation, G is the soil heat flux and H is the sensible heat flux. Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved. The SEBAL and METRIC algorithms solve the energy balance at the earth's surface using satellite imagery. This allows for both actual and potential evapotranspiration to  be calculated on a pixel-by-pixel basis. Evapotranspiration is a key indicator for water management and irrigation performance. SEBAL and METRIC can map these key indicators in time and space, for days, weeks or years. Estimation from meteorological data. Given meteorological data like wind, temperature, and humidity, reference ET can be calculated. The most general and widely used equation for calculating reference ET is the Penman equation. The Penman-Monteith variation is recommended by the Food and Agriculture Organization and the American Society of Civil Engineers. The simpler Blaney-Criddle equation was popular in the Western United States for  many years but it is not as accurate in regions with higher humidities. Other equations for estimating evapotranspiration from meteorological data includes the Makkink, which is simple but must be calibrated to a specific location, and Hargreaves equations. To convert the reference evapotranspiration to actual crop evapotranspiration, a crop coefficient and a stress coefficient must be used. Crop coefficients as used in many hydrological models usually change along the year to accommodate to the fact that crops are seasonal and, in general, plants behave differently along the seasons: perennial plants mature over multiple seasons, and stress responses can significantly depend  upon many aspects of plant condition. Measuring evapotranspiration. There are two primary methods for taking direct measurements of evapotranspiration. Lysimeter. Evapotranspiration can be measured directly with a weighing lysimeter. In a lysimeter, the weight of a plant and associated soil, and any water added by precipitation or irrigation, is measured continuously. The change in storage of water in the soil is then modeled by measuring the change in weight. When used properly, this allows for precise measurement of evapotranspiration. Eddy covariance. In the eddy covariance technique, fast fluctuations of vertical wind speed are correlated with fast fluctuations in atmospheric water  vapor density. This directly estimates the transfer of water vapor (and therefore evapotranspiration) from the surface to the atmosphere. Potential evapotranspiration. Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired by a specific crop or ecosystem if there were sufficient water available. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapor from the ground up into the lower atmosphere. Often a value for the potential evapotranspiration is calculated at a nearby climatic station on a reference surface, conventionally short grass. This value is  called the reference evapotranspiration (ET). Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water. Evapotranspiration can never be greater than potential evapotranspiration, but can be lower if there is not enough water to be evaporated or plants are unable to transpire readily. Some US states utilize a full cover alfalfa reference crop that is 0.5 m in height, rather than the short green grass reference, due to the higher value of ET from the alfalfa reference. Potential evapotranspiration is higher in the summer, on less cloudy days, and closer to the equator, because of the higher  levels of solar radiation that provides the energy for evaporation. Potential evapotranspiration is also higher on windy days because the evaporated moisture can be quickly moved from the ground or plant surface, allowing more evaporation to fill its place. Potential evapotranspiration is expressed in terms of a depth of water, and can be graphed during the year (see figure). Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the soil type for bare soil, and the vegetation. Often a value for the potential evapotranspiration  is calculated at a nearby climate station on a reference surface, conventionally short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and precipitation is used in irrigation scheduling. Average annual potential evapotranspiration is often compared to average annual precipitation, P. The ratio of the two, P/PET, is the aridity index. A humid subtropical climate is a zone of climate characterized by hot and humid summers, and cold to mild winters. Subarctic regions  fall between 50°N and 70°N latitude, depending on local climates. Precipitation is low, and vegetation is characteristic of the coniferous/taiga forest.