<P> Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere . Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies . Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves . Evapotranspiration is an important part of the water cycle . An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator . </P> <P> Potential evapotranspiration (PET), is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop (grass), completely shading the ground, of uniform height and with adequate water status in the soil profile . It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour 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 . 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 . </P> <P> Evapotranspiration is a significant water loss from drainage basins . Types of vegetation and land use significantly affect evapotranspiration, and therefore the amount of water leaving a drainage basin . 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 . </P> <P> Through evapotranspiration, forests reduce water yield, except in unique ecosystems called cloud forests . 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 . </P>

Why is there a seasonal change in the amount of et