Evapotranspiration1 - Evapotranspiration The sum of the...

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1/23/2012 1 Evapotranspiration: The sum of the processes of evaporation and transpiration. On land, it is often difficult to separate out the loss of water by evaporation and transpiration, therefore the two are combined into one term. Evaporation water passes from a liquid to a vapor state. Occurs from the surface of a water body such as the ocean, lake, river, or canal. Also occurs from the land surface (depression storage), upper soil zone, and from water captured on the leaves of plants captured on the leaves of plants. Transpiration : Water loss from plants through their stomata (small openings on the underside of their leaves. Movement of water from the soil or a water body through a plant to the atmosphere. Rate of ET is controlled by 3 factors: •1 ) Meteorological Factors •2 ) Plant morphology Stomatal resistance, leaf area index, plant height, leaf type (broad leaf vs grasses). In general, tall, broad leaf plants tend to have higher ET rates. •3 ) Water availability ET cannot occur if water is not available. Meteorological Factors Energy availability . The more energy available the greater the rate of evapotranspiration. Remember that the latent heat of vaporization requires about 583 calories to vaporize 1 g of water at 25 o C. This is the amount of energy required to break the hydrogen bonds between water molecules. Most of the world’s evaporation occurs within the tropics where the maximum amount of energy from the sun is received. Humidity. Evaporation amount and rate is higher in drier air. Wind speed immediately above the surface. The process of evapotranspiration moves water vapor from ground or water surfaces to an adjacent shallow layer that is only a few centimeters thick. When this layer becomes saturated, ET stops. Wind can remove this layer replacing it with drier air which increases the potential for ET. Physical Properties of Evaporation • Evaporation follows Fick’s first Law: A diffusing substance moves from an area of high concentration to lower concentration at a rate α to the concentration gradient.
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1/23/2012 2 Water molecules are attracted
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Evapotranspiration1 - Evapotranspiration The sum of the...

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