Slide 1

Evaporation, Transpiration, Interception and Depression Storage

Evaporation - is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation.

Aerosolof microscopic water droplets suspended in the air above a hot tea cup after that water vapor has sufficiently cooled and condensed.Water vaporis an invisiblegas, but thecloudsof condensed water droplets refract and disperse the sun light and so are visible.3

Evaporation ProcessHeat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212 F, 100 C) but evaporates much more slowly at the freezing point.

Factors Influencing Evaporation RateConcentration of the substance evaporating in the air - if the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.Concentration of other substances in the air - if the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.Flow rate of air - this is in part related to the concentration points above. If "fresh" air (i.e., air which is neither already saturated with the substance nor with other substances) is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation.

This is the result of theboundary layerat the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.6

Inter-molecular forces - the stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by theenthalpy of vaporazation.Pressure- evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.Surface Area -a substance that has a larger surface area will evaporate faster, as there are more surface molecules per unit of volume that are potentially able to escape.Temperatureof the substance - the higher the temperature of the substance the greater the kinetic energy of the molecules at its surface and therefore the faster the rate of their evaporation.

Transpiration- is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere.Transpiration is essentially evaporation of water from plant leaves.

Transpiration ProcessLeaf surfaces are dotted with pores calledstomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered byguard cellsand their stomatal accessory cells that open and close the pore. Transpiration occurs through the opening of the stomata to allow the diffusion of carbon dioxidegas from the air forphotosynthesis. Transpiration also cools plants, changes osmotic pressureof cells, and enablesmass flow ofmineral nutrientsand water from roots toshoots.

Atmospheric factors affecting transpirationTemperature -transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer due to stronger sunlight and warmer air masses. Higher temperatures cause the plant cells which control the openings (stoma) where water is released to the atmosphere to open, whereas colder temperatures cause the openings to close.Relative humidity-as the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.

Wind and air movement -increased movement of the air around a plant will result in a higher transpiration rate. This is somewhat related to the relative humidity of the air, in that as water transpires from a leaf, the water saturates the air surrounding the leaf. If there is no wind, the air around the leaf may not move very much, raising the humidity of the air around the leaf. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.Soil-moisture availability-When moisture is lacking, plants can begin to senesce (premature ageing, which can result in leaf loss) and transpire less water.

Type of plant - plants transpire water at different rates. Some plants which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants.

Interceptionrefers to precipitationthat does not reach the soil, but is instead intercepted by the leaves and branches of plants and the forest floor.when precipitation reaches the surface in vegetated areas, a certain percentage of it is retained on or intercepted by the vegetation.

Depression Storageis a particular area of land that retains water in its pits and depressions, thus preventing rainwater from flowing.