EVAPORATION evaporation This is the change of state of water from liquid to gas. About 600 calories of heat energy are required to evaporate 1 gram of water. Most of this energy comes the sun. The evaporation that takes place from the drainage basin comes from evaporating bodies such free water surfaces such as dams and lakes, the soil, snow, interception storage, the soil and surface storage depressions Factors influencing the process of evaporation. 1. Insolation Since evaporation is an energy exchange process, the amount of energy received by an evaporating body determines the rate of evaporation. Therefore, evaporation rates vary with:• • • • Latitude Seasons Time of day Sky conditions Latitude Lower latitude areas have high levels of insolation and so experience high evaporation rates. However, high latitude areas receive low levels of insolation, and so experience low evaporation rates. Seasons For the southern hemisphere savanna climate in Zimbabwe, evaporation rates are high in summer because of the longer days and concentrated insolation because of the high angle of the incident ray. However, in winter, evaporation rates in Zimbabwe midyear times, are lower because of the shorter days and low insolation because of the low angle of the incident ray. Time of day During the day, evaporation rates are high because of high insolation levels compared to the night times. Sky conditions When the skies are clear evaporation rates are higher because there is more solar radiation that is supplied than when the skies are overcast. The latter a lot of solar radiation is reflected away which reduces the amount of energy available to evaporate the liquid water. 2. Wind Speed Under gusty conditions, evaporation rates tend to be higher because the water vapour is quickly swept away from above the evaporating surface, creating unsaturated conditions that encourages more evaporation. However, under conditions, the evaporated particles, tend to accumulate above the evaporated body, creating water vapour saturated conditions that discourage further evaporation. Research How do human activities influence evaporation? TRANSPIRATION Is the evaporation of water from the internal structure of the leaf through the stomata. Plants lose their water those moments their guard cells open during photosynthesis and respiration. If the meteorological conditions outside the leaf allow, water evaporates during those moments when the guard cells are open. Transpiration is the principal mechanism by which water falling on continental areas is returned to the atmosphere, because only minute portions of water absorbed by the root systems remain in the plant to make plant tissue. The ratio is 1:800, i.e., 1 out of 800 are used to make plant tissue, 800 are lost as transpiration. Factors influencing transpiration. Vegetation type The Deciduous Savannah vegetation They are said to be deciduous because they shed off their leaves during the dry winter seasons to reduce water losses. They develop quite small leaves to reduce the surface area of the leaves that is exposed to sunlight & transpiration. The Xerophytes of deserts Some desert species have their stoma closed during the day when evaporation rates are high, only opening during the night when evaporation rates are low. Cactus develops thick waxy cuticles which reduce the amount of insolation penetrating the leaf. Some species have their main bodies buried below the soil, with a little tissue exposed to photosynthesise, all to stay away from the sun. Some grass species for example the irises, annual lilies have developed special adaptations for example, they thickened outer layers, their leaves in roll and develop many hairs to reduce the amount of insolation reaching the internal cells and causing transpiration. Meteorological conditions Latitude seasons Time of day Sky conditions Wind speed Influence of human activities on transpiration. Use of anti-transpirants It has been reported in the USA that mixing fatty alcohols into the soil esp. aqueous solution of silicon oil has the effect of reducing transpiration. Unfortunately, the use the chemical has been observed to retard plant growth, making the chemical s not the best transipirant. Deforestation Large scale tree removal for agricultural or industrial purposes has been blamed for the fall in moisture levels in Sub-Saharan regions of Africa. This has enhanced the human desertification. This is because there is a general decline in transpiration levels if massive deforestation takes place. Afforestation Even though there is so much zeal for afforestation or reforestation, there is very little evidence of serious follow ups on the woodlots planted to take care of the young trees. These programs would increase the plant cover density and ultimately increase the amount of transpiration. EVAPOTRANSIPIRATION When studying the hydrological cycle of the whole drainage basin, hydrologists are usually interested with the total water loss from the soil, snow, vegetation interception, free water surfaces and transpiration. This total water loss from the drainage basin to the atmosphere is what is called evapotranspiration. Formulae E= P-R/Q E-evapotranspiration P-precipitation R/Q-runoff Potential evapotranspiration This is the total water loss that would occur from the drainage basin that is fully covered with vegetation and whose soil moisture content is above field capacity. Field capacity is the soil moisture content when the soil is said to be saturated with water. Actual evapotranspiration This the total water loss from the drainage basin whose surface is not necessarily fully covered by vegetation and whose soil moisture content is not necessarily at field capacity. AET is that which is measured at any given point in time at a place. Factors influencing Evapotranspiration. Latitude Seasons Time of day Sky condition Wind speed Type of vegetation Human influence
