Evaporation, Transpiration & Evapotranspiration Lecture Notes PDF

EVAPORATION, TRANSPIRATION & EVAPOTRANSPIRATION  Evaporation is the process during which a liquid changes into gas. Water changes to vapour through the absorption of heat One of the fundamental component of hydrological cycle Essential requirements in the process are 1. The source of...

EVAPORATION, TRANSPIRATION & EVAPOTRANSPIRATION  Evaporation is the process during which a liquid changes into gas. Water changes to vapour through the absorption of heat One of the fundamental component of hydrological cycle Essential requirements in the process are 1. The source of energy to vaporize the liquid water (solar or wind) 2. The presence of gradient of concentration between the evaporating surface and the surrounding air.  Evaporation is defined as a function of the differences in the vapour pressure of the water and the vapour pressure of the air. e ed) f(u) E=( s- E = evaporation es = saturation vapour pressure at the temperature of evaporating surface ed = saturation vapour pressure at the dew point temperature of the atmosphere f(u) = a function of the wind velocity. 1. Degree of saturation of surface 2. Temperature of surface and air 3. Humidity 4. Wind velocity 5. Vegetation cover  PAN EVAPORIMETER PICHE EVAPORIMETER  Transpiration is the process by which water vapour leaves the living plant body and enters the atmosphere. It involves continuous flow of water from soil in to plant and out through stomata (leaves) to the atmosphere. Basically an evaporation process. Transpiration Ratio: The amount of water transpired by a crop in its growth to produce unit weight of dry matter. 1. Climate 1. Light intensity 2. Atmospheric vapour pressure 3. Temperature 4. Wind 2. Soil 1. Availability of water 3. Plant factors 1. Extent and efficiency of root system 2. Leaf area 3. Leaf arrangement & structure 4. Stomatal behaviour  A potometer sometimes known transpirometer is a device used for measuring the rate transpiration. Types: 1. Ganong's Potometer 2. Darwin's Potometer 3. Gaurrea's Potometer 4. farmer potometer POTOMETER  Evapotranspiration (ET) is the quantity of water transpired by the plants during their growth or retained in plant tissue, plus the moisture evaporated from the surface of the soil and the vegetation. Michel (1978) It accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies.  POTENTIAL EVAPOTRANSPIRATION (PET) Theoretical amount of moisture that could be lost from the surface to the atmosphere if it were available. The amount of moisture which, if available, would be removed from a given land area by evapotranspiration. Expressed in units of water depth.  EFFECTIVE EVAPOTRANSPIRATION (EET) Actual amount of water lost due to evapotranspiration from the soil along with actively growing plant or crop. depends upon plant and soil characteristics, and upon the amount of available water in the soil. 1. Lysimeter experiment 2. Field experimental plots 3. Soil moisture depletion studies 4. Water balance/budget method 5. Eddy covariance 6. By using US-open pan evaporimeter 7. Energy balance Lysimeter is adevice in which a volume of soil planted with vegetation is located in a container to isolate it hydrologically from the surronding soil. Having a weighing device and a drainage system, which permit continuous measurement of excess water and draining below the root zone and plant water use, and hence evapotranspiration. The amount of water lost by evapotranspiration can be worked out by calculating the difference between the weight before and after the precipitation input. ET=P+(I–D)+S WHERE, ET = EVAPOTRANSPIRATION P = PRECIPITATION I = IRRIGATION WATER D = EXCESS WATER DRAINED FROM BOTTOM S = INCREASE OR DECREASE IN STORAGE OF SOIL MOISTURE Direct method of measuring evapotranspiration fast fluctuations of vertical wind speed are correlated with fast fluctuations in atmospheric water vapour density. Directly estimates the transfer of water vapour (evapotranspiration) from the land (or canopy) surface to the atmosphere. ETo = KC × E pan where, ETo : reference crop evapotranspiration KC: crop coefficient E pan : pan evaporation Formula – ET=P–Q– S-ΔD where, ΔS= watershed storage variation (mm): Send–Sbeginning P = Precipitation (mm) Q = Stream flow (mm) ΔD = Seepage out – seepage in (mm) ET = evaporation and transpiration (mm) FORMULA- Rn - G - H = λET Where, Rn : Net surface radiation flux density (Wm-2) G : Ground heat flux density (Wm-2) H : Sensible heat flux density (Wm-2) λET : Latent heat flux density (Wm-2) λ : Latent heat of vaporization of water (Jkg-1)  When a surface evaporates, it looses energy and cools itself. It is that cooling that can be observed from space. Satellites can map the infrared heat radiated from Earth, thus enabling to distinguish the cool surfaces from the warm surfaces.  summer winter Satellite map of evapotranspiration of whole world in the season of winter and summer Energy availability - The more energy available, the greater the rate of Evapotranspiration. It takes about 600 calories of heat energy to change 1 gram of liquid water into a gas. Humidity gradient - The rate and quantity of water vapour entering into the atmosphere both become higher in drier air. Water availability - Evapotranspiration cannot occur if water is not available. Wind speed – higher the wind speed, greater will the rate of evapotranspiration. Physical attributes of the vegetation - factors as vegetative cover, plant height, leaf area index and leaf shape and the reflectivity of plant surfaces can affect rates of evapotranspiration. Soil characteristics - Soil characteristics that can affect evapotranspiration include its heat capacity, and soil chemistry and albedo.