evopotranspiration Flashcards
Water balance components
INPUT = OUTPUT
P + I + CR = Rf + E + T + DP + ΔW
EVAPORATION
- Evaporation is defined as “A physical process in which liquid
water is converted into its vapour”. - process by which water moves out of the water surface or soil surface in the form of water vapour to atmosphere due to pressure gradient
- The essential requirement for evaporation process are:
- Source of heat energy to vaporize the water
- concentration gradient between the evaporating surface and surrounding air
IMPORTANCE OF EVAPORATION
It is an important process of hydrological cycle
* Modifies the micro-climate of a crop
* Evaporation from the soil is an important factor in deciding the irrigation requirement of a crop
factors affecting evaporation
- Environmental factors Water temperature
* the rate of evaporation increases with a rise in temperature.
Wind
* Wind velocity is directly proportional to evaporation from a fully exposed surface and vice versa.
Relative humidity
* When the air is dry and has low relative humidity, the evaporation will be greater
Pressure
* The evaporation is more at low pressure and vice versa.
- Water factors
Composition of water
- Dissolved salts and other impurities in water decreases the rate of
evaporation. - The rate of evaporation from the surface of the sea is less than that of fresh water in rivers.
Area of evaporation
- If two volumes of water are equal in two containers, evaporation will be
greater for the one having the larger exposed surface.
MEASUREMENT OF EVAPORATION
Use of ‘Evaporimeters’
- Most common is Open Pan Evaporimeter
- The amount of water lost by evaporation from the pan at any given interval of time is measured by adding known quantities of water to the pan and bringing it to the original level.
TRANSPIRATION
a. loss of water from aerial parts of the plant in the form of liquid/droplets is termed …………………
process by which water evaporates in the form of water vapour from living plant body especially from leaves to atmosphere.
* involves a continuous movement of water from soil to atmosphere
a. guttation
The rate of transpiration depends on:
* Supply of energy to vapourise the water, and
* The water vapour concentration gradient at atmosphere
* Transpiration may occur through three main sites in the
plant : 1) cuticle 2) lenticels and 3) stomata.
Importance
Translocation of water in the plants
* Translocation of minerals in the plants
* Lowering of temperature (cooling)
* Gaseous exchange
FACTORS AFFECTING TRANSPIRATION
- Environmental (external) Factors
Light
- The direct effect of light is on the opening and closing of stomata.
- The indirect effect of light is that increasing light intensity raises the temperature of leaf cells
Atmospheric humidity
* The rate of transpiration is almost inversely proportional to atmospheric humidity.
* However, as the air becomes dry, the rate of transpiration also increases
proportionately.
Air temperature
* Increase in the temperature results in opening of stomata.
* Temperature has significant effect on the permeability of the wall of the guard cells and therefore greatly affects the osmotic phenomenon.
Wind velocity
* The velocity of wind affects the rate of transpiration to a greater extent.
* Fast moving wind and air currents bring fresh and dry masses of air
in contact with leaf surfaces.
* So, higher the wind speed higher the transpiration.
- Plant (internal) Factors
- Some plants modify their structure and some adopt physiological modifications
to check the excess transpiration thereby withstanding drought.
Plant height
* The water need of a crop varies with its height. In general, the rate of transpiration of a tall crop will be more than when the crop is cut or clipped to half.
Availability of water to the plant
* If there is little water in the soil, the tendency for dehydration of leaf causes stomatal closure and a consequent fall in transpiration.
Leaf characteristics
- In some plants like cacti and other desert plants leaves are altogether absent
and their function taken up by the stem itself. - In case of Pines, Firs the leaf size is very much reduced. In such cases
reduction in leaf area brings about reduction in transpiration. - Some Graminaceae family plants roll up their leaves when exposed to bright sun and fast breeze. This causes reduction in the transpiration.
EVAPO-TRANSPIRATION (ET)
It denotes the quantity of water transpired by plants plus the moisture evaporated from the surface of the soil.
* very difficult to separate the losses due to evaporation and transpiration in a cropped field
- two losses are considered as water used for plant
growth
indicates all water consumed by the plants and the water evaporated from bare land and water surface in the area occupied by the crop plants.
Factors affecting ET
Solar radiation supplies energy for ET
* Wind creates water vapour concentration gradient
* Temperature increases ET rate
* Relative humidity which changes the ET rate due to changes in water vapour
gradient
Stage of the crop:
* Emergence and development - ET rate increase rapidly from low value and approaches its maximum.
* Maximum Vegetative phase - ET rate is maximum if abundant soil moisture is available.
* Maturity phase - ET rate begins to decrease.
* Rooting characters of crop plants
Crop (evapo)transpiration
Direct measurement in the field
* Lysimeter
HUMIDITY
Humidity
The amount of water vapour that is present in atmosphere is known as
atmospheric moisture or humidity.
Absolute humidity
The actual mass of water vapour present in a given volume of moist air. It is
expressed as grams of water vapour per cubic meter or cubic feet.
Specific humidity
Weight of water vapour per unit weight of moist air. It is expressed as grams of
water vapour per kilogram of air (g/kg).
Relative Humidity
The ratio between the amount of water vapour present in a given volume of air and the amount of water vapour required for saturation under fixed temperature and pressure. There are no units and this is expressed as percentage
Dew Point temperature
SOURCES OF ATMOSPHERIC MOISTURE
The temperature to which a given parcel of air must be cooled in order to become saturation at constant pressure and water vapour content. In this case, the invisible water vapour begins to condense into visible form like water droplets.
- Major sources are:
a. Evaporation
b. Transpiration
c. Evapo-transpiration
d. Water vapour due to combustion
MEASURING HUMIDITY
Dry bulb thermometer
Wet bulb thermometer
