In this article we will discuss about how to measure evapotranspiration in crop plants.
The concept of PET was given by Thornthwaite (1948) who defined it as evapo-transpiration from a large vegetation covered land surface with adequate moisture at all times. According to Dalton (1882), Evaporation is a function of the difference in the vapour pressure of the water and the vapour pressure of the air.
E = f (V.P.water – V.P.air)
Solar energy arrives at the upper limit of the earth’s atmosphere @ about 2 calories per minute per square centimeter.
The devices used to measure evapotranspiration are as follows:
1. Lysimeters:
It permit the specific measurement of ET, rainfall and water uptake by plants. Lysimeter are tanks, buried in the ground to measure the percolation of water through soils. Lysimeter are the most dependable means of directly measuring the evapo-transpiration rate.
But their installation must meet four requirements:
(i) Lysimeters itself should be very large and deep to reduce the boundary effect and to avoid restricted root development. For short crops lysimeter should be at least one cubic meter in volume.
(ii) The physical conditions within the lysimeter must be comparable to those outside.
(iii) Lysimeter will not be representative of the surrounding area if the crop in the Lysimeter is either taller, shorter, denser or thinner, or if lysimeter is on the periphery of non-cropped area. The effective area of a lysimeter is defined as the lysimeter Evapo- transpiration per unit area of the surrounding field.
(iv) Each lysimeter should have a guard ring area around it maintained under the same crop & moisture conditions in order to minimise the clothesline effect.
Lysimeter is of two types viz. – Non-weighing type lysimeter and weighing type.
Non-Weighing Type Lysimeter:
It is called drainage lysimeter. It operates on the principle that E.T. is equal to the amount of rainfall and irrigation water added to the system minus leaching.
ET = Rainfall + irrigation water – Leaching.
The drainage lysimeter is useful only in determining the PET rate and should be irrigated every four or five days, unless rainfall intervenes.
Weighing Type Lysimeter:
It gives more accurate estimation of ET, capable of measuring ET for a short period, diurnal pattern of ET, the phenomenon of midday wilt, short term variation of energy partition and the relationship between transpiration and soil moisture tension.
It measures the water balances i.e. water added water retained by soil and water lost through all sources – evaporation, transpiration and deep percolation. Effective rainfall is also measured. Therefore weighing type lysimeter are the most direct and accurate instrument for the determination of E.T.
2. Evaporimeters:
These are used for estimating the PET of various crops.
Most commonly used evaporimeters in India are:
(i) US Weather Bureau class. A pan and
(ii) Sunken screen evaporimeter.
(i) USWB Class I Pan:
Most widely used evaporation pan and also written as USPAN (United States pan). Pans have higher rates of evaporation than a large free water surface.
(ii) Sunken Screen Evaporimeter:
Dastane and Sharma (1968) at IARI developed a sunken screen evaporimeter to simplify the measuring device of E.T., they observed-
Consumptive Use of Water:
It is used to designate the losses due to evapotranspiration and the water used by plant for its metabolic activities. Since water used in the actual metabolic process is less than 1% of ET i.e. insignificant, the term consumptive use = Evapotranspiration.
CU = ET + water used in metabolic activities
Amount of water to be applied (in cm)
Water Use Efficiency:
The water utilisation by the crop is generally described as water use efficiency (kg/ha-cm) or q/ha. cm.
It may be defined in two ways:
(A) Crop Water Use Efficiency:
It is the ratio of crop yield (y) to the amount of water depleted by the crop in the process of evapo transpiration.
WUE = Y/ET
(B) Field Water Use Efficiency:
It is the ratio of crop yield (y) to the total amount of water used in the field (WR).
Increasing WUE in Unirrigated Areas or Low Rainfall Areas:
Principal objective is to conserve and utilise a large portion of the available rainfall for crop production.
To increase the productive utilisation of the atmospheric precipitation naturally reaching a given area, the following practices are required to adopt:
1. Maximum absorption of the precipitation into the ground i.e. elimination of surface runoff.
The following practices are:
(a) Runoff control.
(b) Tillage
(c) Increasing infiltration.
(d) Water harvesting and recycling of water
2. Decreasing the agricultural unproductive loss of moisture namely evaporation, consumption of water by weeds and losses by sub-surface flow and deep percolation.
The following practices are:
(a) Weed control.
(b) Timely harvest of rainy season crops.
(c) Mulching.
(d) Tillage.
3. Increasing the Moisture Reserve of the Plants:
The practices which increase effective utilization of rainwater and stored soil moisture:
(a) Growing suitable crops and their varieties.
(b) By adopting suitable cropping sequence.
(c) Increasing root depth of crops by removal of hard pan and fertilization.
(d) Growing intercrops of different rooting depths.
(e) Seeding technique and plant stand- Proper placement of seeds and fertilizers become important in winter season when crops are to grow on stored soil moisture.