Measurement of ET

A. Direct Method:

1. Lysimeter Experiment:

• A lysimeter can be defined as the device in which a volume of soil planted with vegetation is located in a container to isolate it hydrologically from the surrounding.
• Lysimeter studies involves the growing of the crops in large container and measuring their water loss and gains.
• It is of two types:

a. Non-weighing type and

b. Weighing type

• Non-weighing lysimeter gives less accurate results compared to weighing type.
• For accurate and short period estimate, weighing type lysimeters are used.

Limitations of lysimeter

• Reproduction of physical conditions such as temperature, water table, soil texture and density etc within the lysimeter comparable to those outside in the field.
• In some cases, the temperature of lysimeter is raised to such an extent that air conditioning of the whole system becomes a necessity.

2. Field Experiment plots:

• Measurement of water supplies to the field and changes in the moisture contents of field plots are sometimes more dependable for computing seasonal water requirement of the crops than measurement with small tanks (lysimeter).

WR= IR + ER+ £ⁿ _t=1  Ai. Di

Where, IR = total irrigation water supplied , mm

            ER= Seasonal effective rainfall, mm

            Mb_i = Moisture % at the beginning of the season

            Me_i = Moisture % at the end of the season

            Ai= Apparent sp. Gravity of the soil

            Di = Depth of soil within root zone, mm

            n = No. of soil layers in the root zone, D.

Conditions

1. There should be of uniform soil.
2. Ground water doesn’t affect in the soil moisture of the root zone.
3. This method requires that the amount applied to the field is measured accurately.

Limitations

• Doesn’t provide information on intermediate soil moisture conditions, short term use, profile use, deep percolation losses and peak use rate of the crop.

3. Soil Moisture depletion studies:

• It is applied to determine the CU of the irrigated field crops grown on fairly uniform soils when the depth to the ground water is such that it will not influence the soil moisture fluctuation within the root zone.

Conditions

1. Field should be of uniform type soil.
2. Ground water doesn’t affect in the soil moisture of the root zone.

U = £ⁿ _t=1  Ai. Di

Where, U = water use from the root zone for successive sampling period or within 1 irrigation cycle, mm.

             n = no. of soil layers in the root zone depth

             M1_i = Soil moisture % at the time of 1^st sampling

              M2_i = Soil moisture % at the 2^nd sampling

             A_i = Apparent Sp. Gravity of the i^th layer of the soil

             D_i = Depth of the i^th layer of the soil, mm.

Note: Seasonal CU (CU= EU) is calculated by summing the CU values of each sampling interval. A correction is made by adding PET values for accelerated water loss for the interval (s).

4. Water balance method:

• It is suitable for large areas (Water sheds) over long periods.
• It may be represented by the following hydrological equations:

Precipitation = Evapotranspiration + Surface runoff + Subsurface Drainage + change in soil water contents.

B. Estimation of ET from Evaporation data

• The standard US weather bureau class A open pan evaporimeter or the shunken screen open pan evaporimeter may be used for the measurement of ET.
• The relation between evapotranspiration and pan evaporation is given by the crop factor.

Evapotranspiration = Evaporation x Correction factor

• The value of crop factor for any crop depends upon its foliage characteristics, stage of growth, environment and geographical location.
• Consumptive values are low during the early stages of crop growth and increases as the plant approaches maturity.
• There is generally decline during the later periods.

1. Sunken Screen Pan Evaporimeter

• The Sunken screen evaporimeter developed by Sharma and Dastane (1968) provides a simple device to make reasonable estimates of consumptive use.
• As compared to the USWB pan in which the ratio between evaporation and evapotranspiration is 1.2 to 1.5, the same ratio in sunken pan screen evaporimeter was observed to be 0.95 to 1.05.

C. Estimation of ET from Climatological Data.

1. Penman Formula (1948) method.

• It is based on a combination of energy balance and sink strength.

E₀ = (Δ Q_n + YE_a) /( Δ + Y)

Where, E₀ = Evaporation from open surface ; mm/day.

             Δ = Slope of saturation vapour-pressure vs Temperature curve at the mean air temperature; mmhg per ⁰C.

E_a = Saturation vapour pressure of evaporating surface in mm/hg ate mean temperature T_a.

Qn = net radiation (mm of water).

• Since, the penman equation estimates evaporation from a free water surface, the results must be modified to provide evapotranspiration ratio (ET) estimates for crops.

i.e. PET = E₀ x K

where, K = crop use coefficient which value is supposed by penman as follows for short grasses.

2. Blaney- Criddle method:

• They observed that water consumption used by crops during their growing season was closely corelated with mean temperature and daylight hours.

i.e. U = KF = £KF = £ (K+P)/ 100

where, U= seasonal consumptive use of water by the crops for a given period.

            K= empirical seasonal consumptive use coefficient for the growing season

            F = sum of monthly consumptive use factors (F) for the growing season.

Also, F = (t x p)/ 100

Where, t = mean monthly temperature, ⁰C.

             P = Monthly daylight hours expressed as % of daylight hours of the year in metric system.

3. Thornthwalte formula

• The formula was developed for the purpose of a rational classicification of the broad climatic pattern of the world.

i.e. e = 1.6 (10 t / I)^a

where, e = PET, cm/ month (When all months are 30 days at 12 hrs daytime)

            t= mean air temperature, ⁰C.

            I = Annual or seasonal; heat index.

           a = empirical component.

4. Christiansen Method:

• It is used to estimate pan evaporation from climatic data when reliable measure of pen evaporation data is not available.
• The data may not be reliable because of variation in size and shape of pans, their exposure, the presence or absence of algae in water, the specific method of measuring the loss of water from the pans and the protection against use by birds and animals.

E_v = K_ev. R. C_t. C_w. C_h.C_s.C_e.C_m

Where,

Ev = computed pan evaporation equal to class A pan evaporation.

K_ev = empirical constant

R = Extra-terrestrial radiation

C_t = Coefficient of temperature

C_w = ,,               ,,  wind velocity

C_h = ,,     ,,   relative humidity

C_s = ,,   ,, for percent of sunshine

C_e = ,,   ,,    elevation

C_m = monthly coefficient factor (0.9-1.10)