Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil-water balance in lysimeters are required to determine evapotranspiration.
The methods are often expensive, demanding in terms of accuracy of measurement and can only be fully exploited by well-trained research personnel. Although the methods are inappropriate for routine measurements, they remain important for the evaluation of ET estimates obtained by more indirect methods.
Such important indirect methods include:
1. Energy balance and microclimatological methods
2. Soil-water balance
3. Lysimeters
4. Empirical methods (ET computed from meteorological data).
1. Energy Balance and Microclimatological Methods:
Evaporation of water requires relatively large amounts of energy, either in the form of sensible heat or radiant energy. Therefore, evapotranspiration process is governed by energy exchange at the vegetation surface and is limited by the amount of energy available. Because of this limitation, it is possible to predict the evapotranspiration rate by applying the principle of energy conservation. The energy arriving at the surface must equal the energy leaving the surface for the same time period.
All fluxes of energy should be considered when deriving an energy balance equation.
The equation for an evaporating surface can be written as:
Rn – G – λET – H = 0
where, Rn = Net radiation
H = Sensible heat
G = Soil heat flux
λET = Latent heat flux.
The various terms can be either positive or negative. Positive Rn supplies energy to the surface and positive G, λET and H remove energy from the surface. In the above equation, only vertical fluxes are considered and the net rate at which energy is being transferred horizontally, by advection, is ignored.
Therefore, the equation is to be applied to large, extensive surfaces of homogeneous vegetation only. The equation is restricted to the four components: Rn, λET, H and G. Other energy terms, such as heat stored or released in the plant or energy used in metabolic activities, are not considered. These terms account for only a small fraction of the daily net radiation and can be considered negligible when compared with the other four components.
Latent heat flux (λET) representing the evapotranspiration fraction can be derived from the energy balance equation if all other components are known. Net radiation (Rn) and soil heat fluxes (G) can be measured or estimated from climatic parameters. Measurements of sensible heat (H) are however complex and cannot be easily obtained. H requires accurate measurement of temperature gradients above the surface.
Another method of estimating evapotranspiration is the mass transfer method. This approach considers vertical movement of small parcels of air (eddies) above a large homogeneous surface. The eddies transport material (water vapour) and energy (heat, momentum) from and towards the evaporating surface.
By assuming steady state conditions and that the eddy transfer coefficients for water vapour are proportional to those for heat and momentum; evapotranspiration rate can be computed from the vertical gradients of air temperature and water vapour via the Bowen ratio.
Other direct measurement methods use gradients of wind speed and water vapour. These methods and other methods such as eddy covariance require accurate measurement of vapour pressure and air temperature or wind speed at different levels above the surface. Therefore, their application is restricted to primarily research situations.
2. Soil-Water Balance:
An understanding of water balance is necessary to appreciate the role of various management strategies in minimising the losses and maximising the utilisation of water.
Soil-water balance, like a financial statement of income and expenditure, is an account of all quantities of water added, removed or stored in a given volume of soil during a given period of time.
Using the soil-water balance equation, one can identify periods of water stress/excesses which may have adverse affect on crop performance. This identification will help in adopting appropriate management practices to alleviate the constraint and increase the crop yields.
Soil-water balance equation in its simplest form of expression is:
Change in soil-water = Inputs of water – Losses of water
Inputs (addition) of water to the soil:
Water is usually added to the soil in three measurable ways – precipitation (P), irrigation (I) and contribution from the groundwater table (C). Contribution from the groundwater will be significant only if the groundwater table is near the surface.
Inputs of water can be presented as:
Water inputs = P + I + C
Loss (removal) of water:
Water is removed from the soil through evapotranspiration (ET) and deep drainage (D). Further, a part of the rainwater received at the soil surface may be lost as surface runoff (RO).
Losses of water from soil can then be represented as:
Water losses = ET + D + RO
Soil-water balance:
Principle of water balance method is shown in Fig 5.12:
Change in the soil-water content which is the difference between the water added and water withdrawn will be:
Change in soil water = (P + I + C) – (ET + D + RO)
Soil-water refers to the amount of water held in the root zone at a given time. The change in soil-water from one measurement to another depends on the contribution of components in the equation. Suppose the amount of water in the root zone at the beginning is M1 mm and at the end of a given period is M2 mm, thus the equation is expressed as:
M1- M2 = P + I + C – ET – D – RO
or M1 + P + I + C = ET + D + RO + M2
With the help of this equation one can compute any one unknown parameter in the equation if all others are known.
Quantitative data on precipitation (P) evapotranspiration (ET), deep drainage (D) and soil moisture at a given time (M1 or M2) for different locations and for different practices are useful for selecting appropriate water management strategies.
Soil-Water Balance Computation:
An example using the soil-water balance equation to appreciate the usefulness of this model is presented below:
Given:
Soil = Vertisol, crop = sorghum, period = 01 to 31 Aug, area = 2 ha
Soil moisture in the profile on Aug 01 (Ml) = 300 mm
Precipitation or rainfall (P) = 70 mm
Irrigation (I) = nil
Contribution from groundwater (C) = nil
Runoff of 200 cubic m from 2 ha field (R) = 10 mm
Deep drainage (D) = nil
Soil-moisture in the profile on Aug 31 (M2) = 250 mm.
Estimate:
Evapotranspiration (ET) from the field during 01 to 31 Aug?
Solution:
M1 + P + I + C = ET + D + RO + M2
300 + 70 + 0 + 0 = ET + 0 + 10 + 250
ET = 370 – 260 mm = 110 mm.
Thus, evapotranspiration, which is difficult to be measured, could be estimated using the soil-water balance equation.
3. Lysimeters:
Different terms in the soil-water balance equation can be determined with greater accuracy with lysimeters, where the crop grows in isolated tanks filled with either disturbed or undisturbed soil.
The term lysimeter is a combination of the Greek words “lusis” = solution and “metron” = measure and the original aim was to measure soil leaching. The first lysimeter was used by De La Hire in 1688.
Lysimeters are either weighable or non-weighable; weighable lysimeters provide information about the change of water storage W for any time period; non-weighable lysimeters collect only the water percolating from the soil column.
The original purpose—to determine transport and leaching losses of solutes—is not the only one; lysimeters are also used for determining actual evapotranspiration and groundwater recharge and therefore for setting up a water-balance.
In precision weighing lysimeters, where the water loss is directly measured by the change of mass, evapotranspiration can be obtained with an accuracy of a few hundredths of a millimeter and small time periods such as an hour can be considered. In non-weighing lysimeters the evapotranspiration for a given time period is determined by deducting the drainage water, collected at the bottom of the lysimeters, from the total water input.
A requirement of lysimeters is that the vegetation both inside and immediately outside of the lysimeter be perfectly matched (same height and leaf area index). This requirement has historically not been closely adhered to in a majority of lysimeter studies and has resulted in severely erroneous and unrepresentative ETC and Kc data. As lysimeters are difficult and expensive to construct and as their operation and maintenance require special care, their use is limited to specific research purposes.
4. Empirical Methods:
It is necessary to conduct field experiments for precise data on crop water requirements. In view of the difficulties associated with direct measurement of crop water requirements, empirical methodologies have been developed to predict the water requirements, primarily, based on climatological data and crop factors. The FAO group of scientists screened 31 empirical formulae for predicting the ET and recommended four for use under different climatic conditions.
1. Blaney-Criddle method
2. Radiation method
3. Modified Penman method
4. Pan evaporation method.
Three major steps involved in the estimation of ET are:
1. Estimation of PET or reference evapotranspiration (ET0)
2. Determination of crop coefficients (kc)
3. Making adjustments to location specific crop environment.
Input Data Required for Estimating ET0:
The choice of prediction method for the determination of ET0 is primarily determined by the available climatic data. Minimum data requirements for each of the four recommended methods are given in Table 5.5.
During the past four decades, the FAO has played an active role in developing guidelines and methodologies for calculating crop water requirements.
Climatological Nomenclature:
When measured data of climate is not used as input data but only general levels of climatic variables are indicated, the nomenclature to be used is:
Temperature:
Hot. Tmean> 30°C Tmean = (Tmax + Tmin )/2
Cool: Tmean<15°C
Humidity:
RHmin or minimum relative humidity
Low < 20%; Medium 20-25%; High > 50%,
Dry < 20%; Humid > 70%.
RHmin is the lowest humidity during daytime and is reached usually at 14.00 to 16.00 h. From hygrograph or wet and dry bulb thermometer, for rough estimation purposes when read at 12.00 h, subtract 5 to 10 for humid climates and up to 30 for desert climates.
RHmean or nearer relative humidity
Low < 40%, Medium to low 40-45%
Medium to high 55-70%; High > 70%
RHmean is the average of maximum and minimum relative humidity
Or RHmean = (RHmax + RHmin)/2.
For most climates, RHmin varies considerably. RHmax equals 90-100% for humid climates, equals 80-100% for semiarid climates where Tmin is 20-25°C lower than Tmax. In arid areas, RHmax may be 25-40% when Tmin is 15°C lower than Tmax.
Radiation (Ra):
where, n = Daily actual bright sunshine hours
N = Daily maximum possible sunshine hours.
This is used for the adjustment factor (C) of the Blaney-Criddle equation.
n/N 0.8: near bright sunshine all day
n/N 0.6-0.8: 40% of daytime hrs full cloudiness or partially clouded for 70% of daytime hours.
Mean of several cloudiness observations per day on percentage or segments of sky covered by clouds.
4 Oktas: 50 per cent of the sky covered all daytime hours or half of daytime hours the sky is fully clouded.
1.5 Oktas: Less than 20 per cent of the sky covered all daytime h by clouds or each day the sky has a full cloud cover for about 2 h.
The calculation procedure included in FAO IDP 24 and FAO IDP 33 for ETc consists of:
(i) Identifying the crop growth stages, determining their lengths and selecting the corresponding Kc
(ii) Adjusting the selected Kc coefficients for frequency of wetting or climatic conditions during the stage
(iii) Constructing the crop coefficient curve (allowing one to determine Kc values for any period during the growing period)
(iv) Calculating ETc as the product of ETo and Kc.
FAO IDP 24 and FAO IDP 33 provide a standardised range of crop coefficients for a large number of crops.
Blaney-Criddle Method:
The original Blaney-Criddle prediction method for determining ET0 was modified to improve the accuracy.
ET0 = C [P (0.46T + 8)]
where, ET0 = reference evapotranspiration (mm day-1) for the month considered
C = adjustment factor depending on RHmin, daytime wind velocity and ratio of actual sunshine h to maximum possible sunshine h
T = mean daily temperature (°C) for the month under consideration
P = mean daily percentage of total annual daytime h.
Average of forenoon and afternoon relative humidity for this method is classified low with RH less than 20 per cent, 20-50 as medium, more than 50 per cent as high. If wind velocity data are not available, indications in Table 5.6 can be used for rough estimations.
(For rough estimation purposes, sum of several wind-speed observations divided by number of readings in m s-1 are multiplied by 86.4 to give wind run in km day-1).
The n/N ratios are classified as low (< 0.6), medium (0.6 to 0.8) and high (> 0.8). Temperature is the only measured factor for ET0 estimation.
Example:
Radiation Method:
Strong dependence of ET0 on radiation has given rise to formulae based on solar radiation. This method requires direct measurement of duration of bright sunshine hours, general levels of humidity and wind velocity.
The relationship to calculate ET0 from temperature and radiation data is given by:
ET0 = C (W x Rs)
where, Rs = Measured mean incoming short wave radiation (m day-1) or obtained from Rs = (0.25 + 0.50 x n/N) Ra, where, Ra is extraterrestrial radiation (mm day-1), N, maximum possible sunshine duration (h day-1) and n, measured mean actual sunshine duration (h day1)
W = Temperature and altitude dependent weighing factor
C = Adjustment factor made graphically on W. Rs using estimated values of RHmean and U daytime.
Example:
Pan Evaporation Method:
Evaporation from pans provides measurement of integrated effect of radiation, wind, temperature and humidity on evaporation from open water surface. Pan and its environment influence measured evaporation, especially, when it is placed in cropped field instead of open fallow field.
To relate pan evaporation to ET0 empirically derived pan coefficients are suggested to account for climate, type of pan and pan environment. The USWB Class A Open Pan is most commonly used for measuring evaporation.
The ET0 representing the mean value (mm day-1) over the period considered could be obtained by:
ET0 = Kpan x Epan
where, Epan = evaporation (mm day-1) from Class A Pan.
K pan = pan coefficient.
Example:
Month = June
Epan = 9 mm day-1
RHmean = medium
Umean = moderate
Solution:
Pan evaporation method required measured pan evaporation, estimated mean RH, estimated wind velocity (U in km day-1 at 2 m ht) and pan in cropped or fallow field. It is the simplest, inexpensive and fairly reliable for estimating the ET0.
Modified Penman Method:
It gives fairly satisfactory results for predicting the effect of climate on ET0 as it utilises almost all the meteorological parameters associated with evapotranspiration.
Climate data required are:
1. Mean temperature (T°C)
2. Mean relative humidity (RH%)
3. Total wind run (U km day-1 at 2 m ht)
4. Mean actual sunshine duration (n h day-1) or mean radiation (Rs or Rn in mm day-1)
5. Measured or estimated mean maximum relative humidity (RHmax %), and
6. Mean daytime wind speed (U day in ms-1).
The ET0 (mm day-1) representing over the period under consideration can be obtained by:
ET0 = C [W x Rn + (1 – W) x f (U) x (ea – ed)]
where, Rn = Net radiation (mm day-1) or Rn = 7.5 Rs – Rn1, where Rs is incoming shortwave radiation (mm day-1), or obtained from Rs = (0.25 + 0.50 n/N) Ra. Ra is extraterrestrial radiation (mm day-1), n is mean actual sunshine duration (h day-1) and N is maximum possible sunshine duration (h day-1). Rnl is net long wave radiation ((mm day-1) a function of temperature f (T) of actual vapor pressure, f (ed) and sunshine duration f (n/N) or Rnl = f (T) x f (ed)
(ea-ed) = Vapour pressure deficit, the difference between saturation vapor pressure (ea) at Tmean (mb) and actual vapor pressure (ed)
where, ed = ea x RH/100
f (U) = Wind function or f (U) = 0.27 (1 + U/100) with U in km day-1 measured at 2 m ht
W = Temperature and altitude dependent weighting factor
C = Adjustment factor for the ratio U day/U night for RHmax and for Rs.
Example:
Altitude = 15.0°N
Latitude = 200 m
Tmean = 30°C
Day wind velocity = 15 km h-1
Night wind velocity = 12 km hr-1
Mean sunshine (n) h = 8 h day-1
RHmax = 60 per cent
RHmin = 40 per cent
Solution:
ET0 = C x [W x Rn + (1 – W) x f(U) x (ea – ed)]
= 1.05 x [0.78 x 4.85 + (1 – 0.78) x 1.15 x (25.44)]
= 1.05 x [3.783 + (0.22 x 1.15 x 25.44)]
= 1.05 x (3.783 + 6.436) = 1.05 x (10.219) Calc 10.73 mm day-1
Modified Penman and radiation methods offer the best results for periods as short as 10 days followed by pan evaporation method. Blaney-Criddle method is ideal for periods of one month or more in many climates.