Here is a compilation of essays on ‘Evapotranspiration’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Evapotranspiration’ especially written for school and college students.
Essay on Evapotranspiration
Essay Contents:
- Essay on the Introduction to Evapotranspiration
- Essay on the Meaning of Evapotranspiration
- Essay on the Concept of Evapotranspiration
- Essay on the Factors Affecting Evapotranspiration
- Essay on the Empirical Methods for Estimating Evapotranspiration
- Essay on the Instruments used for the Measurement of Evapotranspiration
1. Essay on the Introduction to Evapotranspiration:
While transpiration takes place, land area in which plants stand also lose moisture by evaporation of water from soil and water bodies. In hydrology and irrigation practice, it is found that evaporation and transpiration processes can be considered advantageously under one head as evapotranspiration.
The term consumptive use is also used to denote this loss by evapotranspiration. For a given set of atmospheric conditions, evapotranspiration obviously depends on availability of water. If sufficient moisture is always available to completely meet the needs of vegetation fully covering the area, the resulting evapotranspiration is called potential evapotranspiration (PET).
Potential evapotranspiration no longer critically depends on soil and plant factors but depends essentially on climatic factors. Real evapotranspiration occurring in a specific situation is called actual evapotranspiration (AET). If the water supply to plant is adequate, soil moisture will be at field capacity and AET will be equal to PET.
If water supply is less than PET, soil dries out and the ratio AET/PET would then be less than unity. Decrease of the ratio AET/PET with available moisture depends upon the type of soil and rate of drying. Generally, for clayey soils, AET/PET ≈ 1.0 for nearly 50 per cent drop in available moisture. As can be expected, when soil moisture reaches permanent wilting point, AET reduces to zero.
In addition to physical factors influencing evaporation, as indicated under evaporation, the following additional plant and soil factors play important role in evapotranspiration:
1. Rooting characteristics of plants.
2. Physiological regulations and morphological modifications of plants.
3. Relative distribution of stomata on upper and lower leaf surfaces.
4. Aerodynamic roughness of leaf surfaces.
5. Duration of life cycle of the plant.
6. Available soil moisture in the root zone.
7. Water contribution from subsurface water tables.
8. Extent of exposed soil surface to direct action of solar radiation and wind.
The return flow of water to the atmosphere from moist soil, water bodies and vegetative surfaces is caused by desiccating capacity of air overlying them. The evaporative power of air may be defined as the potential of the air for evaporative losses, given 100 per cent opportunity.
It is the sum total of several factors concerned with evaporation, principally radiation, saturation deficit of air and wind speed. Evaporation loses recorded from pan evaporimeter gives rough indication of evaporative power of air. The evaporative power has significant influence on water needs of crops, fraction of rainfall going into storage and leaf water status for photosynthesis.
2. Essay on the Meaning of Evapotranspiration:
Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. Apart from water availability in the top soil, evaporation from a cropped soil is mainly determined by the fraction of solar radiation reaching the soil surface.
This fraction decreases over growing period as the crop develops and the crop canopy shades more and more of ground area. When the crop is small, water is predominately lost by soil evaporation, but once the crop is well developed and completely covers the soil, transpiration becomes the main process. At sowing nearly 100 per cent of ET comes from evaporation, while at full crop cover more than 90 per cent of ET comes from transpiration.
Evapotranspiration rate is normally expressed in millimeters (mm) per unit time. The rate expresses the amount of water lost from a cropped surface in units of water depth.
The time unit can be an hour, day, decade, month or even an entire growing period or year.
As one hectare has a surface of 10000 m2 and 1 mm is equal to 0.001 m, a loss of 1 mm of water corresponds to a loss of 10 m3 of water per hectare. In other words, 1 mm day-1 is equivalent to 10 m3 ha-1 day-1.
Water depths can also be expressed in terms of energy received per unit area. Energy refers to the energy or heat required to vapourise free water. This energy, known as the latent heat of vapourisation (λ), is a function of the water temperature. For example, at 20°C, λ is about 2.45 MJ kg-1.
In other words, 2.45 MI are needed to vapourise 1 kg or 0.001 m3 of water. Hence, an energy input of 2.45 MJ per m2 is able to vapourise 0.001 m or 1 mm of water, and therefore 1 mm of water is equivalent to 2.45 MJ m-2. Evapotranspiration rate expressed in units of MJ m-2 day-1 is represented by λET, the latent heat flux. Table 5.4 summarises the units used to express evapotranspiration rate and conversion factors.
Converting evaporation from one unit to another (example): On a summer day, net solar energy received at a lake reaches 15 MJ per square meter per day. If 80 per cent of the energy is used to vapourise water, how large could the depth of evaporation be?
From Table 5: 5 MJ m-2 day-1 = 0.408 mm day-1
Therefore, depth of evaporation = 0.8 x 15 MJ m-2 day-1
= 0.8 x 15 x 0.408 mm day-1
= 4.9 mm day-1
3. Essay on the Concept of Evapotranspiration:
Distinctions are made (Fig 5.11) between reference crop evapotranspiration (ET0), crop evapotranspiration under standard conditions (ETC) and crop evapotranspiration under non-standard conditions (ETC adj). ET0 is a climatic parameter expressing the evaporation power of the atmosphere. ETC refers to the evapotranspiration from excellently managed, large, well-watered fields that achieve full production under the given climatic conditions. Due to suboptimal crop management and environmental constraints that affect crop growth and limit evapotranspiration, ETC under non-standard conditions generally requires a correction.
a. Reference Crop Evapotranspiration (ETo):
Evapotranspiration rate from a reference surface, not short of water, is called the reference crop evapotranspiration or reference evapotranspiration and is denoted as ETo. Reference surface is a hypothetical grass reference crop with specific characteristics. Use of other denominations such as potential ET is strongly discouraged due to ambiguities in their definitions.
The concept of reference evapotranspiration was introduced to study the evaporative demand of the atmosphere independently of crop type, crop development and management practices. As water is abundantly available at the reference evapotranspiring surface, soil factors do not affect ET. Relating ET to a specific surface provides a reference to which ET from other surfaces can be related. It obviates the need to define a separate ET level for each crop and stage of growth. ET0 values measured or calculated at different locations or in different seasons are comparable as they refer to the ET from the same reference surface.
The only factors affecting ET0 are climatic parameters. Consequently, ET0 is a climatic parameter and can be computed from weather data. ET0 expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors. The FAO Penman-Monteith method is recommended as the sole method for determining ET0.
FAO Penman-Monteith method:
where, ET0 = Reference crop evapotranspiration (mm day-1)
Rn = Net radiation at the crop surface (MJ m-2 day-1)
G = soil heat flux density (MJ m-2 day-1)
T = Mean daily air temperature at 2 m height (0c)
u2 = Wind speed at 2 m height (m s-1)
es = Saturation vapour pressure (kPa)
ea = Actual vapour pressure (kPa)
es – ea = Saturation vapour deficit (kPa)
Δ = Slope of vapour pressure curve (kPa 0c-1)
γ = Psychrometric constant (kPa 0c-1).
Readers are advised to refer 5.7.4 Empirical Methods for detailed procedure on estimation of ET0.
b. Crop Evapotranspiration under Standard Conditions (ETc):
Crop evapotranspiration under standard conditions, denoted as ETc, is the evapotranspiration from disease free, well-fertilised crops, grown in large fields, under optimum soil water conditions and achieving full production under the given climatic conditions.
The reference crop evapotranspiration (ETc) has been redefined for more accurate estimation of crop water needs.
The redefined FAO Penman-Monteith crop evapotranspiration is:
‘Evapotranspiration from a hypothetical crop with an assumed height of 0.12 m having a surface resistance of 70 s m-1 and the albedo of 0.23, closely resembling the evaporation of an extension surface of green grass of uniform height, actively growing and adequately watered’.
In crop coefficient approach, crop evapotranspiration is calculating by multiplying reference crop evapotranspiration (ET0) by a crop coefficient, Kc:
ETc = ET0 x Kc
Amount of water required to compensate the evapotranspiration loss from the cropped field is defined as crop water requirement. Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration.
Irrigation water requirement basically represents the difference between the crop water requirement and effective precipitation. Irrigation water requirement also includes additional water for leaching of salts and to compensate for non- uniformity of water application.
c. Crop Evapotranspiration under Non-Standard Conditions (ETc adj):
Crop evapotranspiration under non-standard conditions (ETC adj) is the evapotranspiration from crops grown under management and environmental conditions that differ from the standard conditions. When cultivating crops in fields, real crop evapotranspiration may deviate from ETC due to non-optimal conditions such as the presence of pests and diseases, soil salinity, low soil fertility, water shortage or waterlogging. This may result in scanty plant growth, low plant density and may reduce the evapotranspiration rate below ETC.
Crop evapotranspiration under non-standard conditions is calculated by using a water stress coefficient Ks and/or by adjusting Kc for all kinds of other stresses and environmental constraints on crop evapotranspiration.
The effects of soil-water stress are described by multiplying the basal crop coefficient, Kcb by the water stress coefficient, Ks:
ETC adj = (Ks Kcb + Kg) ET0
Where the single crop coefficient is used, the effect of water stress is incorporated into Kc as:
ETC adj = Ks x Kc x ET0
Two other terms frequently mentioned in literature are actual evapotranspiration and potential evapotranspiration.
Actual evapotranspiration (AET or ETa) from a crop depends on weather and stomatal resistance. With decrease in soil water content, the stomata close and resistance to evapotranspiration is increased. Under constant weather conditions, actual evapotranspiration depends on the soil-water content. It would be higher immediately after irrigation (when water is freely available to the plant and the soil surface is wet) and would decline over time as the soil dries.
Potential evapotranspiration (PET or PE) is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply.
4. Essay on the Factors Affecting Evapotranspiration:
The three major factors (Fig 5.10) affecting evapotranspiration (ET) are:
I. Weather/climatic parameters
II. Crop characteristics
III. Management and environmental factors.
I. Weather Parameters:
i. Solar radiation:
Solar or thermal energy is necessary to evaporate water from both soil and plant surfaces. Hence, increased solar radiation increases atmospheric demand and hence increases evapotranspiration.
ii. Ambient temperature:
Increasing temperature increases the capacity of air to hold water (high vapour pressure deficit) which means greater atmospheric demand and hence greater ET.
iii. Relative humidity:
Greater the water content of air (RH), lesser will be the demand for water. As such, ET decreases with increase in relative humidity.
iv. Wind:
Transpiration occurs when water diffuses through stomata. Water vapour builds up around stomata when the air is still leading to water vapour build up on the canopy surface. In other words, diffusion pressure gradient from leaf to atmosphere decreases leading to reduction in ET losses.
v. Precipitation:
In general, ET from the cropped field increases with increase in available soil moisture due to increase in evaporation from the soil surface. However, the effect of soil-water content on ET varies with crop and is conditioned, primarily, by the type of the soil and its water retentive capacity, rooting characteristics and weather conditions determining the level of ET.
II. Crop Characteristics:
Crop characteristics influence the ET by affecting the resistance to water movement from soil to atmosphere.
i. Stomatal opening and closing:
In general, wider the stomata opening higher will be the loss of water through ET. Many factors influence stomatal opening and closing under field conditions, the major being light and soil moisture level. In most crops, light causes stomata to open. A low moisture level in the leaf causes guard cells to loose turgor, resulting in closure of the stomata.
ii. Stomatal number and size:
Majority of crop plants have stomata in both sides of the leaves. Stomatal number and size, which depends on genotype and environment, have much less effect on stomatal transpiration than their opening and closing.
iii. Leaf area:
In general, higher the leaf surface area, higher will be the ET. However, increase in water loss for each unit increase in the leaf area index is less.
iv. Adaptive mechanism:
Many plants exhibit mechanisms that favour reduced ET at times of water deficits. Sorghum and maize exhibit rolling of leaves at the time of water shortage.
v. Rooting depth and proliferation:
Deeper root system increases the water availability while the root proliferation increases water extraction from a unit volume of the soil before PWP is reached. As such, root depth and proliferation increases ET.
III. Management and Environmental factors:
i. Management factors:
Factors such as soil salinity, poor land fertility, limited application of fertilisers, presence of hard or impenetrable soil horizons, absence of plant protection and poor soil management may limit the crop development and reduce the evapotranspiration.
Other factors to be considered when assessing ET are ground cover, plant density and the soil-water content. The effect of soil-water content on ET is conditioned primarily by the
magnitude of the water deficit and the type of soil. On the other hand, too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.
When assessing the ET rate, additional consideration should be given to the range of management practices that act on the climatic and crop factors affecting the ET process.
ii. Environmental conditions:
Cultivation practices and the type of irrigation method can alter the microclimate, affect the crop characteristics or affect the wetting of the soil and crop surface. A windbreak reduces wind velocities and decreases the ET rate of the field directly beyond the barrier. The effect can be significant especially in windy, warm and dry conditions although evapotranspiration from the trees themselves may offset any reduction in the field.
Soil evaporation in a young orchard, where trees are widely spaced, can be reduced by using a well-designed drip or trickle irrigation system. The use of mulches, especially when the crop is small, is another way of substantially reducing soil evaporation. Antitranspirants, such as stomata-closing, film-forming or reflecting material, reduce the water losses from the crop and hence the transpiration rate.
5. Essay on the Empirical Methods for Estimating Evapotranspiration:
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.
(i) Blaney-Cridle method.
(ii) Radiation method.
(iii) Pan evaporation method.
(iv) Modified Penman 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 appropriate adjustments to location specific crop environment.
Input Data Required for Estimating ET0:
The choice of prediction 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 7.20.
Climatological Nomenclature:
Where 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
Humidity:
RHmin or minimum relative humidity
Low < 20%; Medium 20-25%; High > 50%
Dry < 20%; Humid > 70%.
RHmin is the lowest humidity during day time and is reached usually at 14.00 to 16.00 hours. From hygrograph or wet and dry bulb thermometer, for rough estimation purposes when read at 12.00 hour, 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
For most climates, RHmin varies considerably. RHmax equals 90-100% far humid climates, equals 80-100% for semi-arid 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 hour
N = daily maximum possible sunshine hour.
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 day time hour full cloudiness or partially clouded for 70% of day time hour.
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 day time hours or half of day time hours the sky is fully clouded.
1.5 Oktas:
Less than 20 per cent of the sky covered all day time hours by clouds or each day the sky has a full cloud cover for about 2 hours.
(i) Blaney-Criddle Method:
The original Blaney-Criddle prediction method for determining ETO was modified to improve the accuracy.
ET0 = C[P(0.46 + 8)]
where, ET0 = reference evapotranspiration (mm day-1) for the month considered
C = adjustment factor depending on RHmin, day time wind velocity and ratio of actual sunshine hour to maximum possible sunshine hour
T = mean daily temperature (°C) for the month under consideration
P = mean daily percentage of total annual day time hour.
Average of forenoon and afternoon relative humidity for this method is classified low with RH less than 20 per cent, 20-50 per cent as medium, more than 50 per cent as high. If wind velocity data are not available, indications in Table 7.21 can be used for rough estimations.
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 ETO estimation.
(ii) 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 hour, 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 shortwave radiation (mm day-1) or obtained from Rs = (0.25 + 0.50 x n/N) Ra, where, Ra is extra-terrestrial radiation (mm day-1), N, maximum possible sunshine duration (hr day-1) and n, measured mean actual sunshine duration (hr day-1)
W = temperature and altitude dependent weighing factor
C = adjustment factor made graphically on W.Rs using estimated values of RHmean and U day time.
(iii) 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 can be obtained by
ET0 = Kpan x Epan
where, Epan = evaporation (mm day-1) from Class A Pan
Kpan = pan coefficient.
(iv) 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 hr day-1) or mean radiation (Rx or Rn in mm day-1).
5. Measured or estimated mean maximum relative humidity (RHmax%).
6. Mean day time 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 – Rnl, 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 (hr day-1) and N is maximum possible sunshine duration (hr day-1). Rni is net long-wave radiation ((mm day-1) a function of temperature f(T) of actual vapour pressure, f(ed) and sunshine duration f(n/N) or Rnt = f(T) x f(ed)
(ea – ed) = vapour pressure deficit, the difference between saturation vapour pressure (ea) at Tmean(mb) and actual vapour pressure (ed) where ed = ea x RH/100
f(U) = wind function of 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
6. Essay on the Instruments used for Measurement of Evapotranspiration:
(i) Lysimeters:
Lysimeters provide the direct measurement of water flux from vegetative surface. Hence, they provide a standard against which other methods can be tested and calibrated. Lysimeter is a large tank filled with soil and supported on a weighing mechanism (Fig 7.26).
Rectangular units of 4.0 m2 seem practical and satisfactory for most crops. Total depth should be 100 cm for shallow rooted crops and 150 cm for deep rooted crops. In general, 50 per cent available soil moisture depletion in root zone should not be exceeded. Some plants as in the surrounding area are grown in the lysimeter.
To provide reliable measurements of ET, lysimeter should meet certain standards:
1. It should be constructed such that the moisture relationship inside the lysimeter corresponds closely to these of soil under natural conditions.
2. Lysimeter should be sufficiently deep to extend well below the root zone to maintain moisture regime as in surrounding area.
3. Lysimeter should be treated in the same way as that of the buffer surrounding area and should not receive preferential treatment.
4. Ratio of the wall surface area to the enclosed lysimeter area should be small to avoid small scale advection from un-cropped area.
Evaporation measurements are obtained from weight changes of the lysimeter, using water balance equation.
ET = Weight change + water added – percolation
There are two types of lysimeters:
(i) Weighing type and
(ii) Drainage type.
In the drainage type, the inflows and drainage are measured, but changes in storage within the soil are not measured. Weighing type lysimeters allow an evaluation of each element (rainfall, irrigation, runoff, drainage and ET) in water balance equation.
(ii) Evaporimeters:
Evaporation from a free water surface in open pan is widely used as an indicator of crop evapotranspiration. Pans are widely used all over the world because of low investment and easy operation. The USWB Class A Open pan is widely used as standard pan.
Pans installed above ground receive additional energy due to absorption of radiation through the pan walls and transfer of heat between air and pan wall leading to higher ET rates than are calculated from meteorological data. Sunken pans are expected to give more reliable values, but heat exchange between pan wall and surrounding soil limits the accuracy of results.
Example:
At the beginning of a week, depth of water in an evaporation pan of 1.2 m dia was 7.75 cm. Rainfall during the week was 3.80 cm and 2.50 cm water was removed from the pan to keep the water depth within a fixed range. At the end of week, the gauge indicated a depth of 8.32 cm water in the pan. If the pan coefficient is 0.75, estimate the evaporation during the week from the surface of a reservoir under similar atmosphere conditions?
Solution:
Depth at the beginning of the week = 7.75 cm
Rain during the week = 3.80 cm
Total depth = 11.55 cm
Theoretical depth after removing 2.5 cm water = 9.05 cm
Depth indicated by gauge = 8.32 cm
Water lost in evaporation during the week = 0.73 cm
Lake evaporation = 0.73 x 0.75 = 0.55 cm
(iii) Atmometers:
These are porous ceramic or paper evaporating surfaces, generally, coloured either black or white. The evaporating surface is continuously supplied with water. There is linear relationship between water loss from atmosphere and evapotranspiration.