Estimation of Evapotranspiration

Estimation of Evapotranspiration– A key to improve water productivity

N. ManikandanN. ManikandanScientist

ICAR – Indian Institute of Water ManagementBhubaneswar

Email: [email protected]

EVAPORATION EVAPORATION EVAPORATION EVAPORATION

The concept of evaporation put forth byDalton (in 1802), who is credited witha generalformula which predicts evaporationasa function of vapour pressure

E = C (es - ea)C is constant (involving WS)es = SVP at water surface temperatureea = AVP in the air above

• Controls:– Wind speed controls air turnover– Vapor saturation deficit controls uptake

a generalformula which predicts evaporationasa function of vapour pressure

�Process by which the phase of water is changed fromliquid to vapor.

�It occurs at the evaporating surface, the contact betweenwater body and overlaying air.

�Major source of energy - Direct solar radiation; to a lesserextent,ambientair temperature.extent,ambientair temperature.

�Important climatological parameters – Solar radiation, airtemperature, humidity and wind speed.

�If, soil is evaporating surface, the degree of shading of thecanopy and amount of water available at soil surface areimportant factors.

TRANSPIRATIONTRANSPIRATION

� Transpiration is the processby which plants transferwater from the root zone tothe leaf surface, where iteventually evaporates intoatmosphere.

� The vaporization occurswithin the leaf, namely inwithin the leaf, namely inthe intercellular spaces, andthe vapour exchange withthe atmosphere is controlledby the stomatal aperture.

� About 1% of the watertaken up by plants isactually consumed in theplant’s metabolic activity.

Evaporative cooling: The energy used for transpiration is takenfrom the leaf and given to the water molecules that have convertedto highly energetic gas molecules. These gas molecules and theirassociated energy are released into the atmosphere, cooling theplant.

Accessingnutrients from the soil: The water that enters the root

Why do plants transpire?

Accessingnutrients from the soil: The water that enters the rootcontains dissolved nutrients vital to plant growth. It is thoughtthat transpiration enhances nutrient uptake into plants.

Carbon dioxide entry: When a plant is transpiring, its stomata areopen, allowing gas exchange between the atmosphere and the leaf.Open stomata allowwater vapor to leave the leaf but also allowcarbon dioxide (CO2) to enter.

Unfortunately, much more water leaves the leaf than CO2 entersfor three reasons:

a) H2O molecules are smaller than CO2 molecules

b) CO2 is only about 0.036% of the atmosphere (and rising!)

c) CO2 has a much longer distance to travel to reach itsdestination in the chloroplast from the atmospherecompared todestination in the chloroplast from the atmospherecompared toH2O, which only has to move from the leaf cell surface to theatmosphere

Water uptake: Although only less than 5% of the water taken upby roots remains in the plant, that water is vital for plantstructure and function.

Water transport from soil to atmosphere through plant tissues

Factors affecting transpirationFactors affecting transpiration

• Plant parameters – Cuticle, stomata, boundary layer

• Meteorological factors – Radiation, temperature, humidity and wind

• Seasons

• Time of day

• Stage of crop growth

• Soil properties (water holding capacity, roots, depth)

EVAPOTRANSPIRATIONEVAPOTRANSPIRATION

Potential evapotranspiration (PET or ETp)

It is the maximum limit for water that can be lost through the processes ofevaporation andtranspiration in the form of water vapour assuming no control onwater supply.

In 1948 both Thornthwaite and Penman put forth the concept ofpotentialevapotranspiration.

Definition by Thornthwaite : It is the maximum rate of evapotranspiration from alarge areacompletelyand uniformly coveredwith growing vegetationand with anlarge areacompletelyand uniformly coveredwith growing vegetationand with anunlimited moisture supply.

Definition by Penman: Evapotranspiration from the actively growing short greenvegetation completely shading the ground and never short ofsoil moisture availability.

Definition by WMO : It is the maximum quantity of water capable of being lost aswater vapour in a given climate by s continuous stretch of vegetation covering thewhole ground when soil moisture is saturated.

� Generally, ET (or ETactual) < ETp, especially in humid climates

� Exceptions to this generally occur in semi-arid and arid regions

Factors affecting ETp:1. Radiation2. Wind3. Vapour pressure gradient4. Crop height5. Plant resistance to water flow5. Plant resistance to water flow6. Soil water availability7. Crop cover

Other plant effects that includes

a) Row direction i) wind patterns ii) shading patterns iii) Canopy roughness

b) Leaf type, size and shape

c) Leaf coatings i) Pubescences

d) Waxes i) Man-made ii) Natural

Reference crop evapotranspiration (ETReference crop evapotranspiration (EToo))

� Rate of evapotranspiration that occurs froma reference surface i.e.hypothetical grass reference crop with an assumed crop height of0.12 m, a fixed surface resistance of 70 s m-1 and an albedo of 0.23.

� The concept of the reference evapotranspiration was introduced byDorrenbos and Prutteto study the evaporative demand of theatmosphere independently of crop type, crop development andmanagementpractices.managementpractices.

� The only factors affectingETEToo are climatic parameters.

Evapotranspiration measurement

a. Field water balance method

∆S = (P+I+U) – (R+D+ET)

ET= P + I + U –∆S –R - D

Where,Where,• ∆S = Change in soil moisture storage in root zone• P = Precipitation• I = Irrigation• U = Upwind capillary flow of water from below to root zone• R = Run off• D = Downward drainage out of the root zone• ET = Evapotranspiration from crop plants

b) Soil moisture depletion method

n

U = ∑ {(M 1-M2)/100} x Bi x Di +ER

i=1

Where,

U = Moisture depleted from the soil at a certain interval (cm)

M 1= Moisture content (%) in the ith layer at the time of first sampling.M 1= Moisture content (%) in the ith layer at the time of first sampling.

M 2 = Moisture content (%) in the ith layer at the time of next sampling.

Bi = Bulk density of the layer (g/cm3)

Di = Depth of ith layer (cm)

ER = Effective rainfall (cm)

ET = U + CPE

c) U.S.W.B. Class-A Open Pan Evaporimeter

ET0 = Kpan x Eo

ETc = ET0 x KC

Where,

ET0 = Reference crop evapotranspiration

K pan = Pancoefficient (0.35 to 0.85)

Stage Crop Coefficient

2 leaves 0.10

4 leaves 0.18

6 leaves 0.35

8 leaves 0.51

10 leaves 0.88

12 leaves 0.88

14 leaves 1.01

Example of Crop Coefficient for Corn

K pan = Pancoefficient (0.35 to 0.85)

Eo = Pan evaporation

ETc = Crop evapotranspiration

Kc = Crop co-efficient

14 leaves 1.01

16 leaves 1.10

Silks 1.10

Blister 1.10

Dough 1.10

Beginning dent 1.10

Full dent 0.98

Black layer 0.60

Full maturity 0.10

Instruments to measure potential evapotranspiration

Lysimeters� Lysimeter is an instrument for the measurement of water content

change in soil.

� A lysimeter consists of an isolated block of soil, as nearly identicalas possible in its physical and chemical properties with thesurrounding soil.

� Lysimeters can be weighed to determine the amount of water addedto it by precipitation or irrigation and removed byevapotranspiration.

� Lysimeters are expensive devices but, since they provide the onlyabsolute measure of mass transfer froma nearly natural part ofearth’s surface, their use is very important in micrometeorology.

Types of lysimeters1. Gravimetric lysimeter

2. Volumetric lysimeter

1.Gravimetric lysimeter

It consists of large steel tank having a perforated metallic plate atbottom in order to form a hollow chamber below its bottom forfree drainage of excess water.

A tube extending upto the bottomof hollow chamber is installedin lysimeter for removal of percolated water. The tank is placedon the platform of weighing balance, which is installed in soil.

A dummy tank is placed in the gap near the head work ofweighing scale so as to prevent over heating of the sides of themain tank. This whole apparatus is enclosed in an outercontainer, which serve as wall support for the pit in whichlysimeter is installed.

2. Volumetric lysimeter

� Used to measure ET in terms of volume.

� Used for standing water crops while gravimetric lysimeterdoes not required standing water.

� On rain free days, the water added to the reserviour drum� On rain free days, the water added to the reserviour drum(inflow) is measured with the help of measuring jar.

Date Rainfall (mm)

Soil tank (Kg)

Changes in weight (Kg)

ET(mm/day)

Remarks

10.4.1711.4.17

0.0 1125.01116.0

+9.0 9 x 0.6 +0= 5.4

Non-rainy day

12.4.17 5.2 1116.01121.2

-5.2 -5.2 x 0.6 + 5.2= 2.1

Rainy day

Gravimetric lysimeter

ET =∆W x 0.6 + Rainfall

Volumetric lysimeterVolumetric lysimeter

ET = [Inflow – Outflow] x conversion factor + rainfall

Conversation factor 1cc = 0.000694 mm

Ex: Inflow : 3500 cc; Outflow: 2000 cc and rainfall = 0.0 mm

ET = (3500 – 2000) x 0.000694 + 0.0 = 1.04 mm

MINI-LYSIMETER

Hari Mohan Meena et al., 2015

Drum culture technique

Evapotarnspiration estimation

FAO Penman-Monteith method

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 (ms-1)

ea = Saturation vapor pressure (kPa)

ed =actual vapor pressure (kPa)

Δ = Slope vapor pressure curve (kPaoC-1)

γ = Psychometric constant (kPaoC-1)

Improving water use efficiency

Antitranspirants that cause partial stomatal closure will of course, cause somereduction in photosynthesis but should effect a considerably larger reduction intranspiration.

Antitranspirant materials are three types:

(a) Film-forming substances that block the escape of water vapor from the leaf

Examples:�Long chain alcohols (Hexadecanol) – used to reduce evaporation from open water bodies�Thick film antitranspirants – latex, waxes and plastic

(b) Chemical materials that induce stomatal closure

Examples:Type 1: Affecting the turgor of the stomate guard cell - Atrazine, HydroxysulfonatesType 2: Affect the permeability of cell membranes - Phenyl Mercuric Acid (PMA),Alkenylsuccinic acid

(c) Reflectant material that reduce the energy load on the leaf

Examples: Kaolinite, Diatomaceous earths and Aluminum silicates

Mulching to reduce ET

Yield, water use and water use efficiency in wheat under mulches

TreatmentsYield (Kg/ha) Water use

(mm)Water Use Efficiency

(kg/ha-mm)2004-

052005-

062004-

052005-

062004-

052005-

06Limited irrigation, 4199 3728 481 384 8.73 9.7Limited irrigation,No mulch

4199 3728 481 384 8.73 9.7

Transparentpolythene

5019 3870 421 400 12.09 9.65

Black polythene 4079 - 433 - 9.42 -Rice husk 5143 4100 402 398 12.79 10.29Adequate irrigation,No mulch

5675 4139 520 461 10.9 9.36

Debashis Chakrabortyet al., (2008)

Effect of trash mulch on yield, saving in irrigation water and irrigation water use efficiency in sugarcane

TreatmentCane yield (t/ha) Irrigation water applied

(ha-cm)Irrigation water use

efficiency(kg cane/ha-cm)

Demonstration

Farmers practice

Demonstration

Farmers practice

Demonstration

Farmers practice

Trashmulch

80.18 63.8 47.66 65.37 1682.21 975.96

Tuber yield, water use and water use efficiency of potato asTuber yield, water use and water use efficiency of potato asinfluenced by mulching and irrigations

Treatments Yield (Kg/ha) Water use (mm) Water Use Efficiency

(kg/ha-mm)Mulch No mulch Mulch No mulch Mulch No mulch

One irrigation 6000 3455 295 372 20.32 9.31Two irrigations 8990 6750 345 430 26.06 15.66Three irrigations 12805 9003 385 488 33.27 18.41Four irrigations 14925 11200 437 532 34.16 21.05

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