This presentation will provide knowledge on measurement of evapotranspiration by different methods.

1. Measurement of Evapotranspiration
Vijitha Vikneshwaran
Lecturer temporary
Faculty of Technology
University of Jaffna

2. Introduction
• Evapotranspiration (ET) is the combination of two
separate processes whereby water is lost from the,
• Soil surface by evaporation and
• Crop by transpiration
• It is one of the most important components of water
cycle.
• The accurate measurement of ET in field is importance
for,
• Quantifying soil hydrological processes
• Making appropriate decisions regarding irrigation
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3. • Evaporation and transpiration occur simultaneously.
• There is no easy way of distinguishing between the two
processes.
• The evaporation from a cropped soil is mainly
determined by the fraction of the solar radiation
reaching the soil surface.
• This fraction decreases over the growing period as the
crop develops and crop canopy shades most of the
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.
Introduction,cont…..
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4. 4
Introduction,cont…..
Figure 01: Partitioning of ET

5. Factors affecting the ET
Crop factors
• Crop type
• Variety of the crop
• Development stage
• Differences in resistance to transpiration
• Crop height
• Crop roughness
• Reflection
• Ground cover
• Crop rooting characteristics
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6. Weather parameters
• Radiation
• Air temperature
• Humidity
• Wind speed
Management and environmental conditions
• Soil salinity
• Land fertility
• Application of fertilizers
• Presence of hard or impenetrable soil horizons
• Control of diseases and pests
• Poor soil management
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FactorsaffectingtheET,cont….

7. Other factors
• Ground cover
• Plant density
• Soil water content
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FactorsaffectingtheET,cont….

8. Reference crop evapotranspiration (ETo)
• It is the evapotranspiration rate from a reference surface,
not short of water.
• The reference surface is a hypothetical grass reference
crop with specific characteristics such as,
• Height: 0.12 m
• Fixed surface resistance : 70 s m-1
• Albedo: 0.23
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9. • As water is abundantly available at the reference
evapotranspiring surface, soil factors do not affect ET.
• The only factors affecting ETo are climatic parameters.
• Consequently, ETo is a climatic parameter and can be
computed from weather data.
• ETo can be measured by Penman-Monteith method.
• ETo can also be estimated from pan evaporation however
special precautions and management must be applied.
• Any how use of pans to predict ETo for periods of 10 days
or longer may be warranted. (Refer the ppt:
Measurement of evaporation)
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Referencecropevapotranspiration(ETo),cont…..

10. Crop evapotranspirationunder standard(ETc)and
non-standardconditions(ETcadj)
• ETc is the evapotranspiration from disease-free, well-fertilized
crops that grown in large fields under optimum soil water
conditions and achieving full production under the given
climatic conditions.
• ETc adj is the evapotranspiration from crops grown under
management and environmental conditions that differ from
the standard conditions.
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11. • It can be measured by direct or indirect methods.
• Climate and crop data are used to estimate the
evapotranspiration in indirect measurement by theoretical
and empirical equations.
• Direct measurement involves isolating a portion of the crop
from its surrounding and determining ET by measurement.
• It can be measured by four methods:
• Lysimeter method
• Field Experimental plot
• Water balance method
• Soil Moisture Depletion Study
Measurement of Evapotranspiration
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12. Lysimeter
• It is a device used to measure the amount of
actual ET which is released by plants.
• There are two types,
• Weighing type
• Non weighing type
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13. Working principle of lysimeter
• A lysimeter is a device introduced in the ground,
filled with the same soil of the study area and
with vegetation.
• It is used to measure the reference ET or the
crop ET.
• The ET measurement is determined by the water
balance of the devices.
• In weighing type lysimeter, there is usually a
weighting scale at the bottom of the lysimeter.
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14. • Based on the scale, the amount of water
evapotranspirated in the system can be
determined in terms of change in mass.
• In non weighing type of lysimeter, a water drain
system is used to estimate the drainage water.
• So that ET can be determined by deducting the
drainage water from the total water input.
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Workingprincipleoflysimeter,cont…..

15. Methodology
• Needed instruments are,
• Plastic container
• Soil
• Plant
• Tensiometer
• Measuring cylinder
• Water
• Scale
• The designed lysimeter should have,
• Storage tank
• Strainer unit
• Drainage unit
• Collection unit
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16. Experimental Set up-Lysimeter
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Figure 02:Lysimeter

17. Methodology,cont…
Procedure
• Select suitable location to install the lysimeter.
• Select 100 liters of plastic container to use as lysimeter
• Calculate the cross sectional area of the container
• Make drainage hole and connect to the collecting unit
• Prepare the bottom of the lysimeter with the free
drainage system
• Setup strainer unit to prevent the movement of soil
• Fill the container with the soil
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18. • Allow the soil for settlement around two weeks
• Select good healthy plant and transplant into the
lysimeter
• Add known volume water in to the lysimeter and
measure the volume of drained water
• Maintain the tensiometer reading as zero by the
application of known quantity of water
• Allow the set up for 24 hours in the field
• Measure the volume of drainage
• Apply the water balance equation and estimate the
evapotranspiration. 18
Methodology,cont…

19. Water balance equation
• In weighing type lysimeter the water balance equation is,
Input = Output
P + I = ET +RO –D  S
Where,
ET = Evapotranspiration
P = Precipitation
I = Irrigation water
D = Excess water drained from bottom
S = weight change (Increase or decrease in storage of soil moisture)
RO = Zero
• In non weighing type lysimeter the water balance equation is,
Input = Output
P + I = ET +RO –D
Note: Tensiometer reading should be zero always to ensure the field is
in field capacity.
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20. Demerits
• Lysimeters are expensive.
• Operation and maintenance require special care
• The representativeness of the ET measured by lysimeters
is apt to be suspect for,
• Different crop densities
• Crop heights
• Root characteristics
• Soil water and nutrient statuses
• Soil profile structures between inside and outside
lysimeters.
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21. Variations of ET in a lysimeterand field condition
• Compared to large-scale lysimeters, pot experiments and
micro lysimeters are widely used in studies of ET.
• Because these are low-cost and easy-to-use tools.
• But the limited amount of soil available to the roots,
plant growth and ET in pots can be varies from field.
• Reduction in the crop yield or biomass due to water or
nutrient stress can occur more frequently in pots.
• Even a minor difference in crop density between pots
and lysimeters can result in a considerable difference of
ET between them.
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22. Soil water balance method
• This method consists of assessing the incoming and
outgoing water flux into the crop root zone over some
time period.
• Inputs of water to the root zone are Irrigation (I) and
rainfall (P)
• Part of I and P might be lost by surface runoff (RO) and by
deep percolation (DP) that will eventually recharge the
water table.
• Water might also be transported upward by capillary rise
(CR) from a shallow water table towards the root zone
• Some time the water even transferred horizontally by
subsurface flow in (SFin) or out of (SFout) the root zone. 22

23. • In many situations, SFin and SFout are minor and can be
Ignored except under large slopes.
• Soil evaporation and crop transpiration deplete water
from the root zone.
• If all fluxes other than ET can be assessed, the ET can be
calculated by using below water balance equation.
ET = I + P − RO − DP + CR ± ΔSF ± ΔSW
Limitations:
• Some fluxes such as SF, DP and CR are difficult to assess
and short time periods cannot be considered.
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Soilwaterbalancemethod,cont….

24. THANK YOU
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