This document discusses several methods for measuring evapotranspiration (ET), which is the combination of evaporation from soil and transpiration from crops. It describes lysimeters, which isolate a portion of cropped soil to determine water loss. Lysimeters directly measure ET through changes in mass (weighing type) or drainage (non-weighing type). The soil water balance method assesses water fluxes into and out of the crop root zone over time to calculate ET. Other indirect methods use climate and crop data with theoretical equations. All methods aim to accurately quantify ET, which is important for irrigation management and soil hydrological processes.

1. Measurementof 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.
of ET in field is importance
• The accurate measurement
for,
• Quantifying soil hydrological processes
• Making appropriate decisions regarding irrigation
2

3. • Evaporation and transpiration occur simultaneously.
• There is no easy way of distinguishing between the two
processes.
determined by the fraction of the
• The evaporation from a cropped soil is mainly
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…..
3

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
5

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
6
FactorsaffectingtheET,cont….

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

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
8

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)
Referencecropevapotranspiration(ETo),cont…..
9

10. Cropevapotranspirationunderstandard(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.
10

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
Measurementof Evapotranspiration
11

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
12

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.
13

14. • Based on the scale,
evapotranspirated in
the amount of water
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.
Workingprincipleoflysimeter,cont…..
14

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
15

16. Experimental Set up-Lysimeter
16
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
free
• Prepare the bottom of the lysimeter with the
drainage system
• Setup strainer unit to prevent the movement of soil
• Fill the container with the soil
17

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
P + I
= Output
= 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
P + I
= Output
= ET +RO –D
19
Note: Tensiometer reading should be zero always to ensure the field is
in field capacity.

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.
20

21. Variations of ETina lysimeterandfield 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.
21

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.
Soilwaterbalancemethod,cont….
23

24. THANK YOU
24