Estimation of Evapotranspiration through lysimeters

1. “Estimation of Evapotranspiration
through lysimeters”
Course name- Master Seminar
Course No.- AGM-591
Session - 2018-19
Presented by
KHILESH KUMAR SAHU
Dept. of Agrometeorology

2. Contents :-
• Introduction
• Type of lysimeres and its Working principle
• ET Estimation by using gravimetric lysimeter
• ET Estimation by Volumetric lysimeter
• Water Requirement of Different Crops
• Conclusion

3. INTRODUCTION
• EVAPORATION :-
This is the phenomenon in which volume of liquid or solid water is lost from
a water body, moist soil, or any wet surface.
• The volume of liquid or solid water evaporated per unit area in unit time
(mm/day) is defined as evaporation rate.
• TRANSPIRATION:-
Transpiration is a process by which water vapour leaves the living plant body,
principally the leaves and enters the atmosphere. It involve continuous
movement of water from the soil into the roots, through stem and out
through the leaves to the atmosphere.
Fig-1

4. Evapotranspiration
• The combined loss of water from
the plants and surrounding soil
surface through transpiration and
evaporation is jointly called as
Evapotranspiration (ET) .
Fig-2

5. • Potential evapotranspiration (PET):- The maximum
water lost through evaporation from wet soil and
transpiration from a short cut grass, covering ground
completely, under unlimited water supply.
• Reference crop evapotranspiration (ET0):-Rate of
evapotranspiration from an extended surface of 8-15
cm tall green grass cover of uniform height actively
growing, completely shading the ground and not short
of water (mm/day).
• Actual crop evapotranspiration(Eta crop):- Rate of
evapotranspiration equal to or smaller than predicted
ET crop as effected by a level of available soil
water,salinity,field size or other causes (mm/day)

6. This loss of water is determined by various method
•Field experimental plot,
•By soil moisture depletion,
•By water balance method – Ex. Thornthwait water balance
•lysimeter method is the best and convenient for obtaining actual
evapotranspiration(ET) on a day basis. Lysimeter or
Evapotranspirometers are the instrument of tanks filled with field
soil and crops grown on it under natural condition and used to
measure actual evapotranspiration.
There are different type of lysimeres :-
1.Volumeteric lysimeter :- which measures ET by volume. For
example:-drum type lysimeters (used in Paddy crops).
2.Floating lysimeter(used for aquatic crop),
3.Gravimetric lysimeters :-which measures ET by weight.

7. • PRINCIPLE: An isolated block of cropped soil
representative of the field is weighed first to determine
the quantity of water added by precipitation or irrigation
and then after loss by ET is weighed again. The
difference in weight in kg is multiplied with the
conversion factor of 0.6 to give ET in mm.
• CONSTRUCTION: A weighing lysimeter consists of
the following parts:
1.Lysimeter tank
2.Retaining tank
3.Dummy tank
4.Weighing machine

8. • LYSIMETER TANK:- A steel tank of (1.20x1.20x0.9 m)
containing soil in which crop is grown is mounted on a
platform of weighing balance such that its rim is in level
with the soil .The steel tank contains a perforated plate at
depth of 75 cm so as to form a hollow chamber. A tube
inserted through the perforated plate into the bottom of the
hollow chamber. Through this tube percolated water is
removed.
• RETAINING TANK:- the retaining tank is
140x140x112.5 cm isolate the lysimeter and weighing
bridge from the main field and protect it from seepage of
adjoining field.

9. • DUMMY TANK:- A smaller tank of size of 30x30x90
cm is placed in the gap near head work. It prevents
the over heating of the lysimeter tank which is near
the pillar of the weighing machine.
• WEIGHING MACHINE:- it has the capacity to
weigh 2000 kg. Its plat form is 120x120 cm .Its is
erected in the middle of the field on the reinforced
cement concrete foundation , constructed inside the pit
of size 3.5x3.2 m in position the platform of weighing
will be at a depth of about 1 m from the soil surface.
Equation for Estimation of ET
• ET(in mm) = [Different in weights of two
consecutive lysimeter observation (in kg x 0.6) +
[Corresponding rainfall (in mm)]

11. Table.1:- ET Estimation using gravimetric lysimeter
Date Rainfall
(mm)
Soil tank
weight
(kg)
Change in
weight (kg)
ET (mm/day) Remarks
28-jun-1999 1862.5 Non rainy day
29-jun-1999 0.00 1851.3 +11.2 11.2 x 0.6 +0.0 = 6.72
30-jun-1999 5.20 1855.6 - 4.3 (-4.3 x 0.6 )+ 5.2 =2.62 Rainy day
02-jul-1999 1851.3
03-jul-1999 12.00 1865.3 -14.0 (- 14 x 0.6) + 12 =3.6 Rainy day

12. • Volumetric lysimeter is used to measure ET in terms of volume.
• It requires standing water (and hence is used for standing water crops)
while gravimetric lysimeter dose not require the same.
• On rain free days, the water added to the reservoir drum (inflow) is
measured with the help of a measuring jar.
• It is measured by volume in terms of cc. then it is converted into mm
by using the conversion factor 1 cc =0.000694 mm.
Volumetric lysimeter

14. Equation and example of ET Estimation by
Volumetric lysimeter
• Thus,
ET =(inflow-outflow) x conversion factor + rainfall
• Ex. Inflow=3500cc outflow=2000 cc and rainfall=0.0
mm
• ET =(3500-2000) x 0.000694 +0.0 = 1.04 mm

15. S.NO. Crop ET/mm/day Water Requirement
(mm)
1 Rice 4.5 -5.5 1000-1200
2 Wheat 4.41 – 5.86 500-550
3 Sorghum - 450-650
4 Maize 5.17-8.70 500-800
5 Sugarcane 4.5 – 4.6 1500-2500
6 Groundnut - 500-700
7 chickpea 4.7- 5.4 450-700
8 Soybean 5 -8.4 450-700
Table 2. ET/mm/day and Water Requirement of Different Crops
BOOK - irrigation and water management Dr. Oom prakash

16. Conclusion
• Lysimeter is very important instrument with a
wide range of application.
• It is the direct method that helps in
measurement of the actual evapotranspiration .
• It helps to determine Irrigation Scheduling and
the water requirement of the crops.
• Floating type lysimeter is used for the aquatic
crops.