Saturday, June 1, 2019

Understanding the Operating principle of lysimeter

Operating principle of lysimeter
Water is added to the crop daily or every two days interval according to the irrigation interval. The
lysimeter is weighed after the irrigation and reweighed before the next irrigation action in order to
determine the amount of water containing crop due ET. The difference or reduction in the weight is the
water lost due to evaporation. Depth of water lost can be determined from the following equations
Recall that
Pw = Ma Where: Pw = density of water (g/cm3)
Vw Ma = mass of water loss (g)
Vw = volume of water loss due to ET(m3 or L)
Vw = Mw ETc = crop evapotranspiration
Pw Ab = area of the container
ETc = crop evapotranspiration in mm/day
ETc = Vm
Non-weighing lysimeter: This may be usually floating method using liquid. Reduction in the volume of
water is due to water lost. 2. EVAPORATION PAN
The pan that is use for measuring the evaporation rate is called Class A evaporation pan or simple class A
pan. The pan has a diameter of 1.2m and a depth of 250mm (0.25m) and it is usually placed in 150mm
above the ground on a wooden support as shown in the following figures. Evaporation should be located in open space which must not be under vegetation. Water is filled to
200mm level to touch the fixed point gauge. Operation principles or operation
1. Fill the evaporation pan to the level of the pointer on the first day. 2. On the second day after 24 hours. Use a 1000c (1 litre) to add the water to the evaporation pan
continuously to reach the arrow pointer. 3. Determine the total volume that added to reach the pointer. 4. Determine the area of the evaporation pan (A = πd
5. Divide the volume of the water added by the area to get the depth of the water lost due to
6. The depth of water lost per day is the evaporation occurred per day (mm/day)
i.e Epan = Volume
Area of the pan
To convert the evaporation that occurred per day to evapotranspiration ET is by multiplying by a
factor called pan coefficient which varies from 0.70 to 0.75 depending on the relative humidity
(RH) of the location. Other factors influencing the pan coefficient are the location of the pan, surrounding of the ground cover, the distance of the place of measurement from the pan and the
climate particularly the RH. Reference evapotranspiration (ET0) from class A evaporation pan is
ET0 = Kp.Epan
Where ET0 = reference evapotranspiration (mm/day)
Kp = pan coefficient
Epan = pan evaporation (mm/day)
Subtract the depth of water added by rainfall from evaporation pan
Class- A evaporation pan or evaporation pan
Evaporation pan
Evaporation pan with the arrow points.
Example on evaporation pan
A measuring glass cylinder of 1 litre is used to fill the evaporation pan of an internal diameter of 1.2m to
the pointer gauge. The glass cylinder is fill 8 times before the evaporation is filled to the pointer gauge. If
the pan coefficient and crop coefficient of maize are 0.75 and 0.85 respectively
i. Reference evapotranspiration ET0 (mm/day)
ii. Crop evapotranspiration of maize (mm/day)
Area of evaporation pan = A pan
A pan = (A = πd
2) = 3.142 x (1.2)
2 = 1.13112m2
4 4
Volume of water added = Vpan
Vpan = 8 x 1 = 8 litres = 0.008m3
Epan = Vpan = 0.008 = 7.07264 x 10
Apan 1.13112
:- Epan = 7.073mm/day
ET0 = Kp.Epan = 0.75 x 7.03 = 5.305mm/day
ET0 = reference evapotranspiration = 5.305mm/day
ETc = Kc.ET0 = 0.85 x 5.305 = 4.51
:- ETc = 4.51mm/day
Blanney Criddle (1950) observed that the amount of water used by crops during their growing
season. It is widely used and popular because of it’s simplicity. Blanney criddle formula is expressed as: CU=KcP (0.46Tmean + 8.13)……..(1)
The original equation in british unit is given below:
CU =KTP ………. (2) in inch/day
Where for eqn (1)
CU= mean monthly consumptive use (mm/day)
Tmean = mean monthly temperature
P = monthly percent of the total day time hours of the year
Eqn (2): original formula in inch/day
T in
0F now to
0C is commonly used in Nigeria K=Ce
C = F – 32
100 180
F = 180C + 32

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