Page 20 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

 Management factors - Mulching, shading, weeding, irrigation and
fertilizer application

2.2.3 Evapotranspiration (ET) Measurement

i. Direct Measurement

Involve isolating a portion of the crop from its surrounding and
determining ET by measurement

ii. Indirect Measurement

 Calculate from crop and climate data
 Theoretical and empirical equation
 Used for crops where measured ET data are not available

Evapotranspiration is not easy to measure. To calculate evapotranspiration,
specific instruments and precise measurements of several physical
parameters or the soil-water balance in lysimeters are necessary. The
Lysimeter is the most often used device for measuring evapotranspiration.

LYSIMETER

It allows for precise measurements of ET, rainfall, and plant water uptake.
The percolation of water through soils is measured using lysimeters, which
are tanks buried in the ground. The most reliable method of directly
measuring the evapo-transpiration rate is to use a lysimeter.

There are two types of Lysimeter – Non-weighing type lysimeter and weighing
type.

Non-Weighing Type Lysimeter:

It is called drainage lysimeter.
It operates on the principle that E.T. is equal to the amount of rainfall and
irrigation water added to the system minus leaching.

ET = Rainfall + irrigation water – Leaching.

The drainage lysimeter is useful only in determining the PET rate and should
be irrigated every four or five days, unless rainfall intervenes.

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