ByB.Hari Prasad

Computation of Evapo-transpiration by Soil moisture Depletion Studies

Introduction The soil moisture depletion method is

usually employed to determine the consumptive use of irrigated field crops.

Conditions Crops grown on uniform soils

The depth to the ground water is such that it will not influence the soil moisture fluctuation within the root zone.

It involves measurement of soil moisture from various depths at a number of times throughout the growth period.

Greater the number of measurements, more is the information obtained from the studies.

Consumptive use (Cu) is calculated from the change in soil moisture in successive samples fro the following equation:

u = water use from the root zone for successive sampling periods or within one irrigation cycle

n = number of soil layers sampled in the root zone M1i = soil moisture percentage at the time of the first sampling in

the i-th layer M2i = soil moisture percentage at the time of the second sampling

in the i-th layer Ai = apparent specific gravity of the i-th layer of the soil Di = depth of the i-th layer of the soil(mm)

Measurement of soil moistureFeel MethodTensiometersSampling and dryingElectrical Resistance(Gypsum) BlocksThe neutron probe method

Feel Methodestimating soil-water by feeling the soileasy to useresults depend on the experience of the

individual making the measurementReliability is usually poor unless the

operator is very experiencednot generally recommended

TensiometerA tensiometer is a sealed,

airtight, water-filled tube (barrel) with a porous tip on one end and a vacuum gauge on the other

measures soil water suction (negative pressure)

Soil-water tension is commonly expressed in units of bars or centibars

Electrical Resistance(Gypsum) Blocksbest suited for finer-textured soilssince the 1940sThe resistance of the electrode is

proportional to the moisture content of the block.

http://www.ictinternational.com.au/g-blocks.htm

The Neutron Probe MethodNeutrons emitted by the source have an

average speed of about 6,000 miles per second and are called "fast neutrons.“

Slow Neutrons after losing energy travels at a speed of about 1.7 miles per second are detected by neutron detector.

references“Irrigation Theory and Practice” by Michael

http://cru.cahe.wsu.edu/CEPublications/pnw0475/pnw0475.html

http://www.bae.ncsu.edu/programs/extension/evans/ag452-2.html