# By B.Hari Prasad Computation of Evapo-transpiration by Soil moisture Depletion Studies.

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By B.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 M 1i = soil moisture percentage at the time of the first sampling in the i-th layer M 2i = soil moisture percentage at the time of the second sampling in the i-th layer A i = apparent specific gravity of the i-th layer of the soil D i = depth of the i-th layer of the soil(mm)

Measurement of soil moisture Feel Method Tensiometers Sampling and drying Electrical Resistance(Gypsum) Blocks The neutron probe method

Feel Method estimating soil-water by feeling the soil easy to use results depend on the experience of the individual making the measurement Reliability is usually poor unless the operator is very experienced not generally recommended

Tensiometer A 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) Blocks best suited for finer-textured soils since the 1940s The resistance of the electrode is proportional to the moisture content of the block.

The Neutron Probe Method Neutrons 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/ev ans/ag452-2.html