computation of evapo -transpiration by soil moisture depletion studies
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Computation of Evapo -transpiration by Soil moisture Depletion Studies. By B.Hari Prasad. Introduction . The soil moisture depletion method is usually employed to determine the consumptive use of irrigated field crops. . Conditions. Crops grown on uniform soils - PowerPoint PPT PresentationTRANSCRIPT
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ByB.Hari Prasad
Computation of Evapo-transpiration by Soil moisture Depletion Studies
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Introduction The soil moisture depletion method is
usually employed to determine the consumptive use of irrigated field crops.
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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.
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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.
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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)
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Measurement of soil moistureFeel MethodTensiometersSampling and dryingElectrical Resistance(Gypsum) BlocksThe neutron probe method
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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
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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
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Electrical Resistance(Gypsum) Blocksbest suited for finer-textured soilssince the 1940sThe resistance of the electrode is
proportional to the moisture content of the block.
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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.
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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