Download Computation of Evapotranspiration by Soil moisture Depletion Studies

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
 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 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