Soil Moisture Measurement
for Irrigation Scheduling
Sanjay Shukla
Rafael Muñoz-Carpena
Agricultural and Biological Engineering
UF-IFAS
Plant Water Demands
Evapotranspiration (ET) is defined as the amount of
water lost from the ground due to processes:
a)Evaporation: is the physical process by which a liquid
(or solid) changes into a gas. It takes place both from
water surfaces as well as wet soils and vegetation.
b) Transpiration: is the process by which the plant
transports water from the roots to the leaf stomata.
Quantification of Water Demands
To estimate water demands we have basically two kinds of methods
for ET estimation:
a)Hydrological methods: Is the physical process by which a liquid (or
solid) changes into a gas. It takes place both from water surfaces as
well as wet soils and vegetation.
b) Micrometeorological methods: They are based in the fact that
transportation and evaporation are linked to atmospheric conditions,
what has been called the “soil-plant-atmosphere continuum”.
There are also several methods for getting transpiration estimates
Hydrological Methods
This refers to water balance
(budget) methods:
RO: Runoff
P: Precipitation
F: Soil Infiltration
D: Deep percolation
Dq= Soil moisture
Atmosphere
Soil
Aquifer
If RO=0 and all but ET
are measured we can estimate ET
Micrometeorological methods
The atmospheric conditions dictate the amount of water
that can potentially leave the soil surface. Basically there
are three classes of methods:
a) Radiation based methods. Sun energy is the motor for
water evaporation/transpiration.
b) Aerodynamic or transport based. Evaporation from a
surface increases as we move the water away (wind) as it
is evaporated. This is related to “vapor deficit”.
c) Combination: both radiation and aerodynamic terms.
Radiation methods
.
SPACE
SPACE
ATMOSPHERE
OCEAN/LAND
Incoming
solar
radiation
Outgoing earth radiation
Short wave
Long wave
Irrigation vs. Plant Water Demands
We must remember that the final amount of water to give to
the crop is not just the water demands. It has to include also:
a)Losses of the water distribution system
b) Counteract differences in water distribution (irrigation
uniformity or efficiency)
c) Leaching requirements for saline (salty) environments
d) Other water that has entered the soil (namely rainfall)
e)Other losses/restrictions (soil, system, yield potential etc).
Finally we should always keep in mind that the amount will
change with time based on plant phenology (development),
weather conditions, and soil.
Irrigation Scheduling
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Appropriate water at appropriate time
Crop need, soil properties, and weather
condition
Different methods
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Experience
Calendar method (0.8 in every 4th day)
Meteorological (weather based ET estimation
based on weather)
Dynamic water balance – water budget
Soil water measurement based irrigation
Soil Moisture Scheduling
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Self-adjusting: appropriate water at
appropriate time
Opportunity for reduction in water losses
and chemical leaching since soil is kept
below field capacity and deep drainage is
reduced