Fundamentals of Irrigation and On-farm Water Management: Volume 1 || Soil–Water–Plant–Atmosphere Relationship
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Development of sustainable irrigation practices will require that we understand
better the biophysical processes of root-water uptake in soil, and transpiration from
plant canopies. Solar energy is the driving force for most of the biophysical processes
in the plant system and water movement from soil to the atmosphere. Details
regarding soil, water, plant, and atmosphere (weather and climate) are described in
the respective chapters. In this chapter, mostly the interactions among them are
8.1 Concept of SoilPlantAtmospheric Continuum
Movement of water occurs in response to differences in the potential energy of
water (from an area of relatively high-water potential to an area of relatively low-
water potential). Although water uptake by plants is under physiological control, it
is often described as a purely physical process, as a consequence of gradients in
water potential in the soilplantatmosphere continuum (SPAC). The SPAC is the
pathway for water moving from soil through plants to the atmosphere. The SPAC
constitutes a physically integrated, dynamic system in which various flow processes
(e.g., solar energy interception, plant transpiration, water movement through plant
system, and water movement from soil to plant root hair) occur simultaneously and
independently, like links in a chain.
A plant grows in soil and opens to atmosphere. About 99% of all the water that
enters the roots leaves the plants leaves via the stomata without taking part in
metabolism. On a dry, warm, sunny day, a leaf can evaporate 100% of its water
weight in just an hour. Water loss from the leaves must be compensated for by the
uptake of water from the soil. Water movement is due to differences in potential
between soil, root, stem, leaf, and atmosphere. Under normal conditions, the water
potential in soil is higher than that in root saps or fluids. Typical moist soil might
have a water potential of about 0.3 to 1.0 bar, root tissue about 4.0 bar, stemabout 7.0 bar, leaf about 10.0 to 12.0 bar, and typical dry atmosphere about
M.H. Ali, Fundamentals of Irrigation and On-farm Water Management: Volume 1,DOI 10.1007/978-1-4419-6335-2_8,# Springer ScienceBusiness Media, LLC 2010
400 to 600 bar. These differences in water potential (and hence potentialgradient) are the driving force for causing the water movement (Fig. 8.1).
Water potential of the atmosphere is computed as:
Patm RT =MW ln e=e ;
where R gas constant 8.31 J/mol/K, T Temperature (K) C 273, MW molecular weight of water 18 g/mol, and e/e* is the relative humidity.
8.1.1 Some-Related Concepts/Terminologies
22.214.171.124 Water Potential
It is the difference in water energy between two regions containing water. Potential
is defined as the thermodynamic measure of energy with water, and the available
energy to do work. Water potential is usually measured in units of the pressure
needed to stop water movement. Commonly used units are Mega Pascals (MPa),
bar, etc. By convention, pure water is said to have a potential of zero. If any solute
(substance that dissolves) is dissolved in pure water, the water potential of that
solution decreases [and hence negative (lower than zero) water potential], attracting
free water across a semipermeable membrane. Water under pressure will have
positive potential and move to areas of lower pressure. Water that is elevated will
have positive potential and move to lower areas.
Fig. 8.1 Water movement insoilplantatmospheric
374 8 SoilWaterPlantAtmosphere Relationship
8.2 SoilWaterPlantAtmosphere Interrelations
8.2.1 Different SoilWater Coefficients
Irrigation water management at any level requires a thorough understanding of the
relations between soil and water. These relations are governed by intermolecular
forces and tensions, which give rise to the capillary phenomenon. These forces in
unsaturated soils are influenced by soil texture and structure. It is on the basis of the
soil texture and structure that water retention capacity or water holding capacity of
the soil varies.
126.96.36.199 Field Capacity
The soil is at field capacity when all the gravitational water has been drained and a
vertical movement of water due to gravity is negligible. Further water removal for
most of the soils will require at least 7 kPa (7 cbars) tension.
188.8.131.52 Permanent Wilting Point
The permanent wilting point is the point/situation where there is no more water
available to the plant. The permanent wilting point depends on plant variety, but is
usually around 1,500 kPa (15 bars). This means that for the plant to remove water
from the soil, it must exert a tension of more than 1,500 kPa (15 bars). This is the
limit for most plants, and beyond this they experience permanent wilting.
184.108.40.206 Saturation Capacity
It is the moisture content of soil when all the pores are filled with water. In clayey
soil, the porosity is highly variable as the soil alternatively swells and shrinks,
aggregates and disperses, compacts and cracks. Since clayey soils swell upon
wetting, the relative volume of water at saturation can exceed the porosity of the
220.127.116.11 Water Holding Capacity or Water Retention Capacity
Water is held within the soil matrix by adsorption at the surfaces of particles and by
capillarity in the pores. The size, shape, and arrangement of the soil particles and
the associated voids (pores) determine the ability of the soil to retain water. It is
important to realize that large pores in the soil can conduct more water more rapidly
8.2 SoilWaterPlantAtmosphere Interrelations 375
than fine pores. In addition, removing water from large pores is easier and requires
less energy than removing water from smaller pores.
Sandy soils consist mainly of large mineral particles with very small percentages
of clay, silt, and organic matter. In sandy soils, there are many more large pores than
in clayey soils. In addition, the total volume of pores in sandy soils is significantly
smaller than in clayey soils (3040% for sandy soils compared with 4060% for
clayey soils). As a result, much less water can be stored in sandy soil than in the
clayey soil. It is also important to realize that a significant number of the pores in
sandy soils are large enough to drain within a few hours (largely the first 24 h)
because of gravity and this portion of water is lost from the system before the plants
can use it.
18.104.22.168 Effect of Load on Water Retention
Effect of load on soil specimen is that it reduces the void or pore space, and thus it
affects the water retention capacity.
22.214.171.124 MaximumAvailable Moisture orMaximum Plant Available Moisture
Maximum available soil moisture (MASM) is the moisture content between field
capacity (FC) and wilting point (WP), i.e.,
In terms of water depth, it is expressed as:
where dmasm is the maximum available soil water in cm depth, FC is the moistureat field capacity (%), WP is the moisture at wilting point (%), and D is the rootdepth (cm).
For soils having different layers, the total maximum available soil moisture
(TMASM) within the root zone can be expressed as:
where FCi is the soil moisture at field capacity for ith layer (% vol), WPi is thesoil moisture at wilting point in ith layer (% vol), Di is the depth of ith layer (cm),and n is the number of subdivisions/layers of the effective rooting depth.
Maximum available moisture for various soil types is given in Table 8.1.
376 8 SoilWaterPlantAtmosphere Relationship
126.96.36.199 Maximum Readily Available Moisture
It is the portion of the maximum available moisture that is easily extractable by the
plant. It may be 6075% of the maximum available moisture depending on soil,
plant type (crop cultivar), species, and stage of crop.
188.8.131.52 Presently Available Soil Moisture
Presently available soil moisture (PASM) is the moisture currently (i.e., at present
state of the crop and soil) available for plant abstraction. It is equal to the difference
between the present soil-water content (y) and moisture content at permanentwilting point (yPWP) and i.e.,
PASM y yPWP;
where all the terms are expressed in percent volume.
To express the moisture in terms of water-depth (for a particular soil layer), it is
PASM y yPWP100
where PASM is the presently available soil moisture (cm), FC is the soil moisture at
field capacity (by volume percent), y is the soil moisture content (by volumepercent), and D is the depth of the soil layer under consideration (cm).
Total presently available soil moisture (TPASM) is the sum of the available
moisture within the root zone.
TPASM cm Xni1
where yPWPi is the soil moisture at permanent wilting point for ith layer (% vol), yiis the present soil moisture content in ith layer (% vol), Di is the depth of ith layer(cm), and n is the number of subdivisions of the effective rooting depth.
Table 8.1 Maximumavailable moisture for various
Type of soil Available water
Range (cm/m) Average (cm/m)
Sandy soil 510 7.5
loam 1012 11
Silt 1215 13.5