Fundamentals of Irrigation and On-farm Water Management: Volume 1 || Soil–Water–Plant–Atmosphere Relationship

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  • Chapter 8

    SoilWaterPlantAtmosphere Relationship

    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 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. 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. 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. 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

    dry soil. 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. 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. 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:

    dmasm FCWP100


    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:

    dmasm Xni1

    FCi WPi100

    Di; (8.1)

    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

  • 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. 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

    calculated as:

    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


    Di; (8.2)

    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

    soil types

    Type of soil Available water

    Range (cm/m) Average (cm/m)

    Sandy soil 510 7.5

    loam 1012 11

    Silt 1215 13.5

    Clay 1520