**2. Soil moisture potential (ψW)**

Soil moisture potential in the common language is the potential of moisture to do work by its position in soil. ψW is the difference between the activity of the water molecule in pure distilled water and soil solution at normal atmospheric temperature and pressure which might be greater or lesser. In the definition of International Soil Science Society [13], ψW may be defined as "the amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water (at the point under consideration)." Hence, a reference state is a must.

ψW could also be delineated by knowing in a solution of nonelectrolytes, the chemical potential of water which further depends upon mean free energy per molecule and water molecule concentration. The chemical potential of pure water reduces with the addition of salts, which could be expressed as

$$
\mu\text{W} = \mu\text{W} - \mu\text{W}^\* = \text{RT Ln Nw} \tag{1}
$$

where R is the universal gas constant, T is the absolute temperature, and Nw is the mole fraction of water, respectively.

For the simple ionic solution,

$$
\mu\text{W} = \mu\text{W} - \mu\text{W}^\* = \text{RT Ln } \text{aW} \tag{2}
$$

where aw is the activity of the water molecules, which measured how easy the water content may be utilized. Further, *t*he water vapor pressure of the solution expressed as a fraction of the vapor pressure of pure water at the same temperature (or the equilibrium humidity expressed as a fraction) is numerically equal to the activity of the water (aw) in the solution. Eq. (2) is more useful as water always has ions.

When water contains a number of ions, then

$$
\mu\text{W} = \mu\text{W} - \mu\text{W}^\* = \text{RT Ln } \mathbf{e}/\mathbf{e}\_0 \tag{3}
$$

*Delineation of Soil Moisture Potentials and Moisture Balance Components DOI: http://dx.doi.org/10.5772/intechopen.92587*

where ψW is the water potential, μW is the solution's water chemical potential, μW\* is the pure state's water chemical potential, R is the universal gas constant (82 bars cm-<sup>2</sup> ), T is the absolute temperature, and e/eo is the relative vapor pressure, respectively.

ψW could be expressed depending upon the units used for the expression of quantity of water.


Among all the units, weight units are more convenient to use.

However, when all pores are water filled, conducting it under saturated conditions, then the actual and potential vapor pressure is the same, and thus e/eo comes out to be 1 (log 1 = 0). Thus, under saturated soil conditions, ψW comes out to be zero, which is the highest potential of the water, and under unsaturated conditions, it is always expressed as –ve value. Under natural soil environment, soil moisture movement is mainly controlled by the hydraulic potential (ψh), which is the total moisture potential. There is a brief explanation regarding all the components of the soil moisture potential one by one.

#### **2.1 Hydraulic potential**

Buckingham [9] in his classical paper on the capillary potential, while Gardner [10] showed the dependency of water potential on the water content, and Richards [11] prepared a tensiometer for measuring it. Hence, the concept of soil moisture movement is not new but is still difficult to understand by the new budding students and agricultural scientists dealing with agricultural water management. Moreover, quite often research papers published in reputed journals discussed the water balance components without discussing much on their estimation/calculative part, which further confuses the students. Therefore, estimation of the different soil moisture components is a must so as to perform new water management experiments with clear objectives of having higher water productivity under texturally divergent soils. These RCTs are site and situation specific, and a single RCT is not effective equally in all places for improving the water-use efficiency [12]. Therefore, considering above discussions, this chapter focused on the estimation of components of soil moisture potentials and balance components for the proper understanding of the concept by the end users, namely, agricultural students and even budding scientists, for conduction of more region-specific water management experiments under texturally divergent soils for ultimately improving water productivity with-

out affecting the grain yields in water-stressed regions of the globe.

reduces with the addition of salts, which could be expressed as

Soil moisture potential in the common language is the potential of moisture to do work by its position in soil. ψW is the difference between the activity of the water molecule in pure distilled water and soil solution at normal atmospheric temperature and pressure which might be greater or lesser. In the definition of International Soil Science Society [13], ψW may be defined as "the amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water (at the point under consider-

ψW could also be delineated by knowing in a solution of nonelectrolytes, the chemical potential of water which further depends upon mean free energy per molecule and water molecule concentration. The chemical potential of pure water

where R is the universal gas constant, T is the absolute temperature, and Nw is

where aw is the activity of the water molecules, which measured how easy the water content may be utilized. Further, *t*he water vapor pressure of the solution expressed as a fraction of the vapor pressure of pure water at the same temperature (or the equilibrium humidity expressed as a fraction) is numerically equal to the activity of the water (aw) in the solution. Eq. (2) is more useful as water always has

<sup>ψ</sup><sup>W</sup> <sup>¼</sup> <sup>μ</sup><sup>W</sup> � <sup>μ</sup>W<sup>∗</sup> <sup>¼</sup> RT Ln Nw (1)

<sup>ψ</sup><sup>W</sup> <sup>¼</sup> <sup>μ</sup><sup>W</sup> � <sup>μ</sup>W<sup>∗</sup> <sup>¼</sup> RT Ln aW (2)

<sup>ψ</sup><sup>W</sup> <sup>¼</sup> <sup>μ</sup><sup>W</sup> � <sup>μ</sup>W<sup>∗</sup> <sup>¼</sup> RT Ln e*=*eo (3)

**2. Soil moisture potential (ψW)**

*Soil Moisture Importance*

ation)." Hence, a reference state is a must.

the mole fraction of water, respectively. For the simple ionic solution,

When water contains a number of ions, then

ions.

**4**

ψh is the total moisture potential, that is, ψt, which is the sum of other potentials by virtue of its pressure (ψp), attractive forces (ψm), and gravity (ψg) [14]. The ψh/ψt provides direction of the movement of soil moisture; however, if ψh is the same throughout the soil profile (under pounded conditions or under prolonged rainfall), then the water will not move at all in the soils as energy state is the same throughout and moisture only moves under the deviation in the moisture levels/ energy levels. Normally under the unsaturated soils, the water moves from the lesser to higher negative potential. Moisture potential of soil delineation is quite important, as it directs us irrigation timings [9, 14]. Further, hydraulic conductivity of a particular soil having a particular textural class is very important, which is further important for nutrient movements within the plants. The slope of the curve between flux (discharge area<sup>1</sup> time<sup>1</sup> ) and hydraulic gradient decides the hydraulic conductivity itself varied with texturally divergent soils (**Figure 4**). This figure explains why movement of water differs in texturally divergent soils and we could manage our cultivation and management practices so as to increase the water-use efficiency.

#### **2.2 Matric potential (ψm)**

Different adsorption forces prevailing in the soil matrix are responsible for the ψm—the force of attraction of free water with soil particles [14]. The greater the adsorption forces, the more is the matric potential, and thus the water is less free. In other words, water is tightly attached to the soil particles. However, ψm is dependent on many factors, out of which soil texture is important, for example, sandy coarse-textured soils drained out moisture quickly at a smaller suction than clayey fine-textured soils because clayey soils have greater matric adsorption forces which

#### **Figure 4.** *Relationship between flux and hydraulic gradient in three texturally divergent soils.*

hold the water tightly and not allowed the water to drain out quickly. In other words, clayey soil has more –ve values of ψm than that of sandy soils, depicting the higher capacity of former soil water holding capacity of clayey soils. Similarly, the soils with higher organic matter (OM) content have higher water content and thus greater –ve value. *It is very important to understand that the greater is –ve* ψm *value, the higher is the water content as water always moves from the higher potential to lower potential or from lesser –ve to more –ve as more negative values of* ψ*m depict the lower water content.* ψm has been considered as capillary potential. If we consider weight as the unit for expressing the unit quantity of water, then ψm with respect to a particular height in the soil is the distance in the vertical direction between that selected height and level of water in a manometer. Generally, the ψm resulted from the two processes, namely, capillary "wedges" and "films," which cannot be changed without upsetting the others. K under saturated conditions varied in texturally divergent soils, due to attractive forces in soil separates and soil moisture (**Figure 5**). As shown in the picture, saturated hydraulic conductivity of sandy soil is more than of the clayey soil; however, the unsaturated conductivity of sandy soil decreases more steeply with increased suction and decreased from the clayey soils.

Ψm reported to be zero under saturated conditions; hence a –ve sign is always there under the unsaturated conditions which is the most prevalent situation in natural field conditions. Matric potential is always zero at the water level, positive below the water table, and negative above the water table. For measuring the suction or ψm in soils, we used tensiometer in soils (**Figure 6**) and set a particular reading for irrigating the fields.

However, tensiometer could measure the suction <0.85 bar (most prevalent in natural conditions), and pressure plate apparatus and tension plate assembly are used for measuring suctions >0.85 [7]. The graphical behavior of tension of soil moisture with absolute water content is developed through a soil moisture characteristic curve, which delineates the moisture levels that the soils could hold and thus helps in scheduling the irrigation to crops accordingly.

increasing suction values to 2000 and 2400 200 mm reduced the land productivity of the rice than earlier recommendation (2-day interval), which mean drying of soils to certain extent saves significant irrigation water without significantly affecting grain yields. Further, an average of 5-year study delineated (**Table 1**) a saving of up to 30% of irrigation water without adversely affecting the land

For measuring ψm, tensiometers are installed at 15–20 cm depth, because significant rhizosphere's portion of the rice crops retained to upper 15 cm [15], and therefore, tensiometers are placed at this depth, so that farmers could get the exact

productivity [21].

**Figure 6.**

**7**

**Figure 5.**

*Association between saturated hydraulic conductivity and ψ<sup>m</sup> in two soils.*

*Delineation of Soil Moisture Potentials and Moisture Balance Components*

*DOI: http://dx.doi.org/10.5772/intechopen.92587*

idea regarding the exact time to irrigate.

*Soil spec in action measuring soil matric potential [20].*

Under this scenario, the available soil moisture of Indo-Gangetic Plains is described by ψm [15]. Locally fabricated, low-cost tensiometers [16] that could delineate soil matric potential are generally preferred by the farmers for scheduling irrigation more particularly to rice [17, 18]. According to Kukal et al. [19],

*Delineation of Soil Moisture Potentials and Moisture Balance Components DOI: http://dx.doi.org/10.5772/intechopen.92587*

**Figure 5.** *Association between saturated hydraulic conductivity and ψ<sup>m</sup> in two soils.*

**Figure 6.** *Soil spec in action measuring soil matric potential [20].*

increasing suction values to 2000 and 2400 200 mm reduced the land productivity of the rice than earlier recommendation (2-day interval), which mean drying of soils to certain extent saves significant irrigation water without significantly affecting grain yields. Further, an average of 5-year study delineated (**Table 1**) a saving of up to 30% of irrigation water without adversely affecting the land productivity [21].

For measuring ψm, tensiometers are installed at 15–20 cm depth, because significant rhizosphere's portion of the rice crops retained to upper 15 cm [15], and therefore, tensiometers are placed at this depth, so that farmers could get the exact idea regarding the exact time to irrigate.

hold the water tightly and not allowed the water to drain out quickly. In other words, clayey soil has more –ve values of ψm than that of sandy soils, depicting the higher capacity of former soil water holding capacity of clayey soils. Similarly, the soils with higher organic matter (OM) content have higher water content and thus greater –ve value. *It is very important to understand that the greater is –ve* ψm *value, the higher is the water content as water always moves from the higher potential to lower potential or from lesser –ve to more –ve as more negative values of* ψ*m depict the lower water content.* ψm has been considered as capillary potential. If we consider weight as the unit for expressing the unit quantity of water, then ψm with respect to a particular height in the soil is the distance in the vertical direction between that selected height and level of water in a manometer. Generally, the ψm resulted from the two processes, namely, capillary "wedges" and "films," which cannot be changed without upsetting the others. K under saturated conditions varied in texturally divergent soils, due to attractive forces in soil separates and soil moisture (**Figure 5**). As shown in the picture, saturated hydraulic conductivity of sandy soil is more than of the clayey soil; however, the unsaturated conductivity of sandy soil decreases more steeply with increased suction and decreased from the clayey soils. Ψm reported to be zero under saturated conditions; hence a –ve sign is always there under the unsaturated conditions which is the most prevalent situation in natural field conditions. Matric potential is always zero at the water level, positive below the water table, and negative above the water table. For measuring the suction or ψm in soils, we used tensiometer in soils (**Figure 6**) and set a particular

*Relationship between flux and hydraulic gradient in three texturally divergent soils.*

However, tensiometer could measure the suction <0.85 bar (most prevalent in natural conditions), and pressure plate apparatus and tension plate assembly are used for measuring suctions >0.85 [7]. The graphical behavior of tension of soil moisture with absolute water content is developed through a soil moisture characteristic curve, which delineates the moisture levels that the soils could hold and thus

Under this scenario, the available soil moisture of Indo-Gangetic Plains is described by ψm [15]. Locally fabricated, low-cost tensiometers [16] that could delineate soil matric potential are generally preferred by the farmers for scheduling

irrigation more particularly to rice [17, 18]. According to Kukal et al. [19],

reading for irrigating the fields.

**6**

**Figure 4.**

*Soil Moisture Importance*

helps in scheduling the irrigation to crops accordingly.


the quantity of water, then certainly ψp is delineated by vertical space from the considered point and piezometer level of water, connected to that point in question. Pressure potential is always positive and zero below the water level and at and above the water level, respectively. *ψp and ψm are mutually exclusive to each other as*

ψg constitutes an important soil moisture potential component which is not affected by the soil properties [14]. On considering weight as the unit of quantity of water, ψg comes out to be the vertical distance of elevation from a point under consideration to the point in question and is thus considered as the elevation distance from a point under consideration to the level of reference [14]. To raise an object against the gravitational force of attraction, some work must be done which is stored in the form of energy with respect to its gravity. Gravitational potential is zero at, positive above, and negative below the reference level. It does not depend upon soil properties; this is the reason why ψg is not considered while calculating the water potential. However, ψg played an important role and is considered while

Further, ψg is independent on the conditions of soil, water, weather, chemical, and pressure, while elevation levels are affecting it. Hence, height is the only criteria

ψs is an important potential which is there in soil because of the salts in soil water and also due to the presence of the semipermeable layer, which only allowed water entry but not of the salts through it [14]. In soil-water interface, there are mainly two important semipermeable membranes, namely, air-water interface and cell wall in the roots. Air-water interface behaves near to the perfect semipermeable membrane, while cell wall of roots is not a perfect semipermeable membrane as it allows passage of salts as well as water through it. However, while studying liquid water flow in soils, ψs is an unimportant potential due to lack of semipermeable membrane in it, while in plants it is of much importance as plant ease to absorb water is greatly affected by ψs as the more the value of ψs, the higher the energy exerted by plant to pull deep underground water. Consider sodic/saline soil, through which the plants have to exert the water, and then it can exert a ψs equal to the permanent wilting point of soils. Thus determining the value of ψs = -RTCs,

temperature, and solute/salt concentration in soils, respectively, is the most difficult

local levels and allocation efficiency pertaining to water used in the literature [20, 23] for sustainable use of the irrigation water throughout the globe. Further, Allan coined the term "virtual water" for human consumption. Further, published literature also delineate some terms pertaining to crop water, namely, green, blue, gray, and black water [20]. The most important term that pertains to human water use is referred to as "blue water" as it is rain water, which directly enters the lakes and is used by humans. For plants, the most important water term is "green water" as it is there in soil pores and meets the transpiration demands of plants to produce

There are many terminological terms, namely, water-use efficiency at global and

where R, T, and Cs represent universal gas constant (82 bars cm�<sup>2</sup>

as it also includes those species which dissociate into the ions [9].

ψt ¼ ψw þ ψg (4)

), absolute

*if ψp is positive, then ψm is zero, while if ψm is negative, then ψp is zero*.

*Delineation of Soil Moisture Potentials and Moisture Balance Components*

**2.4 Gravitational potential**

*DOI: http://dx.doi.org/10.5772/intechopen.92587*

calculating the total water potential as

**2.5 Osmotic potential (ψs)**

**9**

affecting the gravitational water in one and all [14].

#### **Table 1.**

*Soil matric potential based irrigation water saving viz.-a-viz. yield differences.*

**Figure 7.** *Deviation in Darcy law.*

#### **2.3 Pressure potential**

ψp is a vital constituent of soil moisture potential but under the saturated conditions which seldom exist in nature [22]. Generally, saturated conditions come only when rains up to a considerable duration or continuous irrigation. When saturated flow becomes high enough to be turbulent and lesser enough for not to generate any flux for a prolonged time is there to meet the constant drainage and evaporation and flow in these conditions is basically governed by the force of gravity but these conditions seldom exist in a field or under natural conditions as here all the soil pores are water filled and conducting it [8, 22]. Under this condition, the discharge is governed by Darcy's law, which further has some limitations as shown in **Figure 7**.

Negative pressure potential of unsaturated soil becomes positive in the saturated conditions and is delineated as submergence or pressure potential which is generally measured with a piezometer. A piezometer is a hallow tube open from both ends, passing from the reference point. If we consider weight as the unit of expression of

*Delineation of Soil Moisture Potentials and Moisture Balance Components DOI: http://dx.doi.org/10.5772/intechopen.92587*

the quantity of water, then certainly ψp is delineated by vertical space from the considered point and piezometer level of water, connected to that point in question. Pressure potential is always positive and zero below the water level and at and above the water level, respectively. *ψp and ψm are mutually exclusive to each other as if ψp is positive, then ψm is zero, while if ψm is negative, then ψp is zero*.

#### **2.4 Gravitational potential**

ψg constitutes an important soil moisture potential component which is not affected by the soil properties [14]. On considering weight as the unit of quantity of water, ψg comes out to be the vertical distance of elevation from a point under consideration to the point in question and is thus considered as the elevation distance from a point under consideration to the level of reference [14]. To raise an object against the gravitational force of attraction, some work must be done which is stored in the form of energy with respect to its gravity. Gravitational potential is zero at, positive above, and negative below the reference level. It does not depend upon soil properties; this is the reason why ψg is not considered while calculating the water potential. However, ψg played an important role and is considered while calculating the total water potential as

$$
\mathfrak{w}\mathfrak{t} = \mathfrak{w}\mathfrak{w} + \mathfrak{w}\mathfrak{g} \tag{4}
$$

Further, ψg is independent on the conditions of soil, water, weather, chemical, and pressure, while elevation levels are affecting it. Hence, height is the only criteria affecting the gravitational water in one and all [14].

#### **2.5 Osmotic potential (ψs)**

ψs is an important potential which is there in soil because of the salts in soil water and also due to the presence of the semipermeable layer, which only allowed water entry but not of the salts through it [14]. In soil-water interface, there are mainly two important semipermeable membranes, namely, air-water interface and cell wall in the roots. Air-water interface behaves near to the perfect semipermeable membrane, while cell wall of roots is not a perfect semipermeable membrane as it allows passage of salts as well as water through it. However, while studying liquid water flow in soils, ψs is an unimportant potential due to lack of semipermeable membrane in it, while in plants it is of much importance as plant ease to absorb water is greatly affected by ψs as the more the value of ψs, the higher the energy exerted by plant to pull deep underground water. Consider sodic/saline soil, through which the plants have to exert the water, and then it can exert a ψs equal to the permanent wilting point of soils. Thus determining the value of ψs = -RTCs, where R, T, and Cs represent universal gas constant (82 bars cm�<sup>2</sup> ), absolute temperature, and solute/salt concentration in soils, respectively, is the most difficult as it also includes those species which dissociate into the ions [9].

There are many terminological terms, namely, water-use efficiency at global and local levels and allocation efficiency pertaining to water used in the literature [20, 23] for sustainable use of the irrigation water throughout the globe. Further, Allan coined the term "virtual water" for human consumption. Further, published literature also delineate some terms pertaining to crop water, namely, green, blue, gray, and black water [20]. The most important term that pertains to human water use is referred to as "blue water" as it is rain water, which directly enters the lakes and is used by humans. For plants, the most important water term is "green water" as it is there in soil pores and meets the transpiration demands of plants to produce

**2.3 Pressure potential**

*Deviation in Darcy law.*

**Figure 7.**

*Source: Ref. [12].*

*Soil Moisture Importance*

**Table 1.**

as shown in **Figure 7**.

**8**

ψp is a vital constituent of soil moisture potential but under the saturated conditions which seldom exist in nature [22]. Generally, saturated conditions come only when rains up to a considerable duration or continuous irrigation. When saturated flow becomes high enough to be turbulent and lesser enough for not to generate any flux for a prolonged time is there to meet the constant drainage and evaporation and flow in these conditions is basically governed by the force of gravity but these conditions seldom exist in a field or under natural conditions as here all the soil pores are water filled and conducting it [8, 22]. Under this condition, the discharge is governed by Darcy's law, which further has some limitations

**Year % water saving Yield differences** 29.6–30.7 +0.5–1.5% 25–27.2 At par 18–27.8 At par 16.6–20.8 +0.5–1.0% 11.1–21.4 At par

*Soil matric potential based irrigation water saving viz.-a-viz. yield differences.*

Negative pressure potential of unsaturated soil becomes positive in the saturated conditions and is delineated as submergence or pressure potential which is generally measured with a piezometer. A piezometer is a hallow tube open from both ends, passing from the reference point. If we consider weight as the unit of expression of

biomass [24]. Domestic activities such as bathing and dishwashing constitute the "gray water," while "black water" is the produce of laundry which consists of toilet water. Among all the different categories of water, only gray water has the huge potential of being reused, which further cut off the freshwater demand by 30% in cities [9].

Before sowing and after harvesting the crop, namely, during the intervening periods, profile moisture storage change could be measured, which further played an important role in the cultivation of fodder crops. A soil water balance component provides a way out to identify technologies which improve water productivity. Up to now, this period is the least attended as results of applied treatments evaluated are analyzed during this period [20, 27, 40, 41]. However, the intervening period delineation of soil moisture dynamics helped to assess the residual effects of these RCTs applied during the main crop [40, 41]. Therefore, for sustainable and judicious use of irrigation water, the analysis of the soil water balance component is very important. The following are the important parameters of the soil water balance which needs to be calculated for evaluating the performance of any RCT in any

*Delineation of Soil Moisture Potentials and Moisture Balance Components*

*DOI: http://dx.doi.org/10.5772/intechopen.92587*

where E is the evaporation, T is the transpiration, D is the drainage, S is the seepage, ΔG is the profile moisture change, R is the rainfall, and I is the irrigation. Details along with their calculative/instrumental part are discussed below.

Rainfall is an important soil water balance component which decides the fate of the rainfed crops grown particularly in the submountainous tracts where there is no irrigation facilities, which might be because of the hard subsurface and very deep underground water table [17, 23]. Therefore, its timely quantification is very important for recognizing stressed areas which further helps in rescheduling irrigation plans for improving land and water productivity over here. Received rainfall is estimated using a rain gauge, which is installed permanently at the location/period of experimentation, which is further used in calculating the rainfall water productivity (WPI) [15]. However, one should be very careful that the spot selected for rain gauge installation should be away from huge buildings or any obstacles or any hindrance. Necessary correction factor must be applied, which is the case of the heavy rainfall if rain gauge's cylinder overflowed [39]. Many times, it is observed that rain gauge base is not fixed, which may result in tilting of the gauge while recording the rainfall; thus while installing it, it should be made sure that it should be fixed by using cement and sand mixture, so that no error in calculations will be

Irrigation is the most important for having potential agricultural yields in any area. But generally irrigation water-use efficiency is quite low in spite of the fact that water already is a limiting factor. Further, irrigation is an important input component for soil water solution; however, its exact measurement is generally not there, even in water management experiments. Nowadays we are well equipped with the water measuring meters which accurately measured the water amount which is being applied to a particular plot under any treatment, namely, area velocity flow meter (AVFM 5) which provides a digital reading of water supplied in any plot [15]. Generally, irrigation water depth of 50 and 75 mm in wheat and rice plots supplied which could be measured through the sensor (fitted in the pipe through which water enters a particular plot) of AVFM [27]. GREYLINE is the company manufacturing the Digital flow meter (**Figure 8**) the irrigation water measuring irrigation water device on a quantitative basis. Their sensor has to be fit

E þ T þ D þ S þ ΔG ¼ R þ I (5)

region of the globe:

**3.1 Rainfall (R)**

there [15, 23, 39].

**11**

**3.2 Irrigation water amount (I)**
