**1. Introduction**

The complex nature of the soil pore space and the water held therein makes it difficult to delineate the soil-water interface and moisture advancements in the soil, which is further influenced by soil matrix geometry. Soil moisture is the amount of moisture present in soil pores, which is a must for all important ecological processes and plays a critical and significant role in all the physiological processes. Throughout the globe, water scarcity is an emerging problem that must be worked out for sustaining agricultural growth [1–3]. Different RCTs are recommended for having improved water productivities across the globe [4–6]. The scientists at NASA's Goddard Space Flight Center generate groundwater and soil moisture drought

indicators each week. They are based on terrestrial water storage observations derived from GRACE-FO satellite data and integrated with other observations, using a sophisticated numerical model of land surface water and energy processes. The drought indicators describe current wet or dry conditions, expressed as a percentile showing the probability of occurrence for that particular location and time of year, with lower values (warm colors) meaning dryer than normal, and higher values (blues) meaning wetter than normal (**Figure 1**).

Global analysis based on intermediate population growth rate revealed that water scarcity is a global issue and therefore needs to be addressed for mitigating its adverse effects onto the overall land and water productivities of agricultural crops (**Figure 2**).

Further, in India, the net irrigated area increased from the 1960s and is further projected to increase by 2030 (**Figure 3**), which further increased the installed tube wells and further declined the underground water table of the country, which might be beyond the reach of the poor farmers.

Soil water potential must be understood, and its applications must be applied in field conditions. For measuring the soil water potential, the instrument highlighted as tensiometer is used for irrigating the crops, namely, rice, without affecting the overall land as well as water productivity [7]. Tensiometer measured the soil suction, and when soil dries, then the inner water in the tensiometer via porous cup moves out in the soil. Hence, as a result, the potential reading in tensiometer increased, and at predefined levels of potential, irrigation is applied to crops [1, 7]. After irrigation, water moved back into the tensiometer from the irrigated soil, and water level of inner tube moved back to normal, namely, green level. Soil water potential (as controls moisture movements) is the ultimate technique, under unsaturated conditions when only micropores are water filled, while macropores are air filled for improving the declined water-use efficiency without affecting the grain yields more particularly in global water-stressed regions [7, 8]. However, both macro- and micropores are water filled, and conducting it under saturated soil condition seldom

exists in nature. Gravity and soil water potential are the main driving forces under saturated and unsaturated conditions, responsible for soil moisture movement. Micropores of fine-textured clayey soils are capable of holding water for a longer period of time even at higher value of suction, while macropores of sandy soil drain out the water quickly at a smaller suction. Therefore, generally frequent irrigations resulting in lower water productivity are reported in the sandy soils as compared to the clayey fine-textured soil. In nature, soil moisture has different quantities and forms of energy by virtue of which it moves from one to another point in soil. The potential concept to the soil water in relation to its movement was first given by

*Net irrigated area in India (Source: Food and Agriculture Organization, 2008).*

*) in chief paddy-growing Asian countries viz-a-viz upcoming years*

*(1950–2050) a Estimate based on the population growth trends Source: Modified from [1]).*

*Delineation of Soil Moisture Potentials and Moisture Balance Components*

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

**Figure 2.**

**Figure 3.**

**3**

*Water availability Per capita (m3*

**Figure 1.** *GRACE based global shallow groundwater drought indicators (https://nasagrace.unl.edu/).*

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

**Figure 2.**

indicators each week. They are based on terrestrial water storage observations derived from GRACE-FO satellite data and integrated with other observations, using a sophisticated numerical model of land surface water and energy processes. The drought indicators describe current wet or dry conditions, expressed as a percentile showing the probability of occurrence for that particular location and time of year, with lower values (warm colors) meaning dryer than normal, and

Global analysis based on intermediate population growth rate revealed that water scarcity is a global issue and therefore needs to be addressed for mitigating its adverse effects onto the overall land and water productivities of agricultural crops

Further, in India, the net irrigated area increased from the 1960s and is further projected to increase by 2030 (**Figure 3**), which further increased the installed tube wells and further declined the underground water table of the country, which might

Soil water potential must be understood, and its applications must be applied in field conditions. For measuring the soil water potential, the instrument highlighted as tensiometer is used for irrigating the crops, namely, rice, without affecting the overall land as well as water productivity [7]. Tensiometer measured the soil suction, and when soil dries, then the inner water in the tensiometer via porous cup moves out in the soil. Hence, as a result, the potential reading in tensiometer increased, and at predefined levels of potential, irrigation is applied to crops [1, 7]. After irrigation, water moved back into the tensiometer from the irrigated soil, and water level of inner tube moved back to normal, namely, green level. Soil water potential (as controls moisture movements) is the ultimate technique, under unsaturated conditions when only micropores are water filled, while macropores are air filled for improving the declined water-use efficiency without affecting the grain yields more particularly in global water-stressed regions [7, 8]. However, both macro- and micropores are water filled, and conducting it under saturated soil condition seldom

higher values (blues) meaning wetter than normal (**Figure 1**).

*GRACE based global shallow groundwater drought indicators (https://nasagrace.unl.edu/).*

be beyond the reach of the poor farmers.

(**Figure 2**).

*Soil Moisture Importance*

**Figure 1.**

**2**

*Water availability Per capita (m3 ) in chief paddy-growing Asian countries viz-a-viz upcoming years (1950–2050) a Estimate based on the population growth trends Source: Modified from [1]).*

**Figure 3.**

*Net irrigated area in India (Source: Food and Agriculture Organization, 2008).*

exists in nature. Gravity and soil water potential are the main driving forces under saturated and unsaturated conditions, responsible for soil moisture movement. Micropores of fine-textured clayey soils are capable of holding water for a longer period of time even at higher value of suction, while macropores of sandy soil drain out the water quickly at a smaller suction. Therefore, generally frequent irrigations resulting in lower water productivity are reported in the sandy soils as compared to the clayey fine-textured soil. In nature, soil moisture has different quantities and forms of energy by virtue of which it moves from one to another point in soil. The potential concept to the soil water in relation to its movement was first given by 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 without affecting the grain yields in water-stressed regions of the globe.

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

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

**Expressed units Units of ψW** Mass erg g<sup>1</sup> Volume Dynes cm2 Weight cm, m, mm

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

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

lic 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

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

) and hydraulic gradient decides the hydrau-

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

*Delineation of Soil Moisture Potentials and Moisture Balance Components*

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

), T is the absolute temperature, and e/eo is the relative vapor pressure,

bars cm-<sup>2</sup>

respectively.

quantity of water.

soil moisture potential one by one.

between flux (discharge area<sup>1</sup> time<sup>1</sup>

**2.2 Matric potential (ψm)**

efficiency.

**5**

**2.1 Hydraulic potential**
