**2. Soil moisture**

**1. Introduction**

56 Soil Moisture

ecosystem highly dynamic.

Today, technological advances have favored a better understanding of the circumstances around natural phenomena, being necessary to explain, in order to comprehend the dynamic nature of the climate, all the interrelations of the atmosphere, and the terrestrial surface at determinate time and space. Nicholson [1] described that seasonal time scales define the dynamic predictability, which is explained by atmospheric fluctuations defined by internal and boundary forcing. For the internal forcing, short and medium scales are associated with mechanisms as flow instabilities, non-linear interactions, thermal and orographic forces, fluctuating zonal winds and tropical/ extratropical interactions. Whereas boundary forcing could be associated to a lower boundary condition for heat and moisture fluxes related to external factors such as soil moisture, vegetation, sea-surface temperature, among others, this is also for example, soil moisture feedback on precipitation or quantifying the scales of heterogeneity in surface vegetation and soil, and their dependency with other variables as the leaf area distribution, topographical, and meteorological properties. Thus, in order to understand the different interaction process in the hydrological cycle, it will be necessary to establish the behaviour of parameters such as soil moisture at different levels of aggregation [2, 3]. For instance, on a global scale, soil moisture is important because it maintains a series of interactions with the climatic and terrestrial systems, by serving as a source of water for the atmosphere through evapotranspiration and about 60% of the amount of atmospheric water is returned to the Earth's surface in the form of precipitation [4]. As a regulator of the climate, soil moisture is linked to mass and energy cycles, affecting climatic components such as air temperature and precipitation, whose movements and disturbances presented in the atmosphere at different scales interact with the Earth surface generating heat exchange and, in consequence, supporting the stability of the air near the Earth's surface and its temperature [5]. At the basin scale, the soil moisture content is determined by the soil type and topographic configuration, and influences the partition of precipitation into infiltration and runoff and, therefore, exerts direct control over soil erosion and flooding. At the local scale, local patterns of infiltration and water flow in the soil could affect the surface water quality and groundwater [6]. This means, soil moisture is a variable that directly influences parameters such as precipitation, runoff, evapotranspiration, and infiltration since they depend on the water stored in the soil how defines the degree of modification of the water cycle parameters [5]. Under these terms, soil moisture can be understood as a relevant indicator of the alteration suffered by the climate of a given region due to the interactions with soil, vegetation and the atmosphere affecting directly plants water stress. Also, it is linked to other environmental disturbances such as solar radiation, albedo, surface temperature and water vapour gradients, which was mentioned to control the radiative fluxes between the surface and the atmosphere. This makes soil moisture to show a major complexity, since it has a synoptic condition establishing a two-way land-atmospheric interaction defining spatial patterns and temporal dynamics. Also, having information regarding soil moisture is complex due to its spatial and temporal variability; thus, having a continuous and complete database is difficult. In situ, precise measurements can be obtained but when trying to extrapolate them to a major scale, they are not reliable, which generates uncertainty when trying to use them directly to estimated parameters or in the use of hydrological models. The aim of this chapter is to analyse the database generated for SWC in a complex ecosystem using different methods under different latency and spatially in order to answer that (a) shortage periods could redefine water availability and (b) dielectric methods are a real SWC option in an Arnell [7] defined soil moisture as the amount of water stored in the non-saturated zone, where the soil is made of different layers or horizons (soil profile) each with different properties. These soil properties vary depending on the depth and type of rock that forms it, as well as the time at which the soil has developed and the processes that affect it. As the amount of water present in the soil layers depends on the variation of rainfall intensity and the degree of runoff or infiltration after a storm; areas with rainfall >1800 mm are considered wet, 700–1800 mm are wet-dry and <700 mm are dry. Also, loss of moisture in the soil can be as water vapour by evaporation, extraction of plant roots, transpiration or drainage in deeper layers, being the first two more significant during periods of drought [5, 8]. Within the soil, water presents a dynamic behaviour according to the potential water gradients dominated by hygroscopic and gravity for the saturated moisture content and, by capillarity under drier conditions. The hygroscopic soil moisture is defined as the amount of water that adheres to the surface of the soil particles forming a thin film; this humidity is not available for the root zone. Gravitational moisture is the amount of water that enters from the surface of the soil to the unsaturated zone in a vertical movement. Finally, capillary moisture is the amount of water available to the roots [9].

The state of soil moisture could be described in terms of the amount of water and the energy associated with the forces that hold the water in the soil. Both water content and water potential are related to a particular soil by the physical properties such as plant growth, soil temperature, chemical transport and ground water recharge. The amount of water is defined by water content, and the energy state of the water is the water potential. At this, the terrestrial water balance for a surface soil layer, which includes vegetation but not the lateral exchange between adjacent soil volumes, is expressed as:

$$\frac{dS}{dt} = \left.P - E - R\_s - R\_\chi\right.\tag{1}$$

where \_\_\_ *dS dt* is the change of water content within a layer of soil, which considers soil moisture, surface water, ice and groundwater, *P* is precipitation, *E* is evapotranspiration, *Rs* is surface runoff and *Rg* is underground drainage. As soil moisture is not homogeneously distributed varying vertically and horizontally, it differs based on the soil volume being considered. Following this, the soil water content can be expressed based on its distribution in mass and volume; it is function of the apparent density. In the case of the volumetric water content, it is expressed in units of volume of water per volume of soil [m<sup>3</sup> m−<sup>3</sup> ], while for the content of gravimetric water, there is a relation between the mass of water per unit mass of dry soil [kg·kg−<sup>1</sup> ] [4, 10].

Soil moisture (*θ*) is expressed as the ratio of the total volume of soil that is wet [7]:

$$
\Theta = \frac{V\_v}{V\_r} \tag{2}
$$

where *Vw* is the wet volume and *VT* is the volume of the soil both measured in cm<sup>3</sup> . In practice, only a fraction of the soil moisture is measured, which refers to a volume of soil. In the case of the energy balance in a soil layer, the partition of energy between soil and air is influenced by the presence and order of magnitude of soil moisture, and it could be express as:

$$\frac{dH}{dt} = R\_u - \lambda E - SH - G \tag{3}$$

made. Consequently, there are very large temporary resolutions for extensive measurement networks. Also, it is a time-consuming and impractical way of measuring SWC in large scales [13, 15]. In the second case, the indirect methods estimate the humidity present in the soil by measuring another variable affected by the SWC; thus, any changes observed for this variable represent a change in SWC [15]. This type of measurement could also be sub-divided as in situ and remote methods. In in situ measurements, the instrument registered the variable affected by SWC in direct contact with the ground, whereas in the remote case, instruments are not in contact with the ground, and in fact, instruments are ported in satellite, aeroplanes, or other aerial equipment. In any case, they need to be calibrated through the generation of calibration

Correlation between TDR and FDR Soil Moisture Measurements at Different Scales to Establish…

http://dx.doi.org/10.5772/intechopen.81477

59

There are several indirect in situ methods to estimate soil moisture; one of them is the volumetric. This method determines the volumetric moisture of the soil, and some examples are neutron moderation, nuclear magnetic resonance (NMR), and dielectric. The last one measures the ability of a substance to hold the charge (dielectric permittivity). The dielectric permittivity or constant of the soil determines the speed with which an electromagnetic signal is propagated within the soil. They are based on the principle of reflectometry in various domains such as time and frequency [16–19]. The dielectric constant of the soil (Ka) is dependent on the moisture content and, to a lesser degree, on the texture, temperature of the soil, bulk soil and electrical conductivity (EC). Thus, it is required to consider this dependency in order to select not only the appropriate sensor to be used, but also, those sensor properties such as geometric and electronic features [19, 20]. The value of the soil dielectric constant (Ka) is characterised by the contribution of each of its components in the soil: water (Ka ≈ 81), solid (Ka = 4–16) and air (Ka = 1), and it can be affected by temperature, salinity, presence of organic matter and shape and size of solid soil particles [21, 22]. These differences make the dielectric

*Time domain reflectometry* (TDR) determined soil moisture by measuring the transit time of an electromagnetic pulse launched along a parallel metallic probe buried in the soil. It has been shown that the pulse travel time is proportional to the apparent dielectric constant of the soil [25]. Thus, the dissipation signal is proportional to the electrical conductivity of the soil mass; a higher content of water will provide a better propagation velocity [26]. For that water content estimation, once the instrument was calibrated, it can be related to the travel time or to the apparent dielectric permittivity (εa). The main advantages of this technology are its high accuracy, it can be automated, it provides simple measurements, and it is soil texture-, porosity-, temperature- and salinity independent [25]. For different types of soils, there is a direct relationship between the water content (*θ*) and the apparent dielectric constant (Ka). Some disadvantages are related to the cost of the equipment to install the sensors and to its limited applicability of the sensor in soils with conditions of high salinity or in soils with highly conductive clays [21]. *Frequency domain reflectometry* (FDR) provides a continuous measurement of the SWC, by means of an electromagnetic wave that is transmitted along probes and records the frequency of the reflected wave; it presents variations depending on the dielectric properties of the soil measured through the capacitance [13]. This is because the sensors work as part of a capacitor in which the water molecules are polarised and aligned in a dipolar electric field. The capacitor consists of two hollow cylindrical metal electrodes arranged coaxially but separated by

curves using as base the gravimetric SWC.

permittivity very sensitive to SWC variations [23, 24].

where \_\_\_ *dH dt* is the change of energy within a layer of soil, including vegetation, temperature and change of phases associated with aquifers as part of the water balance; *Rn* is net radiation, which considers the differences between short and long wave solar energy input and output; *SH* is the sensitive heat flow; and *G* is the soil energy flow from deep layers of soil to the surface. Here, soil water potential is an expression of the energy state of water in soil and must be known or estimated to describe water fluxes. The last means the movement of water that occurs within the soil profile, between the soil and plant roots, and between the soil and the atmosphere. This movement throughout the soil is dependent on energy gradients, which includes adhesive and cohesive forces. The magnitude of the forces depends on texture and the physical-chemical properties of the soil solid matter. The differences in water potential between different soil positions cause the water to flow in it, moving from the points where the potential is greatest to those where it is least [11]. The saturated zone corresponds to the surface hydrostatic pressure that is equal to the atmospheric pressure. In the unsaturated zone, the volume occupied by the pores is filled with water and air including the area that starts at the surface of the soil and limits with the saturation zone where the water is suspended by capillary forces. As soil moisture is in the unsaturated zone, it is related to parameters such as field capacity (CC), maximum retention, percentage of permanent humidity, hygroscopic coefficient, permanent wilting point (PMP), soil tension, evapotranspiration, among others [12]. The CC is defined as the amount of water that can be retained by the soil against the outside of gravity, whereas the PMP is the amount of moisture that is not enough to stop the wilting of the vegetation.

### **2.1. Soil moisture measurements**

There are different methodologies to measure soil water content (SWC) at the different scales: local, field, basin, and regional and global scale. Also, the transitional zone between each scale could be monitored in terms of having a better description of the condition in the area. However, one restriction related to the SWC measurement in large scales is the installation of the instruments along the study site in contact with the soil, since they need supervision and maintenance. Generally, SWC can be measured directly or indirectly. In the first case, the amount of water in the soil is determined physically, by measuring its weight as a fraction of the total soil weight by a thermalgravimetric method [13]. Some errors (bias), as well as imprecisions (larger variance), could occur when volumetric water content is calculated using an assumed bulk density or one measured elsewhere or at another time [14]. Additionally, to the direct measurement of SWC obtained, other advantages are the simple of the equipment required and that it is used as a standard method useful for the construction of calibration curves for other instruments. The main disadvantage of them is related to its destructive nature since the soil sample is removed from the field, and in consequence, the medium is destroyed and disturbs the soil profile; thus, no repetitive observations should be made. Consequently, there are very large temporary resolutions for extensive measurement networks. Also, it is a time-consuming and impractical way of measuring SWC in large scales [13, 15]. In the second case, the indirect methods estimate the humidity present in the soil by measuring another variable affected by the SWC; thus, any changes observed for this variable represent a change in SWC [15]. This type of measurement could also be sub-divided as in situ and remote methods. In in situ measurements, the instrument registered the variable affected by SWC in direct contact with the ground, whereas in the remote case, instruments are not in contact with the ground, and in fact, instruments are ported in satellite, aeroplanes, or other aerial equipment. In any case, they need to be calibrated through the generation of calibration curves using as base the gravimetric SWC.

the energy balance in a soil layer, the partition of energy between soil and air is influenced by

tion, which considers the differences between short and long wave solar energy input and output; *SH* is the sensitive heat flow; and *G* is the soil energy flow from deep layers of soil to the surface. Here, soil water potential is an expression of the energy state of water in soil and must be known or estimated to describe water fluxes. The last means the movement of water that occurs within the soil profile, between the soil and plant roots, and between the soil and the atmosphere. This movement throughout the soil is dependent on energy gradients, which includes adhesive and cohesive forces. The magnitude of the forces depends on texture and the physical-chemical properties of the soil solid matter. The differences in water potential between different soil positions cause the water to flow in it, moving from the points where the potential is greatest to those where it is least [11]. The saturated zone corresponds to the surface hydrostatic pressure that is equal to the atmospheric pressure. In the unsaturated zone, the volume occupied by the pores is filled with water and air including the area that starts at the surface of the soil and limits with the saturation zone where the water is suspended by capillary forces. As soil moisture is in the unsaturated zone, it is related to parameters such as field capacity (CC), maximum retention, percentage of permanent humidity, hygroscopic coefficient, permanent wilting point (PMP), soil tension, evapotranspiration, among others [12]. The CC is defined as the amount of water that can be retained by the soil against the outside of gravity, whereas the PMP is the amount of moisture that is not enough

There are different methodologies to measure soil water content (SWC) at the different scales: local, field, basin, and regional and global scale. Also, the transitional zone between each scale could be monitored in terms of having a better description of the condition in the area. However, one restriction related to the SWC measurement in large scales is the installation of the instruments along the study site in contact with the soil, since they need supervision and maintenance. Generally, SWC can be measured directly or indirectly. In the first case, the amount of water in the soil is determined physically, by measuring its weight as a fraction of the total soil weight by a thermalgravimetric method [13]. Some errors (bias), as well as imprecisions (larger variance), could occur when volumetric water content is calculated using an assumed bulk density or one measured elsewhere or at another time [14]. Additionally, to the direct measurement of SWC obtained, other advantages are the simple of the equipment required and that it is used as a standard method useful for the construction of calibration curves for other instruments. The main disadvantage of them is related to its destructive nature since the soil sample is removed from the field, and in consequence, the medium is destroyed and disturbs the soil profile; thus, no repetitive observations should be

*dt* is the change of energy within a layer of soil, including vegetation, temperature

*dt* <sup>=</sup> *Rn* <sup>−</sup> *<sup>E</sup>* <sup>−</sup> *SH* <sup>−</sup> *<sup>G</sup>* (3)

is net radia-

the presence and order of magnitude of soil moisture, and it could be express as:

and change of phases associated with aquifers as part of the water balance; *Rn*

\_\_\_ *dH*

to stop the wilting of the vegetation.

**2.1. Soil moisture measurements**

where \_\_\_ *dH*

58 Soil Moisture

There are several indirect in situ methods to estimate soil moisture; one of them is the volumetric. This method determines the volumetric moisture of the soil, and some examples are neutron moderation, nuclear magnetic resonance (NMR), and dielectric. The last one measures the ability of a substance to hold the charge (dielectric permittivity). The dielectric permittivity or constant of the soil determines the speed with which an electromagnetic signal is propagated within the soil. They are based on the principle of reflectometry in various domains such as time and frequency [16–19]. The dielectric constant of the soil (Ka) is dependent on the moisture content and, to a lesser degree, on the texture, temperature of the soil, bulk soil and electrical conductivity (EC). Thus, it is required to consider this dependency in order to select not only the appropriate sensor to be used, but also, those sensor properties such as geometric and electronic features [19, 20]. The value of the soil dielectric constant (Ka) is characterised by the contribution of each of its components in the soil: water (Ka ≈ 81), solid (Ka = 4–16) and air (Ka = 1), and it can be affected by temperature, salinity, presence of organic matter and shape and size of solid soil particles [21, 22]. These differences make the dielectric permittivity very sensitive to SWC variations [23, 24].

*Time domain reflectometry* (TDR) determined soil moisture by measuring the transit time of an electromagnetic pulse launched along a parallel metallic probe buried in the soil. It has been shown that the pulse travel time is proportional to the apparent dielectric constant of the soil [25]. Thus, the dissipation signal is proportional to the electrical conductivity of the soil mass; a higher content of water will provide a better propagation velocity [26]. For that water content estimation, once the instrument was calibrated, it can be related to the travel time or to the apparent dielectric permittivity (εa). The main advantages of this technology are its high accuracy, it can be automated, it provides simple measurements, and it is soil texture-, porosity-, temperature- and salinity independent [25]. For different types of soils, there is a direct relationship between the water content (*θ*) and the apparent dielectric constant (Ka). Some disadvantages are related to the cost of the equipment to install the sensors and to its limited applicability of the sensor in soils with conditions of high salinity or in soils with highly conductive clays [21].

*Frequency domain reflectometry* (FDR) provides a continuous measurement of the SWC, by means of an electromagnetic wave that is transmitted along probes and records the frequency of the reflected wave; it presents variations depending on the dielectric properties of the soil measured through the capacitance [13]. This is because the sensors work as part of a capacitor in which the water molecules are polarised and aligned in a dipolar electric field. The capacitor consists of two hollow cylindrical metal electrodes arranged coaxially but separated by several millimetres with an insulating plastic, and the use of an electronic oscillator produces a sinusoidal waveform [27]. This allow the capacitor to interact with the soil outside of the tube; thus, the capacitance measured will be affected by the soil bulk electrical permittivity and the dipoles respond to the frequency of the electric field, which can determine the capacitance that leads to know the dielectric constant and, therefore, the estimation of SWC. The relationship between the frequency of oscillation and soil water content is inverse.

can be dissolved more easily, allowing faster erosion, and alluvium [30]. The composition of the soil allows the development of the karstic system, where water is filtered, dissolving the rock and creating underground tunnels where the liquid finally flows. These tunnels create a drainage system that feeds certain bodies of water located west and northwest of the Yucatan Peninsula, and this process favours the formation of cenotes, aguadas, wetlands, basins, cav-

Correlation between TDR and FDR Soil Moisture Measurements at Different Scales to Establish…

http://dx.doi.org/10.5772/intechopen.81477

61

The biodiversity contains ecosystems characterised by their great diversity, wealth and fragility. Vegetation corresponds to high jungle subperennifolia (25–50% of the trees drop their leaves), medium jungle subperennifolia, medium jungle subcaducifolia (50–75% of the trees lose their leaves), low jungle subperennifolia, savanna, aquatic vegetation and thorny scrub. One important aspect is that roots grow horizontally due to the karstic nature of the area [32–34]. The CBR is allocated in one of the hottest and wettest regions of Mexico. The climate in the CBR is warm and sub-humid with summer rainfall. The average annual precipitation in the region is 1092 mm. Rain is distributed in the months of May to October with 75% of the annual sheet, with an extension of this season until November. The months June to September are the ones observing more abundant precipitation with an average sheet from 135 to 184 mm. The dry season includes the months of December to April, during which the precipitation is less than 50 mm, and the month with the lowest precipitation is February, with an average of 33.9 mm. The average annual temperature is 24–28°C due to the vegeta-

García et al. [35] indicated that as two slopes divide the Yucatan Peninsula: the Gulf of Mexico and the Caribbean, and the CBR is allocated in the intermedium area being subject to high scarcity. In addition, there are real water pressure in the surrounding area to use water for social development, which is manifested in the constant colonisation of the area and therefore in the opening of new crop and livestock sites. Virtually, all the rainwater infiltrates, which produces little or no runoff and the local rainfall is concentrated in small superficial storage called "aguadas," which hardly maintain the liquid until the following rainy season. Although legally human activities are restricted in the CBR to a tolerant zone, it is being severely affected by irregular human settlements that eliminate the forest to induce changes

The total region was analysed applying first a regular 500 × 500-m grid resulting in systematic 50 sites distributed within the whole area. Then, a zigzag statistical method was used reducing the sample to 18 sites. The priority was to allocate an aguada with or without human impact and within a town to guarantee its maintenance, thus at the field, as some of them were inaccessible, the final sites were nine. The sites were distributed as: three in the northern zone (Refugio, Flores Magón and Modesto Ángel), three in the southern zone (Carlos A. Madrazo—two sites: La Ceiba and Corosal, and Ley de Fomento) and three in the archaeological zone (Ramonal, Bonfil and Heliport) of the CBR. In the case of Carlos A Madrazo, La Ceiba aguada was only analysed since Corosal presented

erns and springs [31].

tion that regulates it.

in land use.

eutrophication.

**3.1. Selection of point measured sites**
