**1. Introduction**

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 ecosystem highly dynamic.

**2. Soil moisture**

water available to the roots [9].

between adjacent soil volumes, is expressed as:

\_\_\_ *dS*

where \_\_\_ *dS*

[kg·kg−<sup>1</sup>

] [4, 10].

runoff and *Rg*

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

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

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

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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

*dt* is the change of water content within a layer of soil, which considers soil moisture,

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

of gravimetric water, there is a relation between the mass of water per unit mass of dry soil

*VT*

only a fraction of the soil moisture is measured, which refers to a volume of soil. In the case of

is underground drainage. As soil moisture is not homogeneously distributed

surface water, ice and groundwater, *P* is precipitation, *E* is evapotranspiration, *Rs*

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

where *Vw* is the wet volume and *VT* is the volume of the soil both measured in cm<sup>3</sup>

is expressed in units of volume of water per volume of soil [m<sup>3</sup>

*<sup>θ</sup>* <sup>=</sup> *<sup>V</sup>*\_\_\_*<sup>w</sup>*

*dt* <sup>=</sup> *<sup>P</sup>* <sup>−</sup> *<sup>E</sup>* <sup>−</sup> *Rs* <sup>−</sup> *Rg* (1)

m−<sup>3</sup>

is surface

(2)

. In practice,

], while for the content
