**2.2. Soil water potential**

most common methods are shortly explained. More in depth explanation of a given methods

The gravimetric method for the water content measurement represents simplest way to gather accurate soil water content. A soil sample of a known mass is placed in the container, dried in an oven, removed from the container, and allowed to cool in desiccator, then reweighted. The drying procedure is done by placing the sample in convection oven at 105°C for 24 h. From the measurement, gravimetric water content can be calculated and also converted into

**Neutron probe** uses radioactive material for measuring soil water content. A neutron meter is placed at the soil surface above the access tube in which probe is lowered into the soil to the desired measurement depth. The probe contains an americium 241/beryllium pellet that emits fast high‐energy neutrons. These high‐speed neutrons pass through the accesses tube and collide with hydrogen atoms in the surrounding soil and water. When neutrons hit H nuclei, they slow down and some are reflected back to the source tube and counted by the neutron detector. Because soil water is the primary source of hydrogen atoms, the count is directly related to the soil water content. This method can produce reliable results and can be used for measurements at multiple depths in few minutes. Its application might be questionable for shallow measurements (>15 cm), since the neutrons might escape from the soil instead of being detected. However, the main disadvantage is the use of radiative source that can be a potential

Time domain reflectometry (TDR) is a technique that involves measuring the travel time of an electromagnetic wave along a wave guide. The bulk soil properties affecting electromagnetic wave are electrical conductivity and dielectric permittivity. Electrical conductivity is a measure of the free electrons flow when exposed to an electric field. The dielectric permittivity is a measure of the displacement of constrained charges when exposed to an electric field. The speed of the electromagnetic wave in soil is dependent on the dielectric permittivity of soil matrix. The fact that water (80) has much larger dielectric constant than air (1), soil (3–7), or its organic components (2–5) is used to determine the volumetric water content of the soil. The TDR instrument consists of 2–3 parallel rods that are inserted into the soil and act as a waveguides. Electronics in the TDR instrument generate and sense the return of high‐energy signal that travels through the soil along the waveguide, that is, stainless steel rods. The high‐ frequency signal is then converted to the volumetric water content. Readings can be affected by high clay content, high organic matter, or high soil salinity. The rods need to be inserted fully into the soil and have good contact with surrounding soil particles. The TDR probes are

widely used due to its simplicity, accurate readings, and minimal soil disturbance.

**Capacitance devices** are used to determine the resonance frequency of a given soil. The capacitor is connected to an oscillator to form an electrical circuit; changes in soil moisture can

could be found in Refs. [1, 2].

136 Groundwater - Contaminant and Resource Management

volumetric water content using soil bulk density.

*2.1.1.1. Direct measurement*

*2.1.1.2. Indirect measurement*

health hazard for the device operator.

While the knowledge of soil water content is important, it is also important to know its energy potential since water can be held by the force fields of soils particles differently depending on soil type, (soil texture). For example, two soils with identical water content may have different soil water potential, such that one will easily allow plant water uptake (e.g. sandy soil), while in the other type, soil water will be extracted much harder (e.g. clay soil). The total energy state of soil water is defined by its equivalent potential energy, as determined by the various forces acting on the water per unit quantity. In most cases the kinetic energy of water can be neglected, since the flow rates in soil are very slow. Therefore, the energy state of soil water is defined by its equivalent potential energy, which is derived from its position in a force field. Water in the soil will move from area with high soil water potential energy to area of lower potential energy. Driving force for the flow is the change in potential energy with distance (soil water potential gradient).

These driving forces determine the following:


The potential energy of water in the soil is defined relative to its reference or standard state. Standard state water is pure (no solutes), free (no external forces other than gravity) water at a reference pressure (atmospheric), reference temperature, and reference elevation [3]. Soil water potential is defined as the difference in potential energy per unit of volume, mass, or weight of water compared to the standard (reference state). Depending on the choice of unit for quantity, three different systems can be used (**Table 1**.)


**Table 1.** Different unit systems for expressing soil water potential (adapted with permission from [4]).

Total soil water potential is defined as the amount of work per unit quantity of pure water that must be completed by external forces to transfer reversibly and isothermally an infinitesimal amount of water from the standard state to the soil at the point under consideration [3]. The transformation of water from the reference states can be divided into the components caused by each force field acting on soil water. These components are forces caused by gravity, hydrostatic pressure, capillarity, solute, air pressure, and swelling [5].

Following are presented the definitions of the most important components of total soil water potential (*ψ*) which is represented by the sum of its active components:

$$
\Psi = \Psi\_{\underline{z}} + \Psi\_{\underline{p}} + \Psi\_{\underline{s}} + \Psi\_{\underline{m}} + \Psi\_{\underline{a}} \tag{4}
$$

*Gravitational potential* (*ψz*) is defined as the difference in energy per unit volume or weight between standard water and soil water due to gravity. This component quantifies the effect of the gravitational force field on the energy of soil water.

*Hydrostatic pressure potential* (*ψp*) is defined as the difference in energy per unit volume or weight between standard water and soil water due to the pressure exerted by overlying free water. This component quantifies the pressure effect from overlying water on the energy of water.

*Osmotic (solute) potential* (*ψs*) is defined as the difference in energy per unit of volume or weight between standard water and soil water due to the presence of solutes. This component quantifies the effect of solutes on the energy of soil water.

*Matric potential* (*ψm*) is defined as the difference in energy per unit volume or weight between standard water and soil water due to capillarity and adsorption. This component quantifies the effect of the capillarity and adsorption on the energy of soil water.

*Air potential* (*ψa*) is defined as the difference in energy per unit volume or weight between standard water and soil water due to effect of soil air pressure. This component quantifies the effect of the air pressure in soil porous system on the energy of soil water.

Some of the components of water potential can be neglected like osmotic pressure and also the effect of air pressure in most of the cases due to its low effect (and estimation difficulty) on the global soil water potential. Following these assumptions, total soil water potential head or hydraulic head is when expressed per unit weight:

$$H = h + z\tag{5}$$

Thus, hydrostatic pressure (*h*) and gravity (*z*) dominate the potential energy of water under unsaturated condition.
