Measurement of Water Content of Soil

The measurement of water content in the soil is of great importance in many investigations and applications pertaining to agriculture, hydrology, meteorology, hydraulic engineering, and soil mechanics. In the fields of agronomy and forestry, the amount of water contained in the soil affects plant growth and diffusion of nutrients toward the plant roots, as well as acting on soil aeration and gaseous exchanges, with direct consequences for root respiration. Also, continuous monitoring of soil water content can support the setting up of optimal strategies for the use of irrigation water. In hydrology, moisture condition in the uppermost soil horizon plays an important role in determining the amount of incident water—either rainfall or irrigation water — that becomes runoff. Evapotranspiration processes, transport of solute and pollutants, and numerous hydraulic (e.g., retention, conductivity) or mechanical (e.g., consistency, plasticity, strength) soil properties depend on soil water content.

Several methods have been proposed to determine water content in soil, especially under field conditions. Soil water content can be measured by direct or indirect methods

Direct Methods

Direct methods involve removing a soil sample and evaluating the amount of water that it contains. Their use necessarily entails the destruction of the sample and hence the inability to repeat the measurement in the same location.

The most widely used direct method is the thermogravimetric method, often considered as a reference procedure because it is straightforward, accurate, and inexpensive in terms of equipment. This method consists of collecting a disturbed or undisturbed soil sample (usually of about 100-200 g taken with an auger or sampling tube) from the appropriate soil depth, weighing it, and sealing it carefully to prevent water evaporation or the gaining of moisture before it is analyzed. Then, the soil sample is placed in an oven and dried at 105-110°C. The residence time in the oven should be such that a condition of stable weight is attained, and it depends not only on the type of soil and size of the sample but also on the efficiency and load of the oven. Usual values of the residence time in the oven are about 12 h if a forced-draft oven is used, or 24 h in a convection oven.

At completion of the drying phase, the sample is removed from the oven, cooled in a desiccator with active desiccant, and weighed again. The gravimetric soil water content is calculated as follows:

where ww and wd represent the mass of wet and dry soil (kg), respectively, and ta is the tare (kg).

The major source of error using the thermogravimetric method together with (5.95) is related to sampling technique. The fact that the soil cores may contain stones, roots, and voids, as well as certain unavoidable disturbances during sampling, may affect the precision in determining the value of the volumetric water content in soil.

Indirect Methods

Basically, indirect methods consist of measuring some soil physical or physicochem-ical properties that are highly dependent on water content in the soil. In general, they do not involve destructive procedures and use equipment that also can be placed permanently in the soil, or remote sensors located on airborne platforms and satellites. Thus, indirect methods are well suited for carrying out measurements on a repetitive basis and in some cases also enable data to be recorded automatically, but require the knowledge of accurate calibration curves.

The main indirect methods are gamma attenuation, neutron thermalization, electrical resistance, time-domain reflectometry (TDR). Other indirect methods are low-resolution nuclear magnetic resonance imaging and remote-sensing techniques.

A typical nondestructive laboratory method for monitoring water contents in a soil column is based on the attenuation and backscattering of a collimated beam of gamma rays emitted by a radioactive source, such as cesium-137. In case of shrinking/swelling porous materials, a dual-energy gamma-ray attenuation system (usually employing cesium-137 and americium-241 as the radioactive sources) can be used to measure simultaneously bulk density and water content in a soil sample.

Instead, the neutron method often is used for field investigations and enables soil water contents to be determined by the thermalization process of high-energy neutrons colliding with atomic nuclei in the soil, primarily hydrogen atoms [2]. Because hydrogen is the major variable affecting energy losses of fast neutrons, the count rate of thermalized (slow) neutron pulses can be related to soil water content. Actually, the calibration curve linearly relates the volumetric soil water content 6 to the relative pulse count rate N/Nr; that is, where N is the measured count rate of thermalized neutrons, Nr is the count rate under a "reference" condition, and n1 and n2 are parameters. Some manufacturers suggest that the reference count rate Nr be obtained in the same protective shield supplied for the probe transportation, but this value can be highly affected by humidity and temperature of the surrounding environment and by the relatively small size of the shield. More effectively, the value of the reference count rate should be taken in a water-filled tank (e.g., a cylinder of 0.6 m in height and 0.5 m in diameter) on a daily basis during the w = [(Ww + ta) - (wd + ta)]/[(wd + ta) - ta],

investigation. Parameter ni chiefly depends on the presence of substances that play a basic role in the thermalization process, such as boron, cadmium, iron, and molybdenum, whereas the value of parameter n2 is strongly affected by soil bulk density and is nearly zero for very low values of bulk density.

Employing a factory-supplied calibration curve can be inadequate in most situations. It thus is recommended that the calibration curve be obtained experimentally in the field by relating the measured count ratio n/Nr for a soil location to simultaneous measurements of soil water content with the thermogravimetric method. Oven-dry bulk densities are also to be measured. One drawback of the neutron method is the low spatial resolution under certain conditions associated with the thermalization process. Close to saturation, the measuring volume is approximately a sphere 0.15 m in diameter, but under dry condition the diameter of this sphere is about 0.50-0.70 m. Therefore, larger uncertainties are to be expected when the soil profile consists of several alternating layers of highly contrasting soil texture, as well as when measurements are performed close to the soil surface. Moreover, because of the influence of the size of the sphere, the neutron method is not very useful for distances between the measuring depths less than 0.10 m.

In the past decade, indirect estimation of water content by measuring the propagation velocity of an electromagnetic wave is becoming increasingly popular. One method that exploits this principle and that can be employed in laboratory and field experiments, is TDR, which actually determines the apparent dielectric permittivity of soil by monitoring the travel time for an electromagnetic signal (TDR pulse) to propagate along a suitable probe inserted in the soil at the selected measuring depth. Dielectric properties of a substance in the presence of an electromagnetic field depend on the polarization of its molecules and are described by the apparent relative dielectric permittivity e, which is a dimensionless variable always greater than unity and conveniently defined by a complex relation as the sum of a real part, e', and an imaginary part e" of e. The real part of the dielectric permittivity mainly accounts for the energy stored in the system due to the alignment of dipoles with the electric field, whereas the imaginary part accounts for energy dissipation effects [4]. In a heterogeneous complex system, such as soil, essentially made of variable proportions of solid particles, air, water, and mineral organic liquids, it is extremely difficult to interpret dielectric behavior, especially at low frequencies of the imposed alternated electrical field. However, within the frequency range from about 50 MHz to 2 GHz, the apparent relative dielectric permittivity of soil, e^oil, is affected chiefly by the apparent relative dielectric permittivity of water (ewater = 80 at 20°C) because it is much larger than that of air (e^ = 1) and of the solid phase (e^olid = 3-7). Within the above range, it is therefore possible to relate uniquely the measurements of soil relative permittivity e^oil to volumetric water content by means of a calibration curve. Moreover, the employed measurement frequency makes the soil relative permittivity rather invariant with respect to the frequency and hence usually it also is referred to as the dielectric constant of soil.

By examining a wide range of mineral soils, Topp and his colleagues [5] determined the empirical relationship

between the volumetric soil water content 6 and the TDR-measured dielectric permittivity of the porous medium e'm. The regression coefficients a, are, respectively, a1 = 5.3 x 10-2, a2 = 2.92 x 10-2, a3 = 5.5 x 10-4, and a4 = 4.3 x 10-6.

Even though the calibration equation (5.99) does not describe accurately the actual relation 6 (e'm) when e'm tends toward 1 or to the value of the dielectric permittivity of free water; however, it is simple and allows good soil water content measurements within the range of 0.05 < 6 < 0.6, chiefly if only relative changes of 6 are required. For non-clayey mineral soils with low-organic-matter content, absolute errors in determining water content by Eq. (5.99) can be even less than ±0.015m3/m3, whereas an average absolute error of about ±0.035m3/m3 was reported for organic soils.

When absolute values of 6 and a greater level of accuracy are required, a site-specific calibration of the TDR-measured dielectric permittivity e'm to soil water content 6 should be evaluated. In this case, and especially if measurements are to be carried out close to the soil surface, a zone where soil temperature fluctuations can be relatively high during the span of the experiment, one also should take into account the dependence of temperature

where values of the constants b of this polynomial are, respectively, b1 = 87.74, b2 = 0.4001, b3 = 9.398 x 10-4, and b4 = 1.410 x 10-6. However, relation (5.100) strictly holds for free water only and can be considered as acceptable for sandy soils, but it cannot be used for clayey and even for loamy soils.

Finally, note that this device does not lead to point measurements, but rather it averages the water content over an averaging volume that mainly depends on the length and shape of the TDR probe employed.

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