Subsurface moisture content

The moisture content can be expressed as the mass of water (mw) within a mass of dry soil, regolith or rock (ms), mw n _ _w

This is called the mass wetness (0m, gg-1, also called gravimetric wetness; the term avoided here to prevent confusion with the gravimetric method discussed later). This measure is rarely used by hydrologists because of the influence of unmeasured variations in the mass of dry soil, regolith or rock (ms) within undisturbed volumes of soil, regolith or rock (Vs), i.e. the dry bulk density of the soil (gcm-3). Consequently, the most commonly expressed measure of moisture content is the volumetric wetness (0 w, cm cm-3) where the mass wetness is multiplied by the dry bulk density (pd) of the same sample. Given that 1 cm3 of water has a mass of 1 g, this means that the volumetric wetness is equivalent to the volume of water (Vw) within an undisturbed volume of soil (Vs), m m V

The porosity of a soil, regolith or rock (n, cm cm-3) is the total pore space within the porous media (Section 5.4), and is equivalent to the maximum volumetric wetness, i.e. all pore space is filled with water. The saturation wetness (0 , limits 0^1) is the proportion of the pore space that is filled with water,

Direct measurement of moisture content can be achieved by only a few methods, notably the gravimetric method and carbide gas method.

6.1.1 Carbide gas method

The carbide gas method is not commonly used within hydrology, but is commonly used within agriculture. With this method, a known wet mass of soil can be added to a pressure vessel containing a quantity of calcium carbide (CaC2). Once sealed and the two components brought together, water reacts with the calcium carbide to produce ethyne (C2H2, acetylene) gas,

The increased pressure within the vessel (called a 'Speedy Soil Moisture Meter') caused by the production of ethyne is then measured with a pressure gauge, calibrated to values of mass wetness.

6.1.2 Gravimetric method

The gravimetric method can be used to measure the mass wetness, volumetric wetness and saturation wetness. If volumetric wetness is required, an undisturbed soil or regolith core of known volume (Vs) must be first removed from the ground. This sample is then returned to the laboratory where is it first weighed before being transferred to an oven. Following the British Standard (BS1377-3, 1990), the sample is dried in the oven for 24 h at 105° C, cooled in a desiccator and then reweighed to determine by the volume of water lost (Vw) and dry soil or regolith mass (ms). For sands, further drying would not give further moisture loss. However, with clays moisture would continue to be lost, primarily from the structure of the clays. Thus, for clay soils the British Standard is designed to give a reference moisture content rather than an absolute value. Particular consideration needs to be given to the determination of moisture content within organic soils (e.g. peat), as drying above 50°C also results in loss of organic materials by volatilisation. Particular care also needs to be given to the sampling of well-structured soils, as disturbances to dry bulk density (i.e. amount of soil collected in the sample tin) affect the apparent volumetric wetness. Once dried at 105° C, the structure of soil sample will not return to the natural state, so that a sample cannot be returned to the field for re-sampling. The gravimetric method is therefore, described as a destructive sampling method. Many hydrological applications do, however, require repeated sampling of the same soil volume to derive a time series of moisture content dynamics. This need has led to the development of several indirect or analogue methods of determining, typically, volumetric wetness. The most commonly used methods in the UK are neutron moderation (Fig. 6.1), time-domain reflectometry, satellite and airborne radiometry and electrical resistivity. Where undisturbed samples can be taken for gravimetric analysis, it is often recommended that the use of these methods is accompanied by local calibration against the gravimetric method.

6.1.3 Neutron moderation method

Neutron moderation (also called neutron scattering or attenuation) is a technique developed in the 1950s whereby fast neutrons from a sealed americium-241:beryllium source bombard a similarly sized mass, such as hydrogen nuclei in the form of water, to give a cloud of thermalised slow neutrons. The density of this cloud can then be measured with a detector (e.g. boron trifluoride or helium-3). One such device is the Institute of Hydrology neutron probe (Institute of Hydrology, 1979). With this device, the source and detector are lowered into the ground to the required measurement depth using sealed aluminium 'access tubes'. After a day's measurements, a long count is taken within a water reference to correct for the effects of radioactive decay of the source. This device was shown to give reliable measurements of volumetric wetness, except within the topsoil, where neutron losses from the soil surface make the technique very sensitive to the exact depth of sampling. Installation of access tubes and measurements down to 3 m has been possible. From the 1970s to early 1990s, the neutron probe was the preferred method of measuring a time series of soil moisture content by UK hydrologists. Increasing regulation of radioactive sources and the inability to automate large field arrays has meant that most UK hydrologists now prefer to measure volumetric wetness using techniques based upon time-domain reflectometry. The exception to this is the civil engineering community who continue to use neutron moderation to measure near-surface (0-0.3 m) wetness using surface moisture-density gauges (Fig. 6.1).

6.1.4 Time-domain reflectometry method

Time-domain reflectometry (TDR) is based upon the transmission and reflection of electromagnetic (EM) signals along parallel wave-guides. The rate of propagation of a reflection (caused by a large impedance change) from the bottom of the wave-guide to the top depends upon the dielectric properties of the material surrounding the waveguide (i.e. soil and water). For signals of between 50 MHz and 1 GHz, the propagation velocity (vp; ms-1) is simply,

VP = 7~e where c is the speed of light in a vacuum (3 x 108 ms-1) and £ (or Ka) is the apparent dielectric constant or relative permittivity of the material surrounding the wave-guide (dimensionless). If the top and bottom of the wave-guide can be identified in a trace on a time-domain reflectometer oscilloscope, then the velocity of an EM wave returning

Troxler Moisture Density Gauge
Fig. 6.1 A surface moisture-density gauge used for measuring volumetric wetness within surface (0-0.3 m) soil layers. (Reproduced with permission of Troxler Electronic Laboratories.)

from the bottom to the top of the wave-guide is, L

v = — t where L is the length of the wave-guide (m; for standard cable tester, e.g. Tektronix 1502C) and t is the travel time (seconds). Combining the last two equations gives the dielectric constant as,

The distance ct (metres) can be derived directly from the trace on the oscilloscope, manually or by data-logging and processing the data. Where such high propagation velocities are used, the apparent dielectric constant of soil varies largely with changes in moisture content and Topp et al. (1980) give the following relationship for mineral soils, ev = -5.3 X 10-2 + 2.92 X 10-2e - 5.5 x 10-4e2 + 4.3 x 10-6e3

Level 1

Includes multiplexer and TDR100 reflectometer controlled by the data logger

SDM cable(s) for multiplexer control

Level 1

Includes multiplexer and TDR100 reflectometer controlled by the data logger

Level 2

Supports up to eight multiplexers increasing system capacity to 64 probes

SDM cable(s) for multiplexer control

Coaxial cable(s) to probes or multiplexers

Level 2

Supports up to eight multiplexers increasing system capacity to 64 probes

Coaxial cable(s) to probes or multiplexers

Coaxial cable(s) to probes or multiplexers

Level 3

Supports up to 64 multiplexers increasing system capacity to 512 probes

Coaxial cable(s) to probes only

Level 3

Supports up to 64 multiplexers increasing system capacity to 512 probes

Coaxial cable(s) to probes only

Coaxial cable

Coaxial cable

Fig. 6.2 A time-domain reflectometry system for measuring volumetric wetness at several locations, showing a time-domain reflectometer, datalogger, RF-MUX and a wave-guide. (Reproduced by permission of Campbell Scientific Ltd.)

where 9 is the volumetric wetness, and £ is the relative permittivity or apparent dielectric constant (dimensionless). The wave-guides are metal rods, pushed into the ground no more than 0.05 m apart. For sandy soil, wave-guides can be up to 0.7 m in length, before the signal is so attenuated that the distance ct cannot be identified on the oscilloscope trace (or by an analysis program). For clay soils, the maximum wave-guide length may be only 0.3 m. It should be noted that the inflection in the trace produced by the lower end of the wave-guide is not always sharp or simple. Where such conditions arise, significant errors can be introduced into the ct measurement and hence volumetric wetness measurement.

By connecting several wave-guides to radio frequency multiplexors (RF-MUX), the volumetric wetness at many locations in the field or laboratory can be monitored automatically with a data logger (Fig. 6.2).

6.1.5 Simplified time-domain reflectometry method

High cost is the main disadvantage of time-domain reflectometers operating at gigahertz frequencies, though there is also some risk of vermin damage (then water damage) to the coaxial cables connecting the wave-guides to the reflectometer and

Fig. 6.3 Two examples of a simplified soil moisture probe using time-domain reflectometry principles, namely a Theta probe (top: reproduced by permission of Delta-T Devices Ltd), and a CS6I6 probe (bottom: reproduced by permission of Campbell Scientific Ltd).

radio frequency multiplexer (RF-MUX). As a consequence, many hydrologists opt to use simplified sensors using time-domain reflectometry principles that are one-tenth of the cost and lack coaxial cables.

With simplified TDR moisture probes, a continuous 100 MHz outgoing wave is sent down a wave-guide and a reflection generated at the lower end. These outgoing and returning signals interfere, producing a composite standing wave. The ratio of the outgoing wave to the composite standing wave is dependent on the dielectric constant of the soil around the wave-guide and can be calibrated to the volumetric wetness. Two such simplified TDR moisture probes are shown in Fig. 6.3. The main disadvantage of these simplified probes is the greater uncertainty in the volumetric wetness readings partly due to a greater sensitivity to local soil characteristics.

6.1.6 Satellite and airborne radiometry method

As with TDR (and simplified TDR), airborne and satellite radiometry measure changes in the dielectric properties of the ground surface (and the ground surface roughness). With active microwave radiometry, an EM signal produced by a remote power source is propagated to the target ground surface and a proportion is reflected and returns to a remote sensor. The amplitude of the received/transmitted power ratio, also called the backscatter coefficient (dB), is proportional to the dielectric constant of the ground

(Ulaby et al., 1986). An example of a study using active microwave radiometery involves the 5.3 GHz (C-band) AMI-SAR instrument on the ERS-2 satellite platform (Walker et al., 2004). This study was able to map volumetric soil moisture content over the 0.1 km2 Nerrigundah catchment in Australia. Similar radiometers have been used on airborne platforms. For example, the polarimetric scanning radiometer (PSR/CX), was flown aboard a NASA P-3 aircraft to measure moisture content over a large basin in Mexico (Vivoni et al., 2008).

6.1.7 Electrical resistivity method

The problem with all moisture probes using the principles of TDR is the maximum volume of ground that can be sampled with a single wave-guide. The well-established geophysical technique of electrical resistivity can overcome the volume constraint on moisture measurement. The bulk electrical resistivity of the soil, regolith or rock (pb, ^m) is related to moisture content via Archie's law, where py is the electrical resistivity of the pore water, m and n are fitting parameters, sometimes known as the cementation exponent and saturation exponent, respectively. Alternatively, the relation can be expressed as the inverse of resistivity, namely conductivity, where ab is the bulk electrical conductivity (Sm-1; note 1 Sm-1 =1 x 10-5^.Scm-1), and aw is the electrical conductivity of the pore water (Sm-1; Archie, 1942). Given that the volumetric wetness is the product of the porosity and saturation wetness, this relation has been simplified to give the expression, where A and B are alternative fitting parameters (Shah and Singh, 2005; Schwartz et al., 2008).

The bulk electrical conductivity (or bulk resistivity) of the soil, regolith or rock can be determined using various electrode configurations installed on the ground surface or in boreholes. A Wenner configuration uses two current and two potential electrodes; these are equally spaced in a line, with the current being applied to the outer electrodes (Fig. 6.4). The resistance measured between the two potential electrodes usually being most influenced by the ground at a depth equivalent to approximately half

n = 1 *

Fig. 6.4 A Wenner electrode configuration used for measuring subsurface moisture content by electrical resistivity. (Reproduced with permission from Kirsch, R. (2006) Groundwater Geophysics. Springer-Verlag, Berlin.)

the electrode spacing. With this configuration, the bulk resistivity or bulk conductivity is derived from the measured resistance using,

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