Indirect methods to determine soil water content are widely used in research and also in practical applications as an alternative to gravimetrical determination. These methods are called “indirect” because they do not measure soil water content directly, but they measure some other variable from which the soil water content can be calculated. Based on a known relationship between the actual soil water content and this variable, the soil water content can be determined more or less with accuracy.
The advantages of using indirect methods are clear: The indirect methods are non-destructive, measured data are immediately available, the measurement can be repeated several times in the same spot or the measurement can be taken continuously and operated by a computer. When the sensors are well calibrated, the measured data are accurate enough for most applications. The calibration is always carried out by comparison with the gravimetrical method. Below two indirect methods to determine soil water content which are presently widely used are presented. 

TIME DOMAIN REFLECTOMETRY (TDR)

Principle

TDR is based on the evaluation of the speed by which a fast voltage pulse travels along a parallel transmission line inserted in the soil. The higher the dielectric constant of the soil in contact with the transmission line, the slower the speed of the voltage pulse. Because of the big difference between the dielectric constant of water and that of the other soil constituents (soil particles, air, …), the travel time of the pulse is essentially dependent on the volumetric water content of the soil (Topp et al., 1980). 

Materials and tools

TDR TRIME-FM (Imko, GmbH.), installation kit (stainless steel rods with the holder, hammer)

Important notes

– Pay special attention to the installation of the soil moisture sensor, avoid forcible application in the stony (rocky) areas. Use the stainless steel rods to prepare the holes for the measurement rods.
– Repeat the measurement several times. 
– Do not forget to recharge the reading device’s battery before you take it to the field. 

FREQUENCY DOMAIN (FD) CAPACITANCE and FREQUENCY DOMAIN REFLECTOMETRY (FDR)

Principle

The electrical capacitance of a capacitor that uses the soil as a dielectric depends on soil water content θ. When this capacitor, which is made of metal plates or rods imbedded in the soil, is connected to an oscillator to form an electrical circuit, changes in soil moisture can be detected by changes in the circuit operating frequency. These changes form the basis of the Frequency Domain (FD) technique which is used in Capacitance and Frequency Domain Reflectometry (FDR) sensors.
In capacitance sensors, a relative permittivity ε of soil is determined by measuring the charge time of a capacitor made with that soil. In FDR, the oscillator frequency is controlled within a certain range to determine the resonant frequency (at which the amplitude is greatest), which is a measure of water content in the soil (Muñoz-Carpena et al., 2006). Probes consist of two or more electrodes (e.g. plates or rods) that are inserted into the soil. When an electrical field is applied, the soil around the electrodes forms the dielectric of the capacitor to complete the oscillating circuit (see behind Photo 1 – 4 in one line – Frequency domain (FD) capacitance (rods); Photo of ECH2O – EC10 – Frequency domain (FD) capacitance where plates are embedded in a board). The measurements may be affected by temperature, salinity, bulk density and clay content, so a soil-specific calibration is recommended.

Materials and tools

Theta Probe Soil Moisture Sensor ML2x (ADR) and read-out and data storage device Moisture Meter HH2 (Delta-T Devices, Inc.),
ECH20 – EC-5 Soil Moisture Sensor (FDR) and read-out device ECH2O Check (Decagon Devices, Ltd.).

Important notes

– Before the measurement, the settings of the reading device need to be checked. Choose either generic (factory) or your own calibration equation.
– Pay special attention to the installation of the soil moisture sensor (ensure good contact with the soil).
– Repeat the measurement several times. 
– Have a spare battery for the reading device in case you need it.

Example

Indirect methods to determine soil moisture content need to be calibrated before their use for specific soil conditions. Usually, some general calibration equation is provided by the manufacturer for mineral and organic soil. However, for more accurate measurements it is recommended that the soil specific calibration is carried out. With a known calibration equation, it is possible to determine the volumetric soil moisture content for any sensor reading in mV. Soil moisture sensor Decagon ECH2O – EC10 was calibrated in the laboratory. The readings in mV were recorded for given soil moisture contents (Tab 1). Dry bulk density remained the same during the whole calibration procedure. Based on the measured outputs in mV at known volumetric soil moisture contents, parameters a and b of the linear equation y = a x + b can be determined. The parameters can be found either by plotting a linear regression graph (see Fig 1) or by employing the functions “SLOPE” and “INTERCEPT” in MS Excel.

With the found calibration equation it is possible to determine the volumetric soil moisture content for any sensor reading in mV. For example, for the sensor reading 500 mV: 

y = 0.00098 . 500 – 0.232296
y = 0.258 cm3 cm-3

The volumetric soil moisture content adequate to the sensor reading 500 mV is 0,258 cm3 cm-3.