Soil water content
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.