What do soil moisture sensors measure?
Soil moisture sensors determine the water content (also moisture content) of the soil, so the amount of water that is contained in a soil at a specific time. This ratio can be defined by the mass (gravimetric water content) or by the volume (volumetric water content), whereby it is usually calculated as a proportion of the soil volume (volumetric). The measured values are indicated as a proportion (between 0 … 1) or in % Volume.
The gravimetric water content (Θg) results from the ration of the mass of water to mass of soil.
The volumetric water content (Θ) results from the ratio of the volume of the water to the volume of the soil section respectively the soil sample.
With a known dry bulk density Θg and Θ can be converted into each other.
What is the purpose of determination of the soil moisture?
- understanding of the behavior of soils and internal processes taking place
- assessment of soil fertility
- estimation of plant growth, plant water requirements and consumptive use
- irrigation management
- assessment of water storage capacity of soils
- estimation of water infiltration capacity, seepage and leaching of chemicals into the groundwater
- information about other physical properties of soils (plasticity, consistency, carrying capacity etc.)
- data for water balance studies
Which measuring principles are there?
The gravimetric method is the only direct method to determine the water content (Θ). It only works for retained soil samples. The soil sample is weighed, then dried at 105 °C to constant weight and weighed again. The difference of the weight of the sample in the wet and in the dry state provides the mass of water in the sample. To determine the volumetric water content Θ, subsequently the mass of water is converted by means of the density into the volume of the water.
- TDR – Time-Domain-Reflectometry
TDR (Time-Domain-Reflectometry) is an indirect method for determination of the water content. TDR probes measure the relative permittivity (εr) of the soil. It is the sum of the permittivity’s of all soil components (water, soil minerals, air). The relative permittivity of water (εr = 81) is much higher than that of air (εr = 1) or those of minerals (εr = 2-5). Thus, it is the determining factor for both the total permittivity and changes of it. The relationship between the water content and relative permittivity of the soil can be described mathematically by regression formulas. In practice, often the function according to Topp and Davis is used (see FDR).
The measurement of the relative permittivity is based on a speed and runtime measurement of an electromagnetic wave in the soil body. Parallel rods of the same length are introduced to the soil through which highly frequent electromagnetic pulses are conducted. At the beginning and at the end of the rods the pulses are partially reflected and thus, the propagation speed over the double length of the rods is measured. The water content and thus the relative permittivity of the soil strongly affect the propagation speed. εr is calculated from runtime (t), double length of the rods (2l) and light speed (c0).
- FDR - Frequency-Domain-Reflectometry
As for the TDR measurement, the water content is indirectly determined via the relative permittivity (εr) in this method. A FDR probe generates by means of an integrated oscillator a continuous electromagnetic signal with a constant frequency and amplitude, transmitted by a high frequency cable (measuring rods, antennas). That creates an oscillating electromagnetic field around the rods.
The soil is a dielectric, respectively an electrically insulating material, functioning as a resistant. It attenuates and partially reflects the electromagnetic signal. The relative permittivity of the soil is a measure of the strength of the resistant. The attenuation of the original signal results in an amplitude damping. The amplitude damping of the reflected signal is detected and analyzed. By means of regression functions or tables εr is calculated. Then the water content is determined from the relative permittivity, often with the function according to Topp and Davis.
Capacitive sensors determine the relative permittivity (εr) by measuring the capacity (C) of a capacitor. A capacitor is an electronic device for storage of electric charge (Q). It consists of two conductors (parallel plates) with a defined surface area (A) and distance (d). The conductors are separated by a non-conductive region, known as a dielectric or electrically insulating material (air, soil, etc.). The amount of charge stored on the plates is proportional to the applied voltage (V). The capacity is the constant of proportionality between voltage and charge.
A change of the capacity of the capacitor corresponds with a change of the relative permittivity of the soil. According to this relationship εr of the soil is determined. Besides εr also ε0, the dielectric constant (permittivity of vacuum) is needed for the calculation. Subsequently the water content is determined by means of regression formulas (e.g. Topp and Davis).
The UMP sensors are a series of combi sensors developed and manufactured by Umwelt-Geräte-Technik GmbH. A UMP measures the soil water content, the soil temperature, and the electric conductivity of the soil. The principle of the determination of the soil water content is only partly consistent to the conventional TDR or FDR method. The starting point is the relative Permittivity (εr) as well.
The sensor emits a continuous signal of constant frequency of 60 MHz (see FDR). But not the frequency change, but the phase shift in the collected signal is considered for determining the measured value. Thus, basically a time offset to the original signal is measured (see TDR). Each time offset corresponds with an εr, defined in a range of values sorted in a look-up table. Each look-up table of all UMP sensors are individually calibrated. Then by the regression function according to Topp and Davis the water content is calculated and output in % Volume.
Like the UMP-1 also the SMT100 soil moisture sensor is based on an independent developed measuring principle. As for a TDR, the travel time of a signal is measured to determine the relative permittivity (εr) of the soil. A ring oscillator transforms the travel time in a frequency. Thus, a change of the frequency always corresponds with a change of the travel time. The frequency domain is higher than 100 MHz and thus, it is high enough to operate well even in clayey soils.
Measurement uncertainties of the determination of the water content
When installing the soil moisture sensors make sure that the measuring rods have a good contact to the surrounding soil. Air-filled hollow spaces between the soil and the rods lead to distorted measurement results. An insertion tool with thinner rods than those of the probe facilitate the installation and enable a good contact.
Natural soils are very inhomogeneous. Thus, the water retention capacity and therefore also the soil moisture can vary over small distances significantly. The water retention capacity is determined by soil properties, such as soil type (texture), soil structure, organic compounds and infiltration rate. Downward movement (Percolation) is driven by gravity force and upward movement by the suction forces of the atmosphere. Pores of various sizes hold the soil water with forces of different magnitudes. Root growth, animals and weathering processes are constantly changing the soil composition and structure. By plant roots even deeper horizons are directly connected to the atmosphere and thus, the highly dynamic evaporation process.
The user very soon is going to realize, how much the measuring values reflect the variability of natural soils. Point measurements therefore have a limited significance. Multiple measurements averaged over the area of investigation are necessary to obtain realistic results.
It is recommended to generate specific calibrations for soils with high content of clay, organic compounds or high bulk density. The generic calibrations involve too high uncertainties. Because of the high water content in clayey soils it is difficult for probes operating according the FDR principle to detect a clear signal. The attenuation of the electromagnetic wave is too strong. The amplitude and the phase shifting can hardly be measured. The same applies to soils with higher electrical conductivities. The results (e.g. the runtime measurement) are significantly influenced by higher salt contents.
Flühler, H., & Roth, K. (2004). Physik der Ungesättigten Zone. ETH Zürich und Universität Heidelberg.
Lal, R., & Shukla, M. (2004). Principles of Soil Physics. New York, Basel: Marcel Dekker, Inc.
Topp, G., Davis, J., & Annan, A. (1980). Electromagnetic determination of soil water content: Measurement in coaxial transmission lines. Water Resour. Res., S. 16, 574–582.