Article
Effective Calibration of Low-Cost Soil Water
Content Sensors
Heye Reemt Bogena *, Johan Alexander Huisman, Bernd Schilling, Ansgar Weuthen
and Harry Vereecken
Institute of Bio- and Geosciences, Agrosphere Institute (IBG-3), Forschungszentrum Jülich GmbH, 52425 Jülich,
Germany; s.huisman@fz-juelich.de (J.A.H.); b.schilling@fz-juelich.de (B.S.); a.weuthen@fz-juelich.de (A.W.);
h.vereecken@fz-juelich.de (H.V.)
* Correspondence: h.bogena@fz-juelich.de; Tel.: +49-(0)2461-616752
Academic Editor: Gonzalo Pajares Martinsanz
Received: 27 November 2016; Accepted: 16 January 2017; Published: 21 January 2017
Abstract:
Soil water content is a key variable for understanding and modelling ecohydrological
processes. Low-cost electromagnetic sensors are increasingly being used to characterize the
spatio-temporal dynamics of soil water content, despite the reduced accuracy of such sensors as
compared to reference electromagnetic soil water content sensing methods such as time domain
reflectometry. Here, we present an effective calibration method to improve the measurement accuracy
of low-cost soil water content sensors taking the recently developed SMT100 sensor (Truebner GmbH,
Neustadt, Germany) as an example. We calibrated the sensor output of more than 700 SMT100
sensors to permittivity using a standard procedure based on five reference media with a known
apparent dielectric permittivity (1 < K
a
< 34.8). Our results showed that a sensor-specific calibration
improved the accuracy of the calibration compared to single “universal” calibration. The associated
additional effort in calibrating each sensor individually is relaxed by a dedicated calibration setup
that enables the calibration of large numbers of sensors in limited time while minimizing errors in
the calibration process.
Keywords:
soil water content; permittivity; calibration; sensor-to-sensor variability; SMT100 sensor
1. Introduction
Knowledge of soil water content (SWC) is essential, as it represents a key variable in many
hydrological, climatological, environmental and ecohydrological processes. In hydrology, SWC plays
a major role in the water cycle by partitioning rainfall into runoff and infiltration [
1
], and by controlling
hydrological fluxes such as interflow [
2
] and groundwater recharge [
3
]. SWC is also a key variable
of the climate system, as it governs the energy fluxes between the land surface and the atmosphere
through its impact on evapotranspiration [
4
]. Determining the temporal and spatial variability of
SWC is hence essential for a wide range of studies, and a large number of measurement techniques
have been developed in the past decades [
5
–
10
]. Recently, first initiatives started to compare different
soil moisture sensors in the framework of calibration and validation sites for remotely sensed soil
moisture [11].
Besides destructive gravimetric sampling, electromagnetic (EM) methods, such as time domain
reflectometry (TDR) (e.g., [
12
]), time domain transmission (TDT) (e.g., [
13
]), and capacitance [
14
] and
impedance sensors (e.g., [
15
,
16
]), are most commonly used for soil water content measurements at
the point scale. All EM techniques rely on the dependency of the soil dielectric permittivity on the
SWC. As the dielectric permittivity of liquid water is much higher than the dielectric permittivity of
other soil components, SWC is the principal factor governing EM wave propagation in the soil. TDR
and TDT sensors measure the propagation velocity of EM waves along open and closed transmission
Sensors 2017, 17, 208; doi:10.3390/s17010208 www.mdpi.com/journal/sensors
