Evaluation of the ZigBee based wireless soil moisture sensor network SoilNet

A remaining challenge in hydrology is to explain the observed patterns of hydrological behaviour over multiple spacetime scales as a result of interacting environmental factors. The large spatial and temporal variability of soil water content is determined by factors like atmospheric forcing, topography, soil properties and vegetation, which interact in a complex nonlinear way (e.g. Western et al., 2004). A promising new technology for environmental monitoring is the wireless sensor network (Cardell-Oliver et al., 2005). The wireless sensor network technology allows the real-time soil water content monitoring at high spatial and temporal resolution for observing hydrological processes in small water-sheds (0.1-80 sqkm). Although wireless sensor networks can still be considered as an emerging research field, the supporting communication technology for low cost, low power wireless networks has matured greatly in the past decade (Robinson et al., 2008). Wireless environmental sensor networks will play an important role in the emerging terrestrial environmental observatories (Bogena et al., 2006), since they are able to bridge the gap between local (e.g. lysimeter) and regional scale measurements (e.g. remote sensing). This paper presents a first application of the novel wireless soil water content network SoilNet, which was developed at the Forschungszentrum Jülich using the new low-cost ZigBee radio network

Sensor-to-sensor variability of ECH2O EC-5, TE and 5TE sensors used for wireless sensor networks

Towards an improvement of measurement accuracy for the low-budget soil water content sensors ECH2O EC-5, TE and 5TE used in the wireless sensor network SoilNet, the application of a sensor-specific calibration procedure based on dielectric standard liquids reduce the RMSE of approximately 0.010 to 0.015 cm^3 cm^-3 in high soil water content range

Hybrid Wireless Underground Sensor Networks: Quantification of Signal Attenuation in Soil

Wireless sensor network technology allows real-time soil water content monitoring with a high spatial and temporal resolution for observing hydrological processes in small watersheds. The novel wireless soil water content network SoilNet uses the low-cost ZigBee radio network for communication and a hybrid topology with a mixture of underground end devices each wired to several soil sensors and aboveground router devices. Data communication between the end and router devices occurs partially through the soil, and this causes concerns with respect to the feasibility of data communication due to signal attenuation by the soil. In this study, we determined the impact of soil depth, soil water content, and soil electrical conductivity on the signal transmission strength of SoilNet. In a first step, we developed a laboratory experimental setup to measure the impact of soil water content and bulk electrical conductivity on signal transmission strength. The laboratory data were then used to validate a semi-empirical model that simulates signal attenuation due to soil adsorption and reflection and transmission at the soil boundaries. With the validated model, it was possible to show that in the case of a soil layer of 5 cm, sufficient power will remain to ensure data communication over longer distances for most soil conditions. These calculations are fairly simplified and should be considered as a first approximation of the impact of attenuation. In actual field situations, signal transmission may be more complex. Therefore, a field evaluation of signal attenuation is a crucial next step

Sensor-to-Sensor Variability of the ECH2O EC-5, TE, and 5TE Sensors in Dielectric Liquids

Low-budget sensors used in wireless soil water content sensor networks typically show considerable variation. Because of the large number of sensors in sensor network applications, it is not feasible to account for this variability using a calibration between sensor response and soil water content. An alternative approach is to split the calibration into two parts: (i) determination of sensor response-permittivity relationships using standard liquids with a defined reference permittivity, and (ii) site-specific calibration between permittivity and soil water content using a subset of sensors. In this study, we determined sensor response-permittivity relationships for several ECH2O, EC-5, TE, and 5TE sensors by Decagon Devices (Pullman, WA). The objectives of this study were to determine (i) the sensor-to-sensor variability and precision of these sensor types, and ( ii) the increase in accuracy when a sensor-specific calibration is used instead of a single calibration. The results showed that the sensor-to-sensor variability was significantly larger than the measurement noise for each sensor type. When a sensor-specific calibration was used, the RMSE expressed in (equivalent) soil water content ranged from 0.008 cm(3) cm(-3) for the TE sensor to 0.014 cm(3) cm(-3) for the EC-5 sensor in a permittivity range between (similar to)2 and 35. When a single calibration was used, the RMSE was higher and ranged from 0.01 cm(3) cm(-3) for the 5TE sensor to 0.02 cm(3) cm(-3) for the TE sensor. An improvement in accuracy of nearly 0.01 cm(3) cm(-3) can be reached in the high-permittivity range for each sensor type by calibrating each sensor individually