Evaluation of a low cost soil water cont

Evaluation of a low-cost soil water content sensor

for wireless network applications

H.R. Bogena *, J.A. Huisman, C. Oberdo¨rster, H. Vereecken

Research Centre Ju¨lich, Agrosphere Institute, ICG 4, 52425 Ju¨lich, Germany

Received 2 February 2007; received in revised form 29 May 2007; accepted 15 June 2007

KEYWORDS

Soil water

content sensor;

EM sensor

characterization

method;

Wireless sensor

network

Summary Wireless sensor networks are a promising new in situ measurement technology for

monitoring soil water content changes with a high spatial and temporal resolution for large

areas. However, to realise sensor networks at the small basin scale (e.g. 500 sensors for an

area of 25 ha), the costs for a single sensor have to be minimised. Furthermore, the sensor

technique should be robust and operate with a low energy consumption to achieve a long oper￾ation time of the network. This paper evaluates a low-cost soil water content sensor (ECH2O

probe model EC-5, Decagon Devices Inc., Pullman, WA) using laboratory as well as field exper￾iments. The field experiment features a comparison of water content measurements of a for￾est soil at 5 cm depth using TDR and EC-5 sensors. The laboratory experiment is based on a

standardized sensor characterisation methodology, which uses liquid standards with a known

dielectric permittivity. The results of the laboratory experiment showed that the EC-5 sensor

has good output voltage sensitivity below a permittivity of 40, but is less sensitive when per￾mittivity is higher. The experiments also revealed a distinct dependence of the sensor reading

on the applied supply voltage. Therefore, a function was obtained that allows the permittivity

to be determined from the sensor reading and the supply voltage. Due to the higher frequency

of the EC-5 sensor, conductivity effects were less pronounced compared to the older EC-20

sensor (also Decagon Devices Inc.). However, the EC-5 sensor reading was significantly influ￾enced by temperature changes. The field experiment showed distinct differences between

TDR and EC-5 measurements that could be explained to a large degree with the correction

functions derived from the laboratory measurements. Remaining errors are possibly due to

soil variability and discrepancies between measurement volume and installation depth. Over￾all, we conclude that the EC-5 sensor is suitable for wireless network applications. However,

the results of this paper also suggest that temperature and electric conductivity effects on the

sensor reading have to be compensated using appropriate correction functions.

ª 2007 Elsevier B.V. All rights reserved.

0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jhydrol.2007.06.032

* Corresponding author. Tel.: +49 (0)2461 616752; fax: +49 (0)2461 612518.

E-mail addresses: [email protected] (H.R. Bogena), [email protected] (J.A. Huisman), [email protected]

(C. Oberdo¨rster), [email protected] (H. Vereecken).

Journal of Hydrology (2007) 344, 32– 42

available at www.sc iencedirect.com

journal homepage: www.elsevier.com/locate/jhydrol