Laboratory Calibration and Field Validation of Soil Water Content and Salinity Measurements Using the 5TE Sensor

Nessrine Zemni; Fethi Bouksila; Magnus Persson; Fairouz Slama; Ronny Berndtsson; Rachida Bouhlila

doi:10.3390/s19235272

Laboratory Calibration and Field Validation of Soil Water Content and Salinity Measurements Using the 5TE Sensor

Sensors (Basel). 2019 Nov 29;19(23):5272. doi: 10.3390/s19235272.

Authors

Nessrine Zemni^{1

2}, Fethi Bouksila¹, Magnus Persson³, Fairouz Slama², Ronny Berndtsson^{3

4}, Rachida Bouhlila²

Affiliations

¹ National Institute for Research in Rural Engineering, Water, and Forestry, Box 10, Ariana 2080, Tunisia.
² Laboratory of Modelling in Hydraulics and Environment, National Engineering School of Tunis, University of Tunis El Manar (ENIT), Box 37, Le Belvédère Tunis 1002, Tunisia.
³ Department of Water Resources Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden.
⁴ Centre for Middle Eastern Studies, Lund University, Box 201, SE-221 00 Lund, Sweden.

Abstract

Capacitance sensors are widely used in agriculture for irrigation and soil management purposes. However, their use under saline conditions is a major challenge, especially for sensors operating with low frequency. Their dielectric readings are often biased by high soil electrical conductivity. New calculation approaches for soil water content (θ) and pore water electrical conductivity (ECp), in which apparent soil electrical conductivity (ECa) is included, have been suggested in recent research. However, these methods have neither been tested with low-cost capacitance probes such as the 5TE (70 MHz, Decagon Devices, Pullman, WA, USA) nor for field conditions. Thus, it is important to determine the performance of these approaches and to test the application range using the 5TE sensor for irrigated soils. For this purpose, sandy soil was collected from the Jemna oasis in southern Tunisia and four 5TE sensors were installed in the field at four soil depths. Measurements of apparent dielectric permittivity (Ka), ECa, and soil temperature were taken under different electrical conductivity of soil moisture solutions. Results show that, under field conditions, 5TE accuracy for θ estimation increased when considering the ECa effect. Field calibrated models gave better θ estimation (root mean square error (RMSE) = 0.03 m³ m^-3) as compared to laboratory experiments (RMSE = 0.06 m³ m^-3). For ECp prediction, two corrections of the Hilhorst model were investigated. The first approach, which considers the ECa effect on K' reading, failed to improve the Hilhorst model for ECp > 3 dS m^-1 for both laboratory and field conditions. However, the second approach, which considers the effect of ECa on the soil parameter K₀, increased the performance of the Hilhorst model and gave accurate measurements of ECp using the 5TE sensor for irrigated soil.

Keywords: FDR sensor; real time monitoring; sensor calibration and validation; soil pore water electrical conductivity; soil salinity; soil water content.