Dielectric sensors in an automated facility

Current data on salt tolerance of field crops are scattered, partly conflicting, and not well defined. This paper describes a field test facility designed to test the salt tolerance of crops under well drained irrigated conditions. Irrigation is provided to groups of 8 replicas of test fields, at seven different levels of salt concentration expressed in electrical conductivity (EC) ranging from 1.7 dS/m to 35 dS/m. A unique feature of the system is that each of the 56 test fields is equipped with one or two dielectric sensors to allow quasi continuous monitoring of soil volumetric water content (VWC) and bulk electric conductivity (ECb). The aim is to provide a preliminary assessment of the added value of the sensors, based on two consecutive years of testing. Sensor calibration was performed in the laboratory, and different models to relate bulk EC to pore water EC in dependency of the VWC or dielectric permittivity as measured by the sensor were tested and parameterized. Overall, the root mean square error of the tested models with one or two parameters did not differ very much, and was in the range of 0.57-0.59 dS/m in terms of soil bulk EC. However, the models differed in their robustness against inversion to obtain pore water EC from measured bulk EC. A simple one-parameter model was preferred, showing a ratio between bulk EC and pore water EC that is proportional to the VWC as measured by the sensor. The laboratory calibration was then cross validated by comparing sensor EC readings with the EC of pore water extracts obtained from suction cup samples in the field. It appears that the laboratory calibration formula overestimates the pore water EC at low EC (5 dS/m), and underestimates it at high EC (25 dS/m). The extensive data of 2013 revealed that a simple direct proportionality between pore water EC and sensor bulk EC, without any correction for VWC, can circumvent the difficulties with the calibration. This result must be treated with care as in the current field tests the soil water content was always very high. In addition, such relationship can be used only when actual pore water samples are taken, which is not always Received date: 2014-XX-XX Accepted date: 201X-XX-XX * Corresponding author: Prof. G. van Straten, Emeritus Professor of Systems and Control, Department Bio-refinery and Process Dynamics, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands. Email:gerrit.vanstraten@wur.nl. Tel:+31317483331, Fax:+31317484957. Paper – 18 World Congress of CIGR – Beijing – September 2014 practical.Overall, there is a good correlation between the time trajectories of the suction cup samples and the sensor time series, while the latter provide much more details about the dynamic behavior. Fairly large differences in bulk EC between different fields within the same irrigation treatment group were observed in both the sensor readings as well as the samples, suggesting that these differences are real. Also, within a field, sensors my differ, but the time trajectories are very similar. In the drained conditions used here the average pore water salinity was largely imposed by the irrigation EC, despite variations in rainfall and evapo-transpiration over the season. Despite the difficulties with the laboratory calibration, the higher temporal detail provided by the sensors offers excellent opportunities for advanced control of soil salinity to support crop salinity tolerance tests.