Cost-effective autonomous sensor for the long-term monitoring of water electrical conductivity of crop fields

Salinity is a key parameter determining water quality. In agriculture, irrigation water with high salinity levels impacts plant growth and yield negatively hence there is a need to monitor it and irrigate with fresh water whenever salinity surpasses the tolerance threshold of a cultivar. Electrical conductivity (EC) of water is usually measured periodically instead of salinity because of its practicality. This work describes a low-cost low-power (and yet inexpensive) autonomous sensor prototype that is able to compute continuously the EC of water from complex electrical impedance measurements based on synchronous sampling, a technique that lowers cost and power consumption. The sensor is easy to assemble and has been verified in the lab for an EC range from 0.35 dS m(-1) to 6.18 dS m(-1), showing a maximal deviation of +/- 0.03 dS m(-1) from the readings of a commercial reference EC meter. The sensor has also been installed in a rice paddy and left unattended for a whole cultivation season (107 days). The maximum deviation observed during this field test is +/- 0.14 dS m(-1), which is good enough to detect high salinity levels.