SmartWT: An open IoT sensor, datalogger and GPRS data transmission device for monitoring water levels in rice fields, with application to AWD irrigation

Water management is a time-consuming aspect of rice cultivation, particularly when the Alternate Wetting and Drying (AWD) irrigation technique is adopted. This technique requires the ponding water level to be monitored daily to determine the optimal time for flooding the paddy. This study proposes SmartWT, a remote monitoring system for continuously monitoring ponding water levels in flooded rice fields. Based on open-source architecture, the system uses an Arduino Nano in conjunction with a General Packet Radio Service (GPRS) board to acquire and transmit data for remote communication purposes. Distance is measured by an ultrasonic sensor installed on a Water Tube (WT) inserted into the soil, perforated below the soil surface. The entire system has been optimised for this operating environment and features a waterproof sensor and container. The device has been designed to operate using minimal power; it runs on batteries alone, with an approximate lifespan of three months when transmitting every two hours. Several laboratory and field tests have been carried out to verify the sensor's accuracy and investigate the influence of pipe diameter and environmental temperature. Field tests have demonstrated that the sensor can withstand the extremely challenging conditions found in rice paddies, conditions that are too difficult for most instruments to handle. The accuracy error is typically less than 1 cm, which is a reasonable tolerance for ponding water level management. If accurate readings are needed, an accuracy error of a few millimetres can be achieved by adding a temperature sensor to the device.