Development of a Low-Cost Multi-Depth Real-Time Soil Moisture Sensor Using Time Division Multiplexing Approach

Soil moisture (SM) is an integral parameter for scheduling irrigation. The precise and real-time measurement of SM is difficult due to its complex nature. The purpose of this paper was to determine whether the time division multiplexing (TDM) approach would effectively estimate SM content. We developed a sensor to detect SM from four soil depths (10, 20, 40, and 60 cm) simultaneously by using the TDM technique. This is a type of EM sensor that indirectly estimates the SM on the basis of the dielectric properties of soil. Three types of soil samples were used to calibrate the sensor. The calibration models were established using polynomial (3rd order) fitting equations. The performance of the sensor was evaluated both in the laboratory and
in situ
conditions. The sensitivity of the sensor was examined in a micro-control irrigation system. The sensor measured SM contents in a soil box were compared with HYDRUS-2D simulated wetting patterns. The results were in the optimal range, and the coefficient of determination (
$R^{2}$
) ranged from 0.97 to 0.99 and root means square error (RMSE) from 0.72 to 2.01. The TDM detection circuit-based sensor
$R^{2}$
value was 0.97, and its RMSE was 2.78. Whereas the independent detection circuit
$R^{2}$
values ranged from 0.93 to 0.96 and RMSE from 4.09 to 5.07. The results determined that the sensor could be used for continuous SM measurements, which would be advantageous for planning the irrigation practices in arid and irrigated regions.