Development and Performance Analysis of a U-Shaped Fiber Optic Sensor for Soil Water Content Measurement

Accurate soil water content measurement is essential for irrigation management, soil water retention studies, and geotechnical monitoring. This study developed and evaluated a U-shaped silica fiber optic sensor for soil water content measurement using optical transmission responses at 500, 940, and 1450 nm wavelengths. The sensor was fabricated using a bent optical fiber coupled with LED light sources and a photodiode-based detection system. Laboratory tests were conducted on three soil samples under different gravimetric water content (GWC) levels, and the photodiode outputs were normalized to compare sensor behavior across wavelengths and soil types. Under each condition, five consecutive readings were obtained at three probe locations. The five readings were treated as technical repeats and first averaged at the location level, after which the three location responses were averaged to obtain one representative response for each soil water content condition. Linear, quadratic, cubic, and exponential forward models were compared, and the cubic model provided the strongest descriptive fit for the combined-soil dataset, particularly at 1450 nm (R2=0.8956, adjusted R2=0.8799, RMSE = 0.0717). For practical water-content estimation, separate direct inverse linear models were developed and evaluated using grouped leave-one-water-content-level-out cross-validation. The corresponding RMSE values were 3.41, 3.37, and 3.20 percentage points of GWC at 500, 940, and 1450 nm, respectively. The 1450 nm wavelength also showed the highest range-dependent sensitivity. Time-dependent measurements also showed repeatable response patterns during drying, demonstrating the sensor's ability to track water content redistribution. Overall, the developed U-shaped fiber optic sensor showed a measurable and repeatable response to soil water content variation and has potential as a simple optical sensing approach for soil water content monitoring.