Small‐Scale Measurement of Soil Water Content Using a Fiber Optic Sensor

Abstract

Many water flow and solute transport studies require accurate measurement of water content within small soil volumes. We have examined the feasibility of using fiber optic mini‐probes (FOMPs) for in situ measurement of water content in relatively small soil volumes (<15–20 mm3) in real time and on a continuous basis. The system consists of transmitting a constant beam of light through the input leg of a fiber optic mini‐probe to a location of interest within the soil matrix. At the tip, the light exits the probe, interacts with the soil volume directly in front, and partially reflects back into the probe. The reflected signal is transmitted through the output leg to a photodetector and quantified. The output signal, which is constant during steady state (i.e., dry soil), decreases as the water content in soil increases. A calibration is necessary to convert the output light intensity to water content. In developing calibration curves for the three soils used in the study, we consistently found an excellent correlation (r2 > 0.98) between the soil light reflectivity and the water content. Calibration of the FOMP depends on the individual probe, soil type (largely due to texture), and bulk density. The FOMP system may be ideal in situations where water content is dynamic and changing at small spatial scales, especially where these changes have a large impact on other processes.