Small-scale measurement of soil water content using a fiber optic sensor

Many water flow and solute transport studies require accurate measurement o f 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 mm(3)) 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 i nterest within the soil matrix, At the tip, the light exits the probe, inte racts with the soil volume directly in front, and partially reflects back i nto the probe. The reflected signal is transmitted through the output leg t o a photodetector and quantified. The output signal, which is constant duri ng steady state (i.e., dry soil), decreases as the water content in soil in creases. A calibration is necessary to convert the output light intensity t o water content. In developing calibration curves for the three soils used in the study, we consistently found an excellent correlation (r(2) > 0.98) between the soil light reflectivity and the water content, Calibration of t he 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 t hese changes have a large impact on other processes.