An optical reflectance technique for soil moisture measurement. I. Theory, description, and application

Numerous techniques have been recently developed or modified for improved, in-situ soil moisture content determination. Examples of the devices used in these techniques include the time domain reflectometer, the soil moisture gauge, and the heat-pulse probe. This paper discusses the theory, description, and application of an optical reflectance technique for the in-situ measurement of soil moisture content. An encased, simple light source is proposed which produces a reflectance from a coated, porous membrane whose moisture content is a result of being in close contact with a moist section of soil. A photodiode and a photocell detector capable of sensing the 1.4 mm wavelength of light that is reflected by the moist porous membrane surface are used to quantify the moisture content. The device is approximately 125 cm³ and can be pushed into the soil to desired depths. Reflectance measurement techniques such as this have been able to obtain soil moisture content measurements to an accuracy of better than 1%. The optical reflectance methodology presented uses a simple technique that will allow the determination of soil moisture content by using a simple equation which is based on the fraction of reflected light power incident on the detector with a dry and moist porous membrane