Evaluating Surface‐Soil Water Content by Measuring Reflectance

Water's property to absorb certain wavelengths in the near infrared was the basis for developing a reflectometer to measure reflectance of near‐infrared radiation from a soil surface. The reflectometer's essential elements include: source of infrared radiation, optical system, integrating sphere, detector, light chopper, amplifier, and meter system. The radiation from an incandescent lamp was filtered with narrow‐band pass filter, chopped and allowed to strike the test surface, where it was either absorbed or reflected onto the surface of the integrating sphere. The intensity of the reflected radiation was measured with a lead sulfide detector and appropriate amplifier and meter. The reflectance as a function of water content was measured for filter paper and several soils at 1.30, 1.45, 1.65, and 1.95 µm. Although at low water contents soil properties (other than water content) strongly influenced soil reflectance, at 1.95‐µm wavelength—the most prominent absorption band of liquid water—the reflectance—water content relationship tended to be log‐linear.