A near-infrared reflectance sensor for soil surface moisture measurement

Soil moisture is an important soil property that has important functions in various studies and applications, such as agricultural practices, hydrological processes and ecological issues. A near-infrared (NIR) reflectance sensor designed for moisture measurement at the soil surface is based on the reflectance of two light-emitting diodes (LEDs) of different wavelengths, in which one has a wavelength of 1940 nm and a strong water absorption band, whereas the other has a wavelength of 1800 nm and a weak water absorption band, as related to soil moisture reflectance. The algorithm is designed for estimating soil moisture using the relative absorption depth from reflectance data of the 1800 and 1940 nm wavelengths. Laboratory experiments investigate the relationship between soil moisture and surface reflectance by studying four different soils. The results indicate a strong linear correlation between soil moisture and relative absorption depths for the different soils tested, and the reflectance models are dependent on soil type. The soil moistures predicted using the proposed method agree well with measurements obtained by gravimetric method, indicating the feasibility of the proposed method for soil moisture measurement. The NIR LED reflectance sensor developed in this study is potentially useful for soil surface moisture measurement in the laboratory and field.