Estimates of surface soil water content using linear combinations of spectral wavebands

Abstract

Surface reflectance factors from bare field soil were measured to determine the relationship between surface soil water content and spectral reflectance. Reflectance in the six reflective Thematic Mapper (TM) bands plus a 1.15 to 1.30 µm bandpass (referred to as MMR 5) was measured using a groundbased radiometer across a soil water gradient provided by a line source sprinkler system. A spectral index of soil brightness (Brightness) derived using the Gram-Schmidt process and utilizing reflectance information was calculated for each band and for combinations of bands. The results of this study show that TM band 7 (2.05–2.30 µm) provided improved estimates of surface soil water content (0–0.5 cm depth) over estimates using reflectance information from all seven bands. Good correlations were also found between band ratio spectral indices of TM 5 (1.55–1.75 µm) / TM 7, MMR 5/TM 7, and MMR 5/TM 5 and surface soil water content. Results indicate that surface reflectance factors within bandpasses that partially overlap water absorption regions, such as TM 7, are most highly correlated with surface soil moisture. Band ratios utilizing a bandpass partially overlapping a water absorption region such as TM 7 and a non-water-absorbing bandpass such as MMR 5 yield close correlations with surface soil water content.