Understanding soil surface water content using light reflection theory: A novel color analysis technique considering variability in light intensity

Accurate measurement of soil surface water content is vital for analyzing evaporation, partition of solar radiation, infiltration, and runoff. Color analysis of soil has already been found to be a nonintrusive, economical, and feasible approach for the measurement of surface water content. However, color contrast within the soil domain was rarely considered in color analysis approaches demonstrated by previous researchers. This color contrast occurs because of the spatial heterogeneity of light intensity in the soil domain. This study aims to demonstrate and validate a new color analysis technique for overcoming the shortcomings of previous approaches. Six series of tests were conducted to analyze the color of red soil at various surface water contents. In addition, six measured dry densities on the compaction curve were selected for validation of the proposed color analysis technique. Soil has been compacted by hand to the desired state of compaction in a small mold. Images of soil samples were captured using a commercially available camera model (NIKON COOLPIX L29, Nikon, Tokyo, Japan). The brightness variation of soil samples was quantified using the change in mean gray value of the images. A white paper was placed below the mold to consider color contrast within the soil samples. Mean gray values of all test samples were obtained using a public domain image processing program called ImageJ. Color contrast within the soil samples was equalized with respect to reference locations considered on the white paper. Mean gray value was found to decrease with the increase in surface water content. Decrement in mean gray value was found to be more significant (i.e., 14-27 %) in saturation and desaturation zones of soil water characteristic curves (SWCCs) as compared to that in the residual zone (i.e., 3-9 %) with a 3-4 % increase in surface water content. A correlation between mean gray value and surface water content was established for the red soil.