Development of a Predictive Expression for Soil Water Repellency Based on Soil Organic Carbon Content.

The emergence of water repellency (WR) of a hydrophobic soil primarily depends on the water content of the soil at which WR is assessed. Also quantity and quality of soil organic carbon has been identified as a key factor that influences the WR behavior of soils. The WR appears in organic carbon rich soils over range of water contents whereas soils with low organic matter content express only slight to moderate WR over a narrow range of water content. In this study, we investigated the relationship among WR, SOC content and soil water content for a variety of soil samples to develop a model that express the WR- soil water content relationship at various SOC contents. The tested soils were from diverse origins: whole soil sample from five types of vegetation in Denmark; 4 to 6 size fractions of two types of soils from Denmark; soils of for depth classes in a single soil profile of volcanic ash soil in Fukushima, Japan; and soils of eight depth classes in a single soil profile of Cypress forest, Aichi, Japan. Water repellency was determined by applying the Molarity of Ethanol Droplet test for series of volumetric water contents at 0.02 intervals starting from air dried water content to highest water content at which WR disappears. The results revealed that the area under WR and volumetric water content curve has significant positive linear correlation. Further a two parameter s-shaped log equation which expresses the cumulative WR as function of dimensionless volumetric water content, and SOC content was introduced. The ultimate cumulative WR value at unity (total area under WR vs volumetric water content curve) demonstrate a significant positive linear correlation with SOC content. In contrast, the s-shaped log curve shape parameter showed a negative non-linear correlation with SOC content.