Useful soil-water repellency indices: linear correlations

Water repellency (WR) has been classically characterized at fixed (usually oven-dry) soil water content (θg) in terms of the soil water contact angle (CA), α. However, α has been previously reported to depend upon θg in a nonlinear fashion, such that WR increases from a wettable state close to saturation (θg-min) up to a maximum, αmax, decreasing afterward either monotonically or rising again to a second local or absolute α maximum nearby the dried soil state. Hence, a CA versus water content (α-θg) curve may be described in terms of different WR parameters, such as θg-min, θg-max, αmax, or the integrated area below the α-θg curve, S. Based on previous α-θg measurements carried out with the molarity of an ethanol droplet (MED) test, both in mineral and volcanic soils from different world regions, including cultivated and natural forest soils, and textures ranging from clay-loam to sandy, we confirm here the usefulness of the integrated area below the α-θg curve (S) as a WR describing index for a large variety of α-θg curve shapes. We found a simple relationship between S and the soil water content at which WR is triggered, θg-min, such that S = 16.903 θg-min (R = 0.946), which provides an easy method for the rapid characterization of the overall WR degree of soils. S was also linearly correlated with the soil organic matter (SOM) content (R = 0.817) for 1 g (100 g) < SOM < 88 g (100 g), such that the best estimate of S was that obtained by combining linearly both θg-min and the SOM content (R = 0.990). Linear correlations were also found between θg-max, that is, the soil water content at which α is maximum, and S (R = 0.834) or the SOM content (R = 0.705), and consequently between θg-max and θg-min (R = 0.830). In addition, both θg-min and θg-max were found to depend linearly upon the soil water content at −33 kPa and −1500 kPa, respectively. Finally, a mean soil WR may be defined as the ratio S/θg-min. We found that the maximum CA, αmax, and the mean soil WR S/θg-min were positively correlated (R = 0.780), such that a particular soil with high (low) values of maximum CA is expected to exhibit a high (low) WR degree on average across the whole water regimen from −33 kPa down to oven-dry moisture. Such an estimate of the mean WR index S/θg-min was further improved if both αmax and the SOM content were available (R = 0.825).