Approaches to characterize the degree of water repellency

Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. The water drop penetration time (WDPT) is a commonly used measurement. If a water drop does not enter the soil spontaneously, the soil-water contact angle is greater than 90 degrees and the soil is considered to be water repellent. The time for the drop to enter the soil (WDPT) provides an indication of the stability of the repellency. The liquid-air surface tension of an aqueous ethanol concentration series that enters the soil in approximately 5 s is identified as the ninety degree (ND) surface tension, gamma(ND), of the soil. The gamma(ND) number can be used to calculate the solid-air surface tension, gamma(s), by gamma(S) = gamma(ND)/4. The water-soil contact angle can also be calculated from the gamma(S) value by the relationship cos theta = [(gamma(ND)/gamma(W))(1/2) - 1], where theta is the contact angle and gamma(W) the water-air surface tension. The water entry pressure, h(P), which is a function of both the soil water repellency and pole size, is an important parameter for predicting infiltration and the stability of water flow in the field. Measurements of WDPT, gamma(ND) and h(P) provide a complete characterization of the degree of water repellency. (C) 2000 Elsevier Science B.V. All rights reserved.