Approaches to characterize the degree of water repellency

Measurement techniques that quantify the degree of soil water repellency ar e important for research and for the communication of research findings. Th e water drop penetration time (WDPT) is a commonly used measurement. If a w ater drop does not enter the soil spontaneously, the soil-water contact ang le is greater than 90 degrees and the soil is considered to be water repell ent. 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 aq ueous ethanol concentration series that enters the soil in approximately 5 s is identified as the ninety degree (ND) surface tension, gamma(ND), of th e 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 the ta = [(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), whi ch is a function of both the soil water repellency and pole size, is an imp ortant parameter for predicting infiltration and the stability of water flo w in the field. Measurements of WDPT, gamma(ND) and h(P) provide a complete characterization of the degree of water repellency. (C) 2000 Elsevier Scie nce B.V. All rights reserved.