The Role of Drop Volume and Number on Soil Water Repellency Determination

The water drop penetration time (WDPT) test is commonly used to evaluate the persistence of soil water repellency by placing water drops on the soil surface and recording the time to complete infiltration. Currently no standard protocol exists regarding the number or volume of drops applied, and often neither detail is provided when WDPT is reported. This study evaluated how mean WDPT (WDPTM) varies with drop volume and the number of drops required to obtain WDPTM within a given precision. Two naturally water-repellent soils were tested with 416 drops each of 15, 20, 80, and 200 μL. Wettable analogs of each soil were prepared and combined with the repellent soils to create various soil mixtures, to which 20- and 200-μL drops were applied. The WDPTM was found to vary significantly (α = 0.05) with drop number and volume, and the relationship likely depends largely on soil heterogeneity from variations in particle size, organic matter content, and the distribution of hydrophobic compounds within the soil. The results of this study support the following recommendations: (i) a reliable WDPTM (±10%) can be obtained with 95% confidence from 30 drops of 80- to 200-μL size (smaller drops can be used but will reflect microtopographical variability as heterogeneity increases); and (ii) water repellency class can be determined with 95% confidence from six drops and with 90% confidence from one drop, regardless of volume. Standard deviations should always be included to give some indication of the heterogeneity of water repellency within the soil.