Impact of tension infiltrometer disc size on measured soil water repellency index

Hunter, A. E., Chau, H. W. and Si, B. C. 2011. Impact of tension infiltrometer disc size on measured soil water repellency index. Can. J. Soil Sci. 91: 77-81. Accurate measurement of soil water repellency (or hydrophobicity) is important for assessing the hydraulic properties of soils. Water repellency index (RI), a measure of soil water repellency, can be determined using the tension infiltrometer. Little is known about the effects of different infiltrometer disc sizes on measured RI. Furthermore, the impact of method selection in the context of site assessment is unknown. The objective of this study was to determine if the infiltrometer disc size affects the measured RI. Studies were conducted on seven sandy and one clay site in Western Canada in 2008 and 2009. Mini (disc 4.5 cm in diameter) and standard (disc 20 cm in diameter) tension infiltrometers were used to determine RI. There was strong spatial variability in RI values at all sites. Higher RI and greater variance were associated with the smaller disc size due to the smaller zone of influence. Water repellency index values obtained from the mini and standard tension infiltrometers were not statistically different in most cases. We conclude that the mini infiltrometer is an appropriate method for site assessment of RI. The mini infiltrometer RI values were compared with those from the standard infiltrometer, resulting in a 44% accuracy rate with a type I error in 33% of the cases and a type II error in 22% of the cases.