The Soil Water Characteristic Curve: Assessing the Question of Equilibrium in the Centrifuge Method, and Consequences for Numerical Models.

Achieving equilibrium in the Soil Water Characteristic Curve: An ‘effective’ approach Sara E. Vero1,2, Rachel Creamer1, Mark G. Healy2, Tiernan Henry3, Tristan Ibrahim4, Patrick Forristal1, Karl Richards1 and Owen Fenton1 1 Teagasc, Johnstown Castle, Environment Research Centre, Co. Wexford, Rep. of Ireland. 2Civil Engineering, National University of Ireland, Co. Galway, Rep. of Ireland. 3Earth and Ocean Sciences, National University of Ireland, Galway, Rep. of Ireland 4 Dept. for Environment, Food and Rural Affairs, London, United Kingdom Measurement of the soil water characteristic curve (SWCC) requires equilibration of matric potential within a soil sample to applied pressures, as indicated by cessation of dewatering. As low level dewatering is frequently prolonged, discerning equilibrium is challenging, hence, the frequent application of temporal rules, which may or may not adequately describe dewatering. The objective of this study was to establish a methodological framework to identify the appropriate experimental duration, at each pressure step, for the construction SWCCs. Intact soil cores (5 cm3) from arable and grassland sites were subjected to centrifugation at seven pressure steps (0 to -1,500 kPa), for intervals of 24, 48 and 72-hrs (three common temporal rules). The 24-h treatment significantly underestimated dewatering compared to the 48 and 72-h treatments. For the arable site, no significant difference was observed between the 48 and 72-h treatments, however, this was not the case for the grassland site. Hence, a 48-h duration is sufficient to describe the SWCC for the arable site, but a more prolonged duration would be required to establish an effective treatment for the grassland site. This methodological framework can be used by practitioners for to determine the optimum durations for constructing SWCCs, specific to their soils and apparatus, prior to initiation of a measurement campaign.