Millimeter-scale spatial variability in soil water sorptivity: Scale, surface elevation, and subcritical repellency effects

Recent evidence suggests that reduced water infiltration may be linked to small scale microbial and/or chemical processes that cause subcritical water repellency. We measured water sorptivity on the surface of a large intact block of soil (0.9 m wide, 1.3 m long, 0.25 m deep) taken from a grassland site and examined the effects of surface elevation and water repellency on water sorptivity at the millimeter scale. The soil block was partially dried to 0.22 mm3 mm-3, appeared to wet readily, and is not severely water repellent at any water content. Water sorptivity varied from 0.1 to 0.8 mm s-1/2 across the sampling grid with a coefficient of variation (CV) of 0.57. Water repellency, determined by comparing water and ethanol sorptivities, also varied considerably (CV = 0.47). Geostatistical analyses of water sorptivity and repellency measurements found little evidence of spatial autocorrelation, suggesting a high degree of local variability. These data were compared to larger scale measurements obtained with conventional infiltrometers under tension conditions (40 mm contact radius), and ponded conditions (37 and 55 mm radius rings) where macropores influence infiltration heterogeneity. Larger scale tension infiltrometer measurements were less variable with a CV of 0.22, whereas ponded infiltrometer measurements were more variable, CV > 0.50, presumably because of the influence of macropore flow. Data collected on surface elevation showed that ponded infiltration but not tension infiltration was influenced by surface topography. The results suggested that repellency can induce levels of spatial variability in water transport at small scales comparable to what macropores induce at larger scales.