Snowmelt-driven macropore flow and soil saturation in a semiarid forest

Lateral subsurface flow is generally assumed to occur as a result of the development of a saturated zone above a low-permeability interface such as at the soil-bedrock contact, and it is often augmented by macropore flow. Our objective was to evaluate the development of lateral subsurface flow and soil saturation at a semiarid ponderosa pine forest in New Mexico with respect to the conceptual model of saturation building above the soil-bedrock contact. At this site, we have long-term observations of the water budget components, including lateral flow. A 1.5 m deep by 7 m long trench was constructed to observe lateral subsurface flow and development of saturation directly. Our observations are based on flow resulting from a melting snowdrift. The edge of the drift was about 7 m upslope from the trench. Lateral subsurface flow only occurred from root macropores in the Bt soil horizon. Saturation developed and grew outward from flowing root macropores, rather than growing upward from the soil-bedrock interface. This macropore-centred saturation resulted in a highly heterogeneous distribution of water content until enough macropores began flowing and individual macropore saturated zones grew large enough to coalesce and saturate large volumes of the soil. Our observations are based on one snowmelt event and a relatively short hillslope flow path, and thus do not represent a full range of hydrologic conditions. Nevertheless, the observed behaviour did not conform to the traditional model of soil-bedrock control of saturation and lateral flow. Copyright (C) 2004 John Wiley Sons, Ltd.