Freeze-thaw–induced geomorphic and soil changes in vehicle ruts and natural rills
Lawrence W. Gatto, Jonathan J. Halvorson, Donald K. McCool, 2001. "Freeze-thaw–induced geomorphic and soil changes in vehicle ruts and natural rills", The Environmental Legacy of Military Operations, Judy Ehlen, Russell S. Harmon
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Land managers of military training lands must conserve the soil to ensure that training can continue on those lands. However, military maneuvers damage vegetation, break up soil crusts, loosen the surface soil, change soil-surface geometry, compact the soil, and often form ruts in which runoff is concentrated. The constrained flow in ruts can detach and transport far more sediment than can unchanneled, overland flows. Rills often form in ruts as a result. However, natural processes in the soil alter the impacts of maneuvers over time, and our objectives were to measure how soil freeze-thaw (FT) cycling changes compacted soil and the geometry of military-vehicle ruts and how these changes compare to those in natural rills. We established research sites at Yakima Training Center (YTC) in south central Washington and Ethan Allen Firing Range (EAFR) in northwestern Vermont and made field observations and measurements at these sites over two winters. The cross sections of tank ruts at YTC became smoother as soil from rut crests slid into the rut during thaw. Tank ruts at EAFR were shallower than those at YTC and smoothed over the winter, but rills also formed in the ruts over one winter. Scattered soil slumps occurred along the sides of deeper rills at EAFR during spring thaw, but the slumped sediment was removed by subsequent flows. FT at both sites reduced the mean penetration resistance and bulk density of the top 5 cm of soil in ruts. Below 5 cm, resistance and density were statistically greater in than out of ruts at YTC, especially where the soil contained 15% water by volume during maneuvers. Saturated hydraulic conductivity in and out of ruts at YTC was not statistically different when ruts were formed in soil that contained 5% water, was lower in 75% of straight ruts made in soil containing 15% water, and was lower yet in curved ruts. Surface-water runoff at YTC began sooner in ruts than on adjacent, unrutted soil, and runoff rates were 67% to 77% higher due to the persistence of subsurface soil compaction in ruts. Incipient rills formed in tank ruts at EAFR after one winter on the 7% and 18% slopes. In-rut rills up to 11-cm deep formed on the 21% and 31 % slopes. The adjacent, untrafficked soil on any of the slopes showed no new rills. These results can be used to parameterize soil-erosion models used by land managers of military-training lands in cold regions.