Strong effect of coarse surface layer on moisture within gravel bars: Results from an outdoor experiment


We propose that the coarse surface layer found on most gravel bars acts as mulch, decreasing evaporation from the soil surface and maintaining higher moisture levels in the underlying fine matrix, as compared with the case of sand bed streams. Because of the importance that such process would have for vegetation establishment, we conducted an outdoor evaporation experiment on soil columns containing diverse mixtures of fines and gravel, covered with 0, 4, and 8 cm thick gravel layers, as well as columns with homogeneous sand. The columns were saturated, drained, and then exposed to the atmosphere for 58 days. They were weighed and their soil moisture was computed on eight different dates. Both the coarse layer thickness and the texture of the underlying fine matrix, as well as their interaction, had significant effects on soil moisture. The presence of 4 and 8 cm thick gravel layers on the surface increased moisture up to 3.7 and 5.3 times, respectively, as compared with homogeneous sand. Effects of the fine matrix texture were significant only when the fines were exposed to the atmosphere at the top of the columns, but not when a coarse layer was present. Our results show that the presence of a clean gravel layer maintains high moisture levels within the underlying fines, for periods of weeks without water input. We suggest that this rock mulching effect could be fundamental for successful plant colonization of bare gravel bars, so that it should be incorporated into any vegetation establishment model developed for gravel bed rivers. Copyright © 2010 by the American Geophysical Union.

Publication Title

Water Resources Research