Resistance to overland flow on semiarid grassland and shrubland hillslopes, Walnut Gulch, southern Arizona

One hundred and thirty six experiments were conducted on runoff plots on semiarid grassland and shrubland hillslopes at Walnut Gulch, Arizona. Graphs of Darcy-Weisbach friction factor f against Reynolds number Re are positively sloping or convex-upward for the grassland and predominantly negatively sloping for the shrubland. These trends are attributed to the progressive inundation of the roughness elements, with the submergence of the gravel on the shrubland being greater than the submergence of the plants on the grassland. The f-Re relations are of little value as models for predicting flow resistance at locations other than where they were developed because each location has its own unique relation which is a function of the surface properties at that location. Consequently, multivariate models that include surface properties among their predictive variables were derived for the grassland and shrubland hillslopes using multiple regression analysis. On the grassland 69.5% of the variation in f is accounted for by basal plant stem and litter cover, whereas on the shrubland 56.3% of the variation is explained by gravel cover and gravel size. The inclusion of Re improves the explained variation by 5.4% on the grassland and 7.6% on the shrubland. These results suggest that for most purposes the extra effort involved in measuring or modeling flow rate is not worth the small improvement in predictive accuracy and, therefore, that the practice of estimating resistance to overland flow solely on the basis of surface properties is entirely reasonable. A comparison of the two sets of experiments reveals that both infiltration and resistance to flow are higher on grassland than shrubland. Consequently, where shrubland has replaced grassland during the past century, in addition to the ground surface becoming more exposed to raindrop impact, overland flow has increased in volume and velocity. Together these changes have resulted in accelerated erosion in the form of rill development and stripping of the soil A horizon.

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