ARS | Publication request: Slope impacts on concentrated flow hydraulics in rangeland

Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: December 14, 2009
Publication Date: December 14, 2009
Citation: Al-Hamdan, O.Z., Pierson Jr, F.B., Williams, C.J., Kormos, P.R., Nearing, M.A., Boll, J. 2009. Slope Impacts on Concentrated Flow Hydraulics in Rangeland, EOS Transactions, American Geophysical Union 90(52), Fall Meeting Supplement, Abstract EP31B-0595.

Interpretive Summary: Several studies have been conducted to describe rill or concentrated flow hydraulics. However, most of these studies used data obtained from either laboratory experiments or field sites located on gently sloping crop lands. The data sets in the few rangeland field studies conducted did not cover a variety of hillslope angles and generally focused on slope gradients less than 20%. The lack of studies with steeper slopes resulted in misinterpreting the slope gradient impact on concentrated flow hydraulics, as sites with different slopes have different soil and vegetation cover characteristics. This study examines the characteristics of rangeland concentrated flow hydraulics as a function of vegetation and ground cover using field experimental data from diverse vegetated rangeland sites of the western United States. These data span a wide range of slope angles (6%-66%), soil types, and vegetative cover. Many of the sites exhibit some degree of disturbance, such as wild fire, prescribed fire, tree mastication, and/or tree cutting. The data were divided into two sets, gently sloping (<20%) and steeply sloping (>20%). Analyses were performed on each data set separately as well as on the combined data set. For the complete data set, concentrated flow occurred on less than 26% of the gently sloping plots and on more than 70% of the steep plots. The results showed that the Darcy-Weisbach roughness coefficient (f) had no significant correlation with vegetation and ground cover variables on the gently sloping sites. However, roughness coefficient f was positively correlated with vegetation and ground cover on steep rangelands (R2=0.53, n=439). The power relation relating the flow width (w) to the discharge (Q) was statistically different on the gentle and the steep sites. A multi regression equation for estimating the width of the concentrated flow as a function of flow discharge, slope, and vegetation cover was developed (R2=0.62, n=360), where the variation of slope alone explained 39% of the variation of f (partial R2=0.39).

Technical Abstract: Several studies have been conducted to describe rill or concentrated flow hydraulics. However, most of these studies used data obtained from either laboratory experiments or field sites located on gently sloping crop lands. The data sets in the few rangeland field studies conducted did not cover a variety of hillslope angles and generally focused on slope gradients less than 20%. The lack of studies with steeper slopes resulted in misinterpreting the slope gradient impact on concentrated flow hydraulics, as sites with different slopes have different soil and vegetation cover characteristics. This study examines the characteristics of rangeland concentrated flow hydraulics as a function of vegetation and ground cover using field experimental data from diverse vegetated rangeland sites of the western United States. These data span a wide range of slope angles (6%-66%), soil types, and vegetative cover. Many of the sites exhibit some degree of disturbance, such as wild fire, prescribed fire, tree mastication, and/or tree cutting. The data were divided into two sets, gently sloping (<20%) and steeply sloping (>20%). Analyses were performed on each data set separately as well as on the combined data set. For the complete data set, concentrated flow occurred on less than 26% of the gently sloping plots and on more than 70% of the steep plots. The results showed that the Darcy-Weisbach roughness coefficient (f) had no significant correlation with vegetation and ground cover variables on the gently sloping sites. However, roughness coefficient f was positively correlated with vegetation and ground cover on steep rangelands (R2=0.53, n=439). The power relation relating the flow width (w) to the discharge (Q) was statistically different on the gentle and the steep sites. A multi regression equation for estimating the width of the concentrated flow as a function of flow discharge, slope, and vegetation cover was developed (R2=0.62, n=360), where the variation of slope alone explained 39% of the variation of f (partial R2=0.39).