Rangeland hillslope lengths: A case study at the Walnut Gulch Experimental Watershed, southeastern Arizona

Authors: LI, L. - University Of Arizona; Nearing, Mark; Heilman, Philip - Phil; Nichols, Mary; GUERTIN, D.P. - University Of Arizona; Williams, Christopher - Jason

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2022
Publication Date: 12/1/2022
Citation: Li, L., Nearing, M.A., Heilman, P., Nichols, M.H., Guertin, D., Williams, C.J. 2022. Rangeland hillslope lengths: A case study at the Walnut Gulch Experimental Watershed, southeastern Arizona. International Soil and Water Conservation Research. 10(4):597-609. https://doi.org/10.1016/j.iswcr.2022.02.004.
DOI: https://doi.org/10.1016/j.iswcr.2022.02.004

Interpretive Summary: Sediment that is produced from hillslope erosion causes many environmental problems on rangelands. For a purpose of understanding the sediment generation and transport processes on hillslopes, hillslope length needs to be determined. However, information regarding lengths of rangeland hillslopes, and how best to estimate them, is limited, due to the challenges of measurement and limited topographic data sources for these diverse rangeland landscapes. In this study, we estimated rangeland hillslope length, using high-resolution topographic data from the Walnut Gulch Experimental Watershed as an example. Ten watersheds were selected and classified into Group 1, 2, and 3, according to their geology, soil and vegetation characteristics. Group 1 watersheds were at lower elevations dominated by shrubs, Group 3 were at high elevations dominated by grass, and Group 2 were mixed shrub and grass. Three methods were used. We found that the estimated hillslope lengths were different for the three methods, and the algorithm that looks at how the water routes down th esloope during a rainfall was better to estimate hillslope lengths as compared to other two methods. Hillslope lengths that were estimated from the flow routing algorithm for the ten selected watersheds primarily ranged from 30 to 100 m, with a median value of 63 m. We also found that the estimated hillslope lengths were different between the three groups of watersheds. More specifically, Group 3 watersheds exhibited longer hillslopes than did in Group 2, and then Group 1 watersheds. Those differences were attributed to the past geological history, watershed and drainage network morphology, and differences in vegetation characteristics.

Technical Abstract: Rangeland hillslopes provide much of the sediment supplied to channel systems and their lengths exert a fundamental constraint on hillslope diffusive processes. However, information regarding lengths of rangeland hillslopes, and how best to estimate them, is limited. In this study, three groups of watersheds (10 in total) were selected from the Walnut Gulch Experimental Watershed according to their geology, soil and vegetation characteristics. Group 1 watersheds were at lower elevations dominated by shrubs, Group 3 were at high elevations dominated by grass, and Group 2 were mixed shrub and grass. Their hillslope lengths were calculated from 1 m-resolution DEMs using three methods: a flow routing algorithm, slope-area relationships, and inverted relationship with drainage density. Parameters that characterize the current watersheds, including Hack’s exponent and coefficient, watershed shape coefficient, channel concavity and steepness, and surface roughness, were quantified and related to hillslope lengths. Results shows: (1) estimated hillslope lengths were different for the three methods and between the three groups of watersheds; (2) hillslope lengths that measured from the flow routing algorithm for the ten selected watersheds primarily ranged from 30 to 100 m, with a median value of 63.0 m, which was 20% to 50% greater than those derived from slope-area plots or drainage densities; (3) hillslope lengths estimated from the flow routing method were greater in Group 3 watersheds than in Group 2 and then in Group 1 watersheds. We attributed these differences in hillslope lengths to historic epeirogenic pulses, watershed and drainage network morphology, and differences in vegetation characteristics; (4) measured hillslope lengths from the flow routing algorithm were best correlated with hillslope relief, then surface roughness, channel steepness and concavity; (5) the linear correlations between hillslope length and relief suggested the nonlinear dependency of sediment flux on slope gradient in our study area.