Spatial and temporal analysis of hillslope–channel coupling and implications for the longitudinal profile in a dryland basin

Authors: MICHAELIDES, K. - University Of Bristol; HOLLINGS, R. - University Of Bristol; SINGER, M.B. - Cardiff University; Nichols, Mary; Nearing, Mark

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2017
Publication Date: 2/20/2018
Publication URL: https://handle.nal.usda.gov/10113/6471178
Citation: Michaelides, K., Hollings, R., Singer, M., Nichols, M.H., Nearing, M.A. 2018. Spatial and temporal analysis of hillslope–channel coupling and implications for the longitudinal profile in a dryland basin. Earth Surface Processes and Landforms. 43:16085-1621. https://doi.org/10.1002/esp.4340.
DOI: https://doi.org/10.1002/esp.4340

Interpretive Summary: The shape of river channels evolves in response to runoff patterns and sediment supply, which is contributed by both material eroded from hillslopes and that picked up from the channel bed. The net balance of hillslope sediment supply to the channel and bed material evacuation along a river impacts the shape of the channel’s longitudinal profile. Research was conducted on the Walnut Gulch Experimental Watershed in southeastern Arizona to assess this net balance over a multi-decadal period using field measurements, historic records of rainfall and stream flow, and modelling. During flow sizes that occur most commonly, the hillslope contribution generally is balanced with the amount of sediment evacuated from the channel, however during high magnitude flood flows the balance is tipped, and more sediment is evacuated from the main Walnut Gulch channel than is input from the hillslopes. Our results show that in dryland watersheds the balance between hillslope and channel bed sediment could explain the straight long profiles displayed by channels that are dry for most of the year.

Technical Abstract: The long-term evolution of channel longitudinal profiles within drainage basins is partly determined by the relative balance of hillslope sediment supply to channels and the evacuation of channel sediment. However, the lack of theoretical understanding of the physical processes of hillslope-channel coupling makes it challenging to determine whether hillslope sediment supply or channel sediment evacuation dominate over different timescales and how this balance affects bed elevation spatially. In this paper, we develop a framework for inferring the relative dominance of hillslope sediment supply to the channel versus channel sediment evacuation, over a range of temporal and spatial scales. The framework is based on combining distinct local flow distributions on hillslopes and in the channel with surface grain-size distributions, to compute local hydraulic stresses at various hillslope-channel coupling locations within the Walnut Gulch Experimental Watershed (WGEW), a dryland drainage basin in SE Arizona, USA. These stresses are then assessed as a local net balance between hillslopes and channel for a range of flow conditions generalizing decadal historical records. Our analysis reveals that, although the magnitude of hydraulic stress in the channel is consistently higher than that on hillslopes over all flow percentiles, the product of stress magnitude and frequency results in a close balance between hillslope supply and channel evacuation for high frequency flows. Only at less frequent, high-magnitude flows do channel hydraulic stresses exceed those on hillslopes, and channel evacuation dominates the net balance. This result suggests that WGEW exists mostly (~60% of the time) in an equilibrium condition of balance between hillslopes and channels, which helps to explain the straight longitudinal profile. We illustrate how this balance can be upset by climate changes that differentially affect relative flow regimes on slopes and in the channel. Such changes can push the long profile into a convex or concave condition.