Gully processes and modelling

Bull, L. J.; Kirkby, M. J.

doi:10.1177/030913339702100302

Cited by 153 publications

(94 citation statements)

References 52 publications

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“…This in turn implies either an increased free energy export from the hillslope/catchment control volume or an increased depletion of internal driving gradients and thus a faster relaxation of the system back towards local thermodynamic equilibrium Zehe et al, 2013). This common functionality might explain the dominance of rapid flow in different forms of connected networklike flow paths across many scales: locally in vertical macropores Germann, 1982, 2013), in hillslope-scale lateral surface rills or subsurface pipe networks (Bull and Kirkby, 1997;Parkner et al, 2007;Weiler and McDonnell, 2007;van Schaik et al, 2008;Wienhöfer et al, 2009) or in catchment-scale and even continental-scale river networks (Howard, 1990).…”

Section: Are Hrus and Landscape Organization Resulting From Co-evolutmentioning

confidence: 99%

HESS Opinions: From response units to functional units: a thermodynamic reinterpretation of the HRU concept to link spatial organization and functioning of intermediate scale catchments

Zehe

Ehret

Pfister³

et al. 2014

Hydrol. Earth Syst. Sci.

100

103

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Abstract. According to Dooge (1986) intermediate-scale catchments are systems of organized complexity, being too organized and yet too small to be characterized on a statistical/conceptual basis, but too large and too heterogeneous to be characterized in a deterministic manner. A key requirement for building structurally adequate models precisely for this intermediate scale is a better understanding of how different forms of spatial organization affect storage and release of water and energy. Here, we propose that a combination of the concept of hydrological response units (HRUs) and thermodynamics offers several helpful and partly novel perspectives for gaining this improved understanding. Our key idea is to define functional similarity based on similarity of the terrestrial controls of gradients and resistance terms controlling the land surface energy balance, rainfall runoff transformation, and groundwater storage and release. This might imply that functional similarity with respect to these specific forms of water release emerges at different scales, namely the small field scale, the hillslope, and the catchment scale. We thus propose three different types of "functional units" -specialized HRUs, so to speak -which behave similarly with respect to one specific form of water release and with a characteristic extent equal to one of those three scale levels. We furthermore discuss an experimental strategy based on exemplary learning and replicate experiments to identify and delineate these functional units, and as a promising strategy for characterizing the interplay and organization of water and energy fluxes across scales. We believe the thermodynamic perspective to be well suited to unmask equifinality as inherent in the equations governing water, momentum, and energy fluxes: this is because several combinations of gradients and resistance terms yield the same mass or energy flux and the terrestrial controls of gradients and resistance terms are largely independent. We propose that structurally adequate models at this scale should consequently disentangle driving gradients and resistance terms, because this optionally allows Published by Copernicus Publications on behalf of the European Geosciences Union. E. Zehe et al.: HESS Opinions: Thermodynamic reinterpretation of the HRU conceptequifinality to be partly reduced by including available observations, e.g., on driving gradients. Most importantly, the thermodynamic perspective yields an energy-centered perspective on rainfall-runoff transformation and evapotranspiration, including fundamental limits for energy fluxes associated with these processes. This might additionally reduce equifinality and opens up opportunities for testing thermodynamic optimality principles within independent predictions of rainfall-runoff or land surface energy exchange. This is pivotal to finding out whether or not spatial organization in catchments is in accordance with a fundamental organizing principle.

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Section: Are Hrus and Landscape Organization Resulting From Co-evolutmentioning

confidence: 99%

HESS Opinions: From response units to functional units: a thermodynamic reinterpretation of the HRU concept to link spatial organization and functioning of intermediate scale catchments

Zehe

Ehret

Pfister³

et al. 2014

Hydrol. Earth Syst. Sci.

100

103

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“…Mass failures of steep gully walls are a primary mechanism of gully enlargement, particularly in cohesive soils (e.g., Bull and Kirkby, 1997). Mass failures are important because they rapidly move large volumes of colluvium to the bottom of a gully, where the colluvium becomes susceptible to erosion by fluvial action.…”

Section: Introductionmentioning

confidence: 99%

Bank‐collapse processes in a valley‐bottom gully, western Iowa

Thomas

Iverson

Burkart

2008

Earth Surf Processes Landf

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Widening and bank-slope reduction of a valley-bottom gully in western Iowa was correlated to increasing subsurface flow over a 36-year period. To study bank collapse at this gully, we measured rainfall, air temperature, hydraulic head near the banks and bank movement nearly continuously over a 2-year period. Styles of movement ranged from imperceptible creep to rapid slab collapses preceded by the formation of tension cracks parallel to the gully walls. Bank movement was commonly correlated to rainfall or snowmelt and associated head increases in the banks. If the banks are modelled as a twodimensional slab with an adjacent tension crack partly filled with water, measured heads were sufficient to cause bank failures through reduction of frictional support at the base of the slab. During winter months, air temperature variations across 0°C were correlated with bank movement: during mildly subfreezing periods banks expanded, and most, but usually not all, of this movement was recovered during above-freezing periods. This motion is attributed to frost heave followed by thawing. Deformation of the banks by heaving and thawing during winter may weaken them and prime them for failure during spring rains and snowmelt, when the frequency of mass-wasting events is highest.

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“…12). It is mentioned that a gully is relatively deep (>0.6 m), recently formed eroding channel (with ephemeral flow) on valley sides and on valley floors where no well-defined channel previously existed and it has steep sides, low width-depth ratio and stepped profile (presence of knick points), characteristically with a headcut (with plunge pool) at the upslope end, dominated by the processes of surface flow, piping and mass movement [13][14][15][16][17] . Gully initiation by surface hydro-geomorphic processes has been recognized as a threshold phenomenon related to the size of the contribution drainage area and its slope [18][19][20] .…”

mentioning

confidence: 99%