Bias in topographic thresholds for gully heads

ROSSI M. (1,2), TORRI D. (1), SANTI E. (1), 2013, Bias in topographic thresholds for gully heads, 6th International Symposium on Gully Erosion in a changing world (6th ISGE), Iasi, Romania, 06-12 May 2013,

The topographic threshold for overland flow gully head should be re-examined because of the effect of a basic assumption in deriving the threshold equation. The assumption that the gully head catchment (GHC) area is a synonym for runoff brings in several problems. These cause errors when calculating the threshold equation parameters (i.e. exponent and coefficient of the power equation linking critical slope gradient nearby the gully head to the GHC area). The assumption implies that every part of the GHC is connected by runoff to the outlet at the moment of peak discharge. Larger areas require larger concentration times which require a longer duration time of the rainfall. This makes less frequent (i.e. probable to observe) the occurrence of a rainfall intensity of the tight duration to allow a total connectivity inside the GHC. Also the vegetation type and land use (characterized by a type of vegetation) have an effect on the probability that the assumption is verified by the gully generative event. In order to show it, a simple distributed model have been developed in GRASS-R environments and a series of simulations were run showing when the conditions for gully erosion are actually verified. The hydrological part was developed based on the CN method approach, including the simulation of peak discharge with a few modification/adaptation to the spatially distributed environment. A small routine was added to simulate concentrated flow erosion and condition for gully head formation. Then a set of simulations were run using a series of daily rainfall amount. Results show a clear effect of the land use and vegetation on gully head position in the simulated landscapes. Using vegetation-induced modification of soil shear strength with root density and considering the effect of hydraulic roughness on overland flow mean velocity, even discontinuous gully channel were produced. Simulations clearly show that while small watersheds often contribute to runoff from every pixel, only rarely large GHC areas are contributing to the gully head. Substantially, it is as if the contributing GHC area increases only slightly with the topographic GHC area and stops increasing relatively soon. When the GHC topographic area is used as a surrogate for the GHC contributing area a bias is introduced into the trend which causes the apparent exponent to be lower than the real one

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