Use of geomorphic indicators in parameterizing an event-based sediment-yield model

Abstract

This research developed a method for parameterizing a physically-based distributed rainfall-runoff model to more effectively model erosion on small semiarid watersheds. Topographic survey was done to characterize the form of a small watershed near Tombstone, Az. Soil samples were collected to characterize the spatial variability of soils. It was found that a relationship between slope and area can be used to determine the critical source area necessary to initiate a channel. These estimates agreed with the location of channel heads identified in the field. This provided a basis for partitioning the watershed into subcatchment elements based on process-scale rather than topographic map scale. The Engelund and Hansen (1967) transport capacity relationship in the KINEROS2 model can be parameterized using soil particle size data. Soils on the surface of the hillslopes are the least variable, while soils in the channels are the most variable. The coarsest soils on the watershed occur at the transition from hillslope to channel. Particle size in channels were estimated using drainage area and channel slope as predictors, because downstream fining occurs in channels. Particle size distributions on hillslopes were estimated using geostatistics and regression relationships. To see whether these methods improved model estimates, simulations from a simplified 18 element representation of the watershed using lumped parameter estimates were compared to simulations from a 312 element partitioning of the watershed using distributed parameter values. Data from six events could be precisely modeled (model efficiencies > 0.9) which minimized the effect of hydrologic error on erosion modeling. A multiplier was placed in front of the raindrop impact sediment entrainment term, and in front of the transport capacity term, the two primary erosion mechanisms. The model predicted sediment yield for some events well (model efficiencies 0.85 and 0.97). Results from the complex configuration of the watershed were better than the simple configuration. Multiplier values on the splash term were 5 (complex) and 12 (simple). The multiplier on the transport capacity term was between 1.2 and 1.70. These multiplier values are unrealistic suggesting that the multipliers are acting as fitting coefficients, and may not have any physical significance.