Jeffrey G. Arnold

Channel Erosion Estimate for Urbanizing Watersheds: Submerged Jet testing and SWAT DEG

The Soil Water Assessment Tool (SWAT) has been modified to simulate downcutting and widening on small alluvial and threshold streams. An erodibility coefficient from submerged jet testing is multiplied by tractive force to compute downcutting and the channel slope is adjusted accordingly. Widening of the channel is accomplished through local width-depth ratios derived from measurements of streams in the area. Modeled results indicate the temporal change in down cutting and channel adjustment, useful in project planning and channel assessment. The variables for this tool have been kept to the minimum. Modeled results indicate land use and erodibility coefficients are more important than basin size in regulating degradation in small urban basins .Downcutting follows hyperbolic trends and indicates most degradation is accomplished in this North Texas area in 30 years.

Watershed-Scale Impact of Land-use changes for Bioenergy Production

USDA goals for meeting renewable fuels standards by 2022 indicate that 50% of the advanced biofuels to be produced in the U.S. are expected to come from the Southeastern U.S. High net primary productivity of the region from a favorable climate and productive soils make these goals attainable. Meeting these goals will require conversion of row-crops to high-yielding biomass crops. Changes in water resources, both quantity and quality, are anticipated with these changes. Biomass crops provide excellent ground cover, are believed to have lower water use requirements, and have high nitrogen use efficiency. The Soil and Water Assessment Tool (SWAT) was used to examine the long-term impacts of land-cover changes associated with bioenergy production. Simulations indicate conversion of existing production land into grass and forest bioenergy crops will result in: 1) decreased evapotranspiration; 2) increased streamflow; 3) decreased sediment loading; and 4) seasonal shifts in streamflow.

SWAT Revisions for Simulating Landscape Components and Buffer Systems

Methods for simulating different landscape positions within the SWAT model are being examined. A three component system, consisting of the watershed divide, the hillslope, and the floodplain landscape positions, has been developed to address flow and transport across hydrologic response units prior to concentration in streams. The modified SWAT model is capable of simulating flow and transport from higher landscape positions to lower positions within a single river basin. The revision was developed to address variable source areas within watersheds and stream-side buffer systems which exist alongside many streams. The enhanced model will allow for more accurate simulation of natural transport processes within a hillslope. The revision was tested using data collected from a low-gradient watershed near Tifton, Georgia, USA which contains heavily vegetated riparian buffers. The modified model provided reasonable simulations of surface and subsurface flow across the landscape positions without calibration. The application demonstrates the applicability of the model to simulate filtering of surface runoff, enhanced infiltration, and water quality buffering typically associated with riparian buffer systems.

Application of a Simple Headcut Advance Model for Gullys

Gully erosion begins in streambanks and uplands as a consequence of adjustments in driving forces on the landscape imposed by changes in land use or climate. The deleterious effects of gullies worldwide have led to many site-specific studies of gully form and function. In the continental United States, gully erosion in agricultural land has destroyed valuable farmland yet, prediction of gully processes remains problematic on a national scale. This research has proposed a simple method to predict gully headcut advance. When combined with SWAT hydrologic flow routines, the model predicted gully headcut advance with reasonable accuracy on a daily time step for time periods exceeding two decades. The model was tested in two distinct land resource areas of the United States with differing climate, soils, cover and drainage. The inputs for the headcut model have been kept simple as the model will be applied over large areas. Model inputs consist of headcut height, headcut resistance (based on soil erodibility and a root-cover factor), and daily flow. The model is compared to an annual time step model used in assessment of headcut advance and appears to offer a better way to assess gully headcut advance.