INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Automated Mapping of the Potential for Ephemeral Gully Formation in Agricultural Watersheds
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 24, 2010
Publication Date: June 27, 2010
Citation: Parker, C., Bingner, R.L., Thorne, C., Wells, R.R. 2010. Automated Mapping of the Potential for Ephemeral Gully Formation in Agricultural Watersheds. In: Proceedings of the 2010 Federal Interagency Sedimentation Conference, June 27-July 1, 2010. Las Vegas, Nevada. 2010 CDROM.
Interpretive Summary: Ephemeral gullies serve as effective links transferring sediment and associated agrichemicals from upland areas to stream channels. Current erosion prediction technologies often require the exact topographic location of an ephemeral gully to be known, which greatly limits the application of such models. Within a watershed system, there may be hundreds of ephemeral gullies that are present or may be present if management conditions were performed that promotes the formation of ephemeral gullies on the landscape and would require a considerable amount of time to manually identify their locations. An automated approach is needed to help identify watershed ephemeral gully locations and their downstream beginning points that can be used to simulate ephemeral gully erosion by watershed models. The approach adopted builds on previously developed technology to estimate locations of ephemeral gullies at the field level through the utilization of topographic information analyzed within a geographic information system (GIS) interface. A number of field sites were used to compare the predicted and observed locations and extents of ephemeral gullies. The results of the study show that this automated technique works reliably well provided that sufficient resolution and accuracy of the topographic data is available. This technology provides an automated, physically based standard of identifying ephemeral gully locations, including their downstream beginning points in agricultural watersheds that can be used to simulate and track ephemeral gully sediment sources. These tools are necessary to effectively manage the Nation’s soil resources.
Erosion due to ephemeral gullies in cultivated areas contributes significantly to soil loss and sediment yield from arable watersheds. Despite this, no readily applied, automated method for mapping the potential for ephemeral gullies to form currently exists. This presentation outlines how the capability to perform these tasks within a GIS environment adds value to the utility of catchment erosion and sediment yield models, and demonstrates how the potential for ephemeral gully locations may be mapped within the USDA-ARS’s AnnAGNPS model, to support prediction of the impacts of ephemeral gully initiation and subsequent growth.
The approach adopted builds on the results of long-term research performed at the USDA-ARS National Sedimentation Laboratory that began in the 1980s and which used the manual, field-based calculation of a Compound Topographic Index (CTI) as a predictor of ephemeral gullying potential. More recently, joint research between the NSL and Nottingham University has examined the ability of a GIS-based version of the original CTI to correctly predict the locations of ephemeral gullies based gridded altitude data obtained by remote sensing. The new technique has been tested at the field scale through direct comparison with ephemeral gullies observed at a number of experimental monitoring sites. The results of the study demonstrate that the technique developed works effectively where gridded altitude data are available, and with the current rate of increase in the availability of such data the method should become nationally applicable it in the near future. On this basis, the alogorithms necessary to support prediction of potential sites for ephemeral gully formation have been programmed in to the latest version of the AnnAGNPS software.