INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Effect of topographic characteristics on compound topographic index for identification of gully channel initiation locations
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2013
Publication Date: April 29, 2013
Citation: Momm, H.G., Bingner, R.L., Wells, R.R., Rigby Jr, J.R., Dabney, S.M. 2013. Effect of topographic characteristics on compound topographic index for identification of gully channel initiation locations. Transactions of the ASABE. 56(2):523-537.
Interpretive Summary: Gully sediment eroded from agricultural fields has a significant impact on crop productivity and downstream watershed water quality. Identifying where gullies form throughout watersheds can be difficult without technology utilizing the latest remotely sensed information. Knowing where gullies form can be used by watershed models to assess the impact of agricultural practices on reducing gully erosion. This study evaluated the capability of topographic indices from agricultural landscapes to identify the location of concentrated flow paths and potential gully initiation points. Topographic indices were compared to results from variable spatial resolutions from synthetic digital elevation models (DEMs) and from DEMs describing agricultural fields located in Kansas, Iowa, and Mississippi to assess the individual effect of relief variance, overall catchment slope, and DEM raster grid cell size. Results indicate that critical topographic index (CTI) values used to define gully initiation points were linearly influenced by changes in relief variance and overall slope while variations in raster grid cell size caused a non-linear variation of average highest 0.1% CTI values. Standardization of CTI cumulative distributions improved comparisons between the various sites with distinct drainage area sizes and topographic characteristics, providing a possible alternative for investigations of large watersheds. Future investigations in methodologies for identification of areas prone to gully formation should focus on improved topographic representation and incorporation of soil properties into the decision-making process. Utilizing improved gully identification technology can provide action agencies with enhanced information and management tools to assess ephemeral gully erosion control practices critical in the development of effective management plans that reduces sediment loads within watershed systems.
Sediment loads from gully erosion can be a significant sediment source within watershed resulting in major contributions to water quality problems, reduction of crop productivity by removal of nutrient rich top soil, and damaging downstream ecosystems. Areas containing a high probability of forming gully channels in agricultural watersheds are often evaluated by spatially deriving stream power estimates from proxy topographic information combining local slope, upstream drainage area, and planform curvature into a compound topographic index (CTI). The ability to use CTI in identifying zones prone to gully formation is affected by field topographic characteristics and DEM resolution. Evaluation of CTI values, using simulated catchments, provided a mechanism to assess the individual effect of relief variance, overall catchment slope, and raster grid cell size. Results indicate that CTI values were linearly influenced by changes in relief variance and overall slope while variations in raster grid cell size caused a non-linear variation (inverse power) of average highest 0.1% CTI values in addition to changes in the shape of cumulative distributions function. Standardizing CTI values (CTIn) produced merged cumulative distribution curves when varying overall slope, terrain relief variance, and to a lesser degree, DEM resolution. A similar investigation was performed for three sites located in USA with distinct topographic characteristics. Critical CTIn values were determined through comparison of measured gully thalweg location with points obtained by developing threshold CTIn raster grids at different resolutions.
When DEM resolution varied, the differences in critical CTIn values in the same catchment field were significantly reduced when compared to the original critical CTI values, although not fully eliminated. Standardization of CTI cumulative distributions improved comparisons between different sites with distinct drainage area sizes and topographic characteristics, providing a possible alternative for investigations of large watersheds with more than one topographic face. This information is critical in the development of automated techniques to locate the gully initiation points for use in technologies assessing the effect of conservation practices.