GIS technology for spatiotemporal measurements of gully channel width evolution

Abstract

Field observations of ephemeral gully evolution in active croplands have often revealed the presence of a less erodible soil layer that is typically associated with tillage practices (i.e., plowpan). This more erosion-resistant layer limits channel incision forcing the gully channel to expand laterally through basal scour of the bank toe and gravitational mass movement of the gully channel sidewalls. Understanding the role and quantification of widening processes is vital to efforts to quantify soil loss from gullies. One major research challenge is designing laboratory experiments that replicate field conditions while accounting for and accurately measuring spatial and temporal gully channel characteristics. Technology was developed to capture 2-cm-spaced cross-sections along a soil flume at user-defined time intervals. Two off-the-shelf high-resolution cameras were positioned above the soil bed looking as close to nadir field of view as possible. Using open source technology, computer control of the cameras was used to trigger each camera at 10-s intervals and download individual images from the cameras. Out of the two sets of images generated (one set from each camera), only one set of images was selected for further processing based on the quality of image information defined by image clarity/sharpness and the presence/absence of light reflectance in the water. Batch processing scripts were used to geo-reference individual images within an image set based on known coordinates of control points and to re-sample each image into a standard raster grid cell size of 0.25 cm. Custom developed image processing software was utilized to identify image color discontinuities representing channel edges based on water and soil image color reflectance differences. After an additional filtering step, the set of image color discontinuities was converted into GIS polygons, and these polygons were then used to produce a set of cross-sections for each time interval (hundreds of cross-sections for each time interval). The technology offers an inexpensive alternative for collecting data from laboratory experiments and serves as a template for multi-purpose investigations where channel edge discontinuities need to be accurately measured at high temporal resolution.