Using Google Earth Imagery to Target Assessments of Ephemeral Gully Erosion

Abstract

Highlights Tested the utility of Google Earth and other imagery to target the location of ephemeral gullies. Developed a targeting methodology and tested it based on a ground truth obtained using a UAV. Quantified the overlap of targeted areas with observed ephemeral gullies. These publicly available sources of imagery had a high degree of success in identifying where ephemeral gullies form. Abstract. Sustaining civilizations requires preventing the loss of agricultural topsoil through processes such as water and wind erosion. Ephemeral gully erosion contributes an estimated 40% of the total water-erosion soil loss from row-crop fields. The identification and tracking of gullies require monitoring fields over time; Google Earth provides high-quality imagery that can potentially meet both the temporal and spatial criteria for ephemeral gully monitoring. Our primary objective was to determine the probability that an ephemeral gully erosion feature could be reliably identified as an area of concern based on Google Earth imagery and/or other publicly available remotely sensed imagery. To develop the ground truth, we visited 72 fields in seven Missouri counties between mid-April and mid-June in 2018 (n = 26), 2019 (n = 21), and 2020 (n = 25) to verify the presence of erosion features. An unmanned aerial vehicle (UAV) was used to collect aerial imagery with an estimated ground sampling distance of 2.1 to 2.7 cm pixel-1 from all locations. From this imagery, ephemeral gullies were observed in 24 of the fields, and all ephemeral gullies in those fields were delineated. We then reviewed all imagery available in Google Earth from 2010 to 2020 for the 24 fields where ephemeral gullies were observed, delineating ephemeral gully features using a “definitive” and a “less stringent” criterion; we also evaluated 2008 and 2015 imagery from a second public source. In the first analysis, one random erosion feature was chosen from each field, and the percentage overlap of lines derived from publicly available information with the ground truth was determined. Combining all imagery sources, using the less stringent method of delineation, and applying a 15-m buffer resulted in a mean overlap rate of 91%. These results were superior to the definitive approach, as well as using a 3-m buffer. In a second analysis, we tested definitive and less stringent criteria in the field for simple intersection with the ground truth. The less stringent strategy, coupled with using a 15-m buffer, had a true positive rate of 81% and identified 100% of the ephemeral gullies at 63% of the locations. There were false positives in 38% of the fields, with a mean rate across locations of 15%. Adding public data from other sources improved the true positive rate while also increasing the false negative rate. At one location, all publicly available image sources failed to identify the single ephemeral gully in the field. This research represents a proof of concept that Google Earth and other publicly available imagery of sufficient quality can be used to target in-field ephemeral gully assessment in row crop fields in the humid regions of the US. Validation work is needed before this approach can be broadly adopted with confidence, given the many uncontrollable factors that can affect the efficacy of this approach. Keywords: Aerial imagery, Conservation compliance, Ephemeral gully, Remote sensing, Soil erosion.