Comparing the Accuracy of Several Field Methods for Measuring Gully Erosion

Most field erosion studies in agricultural areas provide little information on the probable errors involved. Here, for the first time, we compare the accuracy, time and cost of conventional and new methodologies for gully surveying, and provide a model to estimate the effort required to achieve a specified accuracy. Using a terrestrial LiDAR survey of a 7.1-m-long gully reach as a benchmark data set, the accuracies of different measurement methods (a new 3D photo-reconstruction technique, total station, laser profilemeter, and pole) are assessed for estimating gully erosion at a reach scale. Based on further field measurements performed over nine gullies (>100 m long), a simulation approach is derived to model the expected volume errors when 2D methods are used at the gully scale. All gullies considered were located near Cordoba, Spain. At the reach scale, the field measurements using 3D photo-reconstruction and total station techniques produced cross-sectional area error values smaller than 4%, with other 2D methods exceeding 10%. For volume estimation, photo-reconstruction proved similar to LiDAR data, but 2D methods generated large negative volume error (E-V) values (<-13% for laser profilemeter and pole). We show that the proposed error expressions derived from the model are in line with the reach-scale field results. A measurement distance factor (MDF) is defined that represents the ratio between cross-section distance and the gully length, and thus reflects relative survey effort. We calculate the required MDF for specified values of E-V, illustrating how MDF decreases with increasing gully length and sinuosity.