Comparing the accuracy of several field methods for measuring gully erosion

Most fi eld erosion studies in agricultural areas provide little information on the probable errors involved. Here, for the fi rst 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 specifi ed 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 profi lemeter, and pole) are assessed for estimating gully erosion at a reach scale. Based on further fi eld 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 fi eld 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 (EV) values (<–13% for laser profi lemeter and pole). We show that the proposed error expressions derived from the model are in line with the reach-scale fi eld results. A measurement distance factor (MDF) is defi ned that represents the ratio between cross-section distance and the gully length, and thus refl ects relative survey effort. We calculate the required MDF for specifi ed values of EV, illustrating how MDF decreases with increasing gully length and sinuosity.