Estimation of Error Margin in the Quantification of Large Wood Volumes in River From In Situ and UAV Measurements

Large wood (LW) affects worldwide rivers by adding complexity to their flow structure, their sediment transport dynamics and by contributing to the spatial and temporal occurrences of fluvial morphologies. As a result, LW also promotes biodiversity by increasing the variability and quantity of fluvial habitats. To fully address the role of LW on fluvial habitat and dynamics, it is a common practice to measure the LW volumes in a river corridor and to produce a LW budget at various spatial and temporal scales. The quantification of LW volume and LW budget remains, however, a difficult task for practical and theoretical reasons. In this paper, we aim at providing insights on the uncertainty of LW volume measurements. We tested two methods (unmanned aerial vehicle [UAV] imageries and in situ measurements and duplicate measurements) to estimate margins of error in the quantification of LW volumes and budget on two rivers of the Gaspé Peninsula (30 000 ). A particular situation in 2012 led to the duplication of LW in situ volume measurements accumulated in the Saint‐Jean River. Thus, we were able to compare the volume measurements on 40 identical LW jams (LWJs) twice with only one month of interval. No flood with LW transport capacity was observed between the two field campaigns, which assures that the 40 measurements were made on identical LWJs. In order to improve the data set, four other campaigns of this type were carried out, each time with at least three operators taking the same measurements. Two were done by in situ measurement and two by UAV measurement. The results show high variability in the measurements due to operator bias. This high variability, identified mainly in the in situ measurements (59% for in situ values and 25% for UAV values on the wood budget), was then reduced by applying a new methodology for in situ measurements, which significantly reduced the variability. The estimation of errors in measured volumes is crucial to produce sound and robust LW budget. Our results shed light on the uncertainty of LWJ volume measurement and allow us to propose an equation to estimate the uncertainty of LW budget at various spatial and temporal scales.