Contribution of the UAS to the determination of the water budget in a coastal wetland: a case study in the natural park of the Bay of Cádiz (SW Spain)

Wetlands are sensitive and complex systems whose conservation is a priority. For the correct understanding of their hydrological dynamics, it is necessary to determine the different elements of the water budget and, in particular, the geometry of the wetland basin in order to estimate the variations in storage capacity. This paper presents a novel, low-cost, user-friendly photogrammetric technique to obtain high-resolution datasets using non-metric cameras located in unmanned aerial systems (UAS) and structure-from-motion algorithms for producing high-precision 3D point clouds. The accuracy of the cartographic products obtained is evaluated using 59 checkpoints and comparing with the available LiDAR models. Best results are obtained using a full frame RGB sensor, which results in an orthomosaic with a pixel size of 1.38 cm and a positional RMSE of 3.8 cm in horizontal and a digital surface model (DSM) with a 3.5 cm RMSEZ. From the DSM, eliminating the influence of vegetation through masks, a digital terrain model (DTM) with a 5.9-cm RMSEZ that allows defining the filling curve of the wetland basin is obtained. This curve relates the stored volume and the surface exposed to evaporation with the water level, which allows to perform simulations in the balance models.