Development and application of a hybrid terrain–multispectral image model of floodplain hydroperiod

Abstract Surface water mapping is a crucial aspect of studying river and wetland hydrological, geomorphological, and ecological processes. However, deriving inundation frequency solely from a normalised difference water index does not account for geomorphologically induced flooding dynamics, which reduces model accuracy. This study evaluated spatiotemporal inundation dynamics using a novel approach that combines geomorphologically relevant descriptors from a high-resolution LiDAR-derived digital elevation model with spectral outputs from Sentinel-2 imagery. The hydroperiod for the Nuwejaars River Floodplain was determined by: (1) mapping macro-scale geomorphic features and floodplain topography using a DEM and height above nearest drainage; (2) calculating the percentage of images (127 images with clouds < 20%), in which a particular pixel was flooded using Sentinel-2 imagery (2018 to 2023); (3) applying an unsupervised learning algorithm and thresholding in Google Earth Engine to assign water frequency values to each pixel; and (4) deriving a hydroperiod that integrates geomorphic data and Sentinel-2 imagery to classify inundation patterns (F1 = 0.67, n = 60). Accuracy assessment of the model, incorporating various spectral indices, was performed using a confusion matrix and validated against field data. The terrain-informed hydroperiod model achieved improved accuracy (75.9%), precision (66.7%), and recall (66.7%) statistics. Analysis revealed substantial spatial variation in inundation frequency closely related to floodplain geomorphology, with largest inundation frequencies occurring downstream of a partial avulsion and limited inundation in distal, abandoned channels. The study highlights the importance of integrating hydroperiod mapping with geomorphological insights. This approach is essential for understanding how historical and future changes in hydroperiod affect floodplain ecological processes and ecosystem service provision.