Abstract
River restoration projects, which aim at improved flood safety and increased ecological value, have resulted in more heterogeneous vegetation. However, they also resulted in increasing hydraulic roughness, which leads to higher flood water levels during peak discharges. Due to allowance of vegetation development and succession, both ecological and hydraulic characteristics of the floodplain change more rapidly over time. Monitoring of floodplain vegetation has become essential to document and evaluate the changing floodplain characteristics and associated functioning. Extraction of characteristics of low vegetation using single-epoch remote sensing data, however, remains challenging. The aim of this study was to (1) evaluate the performance of multi-temporal, high-spatial-resolution UAV imagery for extracting temporal vegetation height profiles of grassland and herbaceous vegetation in floodplains and (2) to assess the relation between height development and NDVI changes. Vegetation height was measured six times during one year in 28 field plots within a single floodplain. UAV true-colour and false-colour imagery of the floodplain were recorded coincidently with each field survey. We found that: (1) the vertical accuracy of UAV normalized digital surface models (nDSMs) is sufficiently high to obtain temporal height profiles of low vegetation over a growing season, (2) vegetation height can be estimated from the time series of nDSMs, with the highest accuracy found for combined imagery from February and November (RMSE = 29-42 cm), (3) temporal relations between NDVI and observed vegetation height show different hysteresis behaviour for grassland and herbaceous vegetation. These results show the high potential of using UAV imagery for increasing grassland and herbaceous vegetation classification accuracy.
Highlights
River floodplains are be used for different - often spatially conflicting - purposes, such as water conveyance and storage during peak discharge, nature development, agricultural practices and recreation
Thereby, we implicitly assumed that the February digital surface models (DSMs) represents the Digital Terrain Model (DTM), which is reasonable for herbaceous vegetation in leaf-off conditions
The results show that the temporal trend for Hv,predicted and Normalized Difference Vegetation Index (NDVI) value over the growing season is similar to the trend observed in the field
Summary
River floodplains are be used for different - often spatially conflicting - purposes, such as water conveyance and storage during peak discharge, nature development, agricultural practices and recreation. Restoration projects of floodplain ecology have resulted in a more natural and heterogeneous floodplain vegetation (Göthe et al, 2015). These have enhanced ecological value and biodiversity of the floodplains, but developing natural vegetation results in increasing hydraulic roughness, which lowers the conveyance capacity and increases flood risk. Due to vegetation development and succession, both ecological and hydraulic characteristics of the floodplain change more rapidly over time. To document and evaluate these floodplain characteristics, monitoring of floodplain vegetation has become essential. Reported land cover classifications showed low accuracies for grassland and herbaceous vegetation, due to the spectral and structural similarity of these vegetation types (Geerling et al, 2007; Straatsma et al 2008, Knotters and Brus, 2013)
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