Mapping Vegetation Density in a Heterogeneous River Floodplain Ecosystem Using Pointable CHRIS/PROBA Data

Jochem Verrelst,Erika Romijn,Lammert Kooistra

doi:10.3390/rs4092866

Jochem Verrelst, Erika Romijn + Show 1 more

Open Access

https://doi.org/10.3390/rs4092866

Copy DOI

Abstract

River floodplains in the Netherlands serve as water storage areas, while they also have the function of nature rehabilitation areas. Floodplain vegetation is therefore subject to natural processes of vegetation succession. At the same time, vegetation encroachment obstructs the water flow into the floodplains and increases the flood risk for the hinterland. Spaceborne pointable imaging spectroscopy has the potential to quantify vegetation density on the basis of leaf area index (LAI) from a desired view zenith angle. In this respect, hyperspectral pointable CHRIS data were linked to the ray tracing canopy reflectance model FLIGHT to retrieve vegetation density estimates over a heterogeneous river floodplain. FLIGHT enables simulating top-of-canopy reflectance of vegetated surfaces either in turbid (e.g., grasslands) or in 3D (e.g., forests) mode. By inverting FLIGHT against CHRIS data, LAI was computed for three main classified vegetation types, ‘herbaceous’, ‘shrubs’ and ‘forest’, and for the CHRIS view zenith angles in nadir, backward (−36°) and forward (+36°) scatter direction. The −36° direction showed most LAI variability within the vegetation types and was best validated, closely followed by the nadir direction. The +36° direction led to poorest LAI retrievals. The class-based inversion process has been implemented into a GUI toolbox which would enable the river manager to generate LAI maps in a semiautomatic way.

Highlights

Climate change is expected to have a large impact on water resources and flooding risks of the main rivers in the Netherlands [1]
Most misclassifications occurred between the ‘grasses and low herbaceous vegetation’ and the ‘higher herbaceous vegetation’, because the spectral characteristics of these classes have similarities and mixing of different vegetation types occurred in the pixels (~17 m) of the Compact High Resolution Imaging Spectrometer (CHRIS) image
The remaining part of the area had a heterogeneous land cover with transitions between land cover types on the pixel-level which is characteristic for a natural river floodplain ecosystem

Summary

Introduction

Climate change is expected to have a large impact on water resources and flooding risks of the main rivers in the Netherlands [1]. The capability of the river system to accommodate peak flows has been reduced which leads to increased flooding risks for the floodplains and its hinterland [6]. The water discharge capacity of the river system in the Netherlands has been increased by lowering and widening of the floodplains, removal of hydraulic obstacles in the floodplains and by excavation of secondary channels [7,8]. These newly developed river floodplains serve as nature restoration areas, where succession of vegetation leads to highly valued ecosystems [9].

Methods

Results

Discussion

Conclusion