Abstract
River water extent is essential for river hydrological surveys. Traditional methods for river water mapping often result in significant uncertainties. This paper proposes a support vector machine (SVM)-based river water mapping method that can quantify the extraction uncertainties simultaneously. Five specific bands of Landsat-8 Operational Land Imager (OLI) data were selected to construct the feature set. Considering the effect of terrain, a widely used terrain index called height above nearest drainage, calculated from the 1 arc-second Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), was also added into the feature set. With this feature set, a posterior probability SVM model was established to extract river water bodies and quantify the uncertainty with posterior probabilities. Three river sections in Northwestern China were selected as the case study areas, considering their different river characteristics and geographical environment. Then, the reliability and stability of the proposed method were evaluated through comparisons with the traditional Normalized Difference Water Index (NDWI) and modified NDWI (mNDWI) methods and validated with higher-resolution Sentinel-2 images. It was found that resultant probability maps obtained by the proposed SVM method achieved generally high accuracy with a weighted root mean square difference of less than 0.1. Other accuracy indices including the Kappa coefficient and critical success index also suggest that the proposed method outperformed the traditional water index methods in terms of river mapping accuracy and thresholding stability. Finally, the proposed method resulted in the ability to separate water bodies from hill shades more easily, ensuring more reliable river water mapping in mountainous regions.
Highlights
Water area variation is the most direct reflection of river water regime dynamics
River width, which could be extracted from the river water area, is a key input of river hydrodynamic models and a core parameter for estimating river discharge [1]
Uncertainty often occurs when identifying water pixels by water index values on optical images as it is often difficult to determine whether a pixel denotes water or non-water when its water index value is near the segmentation threshold [9]
Summary
Water area variation is the most direct reflection of river water regime dynamics. River width, which could be extracted from the river water area, is a key input of river hydrodynamic models and a core parameter for estimating river discharge [1]. Numerous studies have proposed a series of methods to map river water, including several water index methods [2,3,4,5] and thresholding methods [6,7] All of these methods inevitably cause uncertainties [8]. Uncertainty often occurs when identifying water pixels by water index values on optical images as it is often difficult to determine whether a pixel denotes water or non-water when its water index value is near the segmentation threshold [9]. The reason for this is twofold: the mixed pixels and the quality of the remote-sensing images. Cloud coverage is another issue that often affects extracting the water area
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