A New Spectral Index for Mapping Water Surfaces in Urban Contexts

This paper proposes a modification of MNDWI called Rescaled Water Index, aiming to enhance the delineation and performance of water body mapping in urban areas using satellite imagery. To evaluate the effectiveness of the proposed index, we compare its performance in mapping water surfaces in different cities in South America with the results obtained by other well-known water indices in the literature (namely, NDWI, MNDWI, AWEIsh, and AWEInsh). We utilized images from the Sentinel satellites (2A and 2B), all acquired in June 2021, focusing on cities located south of the Tropic of Capricorn to obtain images with the highest incidence of shading, whose spectral response complicates the mapping of water surfaces without commission errors. In this scenario, the present study revealed that selecting the best single index for mapping all water surfaces is an arduous task, as their performances varied across the analyzed locations. Overall, the performance of the indices, evaluated by partial receiver operating characteristic curve and non-water misclassification point restrictions, revealed quite close but different results, especially considering the Rescaled Water Index, NDWI, and MNDWI. In the six areas analyzed, the Rescaled Water Index outperformed in three. Additionally, when considering the results for all the cities, the Rescaled Water Index outperformed in all the analyses. Such results, therefore, are interpreted as an essential contribution to water body mapping, considering its practical applications in environmental monitoring and water resource management in urbanized areas.