Since the Normalized Difference Water Index (NDWI) was proposed, water indices have served as useful tools for surface water detection. However, existing water indices are highly influenced by atmospheric and other environmental conditions and suffer from limited performance, especially in urban areas. At the core of the limitation is the sole dependency on the spectral distribution of reflectance signals. To overcome this, we propose to utilize topographic data as additional information for better water detection. Accordingly, the new index, namely Combined Water Index (CWI), is developed as the product of the topographic index and the reflectance-based index. These two indices excellently compensate each other: the former is free from noise issues but invariant over time while the latter can capture temporal dynamics of waterbody extents. The CWI is applied to four study areas of different development levels (natural, medium-sized cities, and megalopolis) in the Han River basin, South Korea. The water detection results of the CWI is promising, particularly in the heavily developed urban setting, demonstrated through visual images as well as various statistical measures.

自从标准化差异水指数（NDWI）提出以来，水指数已成为地表水检测的有用工具。然而，现有的水指数受大气和其他环境条件的影响很大，并且性能有限，特别是在城市地区。限制的核心是仅依赖于反射信号的光谱分布。为了克服这个问题，我们建议利用地形数据作为附加信息来更好地检测水。因此，新的指数，即组合水指数（CWI），是作为地形指数和基于反射率的指数的乘积而开发的。这两个指数很好地相互补偿：前者没有噪音问题，但随着时间的推移保持不变，而后者可以捕捉水体范围的时间动态。CWI适用于韩国汉江流域不同发展水平的四个研究区域（自然、中等城市和特大城市）。通过视觉图像和各种统计措施证明，CWI 的水检测结果是有希望的，特别是在高度发达的城市环境中。