Secchi depth (SD, m) is a direct and intuitive measure of water’s transparency, which is also an indicator of water quality. In 2015, a semi-analytical model was developed to derive SD from remote sensing reflectance, thus able to provide maps of water’s transparency in satellite images. Here an in-situ dataset (338 stations) is used to evaluate its potential ability to monitor water quality in the coastal and estuarine waters, with measurements covering the Zhujiang (Pearl) River Estuary, the Yellow Sea and the East China Sea where measured SD values span a range of 0.2–21.0 m. As a preliminary validation result, according to the whole dataset, the unbiased percent difference (UPD) between estimated and measured SD is 23.3% (N=338, R²=0.89), with about 60% of stations in the dataset having relative difference (RD) ⩽ 20%, over 80% of stations having RD ⩽ 40%. Furthermore, by excluding the field data which with relatively larger uncertainties, the semi-analytical model yielded the UPD of 17.7% (N=132, R²=0.92) with SD range of 0.2–11.0 m. In addition, the semi-analytical model was applied to Landsat-8 images in the Zhujiang River Estuary, and retrieved high-quality mapping and reliable spatial-temporal patterns of water clarity. Taking into account the uncertainties associated with both field measurements and satellite data processing, and that there were no tuning of the semi-analytical model for these regions, these findings indicate highly robust retrieval of SD from spectral techniques for such turbid coastal and estuarine waters. The results suggest it is now possible to routinely monitor coastal water transparency or visibility at high-spatial resolutions from measurements, like Landsat-8 and Sentinel-2 and newly launched Gaofen-5.

中文翻译：

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使用半分析模型检索沿海和河口水域的塞基深度

Secchi深度（SD，m）是水的透明度的直接直观的度量，也是水质的指标。2015年，开发了一种半分析模型，可从遥感反射率得出标准差，从而能够在卫星图像中提供水的透明度地图。这里使用一个现场数据集（338个站）来评估其监测沿海和河口水域水质的潜在能力，其测量范围涵盖珠江（珠江）河口，黄海和东海，其中SD值范围为0.2–21.0 m。作为初步的验证结果，根据整个数据集，估计的和测量的SD之间的无偏差百分比差异（UPD）为23.3％（N = 338，R ²= 0.89），数据集中约60％的站点的相对差异（RD）≤20％，超过80％的站点的RD≤40％。此外，通过排除不确定性相对较大的现场数据，半分析模型得出的UPD为17.7％（N = 132，R ²= 0.92），SD范围为0.2-11.0 m。此外，将半分析模型应用于珠江口的Landsat-8影像，并获得了高质量的制图和可靠的时空净水模式。考虑到与野外测量和卫星数据处理相关的不确定性，并且没有对这些地区的半分析模型进行调整，这些发现表明，从这种浑浊的沿海和河口水域的光谱技术中，SD的检索能力非常强。结果表明，现在可以通过Landsat-8和Sentinel-2和新近发射的Gaofen-5等测量结果，以高空间分辨率例行监视沿海水域的透明度或可见性。