Secchi Disk Depth Estimation from China’s New Generation of GF-5 Hyperspectral Observations Using a Semi-Analytical Scheme

Abstract

Water clarity, commonly measured as the Secchi disk depth ( $Z_{sd}$ ), is an important parameter that depicts water quality in aquatic ecosystems. China’s new generation Advanced HyperSpectral Imager (AHSI) on board the GF-5 satellite has significant potential for applications of more accurate water clarity estimation compared with existing multispectral satellite imagery, considering its high spectral resolution with a 30-m spatial resolution. In this study, we validate the semi-analytical model with various Quasi-Analytical Algorithms (QAA), including $QA{A}_{V5}$ , $QA{A}_{V6}$ , $QA{A}_{L09}$ and $QA{A}_{M14}$ , for the AHSI images with concurrent in situ measurements in four inland water bodies with a $Z_{sd}$ range of 0.3–4.5 m. The semi-analytical method with $QA{A}_{V5}$ can yield the most accurate $Z_{sd}$ predictions with approximated atmospheric-corrected remote sensing reflectance. For 84 concurrent sampling sites, the estimated $Z_{sd}$ had a mean absolute error (MAE) of 0.35 m, while the mean relative error (MRE) was 25.3%. Specifically, the MAEs of estimated $Z_{sd}$ were 0.22, 0.46, and 0.24 m for $Z_{sd}$ of 0.3–1, 1–3, and 3–4.5 m, respectively. The corresponding MREs were 33.1%, 29.1% and 6.3%, respectively. Although further validation is still required, especially in terms of highly turbid waters, this study indicates that AHSI is effective for water clarity monitoring.