Abstract
Abstract. Water clarity serves as a sensitive tool for understanding the spatial pattern and historical trend in lakes' trophic status. Despite the wide availability of remotely sensed data, this metric has not been fully explored for long-term environmental monitoring. To this end, we utilized Landsat top-of-atmosphere reflectance products within Google Earth Engine in the period 1984–2018 to retrieve the average Secchi disk depth (SDD) for each lake in each year. Three SDD datasets were used for model calibration and validation from different field campaigns mainly conducted during 2004–2018. The red / blue band ratio algorithm was applied to map SDD for lakes (>0.01 km2) based on the first SDD dataset, where R2=0.79 and relative RMSE (rRMSE) =61.9 %. The other two datasets were used to validate the temporal transferability of the SDD estimation model, which confirmed the stable performance of the model. The spatiotemporal dynamics of SDD were analyzed at the five lake regions and individual lake scales, and the average, changing trend, lake number and area, and spatial distribution of lake SDDs across China were presented. In 2018, we found the number of lakes with SDD <2 m accounted for the largest proportion (80.93 %) of the total lakes, but the total areas of lakes with SDD of <0.5 and >4 m were the largest, both accounting for about 24.00 % of the total lakes. During 1984–2018, lakes in the Tibetan–Qinghai Plateau region (TQR) had the clearest water with an average value of 3.32±0.38 m, while that in the northeastern region (NLR) exhibited the lowest SDD (mean 0.60±0.09 m). Among the 10 814 lakes with SDD results for more than 10 years, 55.42 % and 3.49 % of lakes experienced significant increasing and decreasing trends, respectively. At the five lake regions, except for the Inner Mongolia–Xinjiang region (MXR), more than half of the total lakes in every other region exhibited significant increasing trends. In the eastern region (ELR), NLR and Yungui Plateau region (YGR), almost more than 50 % of the lakes that displayed increase or decrease in SDD were mainly distributed in the area range of 0.01–1 km2, whereas those in the TQR and MXR were primarily concentrated in large lakes (>10 km2). Spatially, lakes located in the plateau regions generally exhibited higher SDD than those situated in the flat plain regions. The dataset is freely available at the National Tibetan Plateau Data Center (https://doi.org/10.11888/Hydro.tpdc.271571, Tao et al., 2021).
Highlights
Lakes and reservoirs are important aquatic habitats and serve as freshwater water sources for drinking, industrial and agricultural uses (Pekel et al, 2016; Tranvik et al, 2009; Wetzel, 2001)
In the eastern region (ELR), northeastern lake region (NLR) and Yungui Plateau region (YGR), almost more than 50 % of the lakes that displayed increase or decrease in Secchi disk depth (SDD) were mainly distributed in the area range of 0.01–1 km2, whereas those in the Tibetan–Qinghai Plateau region (TQR) and Mongolia–Xinjiang region (MXR) were primarily concentrated in large lakes (> 10 km2)
In situ water clarity data collected in lakes across China during 2004–2018 were used to calibrate and validate SDD models that incorporate top-of-atmosphere reflectance products and Google Earth Engine to map the spatiotemporal dynamics of SDD over a 35-year time span (1984–2018)
Summary
Lakes and reservoirs are important aquatic habitats and serve as freshwater water sources for drinking, industrial and agricultural uses (Pekel et al, 2016; Tranvik et al, 2009; Wetzel, 2001). More than 26 000 lakes (with area > 0.01 km2) and 78 000 reservoirs are distributed across China (Song et al, 2018a), providing multiple ecosystem services Over the last 4 decades, China has made considerable achievements with respect to socioeconomic development but has faced increasing water pollution challenges due to, among other contributing factors, agricultural nonpoint pollution, wastewater discharge, urban expansion and increased water consumption (Han et al, 2016; Qin et al, 2010; Tong et al, 2017). Eutrophication and algal bloom proliferation are the clearest manifestations of these water quality problems, and major efforts have been made (afforestation, conversion of cropland to grassland or wetland) to mitigate these impacts and restore the ecological integrity of inland water systems (Huang et al, 2016; Ma et al, 2020; Tong et al, 2020)
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