Chlorophyll and Suspended Solids Estimation in Portuguese Reservoirs (Aguieira and Alqueva) from Sentinel-2 Imagery

Abstract

Reservoirs have been subject to anthropogenic stressors, becoming increasingly degraded. The evaluation of ecological potential in reservoirs is remarkably challenging, and consistent and regular monitoring using the traditional in situ methods defined in the WFD is often time- and money-consuming. Alternatively, remote sensing offers a low-cost, high frequency, and practical complement to these methods. This paper proposes a novel approach, using a C2RCC processor to analyze Sentinel-2 imagery data to retrieve information on water quality in two reservoirs of Portugal, Aguieira and Alqueva. We evaluate the temporal and spatial evolution of Chl a and total suspended solids (TSS), between 2018 and 2020, comparing in situ and satellite data. Generally, Alqueva reservoir allowed lower relative (NRMSE = 8.9% for Chl a and NRMSE = 21.9% for TSS) and systematic (NMBE = 1.7% for Chl a and NMBE = 2.0% for TSS) errors than Aguieira, where some fine-tuning would be required. Our paper shows how satellite data can be fundamental for water-quality assessment to support the effective and sustainable management of inland waters. In addition, it proposes solutions for future research in order to improve upon the methods used and solve the challenges faced in this study.

Retrieving Inland Reservoir Water Quality Parameters Using Landsat 8-9 OLI and Sentinel-2 MSI Sensors with Empirical Multivariate Regression

“…The optical properties of inland waters are very different between water bodies, and there are also significant differences within water bodies. These problems hinder the development of inland water algorithms and typically limit their applicability to different water bodies [5]. Lai et al retrieved the concentration and distribution of Chl-a in the Guanting Reservoir based on the measured data in different years and Landsat-8 images and 22 algorithm formulas, including SABI, KIVU, Apple, and other vegetation indicators, and found that there was a strong correlation between the pixel values of adjacent reservoirs in the same image, so the Chl-a estimation model can be applied to each other [49].…”

“…Inland water bodies are surrounded mostly by land, and border effects are especially significant in areas with raised, undulating topography around the water body [51]. This means that light from objects around the body of the water can change the radiance reaching the sensor, and large parts of the sky may also be blocked by the ground surface (e.g., vegetation) [5], making it impossible to obtain the true WQPs at the water boundary accurately.…”

“…Reservoirs are a distinct example of inland water bodies and are transitional systems between rivers and lakes formed by the damming of rivers. Reservoirs are also simple targets for waste disposal [5]. The biotic and abiotic variables of these functionally complex ecosystems undergo rapid changes owing to natural environmental changes, changes in watershed land cover and land use, and changes in water demand [6].…”

“…Since the 1960s, remote sensing techniques have been used to monitor aquatic environments by analyzing ocean colors under the assumption that Chl-a (a quantified proxy for phytoplankton biomass) and surface temperature can be estimated remotely [5,10]. Based on this, many researchers have used satellite sensors to evaluate WQPs with optically active parameters, such as total suspended matter, Chl-a concentration, turbidity, phytoplankton pigments, and color-dissolved organic matter (CDOM) [11][12][13].…”

Do espaço ao solo

“…Esta missão é considerada um marco científico, pois revelou a importância de possuir informações de imagens para monitorizar processos biosféricos, e demonstrou o seu valor na gestão ambiental ao acompanhar alterações sazonais e inter-anuais das condições de uso do solo, a nível global 4 . Estas ferramentas também permitem realizar análises de tendências em larga escala, ajudando a compreender efeitos de alterações climáticas e de atividades antrópicas na biosfera, uma vez que as primeiras observações bem como para avaliar a qualidade da água em águas interiores 7 . Os diversos satélites Sentinel-3 centram-se na observação marinha, com ênfase no estudo da topografia da superfície da água, cor e temperatura do oceano 6 .…”