Federica Braga

Evaluation of Multi-Resolution Satellite Sensors for Assessing Water Quality and Bottom Depth of Lake Garda

In this study we evaluate the capabilities of three satellite sensors for assessing water composition and bottom depth in Lake Garda, Italy. A consistent physics-based processing chain was applied to Moderate Resolution Imaging Spectroradiometer (MODIS), Landsat-8 Operational Land Imager (OLI) and RapidEye. Images gathered on 10 June 2014 were corrected for the atmospheric effects with the 6SV code. The computed remote sensing reflectance (Rrs) from MODIS and OLI were converted into water quality parameters by adopting a spectral inversion procedure based on a bio-optical model calibrated with optical properties of the lake. The same spectral inversion procedure was applied to RapidEye and to OLI data to map bottom depth. In situ measurements of Rrs and of concentrations of water quality parameters collected in five locations were used to evaluate the models. The bottom depth maps from OLI and RapidEye showed similar gradients up to 7 m (r = 0.72). The results indicate that: (1) the spatial and radiometric resolutions of OLI enabled mapping water constituents and bottom properties; (2) MODIS was appropriate for assessing water quality in the pelagic areas at a coarser spatial resolution; and (3) RapidEye had the capability to retrieve bottom depth at high spatial resolution. Future work should evaluate the performance of the three sensors in different bio-optical conditions.

Assessing water quality in the northern Adriatic Sea from HICO™ data

This letter focuses on water-quality estimation in the northern Adriatic Sea using physically-based methods applied to image obtained with the Hyperspectral Imager for the Coastal Ocean (HICO™). Optical properties of atmosphere and water were synchronously measured to parameterise such methods. HICO™-derived maps of chlorophyll-a (chl-a) and suspended particulate matter (SPM) indicated low values, in the range of 0–3 mg m−3 and 0–4 g m−3, respectively, correlating significantly with field data (R2 = 0.71 for chl-a and R2 = 0.85 for SPM). The results, on analysis, identify clear waters in the open sea and moderately turbid waters near the coast due to river sediment discharge and organic matter from coastal lagoons. These findings support the use of HICO™ data to assess water-quality parameters in coastal zones and suggest the feasibility of integrating them with future-generation space-borne hyperspectral images.

Sensitivity analysis of a bio-optical model for Italian lakes focused on Landsat-8, Sentinel-2 and Sentinel-3

Abstract We analysed the sensitivity of a Case-2 bio-optical model where the water reflectance is computed as a function of concentrations of three optical water quality parameters (WQPs) of three Italian lakes (Garda, Mantua and Trasimeno) and their specific absorption and backscattering coefficients. The modelled reflectance is computed based on the spectral characteristics of three optical sensors, on-board Landsat-8, Sentinel-2 and Sentinel-3. The variance-based analysis was able to quantify the lake-dependence for all (50,000 runs) the simulated reflectance. The results confirmed that Sentinel-3 water reflectance is sensitive to WQPs in all the trophic conditions investigated.

Shoreline detection: capability of COSMO-SkyMed and high-resolution multispectral images

Abstract This study aims to extract the instantaneous shoreline from remote sensing data acquired with very high resolution multispectral and SAR sensors. The capabilities of IKONOS. GeoEye and COSMO-SkyMed for shoreline detection are tested in the Venice littoral (Italy) by classifying the imagery into its land/water components. GPS measurements synchronously to the COSMO-SkyMed acquisitions are carried out along two transects at different tidal levels and used for validation of satellite derived shorelines. Finally, a collection of instantaneous coastlines at a specific tidal level is mapped for reconstructing the intertidal beach morphologic model.

Imaging Spectrometry of Inland and Coastal Waters: State of the Art, Achievements and Perspectives

Imaging spectrometry of non-oceanic aquatic ecosystems has been in development since the late 1980s when the first airborne hyperspectral sensors were deployed over lakes. Most water quality management applications were, however, developed using multispectral mid-spatial resolution satellites or coarse spatial resolution ocean colour satellites till now. This situation is about to change with a suite of upcoming imaging spectrometers being deployed from experimental satellites or from the International Space Station. We review the science of developing applications for inland and coastal aquatic ecosystems that often are a mixture of optically shallow and optically deep waters, with gradients of clear to turbid and oligotrophic to hypertrophic productive waters and with varying bottom visibility with and without macrophytes, macro-algae, benthic micro-algae or corals. As the spaceborne, airborne and in situ optical sensors become increasingly available and appropriate for aquatic ecosystem detection, monitoring and assessment, the science-based applications will need to be further developed to an operational level. The Earth Observation-derived information products will range from more accurate estimates of turbidity and transparency measures, chlorophyll, suspended matter and coloured dissolved organic matter concentration, to more sophisticated products such as particle size distributions, phytoplankton functional types or distinguishing sources of suspended and coloured dissolved matter, estimating water depth and mapping types of heterogeneous substrates. We provide an overview of past science, current state of the art and future directions so that early career scientists as well as aquatic ecosystem managers and associated industry groups may be prepared for the imminent deluge of imaging spectrometry data.

Bio-optical Modeling of Total Suspended Solids

Total suspended solids (TSS) play a fundamental role in inland waters as different materials including contaminants and pollutants can aggregate to these solids and brought in suspension. This can alter the state of the aquatic ecosystem and the use of freshwater resources. For instance, excessive suspended sediment might condition primary productivity and can hinder water use in agriculture. Suspended solids are one of the most successful parameters that can be measured by means of remote sensing due to the effect of TSS on backscattering and water leaving radiance. Consequently, a variety of applications have been developed since the eighties; they have generally been build on empirical or semi-empirical methods which use reflectance at appropriate wavebands as correlates, or semi-analytical and quasi-analytical approaches such as the spectral inversion procedures which relies on the matching of spectral data to bio-optical forward models. Forward bio-optical modeling is used to show the response of water leaving reflectance depending on inherent optical properties of particles and TSS concentrations. Then, remotely sensed data acquired by different optical sensors are presented to show the performance of state-of-art algorithms for mapping TSS and turbidity in different aquatic systems located in Northern Italy, which include deep clear lakes, a system of fluvial lakes characterized by highly productive waters and a segment of the longest Italian river prior reaching the delta. Overall, the conclusions presented in this chapter encourage the use of remote sensing technology to improve inland water management, although new research efforts remain open to adapt bio-optical modeling to TSS to the variety of sensors used in inland water applications.

Hyperspectral tecniques for water quality monitoring: Application to the “Sacca di Goro” — Italy

The paper presents a comparison between an empirical algorithm and a physics based model for the assessment of water compound concentrations by remote sensing hyperspectral data. At the purpose a series of in situ measurements were carried out monthly, from June to October 2005, to spectrally characterize the water of the “Sacca di Goro” (Italy) at spatial (horizontal and vertical) and temporal (daily and seasonal) scales. The results obtained by the application of the two different methods to the in situ acquired data showed that an appreciable improvement is obtainable by considering the physical approach.

Detection of Low Salinity Groundwater Seeping into the Eastern Laizhou Bay (China) with the Aid of Landsat Thermal Data

ABSTRACT Xing, Q. G.; Braga, F.; Tosi, L.; Lou, M. J.; Zaggia, L.; Teatini, P.; Gao, X. L.; Yu, L. J.; Wen, X. H., and Shi, P., 2016. Detection of low salinity groundwater seeping into the Eastern Laizhou Bay (China) with the aid of Landsat Thermal Data. Low-salinity groundwater storages beneath the sea bottom and submarine groundwater discharges (SGD) are of great importance in environmental management and the use of coastal water resources. A preliminary hydro-morpho- geologic analysis allows for considering the presence of low-salinity groundwater in the offshore of the Laizhou Bay (Southern Bohai Sea, China). In order to detect the potential SGD, the analysis of the Sea Surface Temperature (SST) anomalies was carried out using Landsat thermal images acquired in the seasons characterised by the largest difference in temperature between seawater and groundwater. At the nearshore scale, patchy cold water anomalies occur approximately in the same positions along the intertidal zone of the Southern bay, where the unconfined aquifer discharges, independently of tide conditions. At the embayment scale, cold water anomalies spread out in the Eastern Laizhou Bay where the confined aquifer is likely exposed. Salinity and temperature measured in surficial waters of the eastern Laizhou Bay support the hypothesis of SGD, which can be derived from remote sensing images; the occurrence of macroalgal blooms might also be the consequence of the SGD-enhanced nutrient supply. This work pointed out the need of future oceanographic and groundwater flow modeling along with long-term monitoring of processes associated with the suspected SGD. The outcomes of this preliminary study will be of great help to direct effective in-situ investigations aimed at quantifying the volumes of SGD and to simulate coupled groundwater/surface-water flow.

Quasi-simultaneous measurements of suspended sediments concentration (SSC) of very turbid waters at the Yellow River Estuary with the multi-spectral HJ-1 Imageries and in-situ sampling

Evolution of river delta is highly related to the deposition and re-suspension of sediments. At the interacting zone between fresh water of river discharge and seawater, suspended sediments concentration (SSC) varies sharply from a few mg/L to thousands of mg/L; thus, mapping the distribution of SSC will provide the basic information about sediments transportation. The high spatial resolution (30 m) and high revisit frequency (2 day) of CCD imager on board the Chinese environment-monitoring satellite constellation: HJ-1A and HJ-1B, enable an effective observation of the fine dynamics of suspended sediments. In this work, three intensive cruises in the flooding season and dry season of Yellow River were carried out to explore the SSC retrieval algorithms on the basis of HJ-1 CCD imageries. Quasi-simultaneous in-situ SSC data were collected with the pass of HJ-1 over the Yellow River Estuary and its vicinity waters, and a local empirical retrieval algorithm of SSC was established against the TOA (top of atmosphere) reflectance of HJ-1 CCD bands with the correction of Rayleigh scattering. This algorithm can be applied to very turbid waters with thousands of mg/L of SSC.

Features of turbid waters from Hyperspectral Imager for the Coastal Ocean (HICO): Preliminary results at the Yellow River Delta and the Bohai Sea

The Hyperspectral Imager for the Coastal Ocean (HICO) is the first spaceborne imaging spectrometer optimized for monitoring the coastal ocean, which has about 5 nm spectral resolution and 90 m ground spatial resolution. HICO images and in-situ reflectance spectra were collected for studying the spectral responses of waters at the Yellow River Delta and its vicinity where they are featured by high turbidity. Results show that the reflectance spectra are generally consistent with the in-situ measured ones. The reflectance peak at 817.4 nm ( band No.73 of HICO image) was used as an index of the content of suspended sediments (SS). Results show that the reflectance peak in the spectral region of 450-750 nm shifts toward longer wavelength with the increase of the concentration of SS. This work demonstrates that HICO hyperspectral imagery may be useful for monitoring very turbid coastal waters.