Peter D. Hunter

An experimental approach to the measurement of the effects of water depth and substrate on optical and near infra-red reflectance: a field-based assessment of the feasibility of mapping submerged instream habitat

This study represent an assessment of the feasibility of using optical and near infra‐red wavelengths to map water depth and substrate type which are two of the primary components of river physical habitat. The objective was met by measuring reflectance using field spectroscopy of exposed and then progressively submerged (0–1 m) artificial substrates during rising tides on a tidal reach of the River Forth; this approach provided absolute control for substrate type. The field data were used to simulate multi‐spectral response as measured using Airborne Thematic Mapper (bands 1 to 7; 420nm to 900nm) since airborne remote sensing provides the way forward for synoptic mapping of small streams (<20 m), such as those typically found in Scotland. In the presence of sunlit conditions significant differences in reflectance over a broad range of wavelengths was evident with strong correlations with water depth and substrate. Differences in reflectance were most marked under shallow conditions (<0.75 m) and then progressively diminished. Application of simple linear correlation, band ratios, the Lyzenga algorithm and multiple discriminant analysis all suggested the possibility of mapping shallow river channel water depths. The results also showed that substrate type and the analysis technique selected also affected the optimum spectral wavelength for detection of channel substrate type and water depth. Overall the work suggests mapping channel depth and substrate type is possible using remotely sensed data in optical and near infra‐red wavelengths, using high spatial and spectral resolution multi‐spectral imagery. However it may be restricted to depths of less than 1 m where there is high organic colour and/or a significant periphyton cover.

Strategies for monitoring and managing mass populations of toxic cyanobacteria in recreational waters: a multi-interdisciplinary approach

Mass populations of toxin-producing cyanobacteria commonly develop in fresh-, brackish- and marine waters and effective strategies for monitoring and managing cyanobacterial health risks are required to safeguard animal and human health. A multi-interdisciplinary study, including two UK freshwaters with a history of toxic cyanobacterial blooms, was undertaken to explore different approaches for the identification, monitoring and management of potentially-toxic cyanobacteria and their associated risks. The results demonstrate that (i) cyanobacterial bloom occurrence can be predicted at a local- and national-scale using process-based and statistical models; (ii) cyanobacterial concentration and distribution in waterbodies can be monitored using remote sensing, but minimum detection limits need to be evaluated; (iii) cyanotoxins may be transferred to spray-irrigated root crops; and (iv) attitudes and perceptions towards risks influence the publics preferences and willingness-to-pay for cyanobacterial health risk reductions in recreational waters.

Developments in Earth observation for the assessment and monitoring of inland, transitional, coastal and shelf-sea waters

The Earths surface waters are a fundamental resource and encompass a broad range of ecosystems that are core to global biogeochemical cycling and food and energy production. Despite this, the Earths surface waters are impacted by multiple natural and anthropogenic pressures and drivers of environmental change. The complex interaction between physical, chemical and biological processes in surface waters poses significant challenges for in situ monitoring and assessment and often limits our ability to adequately capture the dynamics of aquatic systems and our understanding of their status, functioning and response to pressures. Here we explore the opportunities that Earth observation (EO) has to offer to basin-scale monitoring of water quality over the surface water continuum comprising inland, transition and coastal water bodies, with a particular focus on the Danube and Black Sea region. This review summarises the technological advances in EO and the opportunities that the next generation satellites offer for water quality monitoring. We provide an overview of algorithms for the retrieval of water quality parameters and demonstrate how such models have been used for the assessment and monitoring of inland, transitional, coastal and shelf-sea systems. Further, we argue that very few studies have investigated the connectivity between these systems especially in large river-sea systems such as the Danube-Black Sea. Subsequently, we describe current capability in operational processing of archive and near real-time satellite data. We conclude that while the operational use of satellites for the assessment and monitoring of surface waters is still developing for inland and coastal waters and more work is required on the development and validation of remote sensing algorithms for these optically complex waters, the potential that these data streams offer for developing an improved, potentially paradigm-shifting understanding of physical and biogeochemical processes across large scale river-sea systems including the Danube-Black Sea is considerable.

Detecting and distinguishing moisture- and salinity-induced stress in wheat and maize through in situ spectroradiometry measurements

The shortage of good-quality water resources has become an important issue worldwide. In arid and semi-arid environments, low-quality water such as agricultural drainage water, wastewater and saline ground water has become an important supplement to agricultural supply, but with inevitable consequences for plant health and crop yield. In this study, greenhouse experiments were undertaken to investigate the potential of remote sensing to detect and distinguish moisture- and salinity-induced stress in wheat (Triticum aestivum L.) and maize (Zea mays L.) to facilitate crop management. In situ spectroradiometry measurements were collected from plant canopies under natural and artificial illumination at different growth stages to establish when in the growth cycle the effects of these stressors can be distinguished. The results showed that plant reflectance characteristics were affected by both moisture- and salinity-induced stress. Penalized linear discriminant analysis (PLDA) was used to demonstrate that it is possible to distinguish moisture- and salinity-induced stress effects in maize, but not in wheat.

Spatial variability of absorption coefficients over a biogeochemical gradient in a large and optically complex shallow lake

In order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass-specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption (aph(λ)) over blue and red wavelengths (aph(440) = 0.11–4.39 m−1, aph(675) = 0.048–2.52 m−1), while there was less variability in chlorophyll-specific phytoplankton absorption (a*ph(λ)) in the lake (a*ph(440) = 0.022 ± 0.0046 m2 mg−1, a*ph(675) = 0.010 ± 0.0020 m2 mg−1) and adjoining wetland system, Kis-Balaton (a*ph(440) = 0.017 ± 0.0015 m2 mg−1, a*ph(675) = 0.0088 ± 0.0017 m2 mg−1). However, in the UV, a*ph(350) significantly increased with increasing distance from the main inflow (Zala River). This was likely due to variable production of photoprotective pigments (e.g., MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption (SCDOM) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by nonalgal particles (aNAP(λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton, or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters.

Atmospheric Correction Performance of Hyperspectral Airborne Imagery over a Small Eutrophic Lake under Changing Cloud Cover

Atmospheric correction of remotely sensed imagery of inland water bodies is essential to interpret water-leaving radiance signals and for the accurate retrieval of water quality variables. Atmospheric correction is particularly challenging over inhomogeneous water bodies surrounded by comparatively bright land surface. We present results of AisaFENIX airborne hyperspectral imagery collected over a small inland water body under changing cloud cover, presenting challenging but common conditions for atmospheric correction. This is the first evaluation of the performance of the FENIX sensor over water bodies. ATCOR4, which is not specifically designed for atmospheric correction over water and does not make any assumptions on water type, was used to obtain atmospherically corrected reflectance values, which were compared to in situ water-leaving reflectance collected at six stations. Three different atmospheric correction strategies in ATCOR4 was tested. The strategy using fully image-derived and spatially varying atmospheric parameters produced a reflectance accuracy of ±0.002, i.e., a difference of less than 15% compared to the in situ reference reflectance. Amplitude and shape of the remotely sensed reflectance spectra were in general accordance with the in situ data. The spectral angle was better than 4.1° for the best cases, in the spectral range of 450–750 nm. The retrieval of chlorophyll-a (Chl-a) concentration using a popular semi-analytical band ratio algorithm for turbid inland waters gave an accuracy of ~16% or 4.4 mg/m3 compared to retrieval of Chl-a from reflectance measured in situ. Using fixed ATCOR4 processing parameters for whole images improved Chl-a retrieval results from ~6 mg/m3 difference to reference to approximately 2 mg/m3. We conclude that the AisaFENIX sensor, in combination with ATCOR4 in image-driven parametrization, can be successfully used for inland water quality observations. This implies that the need for in situ reference measurements is not as strict as has been assumed and a high degree of automation in processing is possible.

Atmospheric correction of Landsat-8/OLI and Sentinel-2/MSI data using iCOR algorithm: validation for coastal and inland waters

ABSTRACT Image correction for atmospheric effects (iCOR) is an atmospheric correction tool that can process satellite data collected over coastal, inland or transitional waters and land. The tool is adaptable with minimal effort to hyper- or multi-spectral radiometric sensors. By using a single atmospheric correction implementation for land and water, discontinuities in reflectance within one scene are reduced. iCOR derives aerosol optical thickness from the image and allows for adjacency correction, which is SIMilarity Environmental Correction (SIMEC) over water. This paper illustrates the performance of iCOR for Landsat-8 OLI and Sentinel-2 MSI data acquired over water. An intercomparison of water leaving reflectance between iCOR and Aerosol Robotic Network – Ocean Color provided a quantitative assessment of performance and produced coefficient of determination (R2) higher than 0.88 in all wavebands except the 865 nm band. For inland waters, the SIMEC adjacency correction improved results in the red-edge and near-infrared region in relation to optical in situ measurements collected during field campaigns.

Response of terrestrial net primary productivity (NPPT) in the Wujiang catchment (China) to the construction of cascade hydropower stations

Abstract The damming of rivers results in hydrological modifications that not only affect the aquatic ecosystem but also adjoining terrestrial systems. Thirteen dams commissioned along the Wujiang River have induced ecological problems, including decreased water turbidity and loss of biodiversity, which potentially influence ecosystem net primary production (NPP) and hence the sequestration, transformation, and storage of carbon. We used terrestrial NPP (NPPT) as a bioindicator to assess the impact of dams on carbon storage in the Wujiang catchment. MODIS satellite and meteorological data were used as inputs to the CASA model to calculate annual NPPT from 2000 to 2014. NPPT was calculated at the catchment and landscape scale to quantify the impact of dams on surrounding terrestrial ecosystems. Mean NPPT was calculated for concentric buffer zones covering a range of spatial extents (0–10 km) from the reservoir shoreline. We found a negligible impact from construction of a single dam on NPPT at the catchment scale. By contrast, the impact of dam construction was scale-dependent, with a stronger landscape-scale effect observed at short distances (i.e., 0–1 km) from the reservoir. Decreases in NPPT were mainly ascribed to the loss of vegetated land resulting from dam impoundment and subsequent urbanization of the surrounding area.

Harmonic analysis of Lake Balaton phytoplankton blooms using 9 years of MERIS-derived chlorophyll-A

This study is the first application of harmonic analysis to remotely sensed chlorophyll-a concentration in lakes. Freshwater phytoplankton phenology can track the development of algal blooms in lakes. The dynamics of algal blooms in Lake Balaton is analyzed over 9 years by harmonic analysis of MERIS-derived chlorophyll-a estimates. An analysis of the significance of the individual harmonic terms at each significant frequency provides the amplitudes and phases. Four lake basins are analyzed and phenological indicators are presented. The basins show distinct algal bloom phenologies.