VU Research Portal Dense Water and Fluid Sand Hommersom, A. 2010 document version Publisher's PDF, also known as Version of record Link to publication in VU Research Portal citation for published version (APA) Hommersom, A. (2010). Dense Water and Fluid Sand: Optical properties and methods for remote sensing of the extremely turbid Wadden Sea. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. E-mail address: [email protected] Download date: 06. Oct. 2021 Uitnodiging - Invitation voor het bijwonen van de openbare verdediging van mijn proefschrift: to attend the public defence of my thesis: Cover : ”Dense water” and “Fluid Sand” Background: MERIS image ”Dense Water” May 4 2006 Optical properties and provided by the European Space Agency and methods for remote sensing of “Fluid Sand” Left: TriOS sensors the extremely turbid Wadden Sea. Optical properties Center: reseach vessel Navicula (NIOZ) ”Dense Water” and “Fluid Sand” and “Fluid Water” ”Dense and methods for remote Right: AC9 instrument in turbid water Annelies Hommersom - IVM- sensing of the extremely turbid Wadden Sea. Maandag 28 juni - Monday June 28 om - at 15.45 in de - in the Aula Vrije Universiteit De Boelelaan 1105, Amsterdam En de receptie na afloop. And the reception afterwards. De Aula is 10 min lopen vanaf station Amsterdam Zuid uitgang VU/Parnassusweg. Komen met de auto is niet aan te raden. The Aula is 10 min walk from station Amsterdam Zuid exit VU/Parnassusweg. Coming by car is not recommended. Annelies Hommersom Sumatrastraat 1 6707 EE Wageningen [email protected] “Dense Water” and “Fluid Sand” Optical properties and methods for remote sensing of the extremely turbid Wadden Sea Annelies Hommersom “Dense Water” and “Fluid Sand” Optical properties and methods for remote sensing of the extremely turbid Wadden Sea Ph.D. thesis, Vrije Universiteit, Amsterdam In Dutch: “Dik Water” en “Vloeibaar Zand” Optische eigenschappen en methoden van remote sensing voor de extreem troebele Waddenzee Proefschrift, Vrije Universiteit, Amsterdam ISBN: 9789086594610 © 2010 Annelies Hommersom This work was carried out at the Institute for Environmental Studies (IVM), Vrije Universiteit, Amsterdam and at the Royal Netherlands Institute for Sea Research (NIOZ), Texel. This work was financed by NWO/SRON Programme Bureau Space Research. Printed on FSC paper by PrintPartners Ipskamp, Enschede, The Netherlands VRIJE UNIVERSITEIT “Dense Water” and “Fluid Sand” Optical properties and methods for remote sensing of the extremely turbid Wadden Sea ACADEMISCH PROEFSCHRIFT ter verkrijging van de graad Doctor aan de Vrije Universiteit Amsterdam, op gezag van de rector magnificus prof.dr. L.M. Bouter, in het openbaar te verdedigen ten overstaan van de promotiecommissie van de faculteit de Aard‐ en Levenswetenschappen op maandag 28 juni 2010 om 15.45 uur in de aula van de universiteit, De Boelelaan 1105 door Annelies Hommersom geboren te Hengelo (ov) promotor: prof.dr. J. de Boer copromotor: dr. S.W.M. Peters Table of contents Chapter 1 Introduction 7 Chapter 2 A review on substances and processes relevant for optical remote sensing 17 of extremely turbid marine areas, with a focus on the Wadden Sea Chapter 3 Spatial and temporal variability in bio‐optical properties of the Wadden Sea 39 Chapter 4 Performance of the regionally and locally calibrated algorithm HYDROPT in a 57 heterogeneous coastal area Chapter 5 Tracing Wadden Sea water masses with an inverse bio‐optical and an endmember model 85 Chapter 6 Spectra of a shallow sea: unmixing for coastal water class identification and 97 monitoring Chapter 7 Synthesis and outlook 115 References 123 Summary and samenvatting 141 Acknowledgements and dankbetuiging 155 Annex 1 Glossary of terms and descriptions used in remote sensing of water quality 159 Annex 2 Abbreviations, acronyms, and symbols 163 Publications 167 Chapter 1 Introduction Chapter 1 1 Introduction 1.1 Optical remote sensing of water quality Remote sensing means “detecting from a distance”. A sensor used for detection can be hand‐held, employed from an airplane (air‐borne remote sensing) or be part of a satellite (space‐born remote sensing) and the technique can be passive or active. Optical remote sensing, as used in this thesis, is passive: it uses reflected sun light in the visible part of the spectrum (~ 300‐800 nm, Figure 1.1). Optical remote sensing techniques can for example be used to examine land use changes (Valbuena et al., 2009), to monitor seasonal dynamics in vegetation (Zuritta‐Milla et al., 2009), to calculate carbon accumulation in peat lands (Scheapman‐Strub et al., 2008), to detect erosion (Vrieling et al., 2007), or to monitor water quality (Robinson et al., 2008). Active remote sensing includes radar (wavelengths <1 mm to 1 m, for example Synthetic Aperture Radar: SAR) and laser (infrared, visible, ultraviolet, for example LIDAR), where signals are sent to earth and the subsequently reflected (radar) or induced (laser) signal is detected. These techniques can be used to detect 3‐dimensional features of objects, waves, and land‐ water boundaries. This thesis addresses optical remote sensing of water quality, which is commonly referred to as ocean colour remote sensing (although lakes and rivers require similar approaches). Figure 1.1. Spectrum of visible light. Ocean colour remote sensing dates back to the 19th century, when explorers noticed large differences in water colour (e.g. Figure 1.2) between coastal waters and oceans (Wernand, 2010). For example Joseph Luksch determined 367 observations of the water colour, measured with the Forel Ule colour scale (Luksch, 1901; Wernand, 2010) during the expedition of the S.M.S. Pola in the Mediterranean and Aegean Sea (1890‐1894) and during the Austro‐Hungarian Expedition to the Red Sea (1895‐1898). Ocean colour remote sensing became popular in the 1960’s and 1970’s with increasing awareness of water quality. Secchi discs and air borne remote sensing were introduced to study turbidity and water colour. The first ocean colour sensor (the Coastal Zone Scanner, CZSC) was launched in 1978 and soon after, other satellite sensors for Ocean Colour research became available. Figure 1.2. Various water colours seen in the Wadden Sea To use airborne or space borne optical remote sensing data of earth surfaces, absorption and scattering by the atmosphere must first be removed from the remote sensing signal. The atmosphere and clouds 8 Introduction reduce the sunlight penetration to the earth surface to ~55% (Pidwirny, 2006). A large portion of reflected light is again absorbed and scattered on the way back to the satellite. Correcting signals for this atmospheric influence is a research field of its own. In remote sensing of land, the light reflected by the (land) surface is the variable that tells something about this surface. In ocean colour remote sensing, interest lies with substances in the water column (ray 5 in Figure 1.3); reflectance from the (water) surface (ray 6 in Figure 1.3) obscures this signal. Also the water itself can influence the reflected signal and, at shallow or clear enough locations, so can the bottom (Figure 1.3). Consequently, only a small portion of the light received by the sensor contains information on the content of the water and can be used for water quality monitoring. Therefore, remote sensing of water requires different techniques than remote sensing of land. Figure 1.3. The most common routes of sunlight on its way to a satellite above the sea. Ray 1 is absorbed by the atmosphere and never reaches the water surface and the sensor. Rays 2 and 3 are respectively absorbed and scattered by the water or its contents and so never reach the sensor. Rays 4, 5, 6 and 7 reach the sensor. However, only ray 5 is interesting for water quality monitoring. This ray is partly absorbed by water or its contents, but enough is scattered over an angle > 90˚ (backscattered) to reach the sensor. Rays 4, 6, and 7 are, respectively, reflected by the sea floor, by the water surface and scattered by the atmosphere. Combinations of these routes can also occur. Substances in the water column that can be detected from optical remote sensing are: the water itself, pigments, suspended particulate matter (SPM), and coloured dissolved organic matter (CDOM). The pigment most abundant in marine phytoplankton, chlorophyll‐a (Chl‐a), is usually taken to represent the pigments. SPM, Chl‐a and CDOM are (indirect) indicators for other water quality parameters such as nutrient concentration, river runoff, resuspension or decay. As SPM, Chl‐a and CDOM have a significant influence on the water colour, these three substances are called optically active substances in this thesis. In the absence of optically active substances, water mainly absorbs red light, while the size of the water molecules leads to scattering of blue light. The colour of pure water is therefore blue. Water absorbs more than it scatters, and so very deep water bodies look dark from above. Chl‐a absorbs blue 9 Chapter 1 and red light, turning water green in high concentration (e.g. the central picture in Figure 1.3).