Hans J Van Der Woerd

Hans J Van Der Woerd
Vrije Universiteit Amsterdam | VU · The Institute for Environmental Studies (IVM)

PhD

About

147
Publications
28,207
Reads
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2,431
Citations
Introduction
Generalist in environmental monitoring with support from Earth Observation. Specialist in water monitoring, water composition, radiation transfer, inverse modelling. International unique knowledge on water monitoring by citizens and smart tools to connect Citizens and Satellites.
Additional affiliations
June 1989 - February 1998
National Institute for Public Health and the Environment (RIVM)
Position
  • Senior Researcher
Description
  • Ozone layer depletion
July 1987 - June 1989
European Space Agency
Position
  • PostDoc Position
January 1983 - June 1987
Netherlands Institute for Space Research
Position
  • PhD Student
Education
September 1975 - December 1982
University of Amsterdam
Field of study
  • Astrophysics

Publications

Publications (147)
Conference Paper
Full-text available
The colors from natural waters differ markedly over the globe, depending on the water composition and illumination conditions. Space-borne " ocean color " sensors are operational instruments designed to retrieve important water-quality indicators, based on the measurement of water leaving radiance in a limited number (5 to 10) of narrow (≈10 nm) ba...
Article
Full-text available
In the European Citclops project, with a prime aim of developing new tools to involve citizens in the water quality monitoring of natural waters, colour was identified as a simple property that can be measured via a smartphone app and by dedicated low-cost instruments. In a recent paper, we demonstrated that colour, as expressed mainly by the hue a...
Article
Full-text available
Rapid and widespread monitoring of inland and coastal water quality occurs through the use of remote sensing and near-surface water quality sensors. A new addition is the development of smartphone applications (Apps) to measure and record surface reflectance, water color and water quality parameters. In this paper, we present a field study of the H...
Preprint
Full-text available
We document the development and public release of a new dataset (1997-2018), consisting of global maps of the Forel-Ule index, hue angle and Secchi disk depth. Source data comes from the 15 European Space Agency (ESA) Ocean Colour (OC) Climate Change Initiative (CCI), which is providing merged multi-sensor data from the mid-resolution sensors in op...
Article
Full-text available
Biomass estimation of multiple phytoplankton groups from remote sensing reflectance spectra requires inversion models that go beyond the traditional band-ratio techniques. To achieve this objective retrieval models are needed that are rooted in radiative transfer (RT) theory and exploit the full spectral information for the inversion. HydroLight nu...
Preprint
Full-text available
Biomass estimation of multiple phytoplankton groups from remote sensing reflectance spectra requires inversion models that go beyond the traditional band-ratio techniques. To achieve this objective retrieval models are needed that are rooted in radiative transfer (RT) theory and exploit the full spectral information for the inversion. HydroLight nu...
Article
Full-text available
We document the development and public release of a new dataset (1997-2018), consisting of global maps of the Forel-Ule index, hue angle and Secchi disk depth. Source data come from the European Space Agency (ESA) Ocean Colour (OC) Climate Change Initiative (CCI), which is providing merged multi-sensor data from the mid-resolution sensors in operat...
Article
Full-text available
Stretching and bending vibrations of water molecules absorb photons of specific wavelengths, a phenomenon that constrains light energy available for aquatic photosynthesis. Previous work suggested that these absorption properties of water create a series of spectral niches but the theory was still too simplified to enable prediction of the spectral...
Article
Full-text available
The European-Commission—funded project ‘Citclops’ (Citizens’ observatory for coast and ocean optical monitoring) developed methods, tools and sensors, which can be used by citizens to monitor natural waters, with a strong focus on long-term data series related to environmental sciences. The new sensors, based on optical technologies, respond to a n...
Article
Full-text available
The Forel-Ule (FU) color comparator scale is the oldest set of optical water types (OWTs). This scale was originally developed for visual comparison and generated an immense amount of data, with hundreds of thousands of observations being gathered from the last 130 years. Since recently, the FU scale is also applicable to remote sensing data. This...
Article
Full-text available
Soil moisture affects the partitioning of water and energy and is recognized as an essential climate variable. Soil moisture estimates derived from passive microwave remote sensing can improve model estimates through data assimilation, but the relative effectiveness of microwave retrievals in different frequencies is unclear. Land Parameter Retriev...
Article
Full-text available
Remote sensing by satellite-borne sensors presents a significant opportunity to enhance the spatio-temporal coverage of environmental monitoring programmes for lakes, but the estimation of classic water quality attributes from inland water bodies has not reached operational status due to the difficulty of discerning the spectral signatures of optic...
Article
Full-text available
Soil moisture affects the partitioning of water and energy and is recognized as an essential climate variable. Soil moisture estimates derived from passive microwave remote sensing can improve model estimates through data assimilation, but the relative effectiveness of microwave retrievals in different frequencies is unclear. Land Parameter Retriev...
Article
Full-text available
In order to increase the monitoring capabilities of inland and coastal waters, there is a need for new, affordable, sensitive and mobile instruments that could be operated semi-automatically in the field. This paper presents a prototype device to measure chlorophyll a fluorescence: the SmartFluo. The device is a combination of a smartphone offering...
Article
Full-text available
Marine processes are observed with sensors from both the ground and space over large spatio-temporal scales. Citizen-based contributions can fill observational gaps and increase environmental stewardship amongst the public. For this purpose, tools and methods for citizen science need to (1) complement existing datasets; and (2) be affordable, while...
Article
Full-text available
Vertical mixing is thought to play an essential role in phytoplankton blooms, yet measurements of mixing properties are very sparse. In this paper, we present a methodology to estimate profiles of the upper ocean vertical mixing from satellite colour observations, using a coupled turbulence-phytoplankton model and data-assimilation based calibratio...
Article
Full-text available
This document presents the WAter COlor from Digital Images (WACODI) algorithm, which extracts the color of natural waters from images collected by low-cost digital cameras, in the context of participatory science and water quality monitoring. SRGB images are converted to the CIE XYZ color space, undergoing a gamma expansion and illumination correct...
Article
Full-text available
The colours from natural waters differ markedly over the globe, depending on the water composition and illumination conditions. The space-borne "ocean colour" instruments are operational instruments designed to retrieve important water-quality indicators, based on the measurement of water leaving radiance in a limited number (5 to 10) of narrow (≈1...
Article
Full-text available
Climate change is affecting the hydrodynamics of the world's oceans. How these changes will influence the productivity, distribution and abundance of phytoplankton communities is an urgent research question. Here we provide a unique high-resolution mesoscale description of the phytoplankton community composition in relation to vertical mixing condi...
Article
Full-text available
The oceans play a pivotal role in the global carbon cycle. It is not practical to measure the global daily production of organic carbon, the product of phytoplankton standing stock and its growth rate using discrete oceanographic methods. Instead, optical proxies from Earth-orbiting satellites must be used. To test the accuracy of optically derived...
Article
Full-text available
Using in situ data of upper ocean vertical mixing profiles along a transect in the North Atlantic and an idealised phytoplankton growth model (PGM), we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. Optical parameters in the PGM are calibrated with observations of light attenuation. The calibratio...
Chapter
Full-text available
Participatory science is not, as perhaps is believed, something of the 21st century. In this manuscript we show that over a century ago it were not only scientists who collected oceanographic data but also merchant sailors. A good example of such globally collected data are Forel-Ule observations, from which the first date back to 1889. This hardly...
Conference Paper
Full-text available
Introduction Framed within the European Project CITCLOPS (Citizens' Observatory for Coast and Ocean Optical Monitoring), the aim of this study is to present a number of tools that can be employed by citizens to estimate the color of natural waters. Firstly, a scale that accurately matches the original Forel-Ule (FU) colors was developed using acces...
Article
Full-text available
To determine diurnal variations in the physical and biological state of Lake Garda in early spring, high-resolution measurements were made of the vertical distribution of temperature and fluorescence in the upper 100 meters during 5–7 March 2014. In this paper, the results of these measurements are presented and a preliminary analysis that focuses...
Conference Paper
Full-text available
Background / Purpose: This presentation introduces the European CITCLOPS project. This propject aims to develop systems to retrieve and use data on colour, transparency and fluorescence of surface waters, using low-cost sensors combined with people acting as data carriers, contextual information (e.g. georeferencing) and a community-based Interne...
Article
Full-text available
The colour comparator Forel-Ule scale has been used to estimate the colour of natural waters since the 19th century, resulting in one of the longest oceanographic data series. This colour index has been proven by previous research to be related to water quality indicators such as chlorophyll and coloured dissolved organic material. The aim of this...
Article
Full-text available
The oceans play a pivotal role in the global carbon cycle. Unfortunately, the daily production of organic carbon, the product of phytoplankton standing stock and growth rate cannot be measured globally by discrete oceanographic methods. Instead, optical proxies from Earth-orbiting satellites must be used. To test the accuracy of opticallyderived pr...
Article
Full-text available
The detection of dense harmful algal blooms (HABs) by satellite remote sensing is usually based on analysis of chlorophyll-a as a proxy. However, this approach does not provide information about the potential harm of bloom, nor can it identify the dominant species. The developed HAB risk classification method employs a fully automatic data-driven a...
Article
Full-text available
Using in situ data of upper ocean vertical mixing profiles along a transect in the North Atlantic and an idealised phytoplankton growth model (PGM), we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. Optical parameters in the PGM are calibrated with observations of light attenuation. The calibratio...
Article
Full-text available
The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers and limnologists since the 19th century, providing one of the oldest oceanographic data sets. Present and future Forel-Ule classifications of global oceanic, coastal and continental waters can facilitate the interpretation of these long-term ocean colo...
Article
Full-text available
Marine primary productivity is an important agent in the global cycling of carbon dioxide, a major 'greenhouse gas', and variations in the concentration of the ocean's phytoplankton biomass can therefore explain trends in the global carbon budget. Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton bi...
Article
Full-text available
Multispectral information from satellite borne ocean colour sensors is at present used to characterize natural waters via the retrieval of concentrations of the three dominant optical constituents; pigments of phytoplankton, non-algal particles and coloured dissolved organic matter. A limitation of this approach is that accurate retrieval of these...
Conference Paper
Constraints on values of biological parameters by observed turbulence in a quasi-2D phytoplankton model of the North Atlantic Session and Session Number: Scaling and complex Physical and Biogeophysical Processes in the Atmosphere, Ocean and climate (NP3.1) Preferred Mode of Presentation: Oral Lisa Hahn-Woernle¹, Henk A. Dijkstra¹ & Hans J. van der...
Article
Full-text available
Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63° N in the northeast Atlantic Ocean. The goal of this study was to identify relation...
Article
Full-text available
The North Atlantic Ocean experiences considerable variability in sea surface temperature (SST, >10 m) on seasonal and inter-annual time-scales. Relationships between SST and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and absorption were assessed in spring and summer from latitudes 30–62° N. Fur...
Article
Full-text available
Small-scale temperature and conductivity variations have been measured in the upper 100 m of the northeast Atlantic during the STRATIPHYT-II cruise (Las Palmas-Reykjavik, 6 April-3 May 2011). The measurements were done at midday and comprised 2 to 15 vertical profiles at each station. The derived turbulent quantities show a transition between weakl...
Article
Full-text available
Multispectral information from satellite borne ocean colour sensors is at present used to characterize natural waters via the retrieval of concentration of the three dominant optical constituents; pigments of phytoplankton, non-algal particles and coloured dissolved organic matter. A limitation of this approach is that accurate retrieval of these c...
Article
Full-text available
We present a method to calibrate and validate observational models that interrelate remotely sensed energy fluxes to geophysical variables of land and water surfaces. Coincident sets of remote sensing observation of visible and microwave radiations and geophysical data are assembled and subdivided into calibration (Cal) and validation (Val) data se...
Article
Full-text available
Small-scale temperature and conductivity variations have been measured in the upper 100 m of the Northeast Atlantic during the STRATIPHYT-II cruise (Las Palmas-Reykjavik, 6 April-3 May 2011). The measurements were done at midday and comprised 2 to 15 vertical profiles at each station. The derived turbulent quantities show a transition between weakl...
Article
Full-text available
We present a method to calibrate and validate observational models that interrelate remotely sensed energy fluxes to geophysical variables of land and water surfaces. Coincident sets of remote sensing observation of visible and microwave radiations and geophysical data are assembled and subdivided into calibration (Cal) and validation (Val) data se...
Conference Paper
Full-text available
In Europe, increasing political interest drives the investigation into the spatial aspects of water quality measurements. Additionally, unprecedented information on patterns in chlorophyll (CHL) and suspended particulate matter (SPM, also known as total suspended matter TSM) concentrations in coastal and marine ecosystems can be discerned from long...
Article
Full-text available
This study presents vertical profiles of turbulence parameters obtained in the upper 100 m of the northeastern Atlantic Ocean along a transect from tropical permanently stratified waters to subpolar seasonally stratified waters in July–August 2009. The focus is to fully characterize the vertical mixing along this transect for further studies relate...
Article
Full-text available
Coastal areas of the North Sea are commercially important for fishing and tourism, and are subject to the increasingly adverse effects of harmful algal blooms, eutrophication and climate change. Monitoring phytoplankton in these areas using Ocean Colour Remote Sensing is hampered by the high spatial and temporal variations in absorption and scatter...
Article
Full-text available
Current protocols prescribe 0.2 µm filters for absorption measurements of Coloured Dissolved Organic Matter (a (CDOM)). However, a large historical dataset of a (CDOM) was obtained over 0.7 µm filters. Since small particles may pass 0.7 µm filters, a (CDOM) was prob-ably overestimated because of residual scattering. Furthermore, current protocols r...
Article
The integration of remote sensing data of suspended particulate matter (SPM) into numerical models is useful to improve the understanding of the temporal and spatial behaviour of SPM in dynamic shelf seas. In this paper a generic method based on the Ensemble Kalman Filtering (EnKF) for assimilating remote sensing SPM data into a transport model is...
Article
Full-text available
The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult to test under natural conditions due to rapid changes in physical and biological forcings and the practical inability to manipulate nutrient conditions and phytoplankton composition in the sea. Therefore, an indoor mesocosm was designed to examine the op...