Water Quality Information for Africa from Global Satellite Based Measurements: The Concept Behind the UNESCO World Water Quality Portal

Abstract

Freshwater as one of the most relevant resources for life is facing increasing human made pressures. Suitable information about the status of the water quality in lakes and rivers is sparse, although required for environmental assessments and impact monitoring: There is a vast demand on actual data in many countries, where water policies and management decisions are based on scarce and unreliable information. Satellite data with newest data analytics technologies can already contribute to this today with regular mapping and monitoring in freshwater systems: Consistent information of valuable water quality products are derived for single applications in small lakes, covering extended river basins or the whole world, as provided by the UNESCO World Water Quality Portal. A number of examples from this first global water quality portal is discussed, addressing ecological and economic issues in Africa. At the conceptual level, UNESCO and EOMAP advocate the long-term consistency of the data of these new measurement capabilities: Both satellite sensing and data processing technologies are rapidly evolving. Hence, nowadays concepts should already ensure that the data products are globally intercomparable and in future, even if the accuracy of information products become better and better. This ensures that the sustainable development goals can be supported with meaningful, comparable indicators over time.

1 Introduction

Freshwater resources are limited and face rising pressures due to increased population, exploitation of environmental resources, or effects through climate changes. The OECD¹ (2012) states that, for example, the deterioration in water quality resulting from eutrophication has reduced biodiversity in rivers, lakes and wetlands by about one-third globally. With the largest losses observed in China, Europe, Japan, South Asia and Southern Africa, there is an urgent demand to enhance status information on water quality at local and global level. Despite having experienced more than 10 years of continuous economic growth, Africa today faces great water resource management challenges. With 10% of the world’s renewable water resources, more than 60 trans-boundary basins, a low level of water development and utilization and increasing population, Africa’s future economic growth will continue to be constrained by the development of its water resources. Today, in many African countries, water policies and management decisions are based on sparse and unreliable information.²

In this challenging context, it is not surprising that the actual agenda of governments demonstrate a paradigm change and a new era: Agencies and industry require improved environmental data, develop their directive monitoring obligations, set up socio-economic full-cost calculations and need to understand upcoming risks through new areas of conflicts over clean water and food.

The targets of the Sustainable Development Goals (SDG) address these and further efforts on sustainability of human life. The anticipated indicators to monitor the progress towards the SDGs with statistical data are under continuous enhancement to include globally tangible measures using new technologies. UNESCO, UNEP³ and environmental agencies believe the answers should also entail the resources of satellite-based Earth Observation measurements (EO): A comprehensive range of satellite-based water quality parameters in surface waters can be measured: Turbidity, Chlorophyll as indicator for the trophic status, and the appearance of harmful Cyanobacteria bloom indicators.

These measurements from space provide unique holistic environmental information over our connected water systems which are not available by any other water analytics method: Area-wide, going back more than 30 years, and nowadays with a spatio-temporal resolution which corresponds better to natural dynamics than any other sampling method. However, the new method comes with restrictions: It can only provide measures from the first meters penetrated by light, the trophic zone, and only under cloud-free conditions. The accessible water quality parameters need to be related to optically active water constituents, with spectrally distinct absorption or scattering features in the visible range of the light. As for the wide range of optical conditions, waterbodies require complex physics based algorithms to convert the spectral measurements of the satellite instruments into globally consistent and comparable measures. Using operational data analytics and an unmatched cost efficiency, the new method provides measurements from over thousands of river kilometres, millions of virtual sampling stations and lakes combined (Fig. 1).

Fig. 1

The UNESCO IIWQ World Water Quality Portal (www.worldwaterquality.org). At the right side of the portal a number of tools are provided in the blue bar. Users can select parameters for visualization, use tools to access values for self-defined virtual stations or access training documents

2 Understanding the Value of Satellite Observations: The UNESCO IIWQ Portal

The UNESCO International Initiative on Water Quality (IIWQ)⁴ addresses capacity building for satellite-based water quality monitoring technologies, especially for remote areas and in developing countries such as Africa where water quality monitoring networks and laboratory capacity lack. Developed by EOMAP, a world leading specialist for aquatic earth observation monitoring services, UNESCO launched its World Water Quality Portal (www.worldwaterquality.org) in 2018.

The comprehensive portal assists governmental institutions and industry with global water quality assessment for streams, lakes, and rivers providing direct and free access to up-to-date space-based measurements for any location in inland and near-coastal surface waters.

The portal allows users to instantly obtain measurements at freely selectable virtual stations for any location worldwide. The underlying concentrations are provided straight instantly in the value box for the selected parameter, place and time. It provides a range of satellite-based water quality parameters such as turbidity, chlorophyll, and indicators for toxic Cyanobacteria blooms. It also includes functionalities to select different time periods: Historic measurements are provided at a 30 m resolution for selected regions of each continent throughout 2016, and can be continued with various spatial and temporal resolutions for every country. For example, reports on a yearly tropic status based on chlorophyll can be directly exported for any user defined station within selected time series regions—with only one mouse click away. Figure 2 provides an example of the spatial distribution of Chlorophyll for such a virtual station in the central Egyptian part of Lake Nasser. The spatial distribution at this date in March 2016 already varies by a factor of 5 in this part of the reservoir with impressive dynamics for the different fractal shaped bights. The annual readings as well show high temporal dynamic and seasonal changes of a factor of 10. These readings are provided after selecting the time series plot in Fig. 3. The easy-to-access report classifies this part of the reservoir as predominant mesotrophic at around 5 µg/l for the mean Chlorophyll concentrations.

Fig. 2

Spatial distribution of phytoplankton and its key pigment Chlorophyll in central Lake Nasser/Egypt in March 7, 2016. The white pointer shows the location of the selected virtual station. The actual water quality value and unit is provided in the lower section of the blue bar

Fig. 3

Temporal dynamics of Chlorophyll at a selected virtual station in central Lake Nasser/Egypt in 2016

The online interface, based on satellite-derived information, gives users access to an easy-to-use tool providing detailed global water quality information and helps to understand the diversity of space-based measurements in space and time. But the World Water Quality Portal will also be a key element in improving awareness, capacity building and acceptance of EO products on a global scale.

3 The Technical Development: Satellite Sensors and Space-Based Observations

Satellites with remote sensing instruments have been monitoring the earth since 1985, when the first US Landsat satellites⁵ were launched. Since then, these data are setting the basis for vast investigations of the changing environments worldwide. Between 2010 and 2020 the monitoring capabilities improved significantly in all directions – sensitivity, spectral, spatial and temporal resolution. Raised public and private investments resulted in dense and regular global earth observations. Nowadays, large capabilities are provided through the EU Sentinel satellites⁶ and numerous commercial satellite systems: All satellites are equipped with different technical specifications and objectives, but many of these are capable to provide water analysis. African countries also invest significantly into earth observation, e.g. South Africa orbited its first satellite in 1999, Nigeria has launched several multimillion-dollar satellites since 2003, and Morocco launched Africa’s first high-resolution imaging satellite in 2017.⁷

Fig. 4

The water colour is linked to the water constituents through their specific spectral absorption and scattering features. The left hand side shows the water colour as visible in a Sentinel-2 satellite record, and the right hand side shows the related turbidity for the different levels of particle scattering in the various water bodies in South Africa. The upper left is the Sterkfontein dam with low turbidity levels between 0, 9 and 5 NTU

In the past years, the data analytics and processing technologies evolved to generate usable information on water quality in lakes and river using optical multispectral sensors. Data processing technologies are of equivalent importance as sensors, as the characteristics and quality of the derived information products are defined by a package of used satellite sensors and data analytics software. Today, those consolidated packages of satellite sensors, data analytics and online service provision systems form a new type of further innovating environmental services.

The UNESCO portal is built on such a system package: Data from different satellite sensors, Landsat-8 and Sentinel-2, are transformed into global water quality measurements and continuously available through the portal stored in a geospatial data base and accessible online via the user-friendly web application.

4 The Advances of Satellite Data Analytics: Globally Harmonized Water Quality Measurements

Multispectral satellite sensors are capable of measuring water constituents using sunlight as it penetrates the atmosphere and waterbody. This light is absorbed and scattered as a function of the particles and dissolved materials in the waterbody. The reflected light spectrum detected by the satellite sensors can be used to analyse the optically active water constituents. In other words: The water colour is used to derived water quality information, as indicated in Fig. 4. However, the satellite signal is strongly modified by a number of further very variable impacts. These originate from varying atmospheric aerosols, water surface reflections, scattered light from adjacent land areas, and the observation geometry. The most accurate correction of all these impacts is thus consequentely a fundamental requirement of the satellite data analysis software.

Fig. 5

Section of the Congo river basin, showing the turbidity through suspended sediments in the Congo, the Kasai river (right hand-side) and Levini river (left hand) entering the Congo

Furthermore, the main strengths of satellite-based measurements can only be exploited under the conditions demanded by the UN-water analytical brief 2016⁸: Provide consistent and reproducible products based on high-scientific standards to ensure product quality and independency on ground-truth data. These requirements can be fulfilled with physics-based data processing methods: Derived water quality information measures and their physical units relate directly to the absorption and scattering properties of the water body. They are physical properties, therefore globally comparable and independent of specific algorithms or sensors. For example, turbidity is linearly related to the physical process of the backscattering of light, while Chlorophyll and organic components relate likewise to its absorption.

UNESCO applied such a physics based data analysis technology, based on the Modular Inversion and Processing System MIP. The systematic development started more than 20 years ago at the German Aerospace Center, and has been further developed by EOMAP since 2006.⁹ Today, it forms the scientific and methodological base for the variety of routine water monitoring programs from space, ranging from multi-year regulatory monitoring of the trophic status, dredging impact monitoring for the offshore industry, or environmental impact analysis after natural disasters such as the Rio Docedam collapse in Brazil. The common requirement for the very different use cases is again the consistency of the water quality information products over space and time and the different utilized satellite sensors. Bearing this in mind, historical reviews covering several satellite generations are possible as well as highest temporal resolutions combining different current sensors. Taking this fact into account, historical reviews over several generations of satellites as well as highest temporal resolutions combining different current sensors are possible.

5 The Benefits of Space-Based Measurements: Selected Use Cases

The huge African continent incorporates an endless number of freshwater issues but also great advancements within thousands of interconnected catchment areas. Space-based measures can support to monitor and evaluate these. It can provide data that wouldn’t be available otherwise, support political decisions on information rather than on guesses or unaffordable surveys.

Highlighted below are some interesting aspects visualized with data that is already accessible through the UNESCO IIWQ World Water Quality Portal.

5.1 Assessing the Economic and Ecological Impacts of Human Interventions on the Sediment Balance

Figure 5 shows a section of the Congo river basin, with the Congo river flowing from the north to the south direction. The Kasai river is entering the Congo from the right hand side with high amounts of suspended matter, causing increased turbidity in the downstream Congo river. The mixing processes of the two water bodies can be followed for 100 km downstream, with the less turbid water on the western side of the river. On the western side of the figure, the Levini river is entering into the Congo, with much lower amounts of sediments: It is obvious that the Imboulou dam traps the sediments from the Levini river. The same effects can be observed at numerous dams worldwide. The spatial-temporal dynamic of turbidity and suspended sediments is accessible with time series data sets in the portal, e.g. for the Aswan Reservoir.

Fig. 6

River Nile inflow into the Aswan Dam on 20 August 2016, with large turbidity gradients from south to north

Figure 6 shows the impact of the basin trapping the sediment plume of river Nile in the upstream part between Sudan and Egypt. The concentration drops from almost 500 NTU to less than 5 NTU in this part of the reservoir in August 2016. This dramatic drop implies that the suspended particles are sedimented and removed from the sediment balance of the river. Such satellite-based measurements can therefore be used to quantify the sediment loss through dams¹⁰ or for the economic and ecological evaluation of new hydropower developments over extended river systems.¹¹ In general terms, the sediment balance, sediment trapping as well as water quality and their seasonal or long-term changes are of highest economic and ecological relevance for the lifetime of reservoirs,¹² the operation costs, but also for their river basins and connected deltas.¹³ Sustainable long-term solutions to reduce those significant risks related with the sediment balance in the river basin including the reservoir and deltas are required,¹⁴ and can now be efficiently supported through satellite-based monitoring.

Fig. 7

Harmful algae bloom indication a and Chlorophyll concentrations b for Lake Manyame and the lakes surrounding the Manyame basin in Zimbabwe, May 13 2016

5.2 Environmental Assessment of Lakes: Trophic Status and Cyanobacteria Blooms

Satellite monitoring services are increasingly used by water agencies to evaluate the trophic status of lakes, using Chlorophyll measurements as shown in Figs. 2 and 3 for Lake Nasser, or as demonstrated with the report functionality provided by the UNESCO portal. In many regions, only a fraction of the relevant water bodies can be accessed with traditional in situ data, with few point measurements per year. In contrast, the satellite-based technology can provide typically between 10 and 100 records per year area-wide. It provides insights of the spatio-temporal dynamics and detects impact sources which could not be traceable with other methods. For agencies with obligations to assess the status of hundreds to thousands of surface water bodies, it offers numerous opportunities to improve their assessments: Cost-efficient compliance with regulatory obligations, support for policy makers for better decision making, SDG reporting, but also earlier warnings on changing environments. For example, Cyanobacteria are of increasing concern in freshwater systems worldwide, favoured by anthropogenic eutrophication and rising temperatures.¹⁵ When they appear in masses on surface waters, they can cause immense damages on fish life, drinking water and with harmful impact human life. Lake Manyame near Harare is an example for the increasingly polluted inland water bodies in Zimbabwe, and known for the appearance of the Microcystis Cyanobacteria.¹⁶ The well investigated lake provides also a good test bed for remote sensing researchers in Africa, e.g. Muchini et al. 2018.¹⁷ The UNESCO portal visualizes the situation of an extended Cyanobacteria bloom in Lake Manyame clearly with the Harmful Algae Bloom indicator HAB in Fig. 7. The lake is surrounded by numerous under-investigated smaller lakes, and those well-resourced missing information can be provided by the portal for adequate decision-making about the landscape.

6 Outlook

Satellite-based monitoring of freshwaters became a global applicable technology, suitable to map global challenges and support local solutions. The technology can contribute to provide the required environmental information that is needed to address economic and ecological relevant water issues in Africa.

Despite the global applicability, it must be noted that the individual demand on service specifications differs significantly over the various user organizations, governments and water managers. Aspects such as temporal or spatial resolution, product accuracy, data aggregation, reporting details are crucial to fulfil actual demands. Still, the growing demand is the main driver for further innovations and improvements of market driven environmental services. Better algorithms and sensors provide more accurate data, more detailed parameters and higher spatio-temporal resolutions year after year.

Users can benefit the most from these rapid developments in consideration of the required standards: Globally intercomparable measures for satellite-based water quality parameters are relatively easy to define when linked to their physical inherent optical properties. Such measures are independent of the specifications of previous or upcoming satellites, and of specific data analytics technologies. Hence, users will benefit from long-term consistent data. Such intercomparable environmental data, generated by many countries and contributors, can be shared through intergovernmental platforms such as GEOSS, accessed through various front ends such as the UNESCO IIWQ portal. Finally, they provide suitable data to monitor the success of the Sustainable Development Goals with long-term comparable measures and indicators.