On 2 February 2020, we celebrate World Wetlands Day to raise global awareness about the vital role of wetlands for people and our planet. This year’s edition highlights the connection between water, wetlands, and life.

Wetlands are transitional areas, either permanently or seasonally inundated, that link land and water supporting both, aquatic and terrestrial species. Inland wetlands include lakes and rivers, underground aquifers, swamps and marshes, wet grasslands, peatlands, flood plain and oases, including other human-made wetlands such as rice paddies, saltpans, and farming ponds. Coastal wetlands include estuaries, deltas and tidal flats mangroves and coastal marine areas as well as coral reefs (Fig.1).

Wetlands are unique ecosystems that hold abundant biodiversity, ensure freshwater availability, play an essential role in food production, provide essential ecosystem services, and help to regulate the global climate. Despite their relevance, the area covered by natural wetlands decreased by approximately 35% between 1970 and 2015, a rate three times as high as the loss rate of forests (Gardner and Finlayson, 2018). Global environmental change and anthropogenic activities, such as agriculture land expansion, drainage, illegal settlement, and overfishing are the main drivers of wetlands degradation and therefore, represent a threat to the balance of the planet.

Earth Observation (EO) data have been used as a strategic tool to provide key information for wetland inventory, assessment, and monitoring. With EO data we can overcome some of the difficulties in data acquisition, which represents an opportunity to advance more quickly towards wetlands restoration and conservation. For instance, initiatives like the GlobWetland Africa project, MedWet, Geowetlands community, Satellite based Wetlands Observation service (SWOS Service), Diars project, HiWET project, and the RhoMéO project are at the forefront of developing and providing tools, workflows and datasets for monitoring, analysis and modeling at the global, regional and local levels.

Satellite observation supports sustainable wetland management by data that in combination with analysis software can be used to perform wetlands delimitation, water extent management, land use and cover change classification and water quality parameters. In the following subchapters these practices will be briefly described.

Wetlands delimitation

Establishing the limits of a wetland and observing seasonal changes is essential to monitor this ecosystem. Many approaches have been used to determine the extent of wetlands with the help of data on biodiversity, rivers and watersheds, aquifers and groundwater bodies (Malak and Espinosa, 2016), and soil moisture dynamics (Globwetland Africa, “Wetland Inventory”) (Fig. 2).

Mapping wetlands is challenged by a number of circumstances. Most importantly, there is a lack of a unique feature that can be used to distinguish wetlands from other land surfaces. Furthermore, wetlands show a very high dynamic in moisture and vegetation conditions in terms of both, their temporal and spatial extent (Gallant, 2015). Remote sensing approaches to delimit wetlands are used to support wetland inventories. For more precise inventories, field validation and local knowledge need to be considered, especially for complex environments with different wetland types.

Surface water extent

Annual variations of wetlands’ surface water extent help to identify permanent and seasonally inundated regions as well as to monitor the dynamics of water retention (Wolf, 2011) (Fig. 3). An analysis of the surface water extent allows to assess how the changes in water regimes affect the overall wetland ecosystem, as well as its capacity to temporarily store floodwaters during high runoff events. This ecosystem service provided by wetlands is particularly important in highly urbanized areas that are more likely to face flood events which can damage infrastructure and habitat downstream. With Change in the extent of water-related ecosystems over time being an SDG 6 indicator (6.6.1) it is of utmost importance to monitor and report on the surface water extent of wetlands.

For a thorough analysis of the evolution of the water extent over time, it is usually necessary to build a hydrological model of the basin(s) that include(s) the wetland (Kittel et al., 2018). Modelling hydrologic processes in watersheds is complex and requires data from different sources: public and private databases (such as temperature, water use and demand), satellite observation (such as land cover, digital elevation model (DEM), vegetation index, snow water, etc.) and in situ data if needed (such as discharge data).

Land use and land cover classification

Information on land use and cover change over time helps to monitor how anthropogenic activities and natural events affect the dynamic and integrity of wetlands. Such Land Use and Land Cover products help to identify the expansion of agriculture and human settlements over these areas, as well as the effects of severe droughts that can cause a fragmentation of the wetland. Further influence of human activities on wetlands’ biodiversity, water quality and extent pointed out by Meng and Dong (2019) include the development of aquaculture, tourism, and the replacement by built wetlands (Fig. 4).

Water quality

Information on water quality parameters such as chlorophyll-a concentrations, total suspended sediments or dissolved organic matters allow to monitor wetland ecosystem contamination, in particular water body eutrophication due to excessive nutrients (Fig. 5). The most common pollution sources are urban and industrial wastewater discharge, agrochemicals, pesticides, and mining activities. Changes on suspended sediments due to deforestation and soil erosion can also be estimated and monitored.

Many ongoing efforts by service providers of space technology, data portals and software to analyze the data contribute to a set of available tools to map and monitor wetlands. The table below provides an overview of relevant data sources, indicators / parameters and reference projects providing for such services.

Vegetation

Remote sensing helps in determining important vegetation biophysical parameters such as the Leaf Area Index (LAI) and aboveground biomass. LAI is defined as the total one-sided area of leaves covering a unit ground surface and is strongly correlated with plant biomass (Belgian Earth Observation, “HIWET: Using satellite images for wetland vegetation monitoring”). Therefore, LAI can be used as an indicator to monitor the status of wetland vegetation. LAI maps can be produced based on PROBA-V imagery (HiWE project). These maps present high spatial and temporal distribution of vegetation parameters (such as NDVI) and provide useful information on the state of the ecosystem functioning and dynamics.

Invasive Plants Species

Invasive alien species are those introduced, accidentally or intentionally, outside of their natural geographic area and can endanger the local ecosystem. They are often introduced as a result of the movement of people and goods, for instance via shipping, consignments of wood products carrying insects, or the transport of ornamental plants to new areas (International Union for Conservation of Nature, “Invasive Alien Species”).

Invasive alien species can represent a threat to wetlands biodiversity. Kumar (2018) developed an exploratory survey on exotic plants on Kuttadan Kole Wetlands, in India. The study revealed that 46% of the vegetal species are non-native. These species threat the ecosystem’s biodiversity by removing native species, occupying their space, and consuming available nutrients.

Remote sensing technology provides a promising avenue to upscaling the level of observations of biological invasion (Diars project). Together with field data, space technologies (hyperspectral images, LiDAR images) help to better demonstrating and characterizing the distribution and impact of invasive species on ecosystems. Furthermore, they can booster management measures for mitigation by supporting monitoring, prediction of spread and risk assessment of invasive plant species.

Many ongoing efforts by service providers of space technology, data portals and software to analyze the data contribute to a set of available tools to map and monitor wetlands. The table below provides an overview of relevant data sources, indicators / parameters and reference projects providing for such services.

Conclusion

Remote sensing proved to be a valuable tool to map and monitor wetlands. It contributes significantly to the protection of these complex ecosystems. Using a remote sensing approach allows mapping large areas while lowering the expenses. Furthermore, it allows to compensate for the lack of in-situ data, which can be a relevant issue, especially in emerging countries. Benefitting from the advantage of open data, using freely available software, tutorials and toolboxes can help various stakeholder groups to address their daily challenges and preserve wetlands, a precious ecosystem for both, people, and the planet Earth.