Seasonal wetlands are common in Mediterranean climates. They flood during rainy seasons in autumn and winter and dry-up in summer. Precipitation changes in these areas have profound effects on the dynamics of wetlands, affecting plants and animals that inhabit them. These wetlands can suffer changes in their hydrology, becoming transformed into permanent lakes or completely drying-up—but Copernicus Sentinels are making a difference.
A hydroperiod is defined as the number of days per year that an area of land is wet, or the amount of time that there is stagnant water at a location. The stability of wetlands is directly related to their hydroperiod – meaning the seasonal shift in surface and sub-surface water levels.
Within the ECOPOTENTIAL project, researchers of Estación Biológica de Doñana (EBD-CSIC, Spain) and the Centre for Research and Technology Hellas (CERTH) worked in close collaboration, using satellite data, to study wetland seasonal dynamics and hydroperiods duration and trends in the Doñana National Park (Southern Spain).
CSIC developed an algorithm to identify flooded areas in data acquired by the Thematic Mapper (TM), Enhanced TM Plus (ETM+) and Operational Land Imager (OLI) sensors of the Landsat satellites. The algorithm uses the short-wave infrared (SWIR) band of these sensors, which is less affected by water turbidity and aquatic vegetation in mapping flooded surfaces (Bustamante et al, 2016; Díaz-Delgado et al., 2016).
Landsat images are first atmospherically corrected by the dark object subtraction method and the time series is normalised to a clear atmosphere reference image. Using a series of flood masks, the researchers estimated the hydroperiod of each pixel in the wetland—how long the pixel remains flooded—and other hydrological parameters, like flooding and dry-up dates.
Incorporating sensors from the Sentinel satellites of the European Union's Copernicus programme to the study of Mediterranean wetland dynamics provides several advantages. The multi-spectral imager (MSI) of Copernicus Sentinel-2 has adequate spectral bands with 10 and 20 m pixel resolution, resulting in a higher spatial accuracy in the estimation of the flood mask - thus allowing a finer identification of the boundaries between flooded and dry areas, and allowing the detection of smaller ponds.
CERTH developed an automated thresholding approach for flooded pixel discrimination applied on Copernicus Sentinel-2 Level 2A data, with excellent results (Kordelas et al., 2018; Kordelas et al. 2019). This approach requires reduced computational time to obtain a flood mask without the need of ground-truth data to train the algorithm. An additional advantage is that the time frequency of acquired information in combination with the one derived by Landsat images increases, which in turn improves the accuracy of hydroperiod estimation.
Copernicus Sentinel-1 is also very useful to identify flooded areas, with the complementary capacity to acquire images under cloudy conditions; hence, further increasing data ensure frequency and hydroperiod estimation accuracy.
Flooded areas can be identified in radar images, but automatic thresholding methods tend to fail when the water surface is rippled by wind or covered by emergent or floating vegetation. In this case, CERTH scientists developed a method using a time series of Copernicus Sentinel-1 images to train multiple pixel-centric classification models for classifying each pixel of a target Copernicus Sentinel-1 image as either flooded or non-flooded (Manakos et al., 2019). Reference training data are retrieved from timely close Copernicus Sentinel-2 derived or other existing inundation maps.
ECOPOTENTIAL partners EBD-CSIC and CERTH have worked side by side with Doñana National Park technicians, managers and other end users since the beginning of the project, to facilitate the use of the generated times series data, tools and models, addressing specific biodiversity monitoring needs, in two main ways. Firstly, conservation problems (for instance, groundwater abstraction pressures) and key ecosystem services (for instance, water purification capacity, food production for grazing livestock) in the protected area, for which hydroperiods are a key ecological parameter, were conceptually modelled in participatory workshops.
This enabled the association of data sources, such as imagery from the Copernicus Sentinels with monitoring needs and management actions, to better support decision-making procedures in biodiversity conservation and the sustainable use of natural resources.
Secondly, both partners are jointly involved in continuous capacity building activities through training courses, for the use of tools and models developed for Doñana National Park by its technicians and managers, and other interested parties, and their best use in conservation decision-making through co-design procedures (for instance, expert-based Bayesian belief networks).
Ioannis Manakos, Associate Researcher at Centre for Research and Technology Hellas, states, "The combination of Copernicus Sentinel-1, Sentinel-2 and Landsat time series data provide high spatial and temporal resolution flood mapping for Doñana's seasonal wetlands, out of which hydrodynamic parameters, such as hydroperiods, can be derived.
"These data inform managers about wetland trends and anomalies in near real time (depending solely on the acquisition time of the satellite image), as these algorithms have been incorporated in the ECOPOTENTIAL virtual lab and the map production process can be initiated at any time," concluded Javier Bustamante, Scientific Researcher at Doñana Biological Station (CSIC).
About the Copernicus Sentinels
The Copernicus Sentinels are a fleet of dedicated EU-owned satellites, designed to deliver the wealth of data and imagery that are central to the European Union's Copernicus environmental programme.
The European Commission leads and coordinates this programme, to improve the management of the environment, safeguarding lives every day. ESA is in charge of the space component, responsible for developing the family of Copernicus Sentinel satellites on behalf of the European Union and ensuring the flow of data for the Copernicus services, while the operations of the Copernicus Sentinels have been entrusted to ESA and EUMETSAT.
This work has been conducted within the framework of the H2020 project ECOPOTENTIAL "Improving future ecosystem benefits through Earth observation" (Grant Agreement n. 641762), by researchers of Estación Biológica de Doñana (EBD-CSIC, Spain) and CERTH (Greece). ECOPOTENTIAL focuses its activities on a target set of pilot European protected areas, combining remote sensing, in-situ data, analysis, and modelling, as well as a general participatory approach for the co-design of decision support tools with managers, technicians and other users. Javier Bustamante, Ioannis Manakos, Diego García, Ricardo Díaz-Delgado, Pablo F. Méndez, and Georgios Kordelas contributed to this article.
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