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High-resolution modelling and monitoring of water and energy transfers in wetland ecosystems (HIWET)

Research project SR/00/301 (Research action SR)


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CONTEXT

Freshwater wetlands are among the most biodiverse ecosystems but also most threatened habitats notwithstanding the enormous amount of ecosystem services they deliver to society (Tharme et al., 2006; Zedler et al., 2005). Inland wetlands are not only an important source for freshwater, they also play a crucial role in maintaining a good environmental quality in river basins, are a source of food and raw materials and a sink for carbon (Bernal & Mitsch, 2012) and provide a unique habitat for many animal species, such as breeding and migratory birds (e.g. Austin & Richert, 2005).
Despite the protected status of many freshwater wetlands, many of these ecosystems are under threat of (further) degradation due to climate change and anthropogenic activities (Zedler et. al, 2005; MEA, 2005). Freshwater wetlands are vulnerable to the changes in quantity and quality of their water supply, and climate change will most probably have pronounced effects on these wetlands through alterations in hydrological regimes (Erwin, 2009; Junk et al, 2013). As processes in wetland ecosystems are highly dynamic and complex (Bornette et al., 1998) and wetlands are often difficult to access, hampering their large-scale monitoring in the near real time, still major knowledge gaps exits. Despite the protected status of many freshwater wetlands, these ecosystems are under threat due to anthropogenic activities and suffer from degradation.
The project will contribute to the monitoring of wetland health status and associated changes in freshwater wetland ecosystem functioning. An output of the project will be the identification of health status indicators that are specific for the main wetland typologies. A better understanding of the reciprocal interactions between hydrology and vegetation also allows making recommendations for wetland management and restoration, especially related to desiccation and eutrophication problems.

THEME

Monitoring of water and energy fluxes is considered as one of the necessities for vegetation health monitoring and assessment of climate and/or anthropogenic effects on natural and agricultural ecosystems.
Evapotranspiration is a dominant component in both the hydrological mass balance and the recently developed surface energy balance methods that make use of both optical as well as thermal imagery. Energy balance approaches have the advantage of making optimal use of the dynamic datasets provided by satellite and/or airborne sensors, in particular thermal data. But the thermal remote sensing datasets suffer from a trade-off between spatial and temporal resolution. Most continuous RS datasets have a coarse resolution. Hydrological modeling has the advantage to operate at fine spatial and temporal resolution and have future simulation capacities.

OBJECTIVES

The HiWet project aims at deriving ecosystem health indicators for freshwater wetland vegetation by producing high resolution evapotranspiration maps estimated from the combination of field observations, remote sensing, hydrological and surface energy balance models. HiWET aims at providing a framework for efficient freshwater wetland ecosystem monitoring and evaluation of ecosystem health, using novel techniques to estimate evapotranspiration (ET) and to derive evaporative stress as an indicator for the health state of the wetland vegetation. The project targets consistent ET retrieval across scales, from the local field scale (fine resolution) to the regional

catchment scale (coarse resolution) derived from combined use of hydrological models and remote sensed energy balances. In addition the project aims to contribute greatly to an increased understanding of the functioning of freshwater wetland ecosystems.

The main research objectives are:
1. Overcome limitations of water and energy balance modeling and reduce uncertainty by integration into a single multi-model approach
2. Obtain consistent ET estimates across scales (multi-scale)
3. Enable consistent thermal/ET monitoring at high spatial and temporal resolution
4. Gain understanding on relation ET and vegetation/ecosystem health in temperate wetlands
5. Combine field and RS based approaches to provide wetland ecosystem health indicators

METHODOLOGY

By combining surface energy balance methods with hydrological models, a new multi-scale water balance modelling framework (for wetlands) is developed that is innovative at two levels:
(1) Dynamic remote sensing data at different resolutions are used as new data sources to feed the hydrological models (in addition, or as replacement to the traditional 'old' static land/soil/topography maps and weather gages) whereby input and model uncertainties are considered and the obtained output uncertainties (evapotranspiration and flow) are minimized.
(2) A new paradigm for distributed river basin modelling is proposed, where landscape elements, such as wetlands, and their interactions in the river basin are explicitly represented and where both hydrological and energy balances are computed and respected.
A ‘multi-model surface energy balance modelling’ will carry out an inter-comparison of different thermal remote sensing based ET retrieval approaches in the wetland ecosystems at multiple spatial and temporal scales. Data assimilation is used to integrate both dynamic modeling approaches and observations to improve simulation. The final outcome is a consistent ET timeseries at the high resolution.
This new approach is especially relevant for wetland management, where evapotranspiration varies dynamically in space and time. A suitable indicator of wetland ecosystem health will be developed based on both classical vegetation analyses and analysis of plant functional traits and types.
Anticipated results and deliverables
The primary output will be a multi-model, multi-scale framework, integrating both hydrological water balance and land surface energy balance modelling.
Satellite/airborne remote sensing is integrated into a data assimilation approach at multiple spatial scales, yielding a consistent ET timeseries across scales.
Validation of water and surface energy fluxes will be carried out against in-situ measurements at various freshwater wetland sites.

The project will deliver consistent surface flux estimates at multiple spatial scales (local – high resolution, regional – medium to low resolution) over different freshwater wetlands. An increased understanding of the complex interaction between soil, vegetation and atmosphere in freshwater wetland ecosystems will be achieved in order to develop a conservation and management strategy to tackle potential anthropogenic and climate threats.


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