Abstract. The remoteness, complexity and heterogeneity of tropical wetlands make the characterisation of their hydrological processes challenging. In particular estimates of evaporative water loss are inherently uncertain. In view of the large influence on the local and regional climate, the quantification of evaporation is essential for the determination of the water balance of permanent and intermittent water bodies. Data for tropical wetlands are scarce where their remoteness impedes direct evaporation measurements. Seasonal inundation dynamics affect evaporation processes in tropical wetlands, which can be analysed in two stages: the first stage during the wet season and the second stage during the dry season. As yet no adequate method exists for determining second stage evaporation without soil moisture data, which are usually unavailable for the remote tropical wetlands. Our study aimed at developing a process-based model to simulate first and second stage evaporation in tropical wetlands. We selected a set of empirical potential evaporation (PET) models of varying complexity, each based on different assumptions and available datasets, and evaluated the models with pan evaporation observations in the Pantanal of South America, one of the largest tropical wetlands in the world. We used high-resolution measurements of surface and groundwater levels at different locations to determine the water available for evaporation. Actual evaporation (AET) was derived by constraining simulated PET based on available water. The model of best fit was applied to different types of water bodies with varying inundation durations and captured first and second stage evaporation. With our new model we could quantify evaporative water loss in the dry and the wet season for different locations in the Pantanal. This new spatially-explicit approach represents an improvement in our understanding of the role of evaporation in the water balance of the Pantanal. We recommend the application of this model in other remote tropical wetlands, since only a minimum of input data is necessary.

Water balance studies with stable water isotopes have rarely been conducted in remote and tropical wetland areas. As such, little is known regarding the water balance and groundwater-surface water interaction in the Pantanal, one of the largest and most pristine wetlands in the world. We applied MINA TrêS, a water balance model utilizing stable water isotopes (δ 18 O, δ 2 H) and chloride (Cl -) to assess the dry-season hydrological processes controlling groundwater-surface water interactions and the water balance of six floodplain lakes in the northern Pantanal, Brazil. Qualitatively, all lakes exhibited similarity in hydrological controls. Quantitatively, they differed significantly due to morphological differences in controlling groundwater inflow and lake volume. Our approach is readily transferable to other remote and tropical wetland systems with minimal data input requirements, which is useful in regions with sparse hydrometric monitoring.Key words Pantanal; dry season; stable water isotopes; chloride; water balance; groundwater-surface water interactions Estimation des composantes du bilan hydrique en saison sèche des lacs des zones humides tropicales du Pantanal (Brésil) Résumé Les études sur le bilan hydrique utilisant des isotopes stables de l'eau ont rarement été menées dans les zones humides et tropicales reculées. En tant que tel, on connaît peu le bilan hydrique et les interactions entre eaux superficielles et souterraines dans le Pantanal, une des zones humides les plus importantes et les plus vierges du monde. Nous avons appliqué le modèle de bilan hydrique MINA Três, en utilisant des isotopes stables de l'eau (δ 18 O, δ 2 H) et les chlorures (Cl -), afin d'évaluer, en saison sèche, les processus hydrologiques contrôlant les interactions des eaux souterraines et superficielles et le bilan hydrologique de six lacs de la plaine d'inondation dans le Nord du Pantanal, au Brésil. Qualitativement, les contrôles hydrologiques de tous les lacs sont similaires. Quantitativement, ils diffèrent sensiblement en raison des différences morphologiques du contrôle de l'écoulement souterrain et du volume des lacs. Notre approche, qui ne demande que très peu de données, est facilement transférable à d'autres systèmes de zones humides tropicales reculées, ce qui est utile dans les régions où le suivi hydrométrique est rare.

Abstract:Modifications of human land use and climate change are known to be a threat for the health and proper functioning of tropical wetlands. They interfere with the seasonal flood pulse, which is seen as the most important driver for biodiversity and directly controls evaporation. In order to investigate the impact of local and upstream changes on wetlands, a regional assessment of evaporation is crucial but challenging in such often remote and poorly gauged ecosystems. Evaporation is the major water balance component of these wetlands and links the flood pulse with the ecosystem. It can therefore be seen as a proxy for their functioning. In the last decades, information from space became an important data source to assess remote wetland areas. Here, we developed a new approach to quantify regional evaporation driven by inundation dynamics as its dominant control. We used three water and vegetation indices (mNDWI (modified Normalized Difference Water Index), NDVI (Normalized Difference Vegetation Index), and EVI (Enhanced Vegetation Index)) from MODIS (Moderate Resolution Imaging Spectroradiometer) surface reflectance products to assess regional inundation dynamics between the dry and wet seasons. Two years of continual in situ water level measurements at different locations in our study area, the OPEN ACCESS Remote Sens. 2015, 7 9770Pantanal wetland of South America, provided the reference to evaluate our method. With process-based modeling that used the inundation dynamics to determine the water available for evaporation, we were able to estimate actual evaporation (AET) on a regional scale. Relating AET to changes in discharge due to upstream flow modifications and on local precipitation over the last 13 years, we found that the Pantanal is more vulnerable to alternated inundation dynamics than to changes in local precipitation. We concluded that coupling ground-and space-based information in this remote wetland area is a valuable first step to investigate the status of the Pantanal ecosystem.

Abstract. Characterizing hydrological processes within tropical wetlands is challenging due to their remoteness, complexity and heterogeneity. In particular, estimates of evaporative water loss are inherently uncertain. In view of the large influence on the local and regional climate, the quantification of evaporation is essential for the determination of the water balance of permanent and intermittent water bodies. Data for tropical wetlands are scarce where their remoteness impedes direct evaporation measurements. Seasonal inundation dynamics affect evaporation processes in tropical wetlands, which can be analysed in two stages: the first stage during the wet season and the second stage during the dry season. As yet no adequate method exists for determining second-stage evaporation in a data-scarce environment that additionally allows for a transfer of simulated actual evaporation (AET) to other locations. Our study aimed at developing a process-based model to simulate first-and second-stage evaporation in tropical wetlands. We selected a set of empirical potential evaporation (PET) models of varying complexity, each based on different assumptions and available data sets, and evaluated the models with pan evaporation observations in the Pantanal of South America, one of the largest tropical wetlands in the world. We used high-resolution measurements of surface and groundwater levels at different locations to determine the water available for evaporation. AET was derived by constraining simulated PET based on available water. The model of best fit was applied to different types of water bodies with varying hydroperiods to capture first-and second-stage evaporation across a range of wetland types. With our new model we could quantify evaporative water loss in the dry and the wet season for different locations in the Pantanal. This new spatially explicit approach represents an improvement in our understanding of the role of evaporation in the water balance of the Pantanal. We recommend the application of this model in other remote tropical wetlands, since only a minimum of input data is necessary.