Abstract
Abstract. Monitoring evapotranspiration over land is highly dependent on the surface state and vegetation dynamics. Data from spaceborn platforms are desirable to complement estimations from land surface models. The success of daily evapotranspiration monitoring at continental scale relies on the availability, quality and continuity of such data. The biophysical variables derived from SEVIRI on board the geostationary satellite Meteosat Second Generation (MSG) and distributed by the Satellite Application Facility on Land surface Analysis (LSA-SAF) are particularly interesting for such applications, as they aimed at providing continuous and consistent daily time series in near-real time over Africa, Europe and South America. In this paper, we compare them to monthly vegetation parameters from a database commonly used in numerical weather predictions (ECOCLIMAP-I), showing the benefits of the new daily products in detecting the spatial and temporal (seasonal and inter-annual) variability of the vegetation, especially relevant over Africa. We propose a method to handle Leaf Area Index (LAI) and Fractional Vegetation Cover (FVC) products for evapotranspiration monitoring with a land surface model at 3–5 km spatial resolution. The method is conceived to be applicable for near-real time processes at continental scale and relies on the use of a land cover map. We assess the impact of using LSA-SAF biophysical variables compared to ECOCLIMAP-I on evapotranspiration estimated by the land surface model H-TESSEL. Comparison with in-situ observations in Europe and Africa shows an improved estimation of the evapotranspiration, especially in semi-arid climates. Finally, the impact on the land surface modelled evapotranspiration is compared over a north–south transect with a large gradient of vegetation and climate in Western Africa using LSA-SAF radiation forcing derived from remote sensing. Differences are highlighted. An evaluation against remote sensing derived land surface temperature shows an improvement of the evapotranspiration simulations.
Highlights
In the past decades, an increasing number of models were developed to monitor evapotranspiration (ET) at different scales using remote sensing measurements
Each monthly Leaf Area Index (LAI) is based on homogeneous pixels, i.e. pixels occupied by exclusively one PFT, and six Land Surface Analysis Satellite Application Facility (LSA-SAF) LAI images per month
The global pattern for ECOCLIMAP-I and LSA-SAF is similar, the most striking difference is seen on the length of the vegetation period that is noticeably shorter for LSASAF at highest latitudes
Summary
An increasing number of models were developed to monitor evapotranspiration (ET) at different scales using remote sensing measurements. Simple empirical or statistical methods to fully detailed physical models have been developed, using a wealth of information provided by various satellites Vegetation has been recognized to be a cornerstone of the evapotranspiration process since plants are the main medium for exchange of water between the soil and the atmosphere. Land surface models are widely used for meteorological and climate studies. They are based on a conceptual and semi-empirical description of, respectively, the physical and physiological processes of heat and water exchanges between soil, plants and atmosphere media. Information usually needed for land surface models is 1) the exact coverage of plant functional types (PFT) (Bonan et al, 2002), information given by a land cover map, 2) the state of the vegetation development, Published by Copernicus Publications on behalf of the European Geosciences Union
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