AbstractsGeography &GIS

The procedure to determine evaporation in hydrological models is considered to be unsatisfactory by some researchers; ‘too’ accurate by others. In this procedure catchment scale evaporation is related to some form of potential evaporation, determined with point scale meteorological data. The main criticism is that the potential evaporation is not representative for the catchment and that spatio-temporal dynamics in vegetation cannot appropriately be expressed with the time-invariant, spatially lumped model parameters in the above mentioned procedure. Using remotely sensed observations, catchment scale estimates on evaporation and vegetation dynamics can be derived. It is hypothesized that by integrating remotely sensed evaporation estimates and additional information on vegetation dynamics in conceptual rainfall-runoff models, we can get more insight into the realism of the modelled evaporation flux and the role of vegetation dynamics. With a more realistic representation of evaporation, the water partitioning can be modelled more accurately, eventually improving our understanding of the catchment behaviour. The way the evaporation estimates can be used depends on the spatial and temporal resolution and the reliability of the products. The hypothesis is tested in the well studied Ourthe catchment, located in Belgium. The climate is Atlantic temperate, the streamflow is characterized by a quick response. Vegetation dynamics in both space and time are investigated in a principal component analysis on multi year MODIS Normalized Difference Vegetation Index (NDVI) data. Areas with similar temporal dynamics are distinguished. The most important temporal dynamics are related to phenology and agricultural growing reasons. Areas with an increasing trend in NDVI are identified as well, but the spatial extent is too small to be relevant for hydrological applications. In a validation study three remotely sensed evaporation products are examined in terms of their reliability and applicability in conceptual models, namely EARS (daily, 4km x 9km), WACMOS (daily, clear days, 1km x 1km) and MOD16 (8-daily, 1km x 1km). EARS and WACMOS are surface energy balance models, based on land surface temperature observations. MOD16 uses the Penman-Monteith equation, with distributed surface characteristics and a Jarvis-like approach to calculate the surface resistance. The remotely sensed products are validated with ground measurements of evaporation from five eddy covariance towers and - if applicable - with a multi year water balance analysis. Mainly based on the water balance analysis, we concluded that the EARS product gives relatively accurate evaporation estimates and can be used in the last step of the research. WACMOS (SEBS) was shown to have an extremely poor correlation between the remotely sensed evaporative fraction and the evaporative fraction determined from eddy covariance measurements and was not further considered. MOD16, often criticized for not taking into account soil moisture constraints on evaporation, did deviate from…