Context and objectives
The correct representation of soil moisture in numeric climate models is of paramount importance since it is a key variable of the hydrological cycle, and with global warming an increased frequency of extreme hydrologic events such as flooding and extended droughts periods are expected to occur. Unfortunately, suitable data sets to validate simulated soil moisture over western Europe are lacking.
The most efficient way to obtain information about surface soil moisture with sufficient spatial and temporal sampling for large enough regions resides in satellite measured brightness temperatures in the microwave range, and the SSM/I is currently the only spaceborne microwave imager that can do so.
Recently, a physically based methodology developed at VITO [Passive Microwave Soil Moisture Retrieval (PSR)] has been demonstrated to be capable to explain observed variations in surface soil moisture.
Although the results obtained with this method are very encouraging, they are valid for a limited area and for a short period only. More work is required to test the feasibility of the method over larger areas and for longer periods.
The subject of this proposal is to extend the PSR methodology, from the site scale to the continental scale. In particular, VITO will:
- assess the performance of the PSR methodology at the scale of western Europe through consistency checks, in particular by comparing SSM/I output with theoretical (PSR-predicted) threshold values, and by comparing PSR output with climatological data referring to manifest wet and dry periods;
- produce a temporal series of maps of surface moisture for the domain and the period of interest to the user partner (western Europe, May-October 1990), together with clear specifications of the final product, the emphasis being on its accuracy.
Expected scientific results
The methodology was applied for Europe and North Africa for the period April to November 1990, producing daily maps of superficial (a few mm’s) soil moisture content. One of the findings was that the methodology could only be successfully applied to a restricted part (21%) of the domain, mainly corresponding to those areas with sparse vegetation. Despite this limitation, the obtained moisture maps contain still sufficient information to be useful as a validation tool in regional climate modelling, especially since the areas where the method works well are generally those where bare soil evaporation constitutes an important component of the surface energy and water balance.