Hydrological aspects of the Mesoscale Alpine Programme: findings from field experiments and simulations

The Mesoscale Alpine Programme (MAP) was a unique initiative to improve the understanding of processes involved in orographically influenced precipitation events and in the related emergence and mitigation of floods in the Alps. This study presents a summary of the results from investigations of hydrological relevance in areas to the south (Lago Maggiore watershed) and north (Ammer watershed) of the Alps. A special focus is given to the 1999 Special Observing Period (SOP) experiment. MAP has been an important dialogue platform between the hydrological and meteorological communities. A common goal has been the verification and the assessment of errors and sources of uncertainty of new coupled techniques for flood forecasting in mountainous environments. The ensemble multi-model error in the hydrometeorological forecast, two days in advance, was 30% of the observed peaks and 15% of the observed runoff volumes, on average. To provide some insight on the relative importance and priority of observation strategies of hydrological factors such as soil moisture and reservoir volume in contributing to the flood intensity, the MAP project ‘Hydrology’ focused also on the role and magnitude of water storage in reservoirs. The presented analyses indicate that for the investigated basins and flood events, the storage reservoirs may retain, on average, up to 10% of the event runoff volume and are key elements contributing to the mitigation of peak runoff. The observed variability of soil moisture is large and therefore it has been confirmed that severe difficulties remain in instrumental monitoring of soil moisture in mountainous environments. The sensitivity of floods to soil moisture in the physio-geographic context analysed here seems, however, to be minor, due to the shallow soil layer covering the underlying rocks and the relatively high permeability of soils which reduce the ‘hydrological memory’ of the soil cover in the mountain basins investigated