Abstract:Situated at the foot of the Pichincha volcano, the city of Quito is frequently subjected to hydroclimatic hazards. In 1995 an 11Ð2 km 2 watershed, located in the vicinity of the city, was equipped with eight rain gauges and two flow gauges to better understand the local rainfall/runoff transformation processes. Rainfall simulation experiments were carried out on more than 40 one-square-metre plots to measure infiltration point-processes. The high density of measurement devices allowed us to identify the origin and nature of the various contributions to runoff for the different physiographic units of the watershed: urban area from an altitude of 2800 to 3200 m; farmland, pasture and forested land, and finally páramo above 3900 m. Runoff occurs mainly in the lower part of the basin and is caused by urbanization; however, the natural soils of this area can also produce Hortonian runoff, which is predominant in a few events. This contribution can be studied through rainfall simulation experiments. In the upper natural zone, the younger and more permeable soils generate less runoff on the slopes. However, almost permanently saturated contributing areas, which are located in the bottom of the quebradas, may generate flood events, the size of which depends on the extent of the area concerned. Variations in the runoff coefficients are related first to the baseflow and second to the amount of rainfall in the previous 24 h. This analysis, which underlines the complexity of a small, peri-urban, volcanic catchment, is a necessary preliminary to runoff modelling in an area where very few experiments have been carried out on small catchments.

Equipe Pluridisciplinaire Plan Loire Grandeur Nature -OrléansThe probabilistic Gradex method was applied to both watersheds of the Loire River at Nevers and the Allier River at Veurdre, and then to the confluence of these rivers at Givry, in order to define a strategy for reducing inundation risks due to more-than-50-year floods on the medium Loire, which is a priority of the Plan Loire Grandeur Nature. The present study is mostly based on daily precipitation values measured by a 108 pluviometer net from 1949 to 1996, as weil as discharge chronicles.lt also benefits from precious historical data ofprecipitation events, and particufarly of the discharge data offloods that have been observed and reconstituted since 1856, especially the major ones in the end of the 19th and the beginning ofthe 20th centuries.This study, beyond the usual Gradex method application field, gave results in coherence with other approaches.A few application examples to big sized watersheds show the interest of studying the spatial rain distribution and of detecting the occurrence of intense and generalized rainfails, towards the calcufation feasibility.