Runoff generation processes in a small Mediterranean research catchment (Vallcebre, Eastern Pyrenees)

This paper analyses the runoff generation processes in a small Mediterranean catchment (Can Vila catchment, 0.56 km2), using limited continuous data on water table and soil water potential dynamics along with rainfall and runoff data collected over 6 years. At daily scale, strong non-linearity between rainfall and runoff volume and the effect of the water table position on how rainfall and runoff volume relate were seen. The higher the water table, the greater the runoff for a given rainfall. The relationship between runoff and depth of water table was not straightforward: water table variations sometimes did not correlate with runoff changes, suggesting somewhat intricate hydrological behaviour. Soil water potential data alongside runoff and water table data showed the relatively frequent development of a perched saturation layer in the profile monitored. Examination of soil water potential and water table dynamics during a collection of representative floods helped to identify three types of characteristic hydrological behaviour during the year. Hydrographs corresponding to type 1 events (dry conditions), type 2 events (wetting-up transition) and type 3 events (wet conditions) had different characteristics: each was associated with different dominant runoff generation processes. Under dry conditions, runoff was generated essentially as infiltration excess runoff in low permeable areas, whereas saturation excess runoff dominated during wetting-up and wet conditions. During wetting-up transition, saturated areas resulted from the development of scattered perched water tables, whereas in wet conditions they were linked to the rise of the shallow water table.