Streamflow response and water-table dynamics in a sub-Mediterranean research catchment (Central Pyrenees)

Rainfall–runoff relationships at the event scale were studied in a small sub-Mediterranean research catchment in the central Spanish Pyrenees (Arnás catchment, 2.84 km 2 in area). The strong non-linearity between these two variables evidenced the high variability in the hydrological response of the catchment, linked to seasonal dynamics of the water reserves. Both the magnitude of rainfall events and the baseflow at the start of each event had a pronounced influence on the hydrological response, while the rainfall intensity played a major role in terms of discharge increment. Water-table data suggested a strong topographic control on saturation patterns and showed high seasonal dynamics, with a dry period during summer and a wet period during winter. The transition between dry and wet periods was characterised by high spatial variability in the water-table pattern. An analysis of streamflow and water-table response during a series of four events revealed the complexity of the hydrological processes operating within the catchment. During dry conditions, infiltration excess over localised areas is likely to be the main runoff process, whereas during wet conditions both saturated excess runoff and subsurface flow are most likely to be the dominant runoff processes. During the wetting-up period, the hydrological response was variable, depending mainly on the depth of the water table before the event and rainfall depth and intensity. In these conditions, the interpretation of runoff processes is not straightforward and requires further research.