Catchment soil moisture and rainfall characteristics as determinant factors for discharge/suspended sediment hysteretic loops in a small headwater catchment in the Spanish pyrenees

The concentration of suspended sediment and discharge generated during flood events are not normally homogenous, and the curve representing sediment concentration vs. discharge through time is often a hysteretic loop. Three types of hysteretic loops were found at Arnas, a Mediterranean headwater catchment in the Central Spanish Pyrenees: clockwise (the most frequent), counter-clockwise and eight-shaped. They are associated with different levels of humidity and rainfall and therefore indicators of different processes of runoff and sediment transport. Clockwise loops are generated under 'normal' stormflow conditions, when the catchment is very moist and runoff generation and sediment supply is limited to areas next to the channel (i.e. sediments are removed, transported and depleted rapidly). Counter-clockwise curves occur under very high moisture and high antecedent rainfall conditions. In this case, flood propagation occurs as a kinematic wave. Sediment sources are incorporated all over the catchment. In both cases, saturation excess overland flow generates the superficial runoff. The eight-shaped loop (partial clockwise followed by counter-clockwise) occurs with low water content. Here, the runoff generation process is supposed to be infiltration excess overland flow, which causes a rapid extension of the contributing areas both near the channel and over the whole catchment. (C) 2003 Elsevier B.V. All rights reserved.