Rainfall–runoff response and event-based runoff coefficients in a humid area (northwest Spain)

Abstract

The hydrological response of a small agroforestry catchment in northwest Spain (Corbeira catchment, 16 km 2 ) is analysed, with particular focus on rainfall events. Fifty-four rainfall-runoff events, from December 2004 to September 2007, were used to analyse the principal hydrological patterns and show which factors best explain the hydrological response. The nonlinearity between rainfall and runoff showed that the variability in the hydrological response of the catchment was linked to the seasonal dynamics of the rainfall and, to a lesser extent, to evapotranspiration. The runoff coefficient, estimated as the ratio between direct runoff and rainfall volume, on an event basis, was analysed as a function of rainfall characteristics (amount and intensity) and the initial catchment state conditions prior to an event, such as pre-event baseflow and antecedent rainfall index. The results revealed that the hydrological response depends both on the soil humidity conditions at the start of the event and on rainfall amount, whereas rainfall intensity presented only a significant correlation with discharge increment. The antecedent conditions seem to be a key point in runoff production, and they explain much of the response. The hydrographs are characterized by a steep rising limb, a relatively narrow peak discharge and slow recession limb. These data and the observations suggest that the subsurface flow is the dominant runoff process.Key words rainfall-runoff events; temporal variability; runoff coefficient; subsurface flow; agroforestry catchment; northwest Spain Réponse pluie-débit et coefficients de ruissellement événementiels dans une région humide (nordouest de l'Espagne) Résumé La réponse hydrologique d'un petit bassin versant agroforestier du nord-ouest de l'Espagne (bassin versant de Corbeira, 16 km 2 ) est analysée, avec un accent particulier sur les événements pluvieux. Cinquante-quatre événements pluie-débit, de décembre 2004 à septembre 2007, ont été utilisés pour analyser les traits hydrologiques principaux et montrer quels facteurs expliquent le mieux la réponse hydrologique. La non-linéarité entre les pluies et le ruissellement montre que la variabilité de la réponse hydrologique du bassin versant est liée à la dynamique saisonnière de la pluie et, dans une moindre mesure, à l'évapotranspiration. Le coefficient de ruissellement, estimé comme le rapport entre le volume de ruissellement direct et celui des pluies, sur une base événementielle, a été analysé en fonction des caractéristiques des pluies (quantité et intensité) et les conditions initiales des états du bassin avant événement, telles que l'écoulement de base pré-événement et l'indice des pluies antérieures. Les résultats ont révélé que la réponse hydrologique dépend à la fois des conditions d'humidité du sol au début de l'événement (indirectement déduites de l'écoulement de base au début de chaque épisode de pluie) et de la quantité de pluie, tandis que l'intensité des précipitations n'a présenté une corrélation significative qu'avec les in...

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