Use of deuterium to understand runoff generation in a headwater catchment containing a dambo

Dambos, seasonally, saturated wetlands, are widespread in headwater catchme nts in southern Africa and play an important role in the regional hydrologi cal cycle. Hoc-ever, the processes influencing runoff from these catchments are poorly, understood. This paper reports an isotopic investigation of ru noff-generating mechanisms within a Zimbabwean catchment containing a dambo . Hydrograph separation using deuterium reveals that, once the dambo is sat urated, up to 70% of total storm flow can be considered 'new' water (i.e. d erived directly from rainfall generating the runoff el-ent). However, both the total proportion and the instantaneous maximum amount of 'new' water in hydrographs are sensitive to rainfall characteristics and antecedent condi tions. These results are (1) compatible with observations made in catchment s in temperate climates when wetlands are present, and contrast with result s obtained when wetlands are absent and (2) consistent with saturation over land flow, generated in saturated regions of the dambo, being the major sto rm runoff mechanism. To reconcile these observations with past perceptions that dambos attenuate flood flows, a dual role for dambos in storm flow pro duction is postulated.