Abstract:This study analyses some hydrological driving forces and their interrelation with surface-flow initiation in a semiarid Caatinga basin (12 km 2 ), Northeastern Brazil. During the analysis period (2005 -2014), 118 events with precipitation higher than 10 mm were monitored, providing 45 events with runoff, 25 with negligible runoff and 49 without runoff. To verify the dominant processes, 179 on-site measurements of saturated hydraulic conductivity (Ksat) were conducted. The results showed that annual runoff coefficient lay below 0.5% and discharge at the outlet has only occurred four days per annum on average, providing an insight to the surface-water scarcity of the Caatinga biome. The most relevant variables to explain runoff initiation were total precipitation and maximum 60-min rainfall intensity (I 60 ). Runoff always occurred when rainfall surpassed 31 mm, but it never occurred for rainfall below 14 mm or for I 60 below 12 mm h À1 . The fact that the duration of the critical intensity is similar to the basin concentration time (65 min) and that the infiltration threshold value approaches the river-bank saturated hydraulic conductivity support the assumption that Hortonian runoff prevails. However, none of the analysed variables (total or precedent precipitation, soil moisture content, rainfall intensities or rainfall duration) has been able to explain the runoff initiation in all monitored events: the best criteria, e.g. failed to explain 27% of the events. It is possible that surface-flow initiation in the Caatinga biome is strongly influenced by the root-system dynamics, which changes macro-porosity status and, therefore, initial abstraction.

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