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Abstract. The modification of raindrops by the canopy of olive trees increases the kinetic energy of the rain per unit area. The kinetic energy computed from the measured drop size distribution under the tree canopy in simulated rainfall experiments is greater than that received in the open, 17.1 J mm-1, as against 15.7 J mm-1 . This causes higher soil detachment and loss than that observed outside the canopy. Tillage treatments of the soil modify its erodibility, accelerate soil detachment and reduce, simultaneously, the velocity of runoff. Both effects reduce the amount of sediment compared to that observed in the non-tilled soil. The average values of soil lost per unit of rain depth and unit area were 5.81 g mm-1 m-2 (conventional tillage) and 4.02 g mm-1 m-2 (zero tillage) under the canopy compared to 0.89 g mm-1 m-2 (conventional tillage) and 0.95 g mm-1 m-2 (zero tillage) in the open.

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During the last two decades there have been several herbicide pollution episodes of both surface and groundwater in areas where olive groves are established. These episodes always coincided with rainy periods. Understanding the evolution of herbicides in this type of plantation depending on the season of application is critical, especially when conventional tillage is used. Field monitoring of terbuthylazine in the first 2 cm of soil and in runoff water and sediment yield was carried out after winter and spring soil applications. Soil disappearance of terbuthylazine was associated with the first important rainfall event in both seasons (41 mm in spring and 30 mm in winter). At the end of the experiment (85 days and 103 mm rain in spring, and 107 days and 148 mm in winter), no terbuthylazine soil residues were found in winter, whereas 15% of terbuthylazine applied remained in spring. A higher percentage of the applied terbuthylazine in runoff water was recovered in winter (0.53%) than in spring (0.16%). However, no differences in terbuthylazine residues in sediment yields were observed (0.45% in spring and 0.46% winter). These data suggest that the dissipation of terbuthylazine from soil is related not only to the runoff process but also to other processes like leaching, which are more relevant at the end of the winter season.