SUMMARY
The rainfall simulation tests on homogeneous hydrological units soil samples of a Medjerda middle valley watershed show that the characteristics of the upper surface soil layer and types of management practices as well as the rain characteristics (duration, intensity frequency) control the infiltration-runoff process and determine later the rain aggressiveness degree.The hydrological responses of the same hydrological unit are different. That was related to the tillage practices types.The increasing of the intensity of agricultural practices causes intense soil biodegradation (low organic matter content less than 2%) and distracts consequently its structural stability, thus favors rain aggressiveness process. Indeed, a plowed soil exposed to rainfall of intensity of 54 mm / h loses 3.31 g of soil corresponding to 0.13 kg / m2 of specific erosion whereas soil sample of olive trees the loss is equal to 2.24 g and the specific erosion is about 0.1 Kg/m2.