Rainfall simulations at different scales have been used for understanding the influence of different factors on runoff generation and erosion. They are an accepted method for calibrating and validating physically based rainfall and erosion models. Nevertheless, it is proven that not only the patterns of rainfall are highly variable over space and time. Soil parameters responsible for runoff generation and erosion – e.g. infiltration capacity, soil moisture, aggregate stability – are highly variable, too. The results of more than 100 plot scale rainfall simulations on abandoned fields in different areas of Spain are analysed for identifying and quantifying factors affecting runoff and erosion processes. The variability of superficial runoff and suspended sediment yield is very high within most of the test areas, reaching values up to a runoff coefficient of 1 and yielding 130 g per experiment. Only within the semi-arid areas we can find a significant and high correlation between vegetation cover and sediment yield (and sediment concentration), whereas most of the other investigated surface parameters show no influence on runoff and erosion. For this, even in simulations on small plots more complex processes can be assumed. The factors controlling them have to be searched, even at detailed scales, within the geo-ecological characteristics of the investigated areas at chorological scales.