The aim of this study was to determine the spatial distribution of rainfall erosivity in a 504 km2 river basin in Eastern Pyrenees and assess its uncertainties. The study area is topographically complex and is dominated by Mediterranean climate with intense rains during summer. Available rainfall dataset includes 14 years spanning from 1991 to 2004 at daily resolution from seven weather stations and sub-hourly from one station. Daily R values were calculated from sub-hourly data, and then by using the relationship between daily R values and precipitation, the R values for weather stations having only daily rainfall resolution were calculated. The error propagation by using such upscaling approach was analyzed. The studied sources of uncertainty of R factor at annual scale were: the use of the relationship between daily R values and precipitation, the temporal averaging of R values and spatial variation. Results have shown that at annual scale the temporal averaging was the largest contributor of uncertainty (70%); however this contribution decreased significantly (14%) when R was estimated for the long term.