The paper deals with precipitation and rainfall erosivity, which are among leading factors influencing soil erosion. In the first part, an overview of water erosion and its processes are given. An overview of precipitation measurement methods is given and the network of raingauges in Slovenia is shown. Annual precipitation value does not indicate annual erosivity R, which is one of the factors in RUSLE Equation. Rainfall erosivity depends on precipitation intensity. Analysis and comparison of precipitation on Letališče Portorož station with participation on local raingauges confirmed the importance of local measurements parallel to field measurements. The main part of the paper is given to measurements and equations of soil erosion. With Rusle 2 program soil detachment was computed and compared with observed data on the Dragonja river basin. Rusle 2 was calibrated for plots in wood with adjustment of crop- management factor C. For bare soil and for grassland in process of overgrowing we used the program with no calibration. On bare soil the content of organic matter was very high, because of previous land use – pasture. Rusle 2 is suitable for use in the Dragonja river basin, but first more field measurements should be done.