International audience ; Muddy oods due to agricultural runo are a widespread and frequent phenomenon in the European loess belt, and particularly in central Belgium. These oods are triggered when high quantities of runo are generated on cropland and cause severe erosion. Three soil surface characteristics are relevant to determine the runo potential of cultivated soils: soil cover by crops and residues, soil surface crust-ing and roughness. These characteristics have been observed on 65 cultivated elds throughout 2005. A heavy rainfall event representative for events triggering muddy oods in the region (60 mm.h −1 during 30 minutes) has been simulated using a 0.5 m 2 simulator on elds with the 17 most observed combinations of soil surface characteristics in central Belgium. In the case of (ploughed) bare uncrusted soils, runo is not observed, nor in the case of soils covered by crops, showing a transitional crust and a moderate roughness (1-2 cm). In the cases where runo has been observed, mean runo coecients ranged from 13% (wheat in July) to 58% (sugar beet or maize in May and June). Grassed buer strips (GBS) and grassed waterways (GWW) show a higher runo coecient (62% for GBS and 73% for GWW) than most cultivated soils (13-58%). Furthermore, it is demonstrated that small plot measurements can be used to estimate runo generation at the eld scale. A classication of runo generation risk based on the surveys of soil surface characteristics has been applied to common crops of central Belgium. February as well as the period between May and September are the most critical for runo at the eld scale. However, it appears from monitoring of a 16ha-catchment that the highest runoff volumes and peak discharges are recorded between May and August after heavy rainfall, explaining why 85% of muddy floods are recorded during this period in central Belgium.