Summary In Mongolia, overgrazing and mismanagement are considered to be the causes of soil erosion and land degradation. However, the field data available on soil erosion and land degradation processes are limited. Two experimental watersheds were selected within the Kherlen river basin in Mongolia to assess the state and cause of soil erosion; this was done by monitoring the water–sediment discharge in two small catchments with different vegetation covers. The two sites—Kherlenbayan-Ulaan (KBU; 6.9 ha) and Baganuur (BGN; 7.6 ha)—have relative heights of 100 m and 150 m, respectively. In addition, one of the two hillslope plots (50 m × 25 m) established in each area was surrounded by a fence. The overland flow was monitored by Parshall flumes; rainfall, raindrop impact, and soil water suction were also recorded. The amount of sediment yields from the BGN and KBU catchments during the observation period were 0.37 t/ha/years and 0.02 t/ha/years, respectively. The surface runoff percentage and amount of sediment discharge show a good correlation with the raindrop impact energy; however, the peak rainfall shows a poor correlation. These data suggest that raindrop impact plays an important role in water and sediment discharge. The long-term erosion rate estimated by the 137 Cs inventory is lower in the BGN catchment than the KBU one, suggesting that the high sediment yield in the former might be due to the susceptibility to erosion by recent overgrazing.