Rainwater storage and effect on flow rate at watershed outflow. This study participates to the quantitative balance of rain water domestic use. It aims towards an optimal design of rain barrels volume, the feasibility of soil infiltration techniques with excess of water and the impact assessment of rain barrel on the decrease of peak flow in a pilot rural watershed of 9 km², considering the chronology of precipitation events. The parameters that are considered are the roofs area, the number of inhabitants in the house, the average daily rainfall water consumption, and the local pluviometry. In order to design the infiltration structure associated with the rain barrel, the volume of rainfall excess from the rain barrel needs to be evaluated. Afterwards, the infiltration structure may be dimensioned according to the soil infiltration characteristics. We assume that 75 l, out of the 115 l daily consumed by each inhabitant, are coming from the rain barrel and the remainder (40 l) is drawn from the drinking water distribution network. Calculations are carried out for roofs of 100, 135, 170 and 200 m², for a consumption of water ranging between 150 and 1,150 l per day per house and for rain barrels of 5, 10 and 20 m³. It appears that if correctly designed private rain barrel allows an opportune water use in households, the effect on flood reduction is, however, limited considering the relative low importance of the collecting surface (1.3% of the watershed surface).