Temporal and spatial variability of rainfalls have a significant impact on the operation of urban drainage systems. Stormwater retention reservoirs are particularly sensitive to these variations. This paper presents the results of research on the impact of the direction and speed of rainfall movement over an urban catchment (of total area 560ha) on required volume of the retention reservoir. Calibrated hydrodynamic model has been used to simulate rainstorms movement (software: SWMM5). The whole catchment was divided into sectors of 600´600m, which facilitated the calculation of time shift between sectors for assumed speeds of rainfall movement (vop= 2, 5 and 10 m/s and stationary storm). Simulations comprise four directions of rainfall movement. Simulations were carried out for various outflow-rates from the storage reservoir - four different values were taken: Q0=2000÷8000 dm3/s. The volume of stormwaters that overflowing the edge of the designed reservoir was used to compare the obtained results. The results make possible to draw the following conclusions: the maximum peak flow-rates for the dynamic conditions can be larger than a few to up to 30% (depending on the td and vop ) higher than for the stationary variant, influence of the direction and speed of rainfall movement on the shape of inflow hydrograph to the reservoir is significant, which has a direct impact on the calculated volume of stored stormwaters. the impact of the movement of precipitation on stored volume is closely related to outflow rate from retention reservoir (i.e. for Q0=2m3/s the difference between static and dynamic variants is less than 10% while for Q0=8 m3/s it reach over 90%). the maximum volume of stored stormwaters occurs with rainfalls whose velocities are close to flow velocity in the conduits (2 m/s) and parallel to general direction of flow in the drainage system.