The temporal dynamic of water repellency in soils has a strong influence on water flow and the appearance of preferential flow paths at potentially water repellent sites. To quantify this effect, field investigations were conducted at a sandy site near Berlin, Germany. A large number of soil samples were collected at 32 different times during a 3-yr period. Additionally, a time domain reflectometry (TDR) array with 63 probes measured water contents hourly along a transect of 130 by 60 cm. On the basis of these sampling campaigns the area share of water repellent soil regions was measured. Water content changes were observed with the TDRs at high spatial and temporal resolution after several rainfall events. Heterogeneities in water content changes were analyzed. To quantify the heterogeneity (i.e., the degree of preferential flow) we propose the use of the effective cross section, ECS, for water flow. This parameter was calculated by fitting the β function to the cumulative values of the water content change over a horizontal cross section at a depth of 25 cm. Sampling and TDR measurements showed similar seasonal dynamics of preferential flow, with the highest occurrence in summer and early autumn and a maximum accessible soil volume in the spring. Preferential flow in the winter month was enhanced not only by water repellency, but also by freezing and melting. We also tried to calculate the ECS from climatic data. It was possible to calculate the ECS using a linear relationship with the initial soil moisture at the 10-cm depth, the duration and amount of precipitation, and the potential evapotranspiration rate.