This article deals with mapping exposure (or external vulnerability) to flood risk in two typical Mediterranean ephemeral streams: the Barranco de Carraixet and the Rambla de Poyo. The floodplains of both streams are within the metropolitan area of the city of Valencia (Spain"s third largest city). Following the Mediterranean model, they are very fertile areas (with intense periurban and highly productive agriculture) which have recently absorbed the great urban expansion of the metropolis. Hydrologically these basins remain dry for most of the year, but become particularly dangerous during flash-flood events. They generate a risk pattern highly dependent on exposure since, in general, the hazard factor is very difficult to map, given the unpredictable nature of flash-floods. This work constitutes a proof of concept based on simple estimators obtained from land uses. External vulnerability or exposure is evaluated as a function of economic land value and human exposure to hazard. Land value is directly taken from the cadastre whilst human exposure is indirectly estimated from the location of population in relation to timeeactivity profiles. The temporal dimension is introduced and three exposure scenarios have been mapped, related to different time periods: working days, nights, weekends and holidays. The results show different patterns of exposure for each plain. In Carraixet floodplain exposure is greater at night and on weekends and holidays than during working hours, on account of the dominant agricultural and residential land uses. In the Poyo plain there are no major contrasts in absolute terms between day and night time, although spatial patterns of exposure vary from day to night: during the day, exposure is higher in industrial and commercial areas (around communication routes) whilst at night residential areas show more exposure.