The character and causes of flash flood occurrence changes in mountainous small basins of Southern California under projected climatic change

Small watersheds (O25 km2) in the mountain regions of southern California comprise the study region. This paper examines changes in flash flood occurrence in southern California resulting from projected climatic change. The methodology synthesizes elements of meteorological modeling, hydrology and geomorphology into an integrated modeling approach to define flash flood occurrence in a systematic and consistent way on a regional basis with high spatial and temporal resolution appropriate for flash flooding. A single climate model with three-dimensional atmospheric detail was used as input to drive simulations for historical and future periods. Results indicate an increase in flash flood occurrence for the study region. For two distributed hydrologic models employed, the increase in flash flood occurrence frequency is on average between 30% and 40%. Regional flash flood occurrence is characterized by near saturation of the upper soil layer, and wider ranges in lower soil layer saturation and in precipitation. Overall, a decrease in the total number of precipitation events was found, although with increased precipitation intensity, increased event duration, and higher soil saturation conditions for the 21st century. This combination could signify more hazardous conditions, with fewer precipitation events but higher rainfall intensity and over soils with higher initial soil moisture saturation, leading to more frequent occurrence of flash floods.