Historical Flash Flood Trends from Hcdn Basins

Observational research has shown that in many regions rainfall intensity is increasing both in magnitude and frequency. This change is consistent with climate model predictions of a warming climate due to anthropogenic influences. Other research has shown associated increases in both the magnitude and frequency of floods. Research to date, however, has been limited to studies that do not distinguish hydrologic response on the basis of basin area. With increasing rainfall intensity occurring in some regions due to a warming climate, we might ask if flash flooding is also more prevalent than the past? To answer this question a subset of small, flash-flood scale HCDN basins are studied to identify regional trends in flash flood frequency and magnitude over the CONUS. Results show both increasing and decreasing trends in flash flood frequency and magnitude, depending on the geographic region, which is consistent with previous studies that do not specifically address flash floods. Neither increasing nor decreasing trends are identified in other areas. The issue of spatial sampling, that limits sample size and, possibly, trend detection, is addressed.