What are depth-area-duration relationships for rainfall?

Depth-Area-Duration (DAD) curves show how average rainfall depth decreases as averaging area increases for storms of given duration. Point rainfall is always maximum; areal average is less due to storm spatial variability. DAD analysis uses isohyetal maps of recorded storms. Index values express areal depth as percentage of point depth. Used for: design storms for large catchments, PMP development, spillway design flood estimation. Longer duration storms have smaller DAD reduction because spatial variation averages out over time.