Intensity-duration curve and depth-area curve

Intensity-Duration Curve

It shows how the maximum average rainfall intensity of a storm changes with duration. The intensity is high for short durations and falls for long durations.

• Prepared from a recorded storm: for each duration $D$ the greatest rainfall in any period of length $D$ is found from the mass curve, and $i_{max} = \text{depth}/D$.
• Plotted as intensity (y) against duration (x). With frequency (return period) added it becomes the IDF curve.

 i
  |*
  |  *
  |     *
  |         *
  +--------------- D

Uses: design storm for drainage, culverts, small dams and rational method calculations.

Depth-Area Curve

It shows how the average depth of rainfall of a storm decreases as the area increases. Rainfall is greatest at the storm centre and falls off outward.

• Prepared from an isohyetal map: the average depth inside each isohyet and the area enclosed are computed, and plotted as depth (y) against area (x). Curves for different durations form the DAD curve.

 Depth
  |*
  |  *
  |     *
  |         *
  +--------------- Area

Uses: to convert point rainfall to areal rainfall (areal reduction), design of large catchments, estimating probable maximum precipitation (PMP).