Methods of presenting rainfall data

Rainfall data can be presented in the following ways:

1. Tables of hourly, daily, monthly and annual rainfall.
2. Hyetograph (bar chart of depth or intensity vs time).
3. Mass curve (cumulative rainfall vs time).
4. Intensity-duration and IDF curves.
5. Depth-area-duration (DAD) curves.
6. Isohyetal maps (contours of equal rainfall).
7. Frequency curves and annual/monthly bar graphs.

Two methods explained

1. Hyetograph A hyetograph is a bar chart of rainfall depth (or intensity) in successive equal time intervals, plotted against time. It is obtained from the mass curve by taking the difference between successive ordinates.

 mm/h
  |      ##
  |   ## ## ##
  | ## ## ## ## ##
  +------------------ time

It shows the storm pattern, the time of peak intensity and the total depth (area under the bars). It is the input for rainfall-runoff analysis such as the unit hydrograph.

2. Mass curve of rainfall A mass curve plots the accumulated rainfall (y-axis) against time (x-axis) from the start of the storm. It is always rising or flat.

 cm
  |        _____
  |      /
  |    /      <- steep = high intensity
  |__/
  +------------------ time

• The slope at any point gives the rainfall intensity.
• A horizontal portion means no rain.
• It is used to find the maximum depth for a duration, to derive the hyetograph, and to prepare intensity-duration curves.