Understanding Hyetographs: Rainfall Intensity Over Time

A hyetograph is a fundamental tool used in hydrology and meteorology to visualize and analyze rainfall events. It provides a graphical representation of how rainfall characteristics change over a specific period.

What a Hyetograph Plots

Specifically, a hyetograph plots the rainfall intensity against time.

• The vertical axis (y-axis) typically represents the rate of rainfall, often measured in units like millimeters per hour (mm/hr) or inches per hour (in/hr).
• The horizontal axis (x-axis) represents time, showing the progression of the rainfall event.

This type of graph helps in understanding the pattern of a storm, such as its duration, peak intensity, and how the intensity changes throughout the event.

Distinguishing Hyetographs from Other Graphs

It is important to differentiate a hyetograph from related hydrological graphs:

• Rainfall Depth and Time: A graph showing cumulative rainfall depth over time is known as a rainfall mass curve or simply a cumulative rainfall graph. It shows the total amount of rain that has fallen up to any given point in time.
• Discharge and Time: A graph that plots the flow rate (discharge) of a river or stream against time is called a hydrograph. It is used to study streamflow response to rainfall or snowmelt.
• Cumulative Rainfall and Time: As mentioned, this represents a mass curve, showing the total accumulation, not the rate (intensity) at specific moments.

Why Rainfall Intensity Matters

The representation of rainfall intensity is key to the definition of a hyetograph. Intensity is crucial for determining factors like runoff generation, potential for flooding, and soil erosion. A graph showing only the total depth might not reveal periods of very high intensity that can cause flash floods, even if the total depth is moderate.

Conclusion

Based on the standard definition in hydrology, a hyetograph is specifically used to depict the variation of rainfall intensity with time during a storm event.