Understanding the Mass Curve of Rainfall

The mass curve of rainfall is a fundamental tool in hydrology used to represent the pattern and total amount of rainfall over a specific period. It is a graphical representation that helps analyze rainfall events.

What is Plotted on a Mass Curve of Rainfall?

A mass curve of rainfall is plotted with two main variables:

• The horizontal axis (x-axis) represents time. This time can be in minutes, hours, or days, depending on the duration of the rainfall event being recorded.
• The vertical axis (y-axis) represents the cumulative or total amount of rainfall that has occurred up to that specific point in time. This is also known as accumulated precipitation.

The curve typically starts at zero accumulated rainfall at the beginning of the time period and continuously rises as rainfall occurs. The slope of the mass curve at any point gives the rainfall intensity at that instant.

Analyzing the Options

Let's look at the given options:

• Discharge: Discharge refers to the volume of water flowing through a channel per unit time. While related to rainfall (as rainfall can cause runoff and discharge), the mass curve of rainfall specifically plots the rainfall itself, not the resulting discharge in a river or stream.
• Amount of Transpiration: Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. This is a component of the water cycle, but it is not what is plotted on a rainfall mass curve.
• Rainfall Intensity: Rainfall intensity is the rate at which rain is falling (e.g., mm per hour). While the slope of the mass curve represents rainfall intensity, the curve itself is a plot of accumulated rainfall, not intensity, against time. A separate graph called a hyetograph plots rainfall intensity against time.
• Accumulated Precipitation: Accumulated precipitation is the total amount of rainfall (or precipitation) that has fallen up to a given time. This matches the definition of the vertical axis of a mass curve of rainfall.

Therefore, the mass curve of rainfall represents the plot of accumulated precipitation against time.