Measuring Rainfall

So far there is little sign of rain in Gauteng this spring. However, the weather forecasters have told us that we will experience slightly better-than-average rainfall in the coming  summer.

But how do we actually measure rainfall?

We casually talk about rainfall in millimetres, but in fact we refer to the number of millimetres that have fallen within a specific amount of time. Unless we state that amount of time – e.g. “we had 10 mm in 15 minutes”, it is assumed that it is within 24 hours. The South African Weather Services (SAWS) measure rainfall at 08.00. The rainfall measured between 08:00 yesterday and 08:00 today is recorded against yesterday’s date on their database.

But how can we measure rainfall when the rain soaks into the ground or runs off it into drains or streams? When we talk about millimetres of rainfall we are in fact referring to 1 mm of rain in an area which is enough to evenly cover the ground in that area with a layer of water 1 mm deep. According to SAWS, “what is meant by 1 mm of rainfall is rainfall equivalent to 1 litre of water in a 1 square meter box with no runoff, infiltration or evaporation.”

A square metre box is not practical to leave outside, especially if you have animals on your plot, so a rain gauge is used.  The standard instrument for the measurement of rainfall is the 100 mm rain gauge. This is essentially a circular UV protected plastic funnel with a diameter of 130 mm which is kept in an open area, so that it collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 100mm of rain.

You can make your own rain gauge out of any can, but then you will have to use a formula to measure the rainfall: the height of the water in the can is equal to the volume of water in the can divided by the area of the opening. For most of us, life is too short to do these calculations.