Français

Script 42.3

Script

Save and edit this resource as a Word document.

A farmer in Guatemala suggests a small experiment to calculate approximately how much land we lose to soil erosion every year. Soil erosion losses vary from field to field, so this experiment does not indicate exact losses. Some soils are resistant to erosion, some streams cause less erosion, and steep lands erode more than flat lands. This experiment illustrates what the farmer has observed in one specific region of Guatemala.

In most of our towns there are streams that run close to our houses during rainfall. Generally these streams carry soil with them. This soil is being taken away from our lands. In this article we are going to calculate the amount of soil that is carried away by one of these streams in only one year.

To do this experiment, we just need a big bowl full of water, a handful of soil, and a scale which can weigh one ounce (30g). Take one ounce (30g) of soil and mix it in the bowl. The water‑soil mixture should be the same colour as the water that runs through the streams behind our houses every year.

How many bowls full of water run every second behind our houses during a heavy rainfall? If the hillside is steep and the stream is medium‑sized, it probably carries at least three bowls full of water every second. This means that every second three ounces (90g) of soil run into the stream, since one ounce of earth is carried by each bowl of water.

If there are three ounces every second, how many ounces would the stream carry in one minute? A minute has 60 seconds, therefore 3 ounces every second for sixty seconds represents 180 ounces (5,000 g) of earth carried by the stream every minute.

For convenience, let’s convert ounces to pounds. There are 16 ounces per pound, therefore 180 ounces divided by 16 is 11 (divide the number of grams by 1,000 to convert grams to kilograms). That means that 11 pounds (5 kg) of earth are carried every minute behind our homes.

Now, let’s assume that in a day of heavy rainfall, the water runs rapidly behind our homes for half an hour, that is 30 minutes. In reality it probably runs for a longer period of time, but let’s calculate it this way. This means that every day the stream runs for 30 minutes carrying 11 pounds of earth every minute. That means that 330 pounds (150 kgs) of soil are carried by the stream every day when there is heavy rainfall.

How many days of heavy rainfall do we have in a year? The rainy season lasts six months which is 180 days. Let’s assume that there is heavy rainfall only 50 days, less than one day out of three. It probably rains more but we will use these numbers. If there are 50 days of heavy rainfall days per year, and a stream carries 330 pounds of earth per day, then 16,500 pounds (7,500 kg) of soil are carried away by the stream in one year.

This means that in a stream of water as described above, 16,500 pounds of soil are washing away every year. You probably don’t like the idea of losing this much soil. Fortunately, there are some things you can start doing in your field right now to save your soil. Always plough on the contour, add lots of organic matter to the soil, plant cover crops, and keep planting trees.

Acknowledgements

This script was adapted and translated from an article written by Eladio Mutute, Chiantla, Guatemala. It was reviewed and edited by Rachel Bezner Kerr, Masters student of Soil Science at the University of Guelph, Ontario.

The production of this script was made possible with the generous support of the George Cedric Metcalf Foundation, Toronto, Canada.