How to Calculate Rainfall Totals
While the National Weather Service and similar entities collect and publish data on rainfall amounts, their data might not be accurate to your home’s specific location. What’s more, most people take the number of inches of rainfall at face value without any prior knowledge of how that number is determined.
Calculation of Rainfall: Preparation
Think about it: What constitutes an inch of rain? Raindrops don’t stack on top of each other, after all. So, how is rain measured?
Farmers and home gardeners often need to know rainfall amounts to care for their plants and crops properly. Others may want to make an environmentally friendly switch to utilizing rainwater for certain necessary household tasks but have no idea how much they can expect to gather. Calculation of rainfall can help in both situations.
You can find containers that have been designed specifically to help you calculate rainfall such as rain gauges, which are readily available at various in-person and online stores. While any container works, buying one specifically for this purpose might allow you to skip some of the next steps. Straight up-and-down containers are typically recommended.
Calculation of Rainfall: Basics
Regardless of the size or shape of your container, you must first place it outdoors. If you’re particularly concerned about the rainfall in a specific section of your property, such as in your garden, then put it there. However, if you want the most accurate overall reading, make sure to place it away from trees, buildings and other structures that might either block the rainfall or dump excess rain into the container.
You’ll need to perform a series of measurements and calculations using your container (a rain gauge typically already has these measurements in place). First, calculate the area of the container. According to USGS, do this by first measuring the container.
However, for a very accurate reading, don't include the edges of the container. If the container has variable diameters, you’ll need to measure all of them. In addition, according to Cuemath, you should also find the radius of the container (you may need this when calculating rainfall).
Calculate the Actual Amount of Rainfall
Next, wait for the rain to fall over the period that you would like to measure. Once it does, measure the diameter of the top of the rainfall (assuming that the diameter isn’t the same throughout the entire container). Divide it by 2 to find the radius in that exact spot. Measure the height or depth of the rain – be sure to use the same units for all your measurements!
Then conduct the following equation: radius x radius x pi (if your calculator doesn’t have a pi button, use 3.14). This will provides the area of the rainfall. Then, divide the volume of the precipitation by this number (the area). That should give you the total rainfall amount in the unit you chose (inches, centimeters, etc.).
To obtain a reading on the total amount of rainfall over time, you’ll need to do these calculations at least once daily – possibly more. This is because moisture will evaporate from your container while it sits.