**Contents**show

## How is rainwater catchment calculated?

Calculating your catchment capacity

Roughly speaking, 1 millimetre of rain over 1 square metre of roof equals 1 litre of water. This capacity can be calculated using the following formula: **Annual rainfall (in millimetres) x Roof surface area (in square metres) = Roof catchment capacity**.

## How do you calculate roof catchment area?

It is simple. Just **use your geometrical skills** to calculate rooftop area or any other catchment area of small size. Using some simple multiplication and additions you can come up with the catchment area value in square meter. Measure length and width of the roof in meters and multiply both to get area in square meters.

## How do you calculate rainfall per area?

Find the average volume of **rain = Depth x radius x radius x 3.14**. Find the area at the top of the bucket (this is the area over which the rain is collected). Divide the rainfall volume by this area to get the rainfall.

## How do you calculate rainwater collection on a roof?

**How Much Rain Water Runs Off Your Roof?**

- Take the dimensions of the footprint of your roof and convert them to inches. (So, a 50′ x 20′ roof is 600″ x 240″.)
- Multiply the roof dimensions by the number of inches of rainfall. (In this example, 600″ x 240″ x 1″ = 144,000 cubic inches of water.)

## How do you calculate rain runoff?

Simple, straightforward methods of calculating runoff can tell you the amount of water that storms bring to the earth. For a given surface area such as a roof or yard, **multiply the area by the inches of rainfall and divide by 231** to obtain the runoff in gallons.

## How do water catchment areas work?

Within a catchment, **water runs by gravity to the lowest point**. The water is called surface runoff if it stays on the top of the land or groundwater flow if it soaks into the ground. When water reaches the lowest point in a catchment, it eventually flows into a creek, river, lake, lagoon, wetland or the ocean.

## How much rain can I catch from my roof?

To find how much rain you can collect in an average rain year multiply this number by the average inches of rain. For example, on a 2,000 square foot roof, you can collect 2,000 x 0.56 = **1,120 gallons/inch of rain**.

## How do you measure rainfall?

The standard instrument for the measurement of rainfall is the **203mm (8 inch) rain gauge**. This is essentially a circular funnel with a diameter of 203mm which collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 25mm of precipitation.

## How many Litres is 1mm of rain?

With a rainfall of 1 mm, **every square metre** receives 1 litre of rain water. A rainfall of 1 mm supplies 0.001 m^{3}, or 1 litre of water to each square metre of the field. Thus 1 ha receives 10 000 litres.

## What does 1mm rain mean?

So 1 mm of rain translates to **1 litre of water in a single metre square**. So 802 mm of rain would mean that a single metre square would receive 802 litres of rain, 140 mm would mean 140 litres of water received in a single metre square, and so on.

## How is annual rainfall calculated?

(i) **Annual rainfall = Sum of rainfall in all twelve months**. Therefore, annual rainfall is 129.2 cm. The total annual rainfall = Sum of rainfall in all twelve months. Therefore, annual rainfall is 128.7 cm.