Calculating Rain Runoff from Y

Posted by Robert Dailey Mar 23, 2006

Parts of the West and Southwest are in the grips of a serious drought that the news media are comparing to the infamous Dustbowl of the last century.

So it's to the benefit of all gardeners in this area, including desert gardeners, to start thinking seriously of harvesting all the water possible.

The average desert area receives between 8 and 16 inches of precipitation per year. Some of this is in the form of rain, the rest from snow or sleet. Since the humidity is so low (usually around 10 percent or less) any other atmospheric water (such as dew) is relatively non-existent.

If you receive eight inches of rain per year, and are not making any attempt to harvest it, then you are losing immense amounts of water - water that could be used to irrigate your garden (and lower your water bill).

How much water are you losing?

Let's assume that the area of your roof is 2,000 square feet. And let's further assume that the area in which you live receives eight inches of rain during a year.

Here's a simple but very accurate formula for calculating how much water is coming off your roof (and how much water you are losing if you are not harvesting it).

Take the area (2,000 square feet) and multiply it by 8 (amount of rainfall). Take that number and multiply it by 623. Divide that amount by 1000, and you have the amount of runoff in a season. If you also want to figure evaporation, then multiply your answer by .92 or 92% (.08 or 8% is the factor generally used to figure evaporation in a desert environment: 100% -8%=92%).

Thus, a 2,000-square-foot roof, receiving 8 inches of rain per year, will have about 9,710 gallons of runoff. Or, that same area receiving one inch of rain will have 1,246 gallons of runoff. If you want to calculate evaporation, then multiply 1,246 by 92% and that will give you a fairly accurate figure.