r

*Welcome to the Rainfall Predictor*

**What is the average annual rainfall at my house?**

We plotted your address on an annual rainfall map and from our extrapolation (estimation) your average annual rainfall is expected to be 35.7 inches per year.

How do we know?

**We used the Mean Annual Precipitation and Precipitation Depth Duration Data (Rantz 1971) as our source and we extrapolated the annual rainfall from the isohyetal (or lines of constant rainfall amounts) maps of Marin County. Figure 1 is a map of annual average rainfall amounts for Marin. **

**What is the range of rainfall during any one season?**

## For any one year, the amount of rain may vary - it won't always be the average and, in fact, may vary from year to year. The average seasonal rainfall variation was calculated based on data from Kentfield, which is the closest site to your house (within a mile and on the same side of Mt. Tam). In the case of Mt. Tam, the topography is quite important to the choice of the nearest station because of its effects on significant areal variability. 67 percent of the time, the range for the annual rainfall for Greenbrae is between 23.21 and 48.19 inches.

How do we know?

## We used the California State Department of Water Resources Depth-Duration-Frequency tables to obtain the standard deviation for the seasonal rainfall which was 16.30 inches. The coefficient of variability was calculated to be 0.346 by dividing the standard deviation by the average for Kentfield which was 46.96 inches. The range for Greenbrae was calculated by multiplying this coefficient by the average rainfall (35.7 inches) and adding and subtracting the result (12.49 inches) from the average.

**What is the 100-year 24-hour event for my address?**

8.52 inches in 24 hours

What is the 100–year 1 hour event for my address?

1.48 inches in 1 hour

How do we know?

There isn’t a rain gauge near your town, so we used published data of nearby stations to estimate these events so we used the State of California, Department of Water Resources, Summary of Short-Duration Precipitation Frequency in the San Francisco Bay Area, the 100 year 1 hour and 24 hour events for Kentfield and extrapolated for Greenbrae (with the factor that the rainfall at Greenbrae is 77 percent of of the rainfall at Kentfield).

**
What exactly is a 100-year or a 1000-year event?**

Return Period

100 year events are examples of statistical jargon that engineers and meteorologists use to describe rainfall intensity and to help design storm water management programs.

When all of the 24 hour totals of rainfall were analyzed statistically for locations in the Bay Area, the results were plotted, and the likelihood (chance) of certain amounts of rainfall were calculated for different time periods (hour, day, year). A 100-year 24-hour event (also called a ‘return period’) means that there is a 1/100 chance that this amount of rainfall can occur over a 24-hour period during a season.

The idea of a “return period” is somewhat misleading – Just because there is a 1/100 chance of an event in a season does not mean that if it happens this season that you’d have to wait another 100 years for the event to occur again. Next season you start over and you’d have a 1/100 chance again. (until the engineers and meteorologists recalculate the statistics!!).

Figure 1 Isohyetal Map