Resampled into 60 min slices:

[davemcphee]

Percentile data on how much rain fell per hour, counting only hours during which more than 0 rain actually fell

count    3182.000000
mean        0.111292
std         0.242054
min         0.010000
10%         0.010000
20%         0.010000
30%         0.010000
40%         0.020000
50%         0.030000
60%         0.050000
90%         0.260000
92.5%       0.340000
95%         0.480000
97.5%       0.750000
99%         1.273800
100%        3.490000
max         3.490000

[ericwg]

So if we had a cistern that captured 1 inch of rain, then slowly released it over 48 hours (let's say it was totally full for the next 48 hours to make it easy), what percentage of the total volume of water would we capture over the ten year period?

[davemcphee]

Just realised my numbers are off as I'm looking at rainfall per hour, not day.

[ericwg]

I also could have been more clear - one inch of rain in an hour and the cistern didn't drain for 48. So, if it kept raining for the next 48 hours, all of the water would leave the property.

[davemcphee]

I think I'd need to know size of lot in sq ft, and size of Imp. Cover that we will assume is all collected into the cistern (until it over flows). Also, the size of the cistern.

Initial calculations of how many gallons will fall onto a 10,000 sq ft lot with 0.5 inches of rain (over 3000) indicate you'd need a fairly decent sized cistern to make a dent.

If the IC is about 20% of the total lot size, then 20% is the max you could catch.

I guess lot size doesn't matter, only IC size.

So how much impact would a cistern have on IC run off?

[ericwg]

So one of the things that we're looking at is a one foot cistern under the slab, so if the slab was 3,000 square feet, and the cistern was one feet high, then it could hold 3,000 cubic feet 22,400 gallons or so. For simplicity's sake, let's assume that thats the only way the water can stay on site.

[davemcphee]

Oooh OK, I was thinking in terms of rain barrels / 350G tanks. 22K gallons is a whole diff ball park.

3000 sq ft, subjected to 3 inches of rain, would collect 5,500G of water.

A 22K gallon cistern would be able to pick up 100% of IC run off even in a 100 year deluge.

It would take 12 inches of rain to fill a cistern that size, which according to a quick google search, only happened once in recorded US history, in 1943 http://www.weather.com/holiday/spring/news/extreme-rainfall-records-united-states-20130313#/4

[davemcphee]

I resampled the rainfall data into double days (48 hours) and removed all entries with 0 rainfall. Percentiles for 48hour segments where it did rain:

count    592.000000
mean       0.598193
std        1.008720
min        0.010000
10%        0.010000
20%        0.030000
30%        0.070000
40%        0.130000
50%        0.200000
60%        0.310000
90%        1.679000
92.5%      2.110250
95%        2.564500
97.5%      3.136500
99%        4.787200
100%      10.440000
max       10.440000
Name: inches, dtype: float64

[davemcphee]

At the 99th percentile, roughly 5 inches in 48 hours, equates to 9429 gallons. No issue for a cistern that size.

That 10.44 inch 48 hour period was on 2015-10-30