Harvested Rainwater Guidelines

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The capacity of a rainwater harvesting system depends on the amount of rainfall, size of collection area, storage capacity, and the household's level of demand for water.
Table 1.0 indicates the gallons of water produced annually for different size roof areas and rainfall amounts.
To determine the square footage of catchment area of a house, use only the house's footprint. (The actual area of roof material will be greater due to the roof slope. However, the amount of rainfall on the roof is not affected by the slope.) In Table 1.0, note that Austin's average rainfall is 32 inches.

Table 1.0
Annual Rainfall Yield in Gallons for Various Roof Sizes and Rainfall Amounts
ROOF SIZE IN SQUARE FEET	RAINFALL IN INCHES
ROOF SIZE IN SQUARE FEET	20	24	28	32	36	40	44	48	52
1000	11236	13483	15730	17978	20225	22472	24719	26966	29214
1100	12360	14832	17303	19775	22247	24719	27191	29663	32135
1200	13483	16180	18876	21573	24270	26966	29663	32360	35056
1300	14607	17528	20450	23371	26292	29214	32135	35056	37978
1400	15730	18876	22023	25169	28315	31461	34607	37753	40899
1500	16854	20225	23596	26966	30337	33708	37079	40450	43820
1600	17978	21573	25169	28764	32360	35955	39551	43146	46742
1700	19101	22921	26742	30562	34382	38202	42023	45843	49663
1800	20225	24270	28315	32360	36405	40450	44495	48540	52584
1900	21348	25618	29888	34157	38427	42697	46966	51236	55506
2000	22472	26966	31461	35955	40450	44944	49438	53933	58427
2100	23596	28315	33034	37753	42472	47191	51910	56629	61349
2200	24719	29663	34607	39551	44495	49438	54382	59326	64270
2300	25843	31011	36180	41348	46517	51686	56854	62023	67191
2400	26966	32360	37753	43146	48540	53933	59326	64719	70113
2500	28090	33708	39326	44944	50562	56180	61798	67416	73034

The average rainfall per month for Austin follows:

For outdoor uses of rainwater, the types of plants, amount of exposure to direct summer sun, soil conditions, presence or lack of mulch, and size of the area will determine how much irrigation water is needed. Large landscapes with large water demands are not readily accommodated by rainwater catchment systems.
Storage capacity for indoor uses of rainwater can typically be more readily gauged; although this is not a precise science due to the vagaries of rainfall and personal habits.
A conserving household may use 25- 40 gallons of water per person per day. Multiply the number of persons in the household by the average use (40 gallons per person is a generous amount, 25 gallons is quite conservative. See the Water Budget section if more precise amounts are needed.) The longest drought in 50 years lasted 75 days in our area. Multiply the total of the number of persons times daily use times 100. This gives a safety factor of 25 days over the worst-case scenario of the last 50 years. The total is the amount of storage capacity required.