Be aware that the peak flow which your storm drain system will be required to handle is related to the amount of rain which falls, but it isn’t a direct correlation. In other words, if four inches of rain falls you won’t get four inches of runoff; and the size, shape, and slope of the watershed dictate the resulting peak runoff. What you are looking for is cubic feet per second, which is a rate, not a volume.
Life’s a math exam when you teach math.
And it was only a silly nitpick for a giggle ;), but here’s how to avoid separate lines and still keep things clear and correct:
1 acre * 1/12 ft = 43560 sq ft * 1/12 ft = 3630 cu. ft
This’d be a lot easier if y’all just converted to metric, did the volume calculation, and converted back. 
Well,this website (I thought I provided this earlier) does all this for us, so that’s easier:
I was curious about how much water is needed to irrigate arid land. The issue of runoff and the ability of the soil to take the water is all important. Also, not every bit of an acre needs water if the crops are spread out (like an orchard vs “drilled” soybeans). Overall, however, if you assume you need roughly 40 inches and there’s a chance of severe drought, it sounds like half a million to 1 million gallons per year per acre puts you in the back of the envelope ball park (to mix metaphors wildly). Sadly that website doesn’t give cubic meters of water per hectare which would be nice. 
Anyway, at about 50 cents (US) per 1,000 gallons (also US) (which is about the cost that desalination plants seem to produce in several places), that’s about US$500 per acre per year for water (not including additional infrastructure). Assuming that’s off, it seems it’s may be up to $1,000 per acre for 40 inches of “rain.”
Bummer, that’s pricey.
1Mil gallon [US] = 3,785.4 cubic meter
1 acre = 0.404 hectare
At the Murray Irrigation Water Exchange, general security water is currently priced at AUD70/ML. This is the bottom of the annual cycle, peak prices have gone over AUD1,000/ML.
http://www.murrayirrigation.com.au/content.aspx?p=20021
Point taken. And I knew the nitpick was meant in jest, but I couldn’t let such a remark against my character go undefended. 
Almost none, really.
Part of the Sonoran Desert in southern California became the Imperial Valley due to irrigation from the Colorado River. It is now a highly productive agricultural area, but still part of a desert area, with almost no actual rainfall.
Google calculator, p’shaw.
Yes. That is the point, how much water (in gallongs, piped) to replace how much rainfall (in inches, gratis).
Awesome link. Thanks! :eek:
Did anyone take into account the curvature of the Earth? That acre-sized rain guage is wider at the top than the bottom.
Can I get the units in Hoppus feet?
That sounds high to me. When you’re irrigating in the western US, you’re not buying ready-to-drink water from desalination plants - you’re taking available natural water (either surface water or well water) and applying it directly to the fields by sprinklers or flood irrigation. Desalination is a thermodynamically expensive process, but you don’t have to pay for that level of treatment if you’re irrigating with fresh water.
Northern Nevada has very low humidity and gets about 7 inches (18 cm) of rain a year - most of that falls in the winter. Alfalfa crops here typically get around 4 acre feet of water a year. Only a fraction of that is actually taken up by the plant - much of it is returned to the environment through either percolation or surface runoff, and an enormous fraction is evaporated. Still, the total cost of irrigation is closer to $100 per acre for surface diversions than $1000 (per University of Nevada Cooperative Extension). That cost includes not only the cost of the water ($25/acre-foot), but also the costs of maintenance for the ditches and equipment as well as fees for the irrigation management agency.
Isn’t irrigation use the (partial) idea behind some desalination plants? I thought it was their use in the mideast?
I guess this report answers my question, but the cost does seem to be falling and in some places subsidies may make sense:
http://www.fao.org/docrep/meeting/009/j4238e.htm#P22_2151
[back to the drawing board]
27154.071529364 gallons