# how many gallons of water equals 1 inch/acre of rain? **URL:** \ **Category:** Factual Questions\ **Created:** [April 8, 2010, 5:16pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432 "2010-04-08T17:16:43Z")\ **Posts on this page:** 19\ **Page:** 2
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [April 9, 2010, 2:49am UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/21 "2010-04-09T02:49:45Z")
> [@UncleFred](#): > > It was important because we’re having discussions about the capacity of our storm and sewage drain systems. 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. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 9, 2010, 12:13pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/22 "2010-04-09T12:13:58Z")
> [@Apex\_Rogers](#): > > I considered splitting all of my operations to separate lines to avoid abusing the equal sign, but given the simple nature of the calculation, I thought it would be obvious what happened. I agree that it’s a bit unclear, but come on, this isn’t a math exam here! Life’s a math exam when you teach math. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 9, 2010, 12:18pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/23 "2010-04-09T12:18:38Z")
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 ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [April 9, 2010, 1:25pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/24 "2010-04-09T13:25:10Z")
This’d be a lot easier if y’all just converted to metric, did the volume calculation, and converted back. 🙂 ---
**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\ **Post date:** [April 9, 2010, 6:51pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/25 "2010-04-09T18:51:45Z")
Well,this website (I thought I provided this earlier) does all this for us, so that’s easier: > **[Annual Rainfall Calculators : Inches of annual rainfall to gallons and square...](http://www.virtualsecrets.com/annual-rainfall-water-calculator.html)** > > Series of calculators which allows you to enter inches of annual rainfall to get the volume of water received in acres or square feet, calculate water differences between geographic regions and calculate water needs of plants, seeds, crops from one... 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. ---
**Author:** ![penultima\_thule](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/penultima_thule/32/3833_2.png) [@penultima\_thule](https://boards.straightdope.com/u/penultima_thule)\ **Post date:** [April 9, 2010, 10:47pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/26 "2010-04-09T22:47:33Z")
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](http://www.murrayirrigation.com.au/content.aspx?p=20021) ---
**Author:** ![Apex\_Rogers](https://avatars.discourse-cdn.com/v4/letter/a/e480ec/32.png) [@Apex\_Rogers](https://boards.straightdope.com/u/Apex_Rogers)\ **Post date:** [April 9, 2010, 10:54pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/27 "2010-04-09T22:54:26Z")
> [@naita](#): > > 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 Point taken. And I knew the nitpick was meant in jest, but I couldn’t let such a remark against my character go undefended. 😃 ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [April 10, 2010, 3:14am UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/28 "2010-04-10T03:14:35Z")
> [@drhess](#): > > Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? 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. ---
**Author:** ![\_cc](https://avatars.discourse-cdn.com/v4/letter/_/e19b73/32.png) [@\_cc](https://boards.straightdope.com/u/_cc)\ **Post date:** [April 10, 2010, 4:30am UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/29 "2010-04-10T04:30:42Z")
Google calculator, p’shaw. > **[one acre by one inch - Wolfram|Alpha](https://www.wolframalpha.com/input/?i=one%2Bacre%2Bby%2Bone%2Binch)** > > Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels. ---
**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\ **Post date:** [April 10, 2010, 2:46pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/30 "2010-04-10T14:46:39Z")
> [@t-bonham@scc.net](#): > > 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. Yes. That is the point, how much water (in gallongs, piped) to replace how much rainfall (in inches, gratis). 🙂 ---
**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\ **Post date:** [April 10, 2010, 2:48pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/31 "2010-04-10T14:48:46Z")
> [@'\[cc](#): > > ;12324671’]Google calculator, p’shaw. > > [one acre by one inch - Wolfram|Alpha](http://www.wolframalpha.com/input/?i=one+acre+by+one+inch) Awesome link. Thanks! :eek: ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 10, 2010, 2:54pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/32 "2010-04-10T14:54:24Z")
Did anyone take into account the curvature of the Earth? That acre-sized rain guage is wider at the top than the bottom. ---
**Author:** ![Bayesian\_Empirimancer](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@Bayesian\_Empirimancer](https://boards.straightdope.com/u/Bayesian_Empirimancer)\ **Post date:** [April 10, 2010, 9:06pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/33 "2010-04-10T21:06:29Z")
Can I get the units in Hoppus feet? ---
**Author:** ![Enginerd](https://avatars.discourse-cdn.com/v4/letter/e/bcef8e/32.png) [@Enginerd](https://boards.straightdope.com/u/Enginerd)\ **Post date:** [April 11, 2010, 12:10am UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/34 "2010-04-11T00:10:30Z")
> [@drhess](#): > > 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. 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](http://www.unce.unr.edu/publications/files/ag/2008/sp0810.pdf)). 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. ---
**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\ **Post date:** [April 18, 2010, 7:02pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/35 "2010-04-18T19:02:09Z")
> [@Enginerd](#): > > 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](http://www.unce.unr.edu/publications/files/ag/2008/sp0810.pdf)). 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? ---
**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\ **Post date:** [April 18, 2010, 7:18pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/36 "2010-04-18T19:18:03Z")
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](http://www.fao.org/docrep/meeting/009/j4238e.htm#P22_2151) ---
**Author:** ![Oslo\_Ostragoth](https://avatars.discourse-cdn.com/v4/letter/o/a9a28c/32.png) [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)\ **Post date:** [April 18, 2010, 9:04pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/37 "2010-04-18T21:04:44Z")
[back to the drawing board] ---
**Author:** ![Oslo\_Ostragoth](https://avatars.discourse-cdn.com/v4/letter/o/a9a28c/32.png) [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)\ **Post date:** [April 18, 2010, 9:11pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/38 "2010-04-18T21:11:09Z")
> [@lazybratsche](#): > > … Most farms in the US use irrigation (some much more than others). In many cases, the irrigation isn’t necessary but it helps improve crop yields… Roughly 12% of USA cropland is irrigated. [Cite](http://www.ers.usda.gov/statefacts/us.htm) That’s a lot more than I would have thought. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [April 18, 2010, 10:05pm UTC](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432/39 "2010-04-18T22:05:16Z")
27154.071529364 gallons [Previous page](https://boards.straightdope.com/t/how-many-gallons-of-water-equals-1-inch-acre-of-rain/535432.md?page=1)