That's non-metric thinking. In the metric system, you could simply use the cubic meter.
A hectare is 10000 square meters. A centimeter is 0.01 meters. Thus, if 1 centimeter of rain falls on a 1 hectare field, then that's simply 100 cubic meters of water.
The nice thing about this is that you can easily compare quantities from different contexts. A cubic meter is a cubic meter, whether the water came from a pipe or from the sky.
But what does it mean to have an acre-foot of piped water? Nobody specifies pipe cross-sections in acres. Or a barrel of rain falling on a 1-acre field. How many inches is that equivalent to?
It's not just water. Look at HVAC. In the US, natural gas is billed in therms, and electricity is billed in kilowatt-hours. In metric countries, both electricity and natural gas are specified in kilowatt-hours.
And why not? Energy is energy. If you have a heat pump with gas backup, then you could use gas and electricity interchangeably to heat your house. If you've got a residential cogen system, like they have in Japan, then you could buy either gas or electricity to run your television.
> It's not just water. Look at HVAC. In the US, natural gas is billed in therms, and electricity is billed in kilowatt-hours. In metric countries, both electricity and natural gas are specified in kilowatt-hours.
It's funny you use that example, because kWh are silly, unnecessarily convoluted units themselves :-) the "proper" unit for this dimension is the joule, and 1 kWh = 3.6 MJ (1000 J/s ∙ 1 hr ∙ 3600 s / hr ∙ 1 MJ / 1e6 J).
There actually is, however, some value in having different units for methane vs electricity, since you have to run methane through a suboptimal furnace to harness its energy. Granted, modern gas-powered furnaces are usually more than 90% efficient, but there's still some loss. And since each customer has a different furnace with a different eta, it's usually not useful to directly compare natural gas tariffs vs electricity tariffs even if you do have a cogen unit.
> There actually is, however, some value in having different units for methane vs electricity, since you have to run methane through a suboptimal furnace to harness its energy. Granted, modern gas-powered furnaces are usually more than 90% efficient, but there's still some loss.
Well, heat pumps transfer more than 1 kWh of heat transfer for every kWh of electricity. You have to account for the performance factor anyway, whether you're using natural gas or electricity.
The point is that, once you've divided by COP or AFUE, you get price per effective kWh, which you can then compare directly. As opposed to getting a price per therm, which you must apply a conversion factor to.
With cogen, you get both electricity and heating out of the unit. If it's all in kilowatt-hours, then you simply add them up and compare to the kWh of the incoming natural gas to get efficiency. But how does a therm of electricity translate to volts and amps?
As for the kilowatt-hour, I think the reason we don't use megajoules is that metric time never took hold. If we had metric hours, minutes, and seconds, then the kilowatt-hour would be as awkward as the hectare-centimeter.
Maybe American farmers should trade in their acres of farmland for hectares. There could be a government program to exchange one for the other.
But really -- read the comments and check out the number of times these units are mentioned and converted:
- "200 million acre-feet of precipitation to California in average years"
- "No doubt when water hits $100/barrel..."
- "1 Barrel = 42 Gallons. It costs $0.75/cubic meter (264 Gallons)"
- "There are 326,000 gallons in an acre-foot. So, Most Farms can't pay much more than 2000/326000, = $0.006/Gallon."
- "$0.75/cubic meter = .75 / 264 = $0.002/Gallon."
- "In 2011, the average californian used about 326 gallons/day of water"
If you are going to have a conversation about how urban residents compete with farmers for water, it helps to have a common unit. People don't drink water in acre-feet and farmers don't consume gallons or cubic feet or whatever of water. But if the conversation were in liters or cubic meters, at least you'd have a fighting chance of talking the same units.
Take this from the comments: "For example, Crystal Springs, a 70k cubic meter reservoir in San Mateo, is entirely for urban use. There is no agriculture competition for it. Every gallon we conserve there, is another gallon conserved for city use."
First, it is not 70 000 cubic meters. It's 70 000 000 cubic meters or 70 000 000 000 liters. [1]
What would that look like? Well, we have 7e7 cubic meters to divide up into three dimensions. It would be a lake like 700 meters across by 10 meters deep by 10 000 meters long (10 km).
Those Californians who consume 326 gallons (1200 liters) per day? One of them could use that reservoir in about 60 000 000 days. Or if you had a million of them, they'd take two months.
Four acre-inches would be like [2] one hectare by 10 cm deep -- 10 000 square meters by 0.1 meters, so 1000 cubic meters or 1 000 000 liters. So you can irrigate 70 000 hectares with that much water at that depth per area of land.
I am saying the math is way simpler and you can pretty much do it in your head. And you can visualize it. So: common unit across multiple uses, easily scaled up, easily visualized.
No argument from me on any of that, totally makes sense. And thanks for the correction of 70,000 m^3 -> 70,000 * 1000 m^3 for Crystal Spring Reservoir.
(I'll admit to having to have Soulver up just to have this conversation - Metric system would make it way, way easier.)
So when you hear 70 000 cubic meters, try to visualize it: 1.4m deep by 50m long by 25m (1750 cubic meters) is about an Olympic swimming pool. So we would be talking about a reservoir made up of 40 Olympic swimming pools, and then we'd say, "Hey, that seems pretty small for a reservoir."
Maybe when wikipedia says that the reservoir is 57,910 acre-feet you have a sense of what that would look like, but I have absolutely no intuition on that, even though I know that an acre is 43 560 square feet and that there are 640 acres to a square mile.
I still prefer Metric, but, I have a very good sense of what an acre, 10 acres, and 640 acres are (I have family that farms, and a "section (640 acres)" is a pretty common unit of measurement.
57,910 acre feet would actually be more "intuitive" to me than 70,000 cubic meters (though much harder for me to do calculations and almost impossible to do unit shifting).
57,910 acre feet is roughly 100' deep of water on a section of land. I can "visualize" what that reservoir would would like.
Or, for more practical/relevant to irrigation purposes, it's 100 sections of land, 1' deep.
But yes, I'm still going to argue for the metric system. I promise.