[ChuckMcM]

That gives a great picture of the current situation. I've spent many weekends camping up around the reservoirs and it is pretty dry. And further south, the monsoonal rains from the hurricanes are causing floods. I heard an interesting question asking why they can ship oil from North Dakota to Richmond CA to be refined but can't ship water. No doubt when water hits $100/barrel it will become profitable to ship it here by train.

[ghshephard]

1 Barrel = 42 Gallons. It costs $0.75/cubic meter (264 Gallons) to desalinate water based on recent desalination plant technology in Singapore.

If we switch to desalination, it will cost about $0.12/Barrel to generate water. Distribution will, of course, dwarf that cost, but production of water is cheap. We just use ridiculous amounts of it, and it's energy expensive to distribute it if you can't take advantage of gravity.

[seanflyon]

It's probably what you meant by "We just use ridiculous amounts of it" but I expect that most farms could not continue to operate while paying $0.12 per barrel.

[ghshephard]

Most Farms stop being economical north of $2000 per acre foot. (http://www.bloomberg.com/news/2014-07-24/california-water-pr...).

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.

It's within range - but, once again, it's not the cost of generation, but the cost of distribution that gets expensive. The Sierra Nevada has the advantage of all that potential energy from its altitude.

http://www.kcet.org/news/rewire/science/in-talk-of-solar-des... - Saltwater reclamation from the ocean in Carlsbad using Reverse Osmosis will cost about $2000/acre foot.

EDIT: For runoff water from farms, there is a cheaper solution http://www.sfgate.com/science/article/California-drought-Sol...

"His solar desalination plant produces water that costs about a quarter of what more conventionally desalinated water costs: $450 an acre-foot versus $2,000 an acre-foot."

Of course, the elephant in the room is always - what do you do with the components you've removed from the runoff water. They are pretty toxic.

[hughdbrown]

Barrels, gallons, acre-inches, acre-feet. This is why I find the metric system so much preferable: only liters and cubic meters (1000 liters).

[refurb]

Really? I find acre-inches pretty intuitive. Maybe convert to hectare-centimeters?

[tanzam75]

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.

[ghshephard]

If you have an acre of farmland, an acre-foot of water is the most relevant way of measuring required water.

[hughdbrown]

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.

[1] https://en.wikipedia.org/wiki/Crystal_Springs_Reservoir

[2] Not because an acre is the same size as a hectare but because you'd have proportionately fewer hectares than acres.