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Comment by alwaysdoit | Hacker News Reader

> All told, it takes about 3460 kilowatts per acre-foot to pump water from Northern California to San Diego; Carlsbad will use about thirty per cent more energy, five thousand kilowatts per acre-foot, to desalinate ocean water and deliver it to households, according to Poseidon’s report to the Department of Water Resources

These units are abominations. Couldn't just say 2.8 Watts per liter vs 4.0 Watts per liter? Or even 10.6 and 15.3 Watts per gallon? I'm not a metric purist, but the only advantage to using imperial units is that they are more familiar to the average American, but when does the average person deal with acre-feet?

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Acre feet is how water is sold and talked about in agriculture.

They're especially useful in agriculture because rainfall, and crop water consumption, are typically measured in inches of water. (Which is really a kind of "cubic inches (of water) per square inch (of land)", but the units cancel out.) So if you know the net water requirements of some crop given the climate, and you know how many acres of land you intend to grow it on, acre-feet are a natural, easy-to-calculate unit to measure the water supply needed.

You also need to know that 1 foot (30.48 cm) is 12 inces (2.54 cm).

Which everyone reading the New Yorker surely does.

Which everyone in the US knows. Even very uneducated people.

I am from Europe, educated (PhD, if it matters), and I had to look it up. That's why I left the comment, to help others.

No offense, but it's an article in the New Yorker about an issue in California. Your comment makes about as much sense as if you had said "yeah but you need to know that California is a state in the US". Europeans might not know it, but the target audience of the article certainly does. It's not exactly an extra bit of knowledge you need to learn, it's something you already know.

Which is fine, but presumably the majority of the New Yorker's audience is not in agriculture.

Nor am I, but I know about acre feet from the thousands of articles that have been written about various droughts and agriculture water usage. It's also pretty easy to deduce on your own.

It doesn't even make sense! The units don't match up. Kilowatts are a measure of power, energy over time, and an acre-foot is a measure of volume, not flow (volume over time).

It's like saying how many horsepower it takes to drive from SF to LA. Makes no sense.

Yes, they must have meant kilowatt hours. Was there no editor?

Scientific mistakes in a paper like the new yorker is like poor grammar in a startup CEO's writing.

It just happens.

Huh. So they are really talking about a pressure.

1 kWh/(acre ft) = 2919 Pa

It makes a great deal of intuitive sense that the quantity that describes how hard it is to move water over a path is a pressure.

It's sad that they mistook kilowatts for kilowatt-hours, but I think acre-foot is a good unit for measuring volumes of water. It helps communicate the scale of water use.

Depends, though I suspect you're right.

But expressing energy requirements as power per unit can be quite useful. As in, kW per person or square meter. Though in that case you'd be expressing water delivery as a rate: acrefoot/day, say.

>when does the average person deal with acre-feet?

An acre-foot of water is approximately the amount of water that a suburban household uses in one year, which is why that unit is used when talking about non-agricultural water usage.

> but when does the average person deal with acre-feet

When villifying almond farms for their water usage

I'm probably just ignorant, but I don't know too much about power (Is my light bulb usually around 80 W-h? 200W? 100kWh? I have no idea.) I think you're right, but I do also think the main point was the relative expenditure, a 30% increase.

Incandescent bulbs are usually 60W but can be 40W for a reading lamp or 100W to light up a whole room. Those haven't been sold in most countries for a while now due to their extreme inefficiency, but other bulb types are often compared to a similar incandescent bulb. For example a 14W fluorescent bulb might say "60W equivalent".

Watt-hours really are as simple as they sound. A 14W bulb that's on for an hour used 14 Watt-hours.

Its also silly, since it takes zero kilowatts to pump water from San Diego to San Diego and if you're desalinating you have to realize that California is all coast line on the left hand side.

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