The amount of water that agriculture and urban cities use is so huge just trying to quantify it in a meaningful way would be nearly impossible if we had to express it in a unit of measurement that we are familiar with.. the gallon.
For instance, if it was reported that the City of Los Angeles water supply had fallen from 203,656,875,000 gallons to 192,252,090,000 gallons that would be a mouthful and pretty difficult for the reader to immediately comprehend in a comparative sense.
So to tame down the numbers a little, water departments express it in acre-feet or the amount of water in gallons that it would take to fill a container that is one acre square and 1 foot deep. 1 acre-foot is equal to 325,851 of gallons so we could now report that LA's water supply had fallen from 625,000 Af to 590,000 Af which is a whole lot easier to say.
Having said that, I do find it quite irritating that water departments bill you for water in hundred cubic feet or HFC instead of gallons. A hundred cubic is equal to 748 gallons.

In Los Angeles, your water departments Tier I residential limit might be 24 HFC. Why not just express it on our water bills as 17,952 gallons? You might even be able to see the reduction of your water use on your monthly bill when you last reduced your watering from 7 minutes to 5 minutes. Yes I know that water meters measure it in cubic feet but they could convert it into something more meaningful to the average homeowner.
Here are a number of ways you might see water measured by:

**Acre-foot (af) - Stored water**
- 1 acre-foot = 325,851 gallons of water.
- 1 acre = 43,560 square feet.
- 1 acre-foot = 43,560 cubic feet.

**Cubic feet per second (cfs) - Moving water**
- 1 cubic foot = 7.4805 gallons.
- 1 cubic foot per second = 7.4805 gallons flowing by a particular point in 1 second.
- 1 cfs = 1.983 acre-feet per day = 646,320 gallons = 2447 cubic meters of water.
- 1 cfs is equivalent to 448.8 gallons of water flowing per minute.
- 1 cfs will produce 724 acre-feet of water per year.
- 1 cfs = 38.4 miner's inches of water.