29 Jan 2009

How Much Water Do Farmers Use?

In my post yesterday, I questioned the meaning of "use" when farmers do not receive all of the benefits of the water they divert.

Today, I want to question the accounting for water diversions.

Say that a farmer applies 10AF of water to his fields. If 2AF are consumed and offgassed by the plants, 3AF run off the property and 5AF sink into the soil, recharging groundwater, then how much water did the farmer use?

Put differently, if farmers "use" 80 percent of developed water supplies, how much double multiple counting is involved? If other farmers apply the 3AF of tailwater and still others pump the 5AF onto their lands, does that mean that farmers as a group are using 20AF or 10F?

I genuinely do not know the answer to this question. If the total is 20AF, then we need to think again about the meaning of "use". If the total is 10AF, then we need to be careful when discussing water rights. I'll let Ray's comment from yesterday illustrate:
Consumptive Use (CU) for your carrot patch is the amount consumed by growing the carrot. It will take a great deal more water to grow the carrot with flood irrigation methods, but much of the water will help recharge the groundwater. Sprikler irrigation of the crop is "more efficient", but the evaporation losses can be high and there is little if any recharge.

In dry regions, one man's use is the next man's water right. With flood irrigation, the secondary use would be from groundwater.

[snip]

Converting from flood irrigation to sprinkler &/or drip irrigation (best management practices) is why groundwater aquifers are eventually depleted...too many straws and not nearly enough historic recharge.
Water accounting is a topic dear to my heart (one reason I want California to adopt comprehensive groundwater monitoring), and I came to the topic by accident. As I discuss in Section 6.3.4 of my dissertation, the Metropolitan Water District (importer of Delta and Colorado river water to SoCal) "forgot" to record its groundwater recharge deliveries. Why does this matter? Because it makes it look like local water supplies (which now include the recharged groundwater) are bigger than they really are, which makes it harder for activists to claim that imported water is encouraging sprawl.

Well, here's the news -- imported water (and the way it's priced) does encourage sprawl in SoCal.

Anyway -- back to farmers and agricultural water. Make the accounting clear so that everyone can track individual water diversions but also net out "reused" water when accounting for total sector use.

Bottom Line: If we mess up the accounting, we'll mess up other things.