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Comment by grapehut | Hacker News Reader

It depends a lot on where the farm is, how much they irrigate, and when they irrigate. But often something like >90% of the water goes back into the ground, where it will eventually make its way back into the aquifers and rivers.

It's why often farm water usage is not nearly as bad as it superficially seems, and why farm water usage of a river can be multiples of the actual river flow

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> It's why often farm water usage is not nearly as bad as it superficially seems, and why farm water usage of a river can be multiples of the actual river flow

It's bad enough that not only is a huge share of river water used, but that the actual ground is sinking in the Central Valley from depletion of groundwater resources:

https://en.wikipedia.org/wiki/Central_Valley_land_subsidence

> It's why often farm water usage is not nearly as bad as it superficially seems, and why farm water usage of a river can be multiples of the actual river flow

This is beside the point.

The issue is the reservoirs. If the reservoirs and sources are being depleted faster than they replenish then their levels go down.

The water crises are about the reservoirs and sources going down. We don’t care how much farmers use in absolute numbers as long as it’s not overwhelming the capacity of the systems to replenish themselves.

> But often something like >90% of the water goes back into the ground, where it will eventually make its way back into the aquifers and rivers.

That's not the case in the lower Colorado basin. If it was, there would not be a problem.

I'm not familiar with that area, but considering it's not coastal it seems hard to believe the water is simply wasted once it goes into the ground in the lower Colorado basin.

Imperial county is not actually that far from the mouth of the Colorado... Except that, for practical purposes, it does not have a mouth any more, on account of all the water being taken from it.

My guess is that most of what sinks into the ground from agriculture makes its way to a saline aquifer. Some of it may end up in the Salton Sea, which itself is very saline and also shrinking.

Sure, 90% is an overestimate of the portion of water applied for irrigation that returns as recharge to the groundwater system.

However that number is not zero in the Lower Colorado River basin. I work in groundwater modeling for various clients in the desert southwest, the number we assume for agricultural return flow recharge varies based on crop type and other factors. 90% is an overestimate, 0% is an underestimate

What's a ballpark figure for alfalfa in Imperial county?

Clearly not the case in the San Joaquin valley, there is massive subsidence and the Kern river and Tulare lake disappeared (notably exception is this year due to exceptional snowfall).

If that is true, then why are there so many hundreds of HN users talking about crazy water usage by farms?

(In this and the past few hundred posts related to this topic that made it to the front page.)

Because this defense doesn't solve the issue that the water is being used faster than it's being replenished. The biggest issue with this is on wells which are sucking so much water out of the ground it's collapsing the ground itself, in ways that are likely permanently lowering the water capacity of the ground. Even if it doesn't the run-off water isn't going back into those deep aquafers.

https://blogs.agu.org/geospace/2019/03/19/western-droughts-c...

Perhaps it's the same reason that in the Yudkowsky vs Hotz podcast yesterday, Hotz didn't know what a gelding was despite being incredibly smart -- zero farm experience.

I mean, farms come in a lot of varieties. Entirely possible for one to be successful at cranberry farming but know little about many/most/all animals (that don’t attack their crops).

To be fair, geldings aren't really a part of modern farm life. Steers and barrows are still common if you're in livestock, maybe wethers, but horses are more a hobby and only coincidentally associated with farms.

Eventually making it back into aquifers could mean that in 500 years it mostly ends up in an aquifer near somewhere 500 miles down river/wind or something similarly unhelpful to the region in the short term.

That’s a good point, to be useful to plants the water has to get into the ground. From there it’s either evaporating or moving into aquifers.

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