18 May 2010

What's your evapotranspiration?

JH asks:
Shouldn't sustainable water management include charging the water users for exactly the water they consume, i.e., evapotranspirate?

We can find this with remote sensing/satellite imagery now.

It is impossible to know exactely what every user extracts from aquifers, it is also impossible to know well how much water the irrigator really uses as with the older techniques a lot of water comes available for downstream users and is not lost...

So maybe we should charge users only for what they consume (evapotranspirate)?
I replied with
That's what they do now [in some places], charge for "consumptive use" -- net water loss.

OTOH, EV water is less than that, since some "consumption" goes to g/w (the environment, other users)...

Although it may make sense to charge for EV only (measurement issues aside), it may encourage people to use wasteful irrigation methods, methods that may NOT allow non-EV "waste" to be used elsewhere...
To which JH replied:
They use some average relation EV / returnflow but of course this depends on soil moisture, season, crop, etc., so actually (remotely) measuring it would be better I think.*

Percolation to g/w could be pumped up by water users downstream which would be charged as well exactely for their consumption = EV. Natural areas (environment) could also consume from this aquifer, depending on catchment characteristics. Question of good water accounting, isn't it?

Anyway, maybe we'll better continue disccusing in the comment section of the post on this issue?
Indeed. So what do you guys think?

Should irrigators be charged for ET, as the BEST method? Please tell us the costs and benefits.

* JH continues: As far as I know, the only place where these things are pilotted so far is in the Hai Basin in China [pdf]. Evapotranspiration quotas, monitored by remote sensing. That's an interesting future. Of course you can think on pricing and trading schemes behind this, but the main problem in agricultural water management is right now that no proper monitoring of water consumption can be done by administration or private owner, it's too easy to cover up groundwater abstractions, no way to control that. If monitoring is ok, quotas can be traded without any problem and a healthy 'water economy' might be viable.

1 comment:

  1. This is interesting in that it appears to offer a two-fer for water conservation AND salinity management. ET (and EV) have been found to be the largest drivers for the increasing salinity in the waters of the central valley (especially groundwater). This is especially problematic where because of very efficient beneficial use (ie, extensive ET with no run-off) there is no "wasted" water left to transport the concentrated salts out of the valley....as nature used to do.

    This sort of pricing targets the problem and allows more of the mitigation to occur. Of course if the freed up water isn't used to manage the salt buildup (ie, to facilitate new consumptive uses within, and even worse, via export, outside the basin)then it just intensifies the in basin salinity problem in the end.

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