Tuesday, April 20, 2010

One More Irrigation Efficiency Technical Point - Evaporation

Also in the irrigation efficiency debate, we must remember that evaporation is a consumptive water use (CU).  Normally crop water use (transpiration) and evaporation (from applying water and from leaf surfaces and soil) are lumped together because it is difficult to separately quantify the two values.  Together these values are called evapotranspiration, and again, are a consumptive water use.  However, of any evapotranspiration value used to descibe an irrigation water balance, transpiration is by far the larger portion of that value and can approach or even exceed 90%. 

Since irrigation efficiency improvements reduce non-consumptive water applications AND evaporation, most assume that these upgrades do in fact reduce CU and thus conserve water.  I have been called on this point before, and technically this is true enough.

However, most continue to deny that CU increases under most of these conversions and continue to point to the evaporation reductions.  Our findings show that CU increases do occur, and easily offset the evaporation reductions - resulting in a net increase of water use.  Normally CU will increase in one or more of three ways:  additional acres are irrigated; a higher water consumptive crop(s) is grown on the same acres; or more of the same crop is produced on the same acres - all made possible by the "saved" water resulting from the increased irrigation efficiency.

Now, to be perfectly honest, not every irrigation conversion to a higher efficiency system increases CU.  Some irrigators can't add acres, don't grow different crops, or don't manage to increase yields.  But most do.  So again, we find that the net change of a number of conversions is in fact an increase in CU.  While it's not an obscene increase (kinda subtle, actually) my point is that the amount of money applied to doing this in the name of "saving water" in water short areas is obscene - especially when it actually makes the problem just a tad worse.  The money can almost always be better spent on other solutions to reducing water use.

So wrapping up:  1) irrigation efficiency conversions can in the exact right hydrologic conditions conserve water, but these conditions don't exist very often (see April 19, 2010 blog); 2) they often reduce evaporation (consumptive) water losses, but these savings are small and are most often easily offset by practices that increase CU; and 3) the cost of this approach to "save water" is more often than not very poorly spent money.  Comments?

No comments:

Post a Comment