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<br />AS 1I0UGAIOHS, \\t:'rt' Le-
<br />ginning to talk more and more in
<br />terms of "irrigation efficiency".
<br />If we livc and farm in an area of
<br />ob,'iously diminishing w:Her sup-
<br />ply, our chief persuasion for
<br />thinking dficit'/lcy might he con.
<br />scn'ation. But lihh' as not, let's
<br />face it, most of us art. motivated
<br />into efficiency hy our desire for
<br />incrcasnl crop yields and/or
<br />pared down production custs,
<br />Whatt'\Tr our persuasion, the
<br />attainmcnt of irrigation dfi-
<br />ciene)' requires com.:crncd calcu-
<br />lation. We must know I) when to
<br />irrigatc, and 2) how much water
<br />to apply. And, to bc any \....hcrc
<br />near field-scale practical, we
<br />must be able lO balance when to
<br />irrigate and how much water to
<br />apply with the li'llitations of our
<br />managerial capabilities.
<br />What is an efficient irrigation,
<br />anyway? Well, we might envision
<br />it as a 100 percent agreement on
<br />amount of watcr' applied with
<br />amount of water depicted from
<br />the soil at watering time, without
<br />losses. Under field-scale applica-
<br />tion, a 100 percent efficient irri-
<br />gation seems next [0 impossible
<br />lO attain. But we can approach
<br />that perfection,
<br />
<br />Rather erroneously. some of
<br />us have let ourselves become
<br />conditioned to judging our wa.
<br />tering efficiency in terms of sur-
<br />face runoff alonc. Tail water is
<br />one of the contribulOrs to "less
<br />than 100 perccnt irrigation effi-
<br />ciency:' but there arc other
<br />culprits roo. And they're signif.
<br />icant. "Surface runoff is the onlr
<br />visible loss and therefore oftc~
<br />receivcs the most attention. Data
<br />collected in the 'Use of Water'
<br />study indicate, howen:r, th:ll the
<br />combin:nions of I) coll\'eyance
<br />and distribution sntem 105ses
<br />(mainl)" seepage). plus 2) on-the.
<br />farm deep percolation, comprise
<br />the major losses - often amount-
<br />ing [0 twice the surface runoff,"
<br />state Langley and Robb.
<br />They stress that the major loss
<br />to deep percolation occurs when
<br />all parts of a field recei\"C' more
<br />water than the soil root lone can
<br />hold, And, too, non.uniformity
<br />
<br />8
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<br />II we irrigate when 70 percent of the soil moisture is depleted. we need to re-
<br />place 3.5 inches of moisture, i,e.. moisture.holding capacity is 5 inches; 70 p(:r.
<br />cent of the moisture depleted; 70 percent of 5 inches'" 3.5 inches of moisture
<br />we need to replace. Here is what happens if we deliver 6 inches of water to this
<br />situation:
<br />
<br />(3,5 inches depleted soil moisture)
<br />Delivery
<br />Distribution losses
<br />Surface runoff
<br />Deep percolation
<br />Soil moisture added
<br />
<br />Pert;m1t
<br />delivery
<br />100
<br />
<br />Inches
<br />6,0
<br />0.3
<br />1.2
<br />1.0
<br />3,5
<br />
<br />5
<br />
<br />20
<br />17
<br />53
<br />
<br />Wilh all other losses accounted for - distribution and surface runoff _ we've
<br />lost 1.0 inch of our application (or 17 percent of our total delivery) to deep
<br />percolation. In the final tally. we find thaI the 3.5 inches we do get into the soil
<br />represents 58 peru'nt of the total water applied. Thus we say our irrigation
<br />efficipncy is 58 percent.
<br />1"':'",<." ,I . "'::.: ".. n: -, - " :-=:J
<br />Now let's use the very same soil type and set of field circumstances but let our
<br />soil moisture deplete to 30 percent (instead of 70 percentl before we water_ So,
<br />moisture.holding capacity is 5 inches; 30 percent depIcted; 30 percenl of 5
<br />inches'" 1.5 inches of moisture we need to replace. In the final tally. we've lost
<br />3.0 inches of our application (or a whopping 50 percent of our deli...ery) to deep
<br />percolation. We have a resulting 25 percent efficiency.
<br />
<br />(1.5 inches depleted soil moisture)
<br />Delivery
<br />Distribution Ios~s
<br />, Surface runoff
<br />Deep percolation
<br />tt;itttsture added
<br />
<br />Percent
<br />ddivcry
<br />103
<br />5
<br />>0
<br />50
<br />25
<br />
<br />Inches
<br />60
<br />0_3
<br />1.2
<br />3,0
<br />1.5
<br />
<br />Irrigation fv;Je
<br />
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