Laserfiche WebLink
<br />~ <br /> <br />1 food on irrigated land, is even more threatened by encroaching water- <br /> <br />2 logging and the resulting soil salinity. High water tabies contribute <br /> <br />3 to soil salination by permitting capillary movement of water from them <br /> <br />4 to the soil surface. As the water evaporates, salt contained in this <br /> <br />~. <br />5 water is left on the soil surface. The magnitude ofWthe problem of poor <br /> <br />6 water management is underscored by a recent survey by the International <br /> <br />7 Commission on Irrigation and Drainage (1), which showed that only 25% of <br /> <br />8 the water diverted into continuous flow canal systems was beneficially <br /> <br />9 used by crops. Pipeline systems increased this efficiency to 50%. <br /> <br />10 In addition to fanners not-having cont;roJ.over irrigation timing, <br /> <br />11 another reason for this poor record of water use is that most farms are <br /> <br />12 irrigated with surface distribution systems. Surface irrigation imposes <br /> <br />13 two fundamental constraint9 on irrigation management: (i) it depends on <br /> <br />14 flow over the soil surface to distribute water from one end of the field <br /> <br />15 to the other, which requires a minimum depth of water simply to achieve <br /> <br />16 coverage; and (ii) a fixed labor cost is associated ,nth each applica- <br /> <br />17 tion (3). Both of these constraints tend to make it advantageous to <br /> <br />18 decrease the number of irrigations required in a season. This is done <br /> <br />19 by storing as much water 'as possible during each irrigation and then <br /> <br />20 using as much of this stored water as pos$ible before the next. Irri- <br /> <br />21 gating just to fill the available soil storage capacity, and no more, <br /> <br />22 requires a knowledge of soil properties and conditions that practically <br /> <br />23 no fanner has. Add to this the fact that both water storage capacity <br /> <br />24 and infiltrability (the rate at which the soil absorbs water whea sup- <br /> <br />I <br />25 plied freely to its surface) can vary widely from place to place in a <br /> <br />26 single field, and it is obvious that surface <br /> <br />irrigatio.n i~ J><;one to <br />UC,dJ17 <br />field before others have <br /> <br />27 su?ply far too much ':Jate-c to some areas of a <br />