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<br />"l" <br /> <br />CUOl 64 <br /> <br />~ ,. ". <br /> <br />. -~' . <br /> <br />. <br /> <br />Soil intake rate. The water intake rate of the soil influences, to <br /> <br /> <br />a great extent, water losses from the field due to both runoff and deep <br /> <br /> <br />percolation. If the intake rate is excessively low, it is difficult to <br /> <br />irrigate without "tail water" running off the lower end of the field. <br /> <br /> <br />The time that water must cover the soil in order to apply the required <br /> <br /> <br />amount to recharge the root zone is increased by low intake rates, 'and, <br /> <br /> <br />unless very special precautions are taken, this cannot be accomplished <br /> <br /> <br />without runoff using surface irrigation methods. If the intake rate <br /> <br /> <br />is excessively fast, it is difficult to uniformly recharge the soil <br /> <br /> <br />moisture from one end of the field to the other without excessive appli- <br /> <br /> <br />cation, and resultant leaching, at the upper end of the field. The use <br /> <br /> <br />of sprinkler irrigation is usually the only procedure for preventing <br /> <br /> <br />low field irrigation efficiencies when intake rates are very high. Intake <br /> <br /> <br />rates are influenced to a great extent by surface soil texture but also <br /> <br /> <br />by other factors influencing soil structure, compaction, aggregate disper- <br /> <br /> <br />sion and soil cracking. Management practices can be used to control, <br /> <br /> <br />within limits, the intake rate of soils, but it is impossible to prevent <br /> <br /> <br />some change in this property throughout the irrigation season as well as <br /> <br /> <br />between years when different crops are grown and different tillage prac- <br /> <br /> <br />tices are required. <br /> <br /> <br />Surface contour and slope <br /> <br /> <br />Deep percolation and field runoff of irrigation water are both influ- <br /> <br /> <br />enced by land topography. If the land is nearly level, as may be the <br /> <br />case on alluvial soils near the river, it is difficult, with the small <br /> <br /> <br />water-flow rates usually available in the lower South Platte Valley, to <br /> <br /> <br />apply sufficiently large streams in furrows or border strips to cover the <br /> <br /> <br />"set" before excessive penetration takes place at the head ditch end of <br /> <br />the field. On the other hand, if the slope is steep, runoff at the lower <br /> <br /> <br />end is essentially impossible to avoid before adequate water has been <br /> <br /> <br />applied. Non-uniform slopes and land with high spots or swales make <br /> <br /> <br />attainment of high field efficiencies difficult unless sprinkler or trickle <br /> <br /> <br />irrigation is used or the land is reformed for efficient surface irrigation. <br /> <br /> <br />This latter approach can be quite beneficial. <br /> <br />'I <br /> <br />. <br /> <br />. <br /> <br />-7- <br />