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<br />J.D. RHOADES ET AL.
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<br />tices. Most irrigation projects have a developed supply ofrelatively low salinity
<br />water. Typically, forty percent of it is discharged as drainage water (Van
<br />Schilfgaarde, 1974). This drainage water often moves to the underlying shal-
<br />low groundwater. A proposed management strategy which applies to such proj-
<br />ects, and which meets the above-mentioned farmer requirements, is to substi-
<br />tute saline water (drainage or shallow groundwater) for the non-saline water
<br />when irrigating salt-tolerant crops grown in the rotation when they are in a
<br />non-sensitive growth stage; and to use the non-saline water at the other times. .
<br />The timing and amount of substitution will vary with the quality of the two
<br />waters, the cropping pattern, the climate, and the irrigation system. Obviously,
<br />the maximum soil salinity in the rootzone that can result from continuous use
<br />of saline water will not occur when such water is used for only a fraction of the
<br />time. Simple calculations will show that a soil will not generally become unduly
<br />saline from use of a saline water for a part of a single irrigation season and
<br />often not for several seasons. Whatever salt buildup occurs in the rootzone
<br />from irrigating with the saline water is alleviated in the subsequent cropping
<br />period, when a more sensitive crop is grown using the low-salinity water for
<br />irrigation. Furthermore, the yield of the sensitive crop should not be reduced,
<br />if proper preplant irrigations and careful management are used during germi-
<br />nation and seedling establishment to leach salts out of the shallow soil depths.
<br />Subsequent irrigations given after the establishment of stand will leach these
<br />salts farther down in the soil profile ahead of the advancing root system and
<br />"reclaim" the soil in preparation for use of the saline water again to grow a
<br />suitably tolerant crop. This cyclic use of "low" and "high" salinity waters pre-
<br />vents the soil from becoming excessively saline while permitting, over the long
<br />period, substitution of saline water for a low-salinity water for a large fraction
<br />(up to about 50% ) of the crop waterrequirements (Rhoades, 1983, 1984, 1985).
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<br />FIELD TEST OF SALINE DRAINAGE WATER FOR IRRIGATION
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<br />The crop/water management strategy described above was recently tested
<br />for four years under actual commercial-farming conditions in a 20 ha field
<br />located in the Imperial Valley of California. Two cropping rotations were used.
<br />One was a two-year rotation of wheat, sugarbeets, and cantaloupe melons, which
<br />was repeated for a second cycle. This rotation will be referred to as the "suc-
<br />cessive crop" rotation. In this rotation, Colorado River water (- 900 mg/L
<br />TDS) was used for the preplant and early irrigations of wheat and sugarbeets
<br />and for all irrigations of the melons. The remaining irrigations were with
<br />drainage water (- 4.0 dS/m EC). The compositions of these two waters are
<br />given in Table 1. The other rotation consisted of two crops of cotton followed
<br />by one crop of wheat and then by almost two years of continuous alfalfa. This
<br />rotation is referred to as the "block" rotation. In this rotation, drainage water
<br />was used for the irrigation of both cotton crops after seedling establishment,
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