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Ground-Water Quality Related to Irrigation with Imported Surface or Local Ground Water¹

ABSTRACT

Ground-water quality in an arid irrigated area that imports high quality surface water was compared with an adjacent up-gradient area that uses local pumped ground water. A large irrigation canal separates the two areas which are up-gradient from the pumping depression of the Fresno-Clovis, California metropolitan area, which is dependent on ground water. Intensive sampling (154 wells) was done in the fall of 1972 after it was shown by previous sampling that the ground-water electrical conductivity (EC), NO₃^–, and Cl^– content had not significantly changed since 1967. Diagrammatic distribution maps for ground-water quality showed great variability. The Wilcoxon-Mann-Whitney nonparametric statistical tests showed the significance probabilities for EC, NO₃^–, and Cl^– differences above and below the canal were 0.093, 0.114, and < 0.00006, respectively. The significance of these differences are determined by the reader or decision-maker by his chosen error probability, the error risk he is willing to take in rejecting the null hypothesis of no difference when it is true. Areas of higher ground-water NO₃^– and Cl^– were generally related to soil drainage-recharge and agricultural use. The use of local ground water without supplemental surface supplies above the canal has led to ground-water EC, NO₃^–, and Cl^– concentrations to be 9.5, 18.6, and 91.8% higher, respectively, than below the canal which uses mostly high quality surface water.

Key Words: ground-water NO₃^–, Cl^–, and salinity • water reuse • data evaluation philosophy • irrigation practices

¹ Contribution from Water Management Research, Western Region, Agr. Res. Ser., USDA.

² Soil Scientists, Water Management Research, Agr. Res. Ser., USDA, Fresno, Calif. 93726.

Received for publication November 5, 1973.