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Sampling the Unsaturated Zone of Irrigated Lands for Reliable Estimates of Nitrate Concentrations¹

ABSTRACT

Data from a study of 56 field sites were used to examine the variability of nitrate-nitrogen concentrations in the unsaturated zone underneath the root zone. The specific data were nitrate-nitrogen concentrations in soil water in samples taken at 0.9-m intervals from the 4.5- to 11.9-m depth from four holes per site. Analyses of variance of these data provided estimates of variances that were used to predict the depth sampling interval and the number of holes required for site means to be within confidence limits of 10, 20, and 30%. Calculations were based on sampling intervals of 0.9, 0.6, and 0.3 m providing 9, 13, and 26 samples per hole, respectively. Relationships developed between number of sites, number of holes required, sampling interval, and confidence limits were used to test selected sampling plans for degree of precision in comparison with the actual sampling procedure. A cost analysis of the field study indicated that it would cost three to four times more than the field study to use a sampling plan which would reasonably ensure that 75% of the sites' true means fall within 20% of the measured means at a confidence level of 95%.

¹ Contribution from the Dep. of Soil Science and Agric. Eng., Univ. of California, Riverside, CA 92502. Financial support of the Research Applied to National Needs division of the National Science Foundation through grant GI-34733X1 is hereby gratefully acknowledged.

² Extension Area Soil and Water Specialist, Statistician, Professor of Soil Science and Assistant Professor of Soil Science, respectively, of the University of California, Riverside.

Received for publication July 21, 1975. Accepted for publication March 31, 1976.