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Spatial Variability of Soil Salinity in Furrow-irrigated Torrifluvents¹

ABSTRACT

Distribution characteristics of salinity and sodicity in furrow-irrigated fields have not been adequately evaluated. This study examined the pattern of salt accumulation within crop beds and its spatial variation along or across crop rows, mainly for developing sampling guides for routine salinity appraisal. Soil samples were collected at spacings ranging from 1.5 to 6 m in four furrow-irrigated fields of the middle Rio Grande Basin and were analyzed for salinity of the saturation extract and in selected cases for the sodium adsorption ratio (SAR). Salinity varied two to sixfold within the row lengths of 110 to 190 m, and its variation generally increased with increasing sampling depth and numbers of irrigation. The analyses of autocorrelation showed that salinity readings were spatially dependent at one of the four tested fields, and the distance of dependence reached 46 m. This field showed a gradual change in soil texture (from silty clay loam to silty clay), and provided a semivariogram conforming to a generalized linear model. When samples were collected from an area consisting of a single soil type or when salinity values were stratified by soil type, the distance of dependence decreased to <15 m. The frequency distribution of both salinity and sodicity within the same soil type followed a normal distribution with the standard error of estimates of mostly <8%. The sampling sites required to obtain a mean value within 15% of the true mean ranged from 3 to 11 per test row when sampled at 0- to 0.20-m depth prior to preplant irrigation, and the required number increased with increasing sampling depth and numbers of irrigation. For routine appraisal of salinity in the fields similar to examined here, soil samples can be collected on the basis of soil type, and salinity data can be analyzed by conventional statistics with a reasonable assurance that salinity values are spatially independent.

¹ Contribution from the Texas Agric. Exp. Stn., Texas A&M Univ. System. This project was supported in part by the Expanded Research Area (ERA) fund of the Texas Agric. Exp. Stn., and the Binational Agricultural Research and Development (BARD) fund.

² Associate Professor and Assistant Programmer, respectively. Texas A&M Univ. Agricultural Research Center, 1380 A&M Circle, El Paso, TX 79927.

Received for publication November 24, 1986.