ARS | Publication request: A short note on calculating the adjusted SAR index

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2009
Publication Date: March 1, 2009
Repository URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2291.pdf
Citation: Lesch, S.M., Suarez, D.L. 2009. A short note on calculating the adjusted SAR index. Transactions of the ASABE. 52(2):493-496.

Interpretive Summary: The sodium adsorption ratio (SAR) is commonly used as an index for evaluating the sodium hazard associated with an irrigation water supply. Irrigation waters having high SAR levels can lead to the build-up of high soil Na levels over time, which in turn can adversely effect soil infiltration and percolation rates (due to soil dispersion). The standard SAR formula represents a suitable sodium hazard index for typical irrigation waters, but and adjusted SAR index is required for irrigation waters having appreciable concentrations of calcium (Ca) and/or bicarbonates (HCO3). The formula derived by Suarez (1981) is generally recognized as the most applicable technique for determining the adjusted SAR hazard index, but an exact calculation requires the use of numerical software and accurate approximations can only be obtained using the numeric look-up table (Table 1) in Suarez (1981). In this note we describe and demonstrate a computationally simple technique for calculating the adjusted SAR formula of Suarez (1981), without the use of either a look-up table, custom computer software or the need to compute complex activity coefficients. This simple technique is amenable to Excel spreadsheet calculations and thus can be conveniently used by extension specialists and practitioners. This technique should facilitate a wider adoption of the adjusted SAR index, particularly by the commercial laboratories used by such extension personnel. In turn, this should lead to the more accurate assessment of both domestic and international irrigation water supplies.

Technical Abstract: A simple algebraic technique is presented for computing the adjusted SAR Index proposed by Suarez (1981). The statistical formula presented in this note facilitates the computation of the adjusted SAR without the use of either a look-up table, custom computer software or the need to compute exact activity coefficients via the use of the extended Debye-Huckel equation. A short data analysis example is also presented that demonstrates the application of this technique.