An Alternative Approach for Estimating the Sodium Adsorption Ratio of Irrigation Water

Irrigation water with high sodicity risk can lead to soil dispersion, poor drainage, and groundwater contamination. The sodium adsorption ratio ($SAR$) of soil water is influenced by water loss through evaporation. Evaporation concentrates sodium (Na) and magnesium (Mg), and its impact on soluble calcium (Ca) and alkalinity is more intricate. This study presented a refined sodicity hazard assessment that quantifies the influence of evaporative water loss and calcite precipitation on drainage water. Specifically, the initial equivalent concentration of alkalinity and Ca predominantly determines the potential sodicity risk of drainage water. The alternative approach projects two pathways for potential drainage water $SAR$, which include limiting the sodium adsorption ratio ($LSAR$) to represent the upper limit boundary and evaporated-based sodium adsorption ratio ($ESAR$) to represent the lower limit boundary. When irrigation water alkalinity exceeds the soluble Ca concentration, soluble Ca in the drainage water is limited as calcite precipitates and the drainage water is dominated by Na and Mg. The $SAR$ approaches an upper limit ($LSAR$) determined by the initial relative concentration of Na and Mg. Conversely, if irrigation water alkalinity is less than the soluble Ca concentration, minimal calcite precipitation occurs, and drainage water is dominated by Na, Mg, and Ca. Then, the $SAR$ approaches a lower limit ($ESAR$) determined by the initial Ca, Mg, and Na concentrations. To validate the accuracy of this new sodicity risk assessment method, this paper analyzed data extracted from previously published lysimeter studies. Water composition boundaries for each source water were plotted, and these boundaries were compared to the recorded drainage water composition in the lysimeter studies. As salinity increased through evaporation, the drainage water followed a distinct salinization path but remained within the $LSAR$ and $ESAR$ boundaries. This information is essential for irrigation managers to quickly assess water sodicity levels and make timely management decisions.

The application of irrigation water with high sodicity risk can lead to soil dispersion. Therefore, an accurate and straightforward approach to assess the sodicity level of the source water is crucial for irrigation management. The practical implementation of the proposed approach for evaluating the sodium (Na) hazard of an irrigation water source begins with source water analysis, considering soluble Na, calcium (Ca), magnesium (Mg), alkalinity, and electrical conductivity ($EC$). The irrigation water source is then categorized based on its Ca to alkalinity ratio: an alkalinity-rich water source is where the ratio of ${\text{Alk}}_{dw}^{-}/{\mathrm{Ca}}_{dw}^{2+}\ge 1$, and an alkalinity-poor water source is where ${\mathrm{Alk}}_{dw}^{-}/{\mathrm{Ca}}_{dw}^{2+}<1$. As soil water salinity or $EC$ increases through evaporation, the alkalinity-rich water is likely to approach the upper boundary, which is the limiting sodium adsorption ratio $(LSAR)=\sqrt{F_c}·{\mathrm{Na}}^{+}/\sqrt{{\mathrm{Mg}}^{2+}/2}$, and $F_c=E{C}_{dw}/E{C}_{iw}=1/LF$, where $E{C}_{dw}$ represents the $EC$ of irrigation water, $E{C}_{iw}$ represents the $EC$ of drainage water, and the leaching fraction ($LF$) represents the leaching fraction. On the other hand, alkalinity-poor water is likely to remain close to the lower boundary, which is the evaporation-adjusted sodium adsorption ratio $(ESAR)=\sqrt{F_c}\times {\mathrm{Na}}^{+}/\sqrt{({\mathrm{Mg}}^{2+}+{\mathrm{Ca}}^{2+})/2}$. This information can help managers determine the suitability of a water source or make sodicity management decisions if poor quality irrigation water is used.