| Summary: | Ionic strength (<i>I</i>; mol·L<sup>−1</sup>) acts as one of the most important parameters of natural waters. It is indispensable for obtaining ion activities and thus is crucial for describing chemical processes in water solutions. Limnology, <i>I</i>, has many applications, but calculating the partial pressure of CO<sub>2</sub> (<i>p</i>CO<sub>2</sub>) and the carbonate saturation index (SI) are among the most important examples. The determination of <i>I</i> requires the full ion composition of water to be recognized, and when the concentration of some major ion(s) is/are missing altogether, the <i>I</i> value remains unknown. Because historical and monitoring data are often incomplete, it seems useful to provide a method for the indirect assessment of <i>I</i>. In this paper, we developed and tested an empirical model to estimate <i>I</i> on the basis of electric conductivity at 25 °C (<i>EC</i>). Our model consists of two linear equations: (i) <i>I<sub>mod</sub></i> = 15.231 × 10<sup>−6</sup>·<i>EC</i> − 79.191 × 10<sup>−6</sup> and (ii) <i>I<sub>mod</sub></i> = 10.647 × 10<sup>−6</sup>·<i>EC</i> + 26.373 × 10<sup>−4</sup> for <i>EC</i> < 592.6 μS·cm<sup>−1</sup> and for <i>EC</i> > 592.6 μS·cm<sup>−1</sup>, respectively. We showed that model performance was better than the hitherto used <i>EC</i>–<i>I</i> relationships. We also demonstrated that the model provided an effective tool for limnological monitoring with special emphasis on the assessment of CO<sub>2</sub> emissions from lakes.
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