RESEARCH ARTICLE Evaluation and analysis of different regression models for estimation of ECe from EC 1:5 —With a case study from Buin- Zahra, Iran Mehrdad Hassannia | Bijan Nazari | Abbas Kaviani | Abbas Sotoodehnia Department of Water Sciences and Engineering, Imam Khomeini International University, Qazvin, Iran Correspondence Bijan Nazari, Department of Water Sciences and Engineering, Imam Khomeini International University, Qazvin, Iran. Email: b.nazari@eng.ikiu.ac.ir Abstract Soil salinity is an important parameter in irrigation, drainage, and environ- mental studies. Determining the electrical conductivity of soil saturated paste extract (ECe) is a well-known method, but its use is limited because its produc- tion process is time-consuming and difficult. The 1:5 solution electrical con- ductivity (EC 1:5 ) is an alternative simplified method. The aim of this study was to evaluate ECe-from-EC 1:5 conversion models. A total of 123 samples from 3 soil layers were analysed. The research was planned in two phases: 1) model evaluation in soil layers, 2) model evaluation in ECe categorized data. Results show that the linear model was the best model in the second layer (R 2 = .66, mean absolute percentage error [MAPE] = 0.058), and the exponential model was the best in the other layers (R 2 = .65 to 0.67, MAPE = 0.085 to 0.060). Also, exponential and linear regressions were the best ECe estimation models in the ECe < 4 and ECe > 4 categories, respectively. In addition, the conversion fac- tor (f ) for converting ECe and EC 1:5 was obtained as 3.7–13.9, 2.8–5.0, and 3.4–5.8 for three soil layers, respectively. Results show that the use of general recommendation tables for f will lead to considerable error, and the factor must be determined specifically for each region. KEYWORDS irrigation, salinity management, salinity test, sustainable salinity RĖSUMĖ La salinité du sol est un paramètre important dans les études sur l'irrigation, le drainage et l'environnement. La détermination de la conductivité électrique de l'extrait de pâte de sol saturé (ECe) est une méthode bien connue, mais son utilisation est limitée en raison de difficultés et de temps. La conductivité élec- trique en solution 1:5 (EC 1:5 ) est une méthode simplifiée alternative. Le but de cette étude était d'évaluer l'ECe à partir de modèles de conversion EC 1:5 . Un total de 123 échantillons provenant de 3 couches de sol ont été analysés. La recherche a été planifiée en deux phases: 1) évaluation des modèles dans les couches de sol; 2) modèles d'évaluation dans les données catégorisées ECe. Les résultats montrent que le modèle linéaire était le meilleur modèle dans la pre- mière couche (R 2 = 0,66, MAPE = 0,0583) et le modèle exponentiel était le Received: 18 June 2019 Revised: 25 May 2020 Accepted: 25 May 2020 DOI: 10.1002/ird.2488 Irrig. and Drain.. 2020;1–12. wileyonlinelibrary.com/journal/ird © 2020 John Wiley & Sons, Ltd. 1