The maximum upward flux (Eₘₐₓ) is a control condition for the development of groundwater evaporation models, which can be predicted through the Gardner model. A high-precision Eₘₐₓ prediction helps to improve irrigation practice. When using the Gardner model, it has widely been accepted to ignore parameter b (a soil-water constant) for model simplification. However, this may affect the prediction accuracy; therefore, how parameter b affects Eₘₐₓ requires detailed investigation. An indoor one-dimensional soil-column evaporation experiment was conducted to observe Eₘₐₓ in the presence of a water table of depth 50 cm. The study consisted of 13 treatments based on four solutes and three concentrations in groundwater: KCl, NaCl, CaCl₂, and MgCl₂, with concentrations of 5, 30, and 100 g/L (salty groundwater); distilled water was used as a control treatment. Results indicated that for the experimental homogeneous loam, the average Eₘₐₓ for the treatments supplied by salty groundwater was larger than that supplied by distilled water. Furthermore, during the prediction of the Gardner-model-based Eₘₐₓ, ignoring b and including b always led to an overestimate and underestimate, respectively, compared to the observed Eₘₐₓ. However, the maximum upward flux calculated including b (i.e. Ebₘₐₓ) had higher accuracy than that ignoring b for Eₘₐₓ prediction. Moreover, the impact of ignoring b on Eₘₐₓ gradually weakened with increasing b value. This research helps to reveal the groundwater evaporation mechanism.