Simplification of the Gardner model: effects on maximum upward flux in the presence of a shallow water table

The maximum upward flux (E (max)) is a control condition for the development of groundwater evaporation models, which can be predicted through the Gardner model. A high-precision E (max) 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 E (max) requires detailed investigation. An indoor one-dimensional soil-column evaporation experiment was conducted to observe E (max) 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 E (max) for the treatments supplied by salty groundwater was larger than that supplied by distilled water. Furthermore, during the prediction of the Gardner-model-based E (max), ignoring b and including b always led to an overestimate and underestimate, respectively, compared to the observed E (max). However, the maximum upward flux calculated including b (i.e. E (bmax)) had higher accuracy than that ignoring b for E (max) prediction. Moreover, the impact of ignoring b on E (max) gradually weakened with increasing b value. This research helps to reveal the groundwater evaporation mechanism.