Salt precipitation and evaporative flux on sandy soil with saline groundwater under different evaporation demand conditions

Context Salt precipitation and its influence on evaporation have been widely studied in recent years. However, the evolution process of salt precipitation and evaporative flux is poorly understood under various evaporation demand (ED) rate condition, which is defined as the evaporation rate of distilled water from soil under constant radiation conditions. Aims and methods This study investigated the evolution of salt crust and evaporative flux on sand soil columns with fixed saline groundwater at a depth of 20 cm under four ED conditions (29.5, 21.5, 9.0 and 4.0 mm day(-1)). Key results Evaporation rate significantly decreased in all treatments because the salt crust was elevated and salt domes formed, but the salt patterns of salt precipitation and evaporation exhibited significant differences between different EDs. The homogeneous fine powder crystals precipitated under relatively high ED conditions (29.5 and 21.5 mm day(-1)), and tended to aggregate and form an elevated salt crust in the initial period of salt precipitation. Consequently, it resulted in a sharp decrease in evaporation during the initial period of salt precipitation. In contrast, discrete and large crystals observed under low ED conditions (9.0 and 4.0 mm day(-1)), resulted in a stable evaporation stage during the initial period of salt precipitation. The highest relative evaporation rate was observed under the lowest ED condition when the evaporation rate reached stability, which was attributed to the formation of small and discrete salt domes, indicating that both the upward and lateral growth of salt precipitation were influenced by ED. Conclusions and implications The physics of salt crust formation needs to be considered in understanding how salt precipitates on the soil surface.