On the evolution of salt lakes: Episodic convection beneath an evaporating salt lake

The transient evolution of salt lake brines and the accompanying underlying groundwater salinity distribution with depth is discussed. A convective‐diffusive equation with a novel surface source term modeling the evaporative process is introduced, and an approximate analytical solution of this equation is obtained using a perturbation approach. A linear stability analysis of the resulting solution is carried out. For typical field‐scale parameters, the model predicts that instability may initially occur within timescales on the order of weeks or decades and that a near‐surface layer of salty fluid whose thickness is on the order of decimeters is responsible for the initial onset of convection. Furthermore, the analysis indicates that when convection starts, the decimeter layer mixes, and this results in the loss of the destabilizing force. In this period, the salty fluid diffuses downward. This results in a return to a hydrologic system, where the salt profile is then well mixed throughout, but now with a background concentration with a higher average salinity. The process can then repeat for an extensive but finite period as the surface layer and subsurface groundwater concentration eventually become hypersaline and where salt precipitation occurs. We have called this phenomenon episodic convection. Variable‐density groundwater flow and solute transport modeling experiments of a field‐based salt lake system confirm the existence of episodic convection and semiquantitatively validate the findings of the analytical approach.