Evaporation driven by Atmospheric Boundary Layer Processes over a Shallow Salt-Water Lagoon in the Altiplano

Abstract

The Chilean Altiplano is a region composed by endorheic basins immersed in a complex topography. These basins are predominantly characterized by desert surfaces in which small-scale heterogeneities can be found in the form of salt flats with shallow, salt water lagoons, that act as preferential pathways for evaporation (E). Thus, understanding the processes that control E in the lagoons is essential for water balance predictions, and to understand the impacts of climate change on the region. In addition to the local saline lagoon and desert conditions, the atmospheric boundary layer (ABL) and its interaction with large-scale forcing, play a key role in regulating E. Observations over a salt water lagoon in the Salar del Huasco basin show that in the morning E is virtually zero, with turbulence as a limiting factor due to the absence of wind. Under these conditions, a shallow, stable ABL is formed over the water. In the afternoon, E is triggered by the entrance of a thermally driven and topographically enhanced regional flow characterized by strong winds. Simultaneously, the ABL turns into a deep mixed layer similar to the one observed over the surrounding desert. In this research we investigate the coupling between the ABL and E drivers using a land atmosphere model, observations and a regional model. We also analyze the ABL interaction with the aerodynamic and radiative components of E using the Penman equation adapted to salt water. Our results demonstrate that the morning ABL is controlled by the local advection of warm air (∼5 Kh-1), resulting in a shallow (<350 m), stable ABL, with virtually no mixing and no E (<50 Wm−2). The warm air advection ultimately connects the ABL with the residual layer above, sharply increasing the ABL height by ∼1 km around midday. During the afternoon, the regional flow arrives to the lagoon, causing an increase in wind (∼12 ms-1) and an ABL collapse due to the entrance of cold air (∼-2 Kh-1) with a shallower ABL (∼-350 mh-1). The turbulence produced by the wind decreases the aerodynamic resistance and mixes the water body releasing the energy previously stored in the lagoon. The ABL feedback on E through the vapor pressure enables high E values (∼450 Wm-2). These results are exemplary to E of water bodies in semiarid conditions and emphasize the importance of understanding ABL processes when describing E drivers.