干旱区生态环境知识资源中心

Extremely shallow flows of a few centimeters depth are common in the arid region of the Andes Mountains of northern Chile, Argentina, Bolivia and Peru; and corresponds to shallow and salty water bodies formed by small volumes of upwelled groundwaters, in which the wind is the main external source of energy able to enhance vertical transports of energy, mass and momentum. Benthic primary production in these salty lagoons is large enough such as sustaining large bird populations which annually arrive to this region for feeding and reproducing, so that, in order to develop management tools for these ecosystems, mass exchanges across the water sediment interface are required to be quantified for this flow conditions. Particularly, dissolved oxygen (DO) exchanges across the water sediment interface are determined by transport processes across the diffusive sublayer whose thickness is determined by flow conditions in the turbulent water column, which in turn is controlled by wind action. The aim of this article is to present experimental study conducted in a wind tunnel that blows air over a 400x40x50cm (lengthxdepthxwidth) water tank placed in the downwind end of the experimental facility. The tank contains a false-bottom of acrylic which is placed in a depth that varies in a range of 3 to 9 cm and has compartments where natural sediments are placed. Upwind and downwind ends of the false-bottom are opened, so the dragged water by the wind is allowed to recirculate bellow the false-bottom, and the flow induced above the false-bottom can be characterized as turbulent Couette-like unidirectional flow, modified by a contrary pressure gradient that partially counteracts the wind shear stress. For a particular experimental configuration of wind speed and water depth, velocity profiles in the water were measured with a 2D side-view Sontek ADV, and in the air were recorded with an Extech hot-wire anemometer. In this way, dissolved oxygen is allowed to be consumed, and the DO vertical profiles through diffusive boundary layer thickness were measured with OX-25 Unisense micro-electrodes. As a result of these measurements, the wind-induced flow is characterized and quantified, being the current speed and diffusional mass transfer coefficient related to both the wind shear stress and water depth. These relationships are also used to understand measurements of flow speed and diffusional mass transfer coefficient conducted in October of 2012 in Salar del Huasco (20.274oS, 68.883oW, 3800 m above sea level), Northern Chile.