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Infrared thermography of evaporative fluxes and dynamics of salt deposition on heterogeneous porous surfaces

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https://ir.library.oregonstate.edu/concern/articles/p5547s03w

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  • Evaporation of saline solutions from porous media, common in arid areas, involves complex interactions between mass transport, energy exchange and phase transitions. We quantified evaporation of saline solutions from heterogeneous sand columns under constant hydraulic boundary conditions to focus on effects of salt precipitation on evaporation dynamics. Mass loss measurements and infrared thermography were used to quantify evaporation rates. The latter method enables quantification of spatial and temporal variability of salt precipitation to identify its dynamic effects on evaporation. Evaporation from columns filled with texturally-contrasting sand using different salt solutions revealed preferential salt precipitation within the fine textured domains. Salt precipitation reduced evaporation rates from the fine textured regions by nearly an order of magnitude. In contrast, low evaporation rates from coarse-textured regions (due to low capillary drive) exhibited less salt precipitation and consequently less evaporation rate suppression. Experiments provided insights into two new phenomena: (1) a distinct increase in evaporation rate at the onset of evaporation; and (2) a vapor pumping mechanism related to the presence of a salt crust over semidry media. Both phenomena are related to local vapor pressure gradients established between pore water and the surface salt crust. Comparison of two salts: NaCl and NaI, which tend to precipitate above the matrix surface and within matrix pores, respectively, shows a much stronger influence of NaCl on evaporation rate suppression. This disparity reflects the limited effect of NaI precipitation on matrix resistivity for solution and vapor flows.
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  • Nachshon, U., E. Shahraeeni, D. Or, M. Dragila, and N. Weisbrod (2011), Infrared thermography of evaporative fluxes and dynamics of salt deposition on heterogeneous porous surfaces, Water Resources Research, 47, W12519, doi:10.1029/2011WR010776.
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  • 47
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  • W12519
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  • This work was mostly funded by the Binational Science Foundation (BSF), contract 2006018. ES and DO gratefully acknowledge funding by the Swiss National Science Foundation project 2000021-113676/1, by the German Research Foundation DFG project (FOR 1083) Multi-Scale interfaces in Unsaturated Soil (MUSIS).
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