[1] While soil evaporation studies have typically focused on pure or low salinity water evaporation, higher salinity soil conditions are becoming more prevalent. This work explores the combined effect of matrix heterogeneity and salt precipitation on evaporation from soils. Long-term evaporation processes were studied using sand columns, in which heterogeneity consisted of two layers with different grain sizes, and X-ray computed tomography (CT) scanning to quantify salt deposition within pores. For saline solutions, three new stages of evaporation were defined: SS1, SS2, and SS3. SS1 exhibits a low and gradual decrease in evaporation rate because of increasing osmotic potential. During SS2, evaporation rate falls progressively because of salt-crust formation. SS3 is characterized by a constant low evaporation rate. Even though phenomenologically similar to the well-defined classical evaporation stages for pure water, these saline stages correspond to different mechanisms. It is shown that SS2 and SS3 take place while matrix water content can still support first-stage evaporation. Salinity suppressed evaporation more strongly in homogeneous rather than in heterogeneous media. CT scans indicated preferential salt precipitation in the fine-textured regions. Heterogeneous spatial distribution of salt precipitates within the media enabled vapor transport via large pores, while small pores were clogged with precipitated salts. A mathematical model was formulated that simulates evaporation for saline solutions from homogeneous and heterogeneous soils. The model was used to differentiate and quantify the mechanisms controlling each stage of the evaporation process.Citation: Nachshon, U., N. Weisbrod, M. I. Dragila, and A. Grader (2011), Combined evaporation and salt precipitation in homogeneous and heterogeneous porous media, Water Resour. Res., 47, W03513,

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[1] 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.

If we are to understand global warming, and in particular global water‐cycling, then it is vital to explore the links between atmospheric conditions, earth processes and major global cycles. One arena that has been heretofore ignored is the effect on global dynamics of earth fractures that are open to the atmosphere. Historically, these fractures have been studied merely as participants in aquifer recharge or aquifer contamination during periods of infiltration. In general, they are considered inactive when there is no precipitation. This paper puts forward in‐situ continuous field measurements demonstrating that during no‐flow periods, fractures breathe via convection on a daily basis, enhancing atmospheric exchange by several orders of magnitude compared to the non‐fractured crust. We quantify the timing, persistence and characteristics of this mechanism. The convective exchange mechanism is pervasive, occurring daily with peak flux exchange at night and in winter, the reverse of most other surface processes.

The impact of salt precipitation, within or on top of porous media, on water evaporation from the media is an important issue with practical implications for agricultural practices, civil engineering and construction materials, the food industry and more. Even though the evaporation of saline solutions from porous media has been widely studied in recent years, there are still many uncertainties about the associated physical and chemical mechanisms. Moreover, most studies have focused on the impact of efflorescent salt precipitation on evaporation. This work focuses on subflorescent salt precipitation and its connection to evaporation from porous media. The different impacts of subflorescent and efflorescent salt precipitation on porous media evaporation are discussed, and the results of experiments that measured and quantified the impact of crystallization patterns on evaporation and porous media hydraulic conductivity are presented. A simple model was used to better understand the limiting factors that control liquid transport within porous media, with and without crystallized salts. The experimental results showed a major impact of efflorescent salt crust on evaporation, while for the subflorescent salt precipitation, the measured evaporation rates were similar to those measured for salt-free conditions. The model indicated that even though subflorescent-precipitated salt reduced the hydraulic conductivity of the medium by several orders of magnitude, it was not the limiting factor for evaporation as the diffusive vapor transport between the matrix surface and atmosphere was the slowest transport mechanism in the system, thus controlling evaporation rates.