Evaporation rate from bare sandstone surfaces in humid climate

Evaporation from porous rock is not only a significant part of the earth-atmosphere water balance but it also plays a crucial role in weathering processes. In the case of salt weathering, the evaporation rate directly influences the amount of precipitated aggressive salts. Evaporation also strongly affects frost, hydric and biogenic weathering, since they are influenced by water content and its temporal changes. Without proper quantification of the evaporation loss, it is not possible to thoroughly explain and/or predict the development of moisture content and its spatial distribution within natural rock outcrops. Despite its importance, the study of evaporation from porous rocks has seen little scientific focus so far. In our study (Slavík et al., 2020), we measured the evaporation rate from bare surfaces of sandstone under field microclimate on a roughly monthly interval for about one year. The measurement was performed using sandstone cores with a set depth of the vaporization plane (i.e. the area where most of the phase change from liquid to vapour occurs in the subsurface) and we used a simple Fick’s law of diffusion for calculations of the evaporation rate from the cores. The calculations required only a laboratory-measured water-vapour diffusion coefficient of the sandstone, in-situ seasonally measured vaporization plane depth, and logs of air humidity and temperature. The analysis of measured and calculated evaporation rate revealed that far the most important single factor influencing the evaporation rate is the depth of the vaporization plane. Other factors such as the microclimate characterised by temperature and relative humidity were of lesser importance and the calculated evaporation rate reasonably follows measured values with Pearson correlation coefficient r > 0.81. The experimental setup of evaporation rate measurement, for its simplicity and price, should find use in studies with high-number measuring sites or even locations with a risk of apparatus damage. Our measurements were performed in a humid continental climate and the suggested approach should be verified in more arid environments.

Figure: Goodness‐of‐fit between measured and calculated evaporation rate. Dashed line represents the identity line (calculated values equal to measured values).

This research was funded by the Czech Science Foundation [GA19-14082S].

References: Slavík, M., Bruthans, J., Weiss, T., Schweigstillová, J. (2020): Measurements and calculations of seasonal evaporation rate from bare sandstone surfaces: Implications for rock weathering. Earth Surface Processes and Landforms 45, 2965–2981.