Effect of wetness patchiness on evaporation dynamics from drying porous surfaces

An important aspect of evaporative fluxes from porous surfaces is vapor emission through discrete pores giving rise to nonlinear dependency on surface water content (i.e., the flux is not proportional to surface water content). This nonlinearity is attributed to interactions among pore sizes and spacing, and the thickness of air boundary layer. The study examines effects of pore clustering on evaporation rates, addressing the question: do pore clusters behave like effective large pores? To answer this question, evaporation fluxes from surfaces with distributed or clustered pores (similar pore numbers and sizes) for various boundary layer thicknesses were computed numerically and analytically. Results show consistent suppression of clustered surface evaporation rates (relative to distributed pores configuration) with increasing cluster size. For cluster sizes of the order of boundary layer thickness (approximate to 10(-2) m) the relative evaporation rate becomes proportional to the fraction of cluster surface area resulting in maximum evaporation suppression. An analytical expression for evaporative fluxes from patterned surfaces was derived based on superposition of diffusive fluxes from individual pores accounting for enhanced flux along cluster perimeter. Results may assist design of engineered porous surfaces for efficient evaporation suppression, and for interpretation of effects of coordinated stomatal clustering on transpiration fluxes from plant leaves.