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Abstract An experimental apparatus was designed to study the impacts of wettability on evaporation of water from Ottawa sand. Evaporation rates were measured for: (1) a 5.7-cm-thick layer of hydrophilic Ottawa sand; (2) a 5.7-cm-thick layer with 12% hydrophobic content, consisting of a 0.7-cm-layer of n-Octyltriethoxysilane-coated hydrophobic sand buried 1.8 cm below the surface of hydrophilic sand; and (3) a 5.7-cm-thick layer with mixed wettabilities, consisting of 12% n-Octyltriethoxysilane-coated hydrophobic sand mixed into hydrophilic sand. The sand–water mixtures experienced forced convection above and through the sand layer, while a simulated solar flux (i.e., 112±20 W/m2) was applied. Evaporation from homogeneous porous media is classified into the constant-rate, falling-rate, and slow-rate periods. Wettability affected the observed evaporation mechanisms, including the transition from constant-rate to falling-rate periods. Evaporation entered the falling-rate period at 12%, 20%, and 24% saturations for the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture, respectively. Wettability affected the duration of the experiments, as the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture lasted 17, 20, and 26 trials, respectively. Both experiments with hydrophobic particles lasted longer than the all hydrophilic experiment and had shorter constant-rate evaporation periods, suggesting hydrophobic material interrupts capillary action of water to the soil surface and reduces evaporation. Sand temperatures suggest more evaporation occurred near the test section inlet for higher saturations and the hydrophobic layer experienced more evaporation occur near the outlet. Evaporation fluxes were up to 12× higher than the vapor diffusion flux due to enhanced vapor diffusion and forced convection. 

The primary source of water for crops and livestock in the United States Central High Plains is irrigation from the Ogallala Aquifer. Due to the semi-arid climate of this region, little rainfall contributes to watering crops, thereby resulting in water scarcity. Reducing the evaporation from soil is one approach to conserve the water. In this study, a soil evaporation chamber was designed and constructed to study the impacts of environmental conditions on evaporation from Ottawa sand. Prior to entering the sand test section, compressed air flow was dried in a desiccator then split in two flows before entering the 57mmx228mmx838mm test section, with one airflow flowing above the 57mm thick sand layer and the other below and, subsequently, flowing through the moist sand layer. The percent relative humidity (RH) was measured at the entrance and exit to record the change in relative humidity and, therefore, water content removed from the sand. Using inlet air mass flow rates of air of approximately 1E-4kg/s–2E-4kg/s, temperatures of 28–31oC, and dry air (i.e. 0–1%RH), exit flows of 19–20oC and 80–85%RH were measured. Measured evaporation rates ranging from 3.0E-6kg/s to 5.0E-6kg/s for soil saturation levels of 55–80.5%.