Impact of ambient conditions on evaporation from porous media

The complexity of soil evaporation, depending on the atmospheric conditions, emphasizes the importance of its quantification under potential changes in ambient air temperature, T-a, and relative humidity, RH. Mass loss, soil matric tension, and meteorological measurements, carried out in a climate-controlled laboratory, were used to study the effect of ambient conditions on the drying rates of a porous medium. A set of evaporation experiments from initially saturated sand columns were carried out under constant T-a of 6, 15, 25, and 35 degrees C and related RH (0.66, 0.83, 1.08, and 1.41 kPa, respectively). The results show that the expected increase of the stage 1 (S1) evaporation rate with T-a but also revealed an exponential-like reduction in the duration of S1, which decreased from 29 to 2.3 days (at T-a of 6 and 35 degrees C, respectively). The evaporation rate, e(t), was equal to the potential evaporation, e(p)(t), under T-a=6 degrees C, while it was always smaller than e(p)(t) under higher T-a. The cumulative evaporation during S1 was higher under T-a=6 degrees C than under the higher temperatures. Evaporation rates during S2 were practically unaffected by ambient conditions. The results were analyzed using a mass transfer formulation linking e(t) with the vapor pressure deficit through a resistance coefficient r. It was shown that r(S1) (the resistance during S1) is constant, indicating that the application of such an approach is straightforward during S1. However, for evaporation from a free water surface and S2, the resistances, r(BL) and r(S2), were temperature-dependent, introducing some complexity for these cases.