Borehole Diameter Controls Thermal-Induced Convection and Evaporation From a Shallow Water Table

Understanding air exchange via boreholes, shafts, and wells is important for Earth and environmental sciences because of their potential role for increased evaporation and greenhouse gas emissions. Here, we investigated the effect of atmospheric temperature variability on air transport via a borehole as a function of its diameter. Field experiment included five boreholes with diameters ranging from 0.10 to 0.38 m and a depth of 3 m with an artificial water table at −2.7 m. Temperature was recorded in each borehole at 1-min intervals for 1 year. Diurnal and seasonal air temperature changes triggered thermal instability, mainly during winter nights, which initiated thermal-induced convection within the boreholes. The thermal-induced convection penetrated deeper into the borehole as the diameter increased, doubling the water table evaporation rate for the largest borehole relative to the smallest. Borehole diameter plays an important role in controlling air exchange rates between the subsurface and the atmosphere.