Short- and long-term variations in groundwater temperature caused by changes in vegetation cover

Abstract

Several comparative studies of the earth's surface provide evidence that vegetation and other bio-physical processes at the earth's surface can directly affect the atmospheric boundary layer, leading to changes in temperature and precipitation patterns. In this study, we demonstrate how vegetation cover can be responsible for the subsurface temperature variation as well as how this temperature variation can be related to past events. A linear decrease of 0.0407 K/year was estimated, and a decrease of 2 mK was observed in subsurface temperature when the surface temperature exceeded 9 ^oC. This diurnal temperature variation occurs during the phenological growing season of the vegetation. The transient temperature shows an annual cycle at a depth of 40 m. Model calculation applying a linear decrease in surface temperature of 2 K as a boundary condition was simulated. Comparing the results with the trend it is realistic to assume that when an apparent thermal diffusivity of 1.8*10^-6 m²/s is applied an event starting between 10 and 20 years ago is responsible for the detected decrease in temperature. However, with this thermal diffusivity the conductive annual temperature variation reaches an amplitude of 1.1 mK instead of the measured 5.4 mK at 40 m. In conclusion, beside the vegetation causing additional convective heat transport triggered by the annual surface temperature, the influence of reduced solar incoming heat radiation reaching the ground caused by the increased shadowing effect of vegetation cover might be responsible for a continuous decrease in local temperature of 2 K being active approximately 20 years after plantation.