Effects of heat waves on soil temperatures in Slovenia

Soil temperature regulates the rate of plant growth and tells us much about the climatic characteristics of a particular site. Climate variability and extremes need to be studied and there is a large gap in knowledge about soil temperature during heat waves. Agricultural land is highly dependent on heat waves, which are becoming longer, more intense and more frequent, and it is important to monitor soil temperatures in situ to understand their changes during heat waves. Therefore, the aim of this work was to investigate how soil temperatures change at different depths during and after heat waves. Average daily air and soil temperature data for the 25-year period 1992-2016 were evaluated at four agrometeorological stations in three climate zones in Slovenia and analyzed during heat waves determined according to the Slovenian definition. During the period 1992-2016, 53 (Lesce) to 76 (Ljubljana) heat waves were identified. Analysis of average air and soil temperatures before, during and after heat waves showed higher responsiveness of the upper part of the soils and an increase in the time lag between maximum air temperature and maximum soil temperature with depth. The maximum temperature during the heat wave was reached on average in three to nine days, depending on the depth. Only in Moderate climate of the hilly region, the average daily temperatures at a depth of 100 cm remained below 20°C during and after the heat wave. The temperature rise in the deeper layers of the soil lasts longer than in the shallower layers.