Effects of thermal conductivity and diffusivity of a volcanic ash soil in central Ethiopian Rift Valley on soil temperatures

Abstract

Soils originating from volcanic ashes (i.e. Andosols) are deposited around the Ethiopian Rift Valley. As a result of insufficient land management, the region has experienced deforestation and subsequent soil degradation. Although soils from the Ethiopian Rift Valley are exposed to intense solar radiation, the soil temperature regimes of the volcanic ash soils are largely unknown. Volcanic ash soils generally have unique physical properties compared to other mineral soils. Therefore, we hypothesized that volcanic ash soils in the Ethiopian Rift Valley would also have unique soil thermal characteristics that control soil temperatures. The objectives of this study were to investigate the characteristics of thermal conductivity and diffusivity of a volcanic ash soil from the central Ethiopian Rift Valley, and to evaluate the effects of the thermal characteristics on soil temperatures. The results demonstrated that the thermal conductivity of the volcanic ash soil from the central Ethiopian Rift Valley was low compared to non-volcanic mineral soils due to both the low thermal conductivity of the solid phase and the high porosity of the soil. The thermal characteristics of the Ethiopian volcanic ash soil were similar to that of volcanic ash soil from Japan. The low thermal diffusivity was associated with low thermal conductivity of the soil, and this impeded diurnal soil temperature fluctuations to propagate to deeper depths. Consequently, temperatures of only vicinity of the soil surface rose to higher levels due to the intense solar radiation, whereas the temperature amplitude of the subsurface soil was slight.