Annual Soil Temperature Analysis in Iran Using Singular Spectrum

Soil temperature (ST) variation can affect the earth energy balance. Moreover, the awareness of the soil thermal regime and its thermal fluctuations can prevent possible damages to agriculture and can increase crop productivity. In this study, using the singular spectrum analysis (SSA), trends and oscillation components, as well as the degree of the coincidence of the soil temperature (ST), air temperature (AT) and precipitation time series were investigated in three thermal regime classes namely: Mesic, Thermic and, Hyper thermic, in 28 high quality weather sites during 1993-2017. The results showed that the highest and lowest rates of ST increases have occurred in the Mesic and the Thermic thermal regime, respectively. The precipitation fluctuations were in the opposite phase with the ST fluctuations. The dominant return periods in the annual series were 2.3 and 11-12 years that could be related to quasi-biennial oscillation (QBO) variations, and 11-year cycles of sunspots. By the implementation of the coincidence which exists between the short and long term oscillations of ST and AT time series, one can generate and reconstruct ST data gaps based on AT.