Determination of Soil Temperature Change Trend in Different Climates of Kermanshah Province

Determining the trend of soil temperature change is of special importance due to the close relationship with sustainable agricultural management. In this study, in order to investigate the trend of soil temperature change in three stations of Kangavar, Kermanshah and Sarpolzahab, which have different climates, a series of daily soil temperature data for 25 years (1993 to 2018) of these stations were obtained from the meteorological organization of the country. Using daily averages of soil temperature, the averages of warm period (spring and summer) and cold period of the year (autumn and winter) and the averages of annual were adjusted and calculated based on the solar calendar. Mann-Kendall and Sen’s slope estimator tests were used to analyze the soil temperature trend at 95 and 99% confidence levels. Analysis of Kangavar synoptic station data series showed a significant upward trend using the both tests for annual soil temperature averages. In Kermanshah station, despite the increase in soil temperature values ​​at all depths, no significant trend in the average annual soil temperature was observed. Sarpolzahab station also did not show any trend. In Kangavar and Kermanshah stations, the increasing trend in soil surface temperature showed a higher value, which is due to the greater influence between soil surface temperature and air temperature. The average of temperature increase in 5-cm soil depth for Kermanshah and Kangavar stations were estimated to be 0.04 and 0.10 degree of Celsius per year, respectively. The increase in the average annual soil temperature of stations with increasing trend was recognized in the warm period of the year, mainly due to heat storage.