# Estimation of Thermal Diffusivity for Greenhouse Soil Temperature Simulation
## 3 Results and Discussion
### Analysis of Greenhouse Data
#### 3.1.2 Heating Season  
Figure 2 shows the air temperature inside and outside the greenhouse and soil temperatures at depths of 0.0 m, 0.1 m, 0.2 m, and 0.3 m in the heating season. The greenhouse air temperature was maintained above the heating set points throughout the experiment. To study the effect of the soil temperature at different air temperatures, the heating set points were 21 \({}^{\circ}\)C, 14.4 \({}^{\circ}\)C and 4.4 \({}^{\circ}\)C from 1 December to 6 December, 7 December to 13 December, and 14 December to 20 December 2016, respectively. The outdoor temperature was near or below 0 \({}^{\circ}\)C during this period.  
Figure 2a shows that at the temperature set point of 21 \({}^{\circ}\)C, the greenhouse air temperature was accurately maintained at 21 \({}^{\circ}\)C. When the greenhouse set point was set as 14.4 \({}^{\circ}\)C, the greenhouse air temperature occasionally went above 14.4 \({}^{\circ}\)C and was sometimes higher than 20 \({}^{\circ}\)C under clear-sky conditions (7 December). When the temperature set point was set at 4.4 \({}^{\circ}\)C, the greenhouse air temperature showed a steady sinusoidal pattern under clear-sky conditions (15, 19, and 20 December). On the other hand, for rainy conditions (16 December), the sinusoidal pattern was less pronounced. The greenhouse air temperature was higher and more stable than the outside air due to the low solar radiation load days, absorption of solar radiation by the soil and insulation form the greenhouse structure. The soil temperature was higher than the air temperature; the deeper the soil was, the higher the soil temperature. This finding suggests potential heat transfer from the soil to the greenhouse air.  
Figure 1: Diurnal variation of greenhouse air temperature (**a**) and greenhouse soil temperature (**b**) during the cooling season.  
The soil temperature in Figure 2b shows diurnal variations in sinusoidal behavior with reduced amplitude and reduced phase shift. When the greenhouse temperature was set to 21 \({}^{\circ}\)C and at 14.4 \({}^{\circ}\)C, the outside air temperature was close to 0 \({}^{\circ}\)C. This outside temperature affected the soil temperature, especially at the soil surface. The diurnal greenhouse soil temperature pattern is easier to see but not the greenhouse air temperature pattern. Using an energy balance at the greenhouse floor measured by [PERSON] (1999) suggested that when the air temperature was maintained at a constant value [19], the diurnal variation in soil surface temperature is mainly affected by solar radiation. When the greenhouse air temperature was set to 4.4 \({}^{\circ}\)C, which was close to the outside air temperature, the soil temperature varied with the greenhouse air temperature. The soil temperatures at deeper depths were higher than those at the surface. The analysis of the cooling and heating seasons shows that the greenhouse soil temperature was affected by the greenhouse air temperature, solar radiation, and outside air temperature. The soil temperatures were lower than the greenhouse air temperature during the cooling season, while the effect was reversed during the heating season.