# Estimation of Thermal Diffusivity for Greenhouse Soil Temperature Simulation
## 5 Conclusions  
Heat transfer in greenhouse soil is an important variable affecting the greenhouse climate. In this study, we established and validated a greenhouse soil temperature model by estimating the thermal diffusivity of the soil. Our results showed that in the greenhouse, the diurnal soil temperatures at different depths have a steady sinusoidal behavior. The soil temperatures at different depths have decreasing amplitudes and hysteretic phase shifts with increasing depth. An evaluation of the five thermal diffusivity estimation methods showed that the amplitude method and the min-max method have the most consistent outcomes for estimating the greenhouse soil thermal diffusivity under  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Environmental Conditions**} & \multicolumn{3}{c}{**Soil Depth**} \\ \cline{2-5}  & **0.1 m** & **0.2 m** & **0.3 m** \\ \hline \multirow{3}{*}{Cooling season} & Clear sky (w/cooling) & 0.95 & 0.80 & 0.47 \\  & Clear sky (w/o cooling) & 0.88 & 1.29 & 1.30 \\  & Rainy & 0.81 & 0.71 & 0.90 \\ \hline \multirow{3}{*}{Heating season} & Set point: 21 \({}^{\circ}\)C & 0.31 & 0.11 & 0.24 \\  & Set point: 14.4 \({}^{\circ}\)C & 0.43 & 0.32 & 0.18 \\ \cline{1-1}  & Set point: 4.4 \({}^{\circ}\)C & 0.25 & 0.12 & 0.24 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance of the soil temperature prediction model under different environmental conditions. The values in the table are the Root Mean Square Error (RMSE) of the model predictions and actual measurement.  
different greenhouse and outdoor weather conditions. In addition, we found that the greenhouse soil temperature was affected by different factors during different seasons. The soil temperature is greatly affected by the greenhouse air temperature during the cooling season, while the soil temperature was mainly affected by the outdoor air temperature during the heating season due to the lower outdoor air temperature compared to the greenhouse air temperature. The soil temperature and daily average temperature at different depths can be predicted according to the temperature based on the soil thermal diffusivity. It should be noted that there are limitations to this study. First, the soil was assumed to be infinitely deep and second, we assumed that heat transfer only occurred downward in a one-dimensional isotropic medium. In reality, with the limited dimensions of the greenhouse, heat transfer through the soil is three-dimensional, and it is affected by the temperature gradient between the soils inside and outside the greenhouse. Second, the greenhouse air temperature is affected by more than just the outdoor air temperature. It is also affected by incoming solar radiation, crop growth and greenhouse climate control. To obtain a more comprehensive model, the convective heat transfer between the greenhouse air and the soil as well as the influence of different crops at different growing stages must be considered in a comprehensive energy balance model.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] analyzed the data and wrote the paper; [PERSON] and [PERSON] edited the paper. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Key R&D Programs of Jiangsu Province (BE2018321), the Natural Science Foundation of Jiangsu Provincial Department of Education (17 KJA416002), the China Postdoctoral Science Foundation (2015M580400), the Jiangsu Province Postdoctoral Science Foundation (1501112B), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.  
The authors are grateful to the Ohio Agricultural Research and Development Center (OARDC), Ohio State University, Wooster, OH, USA, for providing support to conduct the experiments.  
The authors declared that they have no conflicts of interest to this work.