Mechanism Of Heat And Water Transfer At The Land-Atmosphere Interface And Its Effects On The Heat And Water Flow In Vadose Zone In An Arid Region

As the connection of the atmosphere and soil, the mass and energy transfer at the land-atmosphere interface was not only restricted by the radiation, atmospheric circulation driving and the complex soil properties, but also effecting the local climatic change and the water, heat and salt circulation. Because of its complexity, the study on land-atmosphere has been still on the elementary stage. Therefore, it is necessary to do further study on water and heat transfer on the land-atmosphere interface and to clarify its effects on water transportation in the vadose zone for evaluating the groundwater quality and improving the accuracy of the atmosphere models. In this paper, the transfer of water and heat at the land-atmosphere and its effects on the vadose zone were analyzed, by field experiments and mathematical models. The results showed that:1. The laws of the water and heat transfer at the land-atmosphere were discovered under different weather conditions. Soil moisture and temperature at the land-atmosphere interface were sinuous in clear sky. In summer, the peak of soil heat flux was over 200W/m~2 at 13:00 pm under clear condition, but the peak of soil heat flux was much smaller under overcast and rainfall conditions. It would yield 30% error ignoring the surface soil heat flux during the estimation of evaporation at regional zone.2. The water and heat transfer on the land-atmosphere interfac was affected by the time scale of obvervatione. Soil moisture and temperature appeared obvious diurnal fluctuation, and the monthly soil temperature presented standard sine function. Soil surface soil heat flux was different hourly and was positive value from 9:00 to 18:00, and the monthly value ranged from-10 to 20 W/m~2. The soil moisture at the land-atmosphere interface was changeable on different seasons, with lower value in summer and higher value in winter, which was influenced by groundwater level. The soil temperature on the land-atmosphere interface was higher in summer and lower in winter. The soil surface heat flux peaked at 25W/m~2 in summer and reached the valley at-5W/m~2 in winter.3. The relationship was clarified between the surface soil temperature and the micro-meteorological factors. Specifically, soil temperature has positive correlation with solar radiation and air temperature, and has negative correlation with air pressure and relative humidity, and has complex relationship with gusts. During the heat transfer downwards with the depth increase, the amplitude of the soil temperature is decreased and the phase is lagged. When the depth increased,the amplitude decreased about exp(-τ) and the the phase lagged about t, which was influenced by the depth and the soil thermal diffusion speed simultaneously.5. There is no single method to accurately estimate the thermal diffusivity, k, under differing weather conditions. Under clear-sky conditions, the harmonic method has a greater accuracy in estimating k than the amplitude and the phase shift methods in dry soils while the convection-conductivity method generates a major error in estimating k and heat flux when the soil is too dry. When the soil is wet, the convection-conductivity method is more accurate in estimating k than the amplitude, the phase shift and the harmonic methods under rainy or overcast conditions, while the accuracy of the harmonic and the amplitude methods is poor in estimating k under rainy conditions. For a simple calculation, the phase shift is most suited to estimate k as a substitute for the convection-conductivity method under high soil moisture content conditions with low liquid movement.6. Under clear conditions, harmonic method worked very well on clear days, but it would generated large errors under cloudy and rainy days when soil temperature failed to form a steady sine wave on one day. The force-restore method was not recommended on rain-free days because of its poor estimation of soil surface heat flux. Under rainfall conditions, the force-restore method was a substitution model if only the soil temperature information was available. In contrast, the convection-conductivity method can only be used for rain-free days. During rainfall events, it yielded worst estimation compared with other models. The plate calorimetric method generally was the best choice in field due to its well estimation under three different weather conditions.7. The best palcement of the plate was determined to estimate surface soil heat flux. When the plate placed at the inflection point of soil temperature, the estimation of soil surface heat flux would yield major error. Therefore, the plate should be avoided this inflection point. It showed that Philip correction method can improve the estimation of soil surface heat flux when the plate was buried at the shallow soil layer. If the plated buried deeper than 5cm, Philip correction would failed to improve the estimation accuracy of the surface heat flux and the correction increased the estimation error.8. The transfer of heat and water at the land-atmosphere affected the heat and water transfer in the vadose zone. The changeable water flux has influenced the distribution of moisture and temperature in the vadose zone, the water impact the penetration depth and penetration time of soil temperature. The changeable temperature at the land-atmosphere is the main reason resulting in the differences in temperature distribution, but its influence on the the daily variation of liquid water is not significant in the vadose zone.