The Law Of Potential Evaporation Between Different Saturated Soils

The land-atmosphere interface referring to the place where the top of the vadose zone had an connection with the atmosphere,it's the upper of vadose zone,the bottom of atmospheric system.With the influence of solar radiation and air turbulence,soil temperature increased,water changed into vapour then escape from soil surface,at this time evaporation happened.Evaporation rate is restricted by air temperature,radiation,wind speed.Potential Evaporation is the biggest evaporation rate that only influenced by meteorological elements.It is the basis of heat and moisture flux at land-atmosphere interface,also one of the most important parameters in calculating groundwater evaporation.The value of potential evaporation is closely related to land surface physical properties,roughness and saturation etc.Saturated bare soils have great liquid phase continuity and capillary driving forces,can be regard as the best initial reference on potential evaporating research.While researchers always make open water and wet land with short plants' evaporation equals to potential evaporation,there researches are lake of field experiments in measuring saturated bare soil evaporation.In this paper,we compared potential evaporation rates in different climate,different column size between different sand types and open water,also their relationship with meteorological elements.1.One of the field experiment took place in semiarid semi-humid region,we compared the daily average potential evaporating rate values in July between gravelly sand,coarse sand,medium sand,fine sand and open water in different column size,also there relationship with meteorological elements.The results are as follows:Comparing the daily average potential evaporating rates in large-size column,from largest to smallest,there sorting is: fine sand? medium sand >coarse sand > open water?gravelly sand.The rates of fine sand and medium sand is 26~27% over than gravelly sand,coarse sand is 14%over than gravelly sand.The value of open water is a little bigger than gravelly sand,but 3~20% smaller than other sands.All in all,for the sands that can use “The principle of connected vessels” to measured the evaporating rates,their potential evaporation rates are proportional to their water content.Also the daily potential evaporation rates of wateris smaller than all the saturated sands besides gravelly sand.Analyses the diurnal potential evaporation rates' fluctuation curves,we find they all in sine trend,but has different characters.For gravelly and coarse sands,the amplitude of the curve is bigger,that means their evaporation rates in daylight is bigger,they had a big temperature difference between day and night,their cycle signal of sine trend is weak.In comparison,the characters of medium and fine sands are in the opposite direction,the curve is smooth,their cycle signal of sine trend is obvious.The trend of open water is stable,evaporating value in daylight is smaller than four sands,had a small difference between day and night.The potential evaporation rates are in positive correlation with wind speed,air temperature,net radiation,correlation analysis shows air temperature has the maximal relationship.Evaporation rates are in negative correlation with humidity,vapour pressure.Besides that,regressive analysis verified the hysteresis phenomenon of potential evaporation.Compared with the change of air temperature,gravelly sand,coarse sand,medium sand had0~1 hour's lag time,fine sand lag 2 hours,open water lag 5~6 hours;Compared with the change of net radiation,gravelly sand,coarse sand,medium sand and fine sand had 2~3hours' lag time,open water lag7~8 hours.Comparing the diurnal potential evaporating rates in small-size column,from largest to smallest,there sorting is: In sunny daylight,open water ?fine sand>medium sand>?coarse sand>gravelly sand(>?means slightly bigger),in sunny night :open water>fine sand?medium sand? coarse sand? gravelly sand;In cloudy daylight:open water? fine sand>medium sand>coarse sand>gravelly sand,in cloudy night :open water>gravelly sand>coarse sand? medium sand? fine sand.The rules of small column have two differences between the large one,first is that water has the biggest evaporation rates,the second is that gravelly sand and coarse sand have bigger evaporation rates in night.These two differences are caused by profile temperature variation characters of the small-size column,the characters is restricted by the column size,it is the phenomenon of Scale Effect.2.Another field experiment took place in arid semi-arid region,we compared the potentialevaporating rate values in July between sand loam,aeolian sand,weathered sandstone and open water in different column size.The results are as follows:Comparing the daily average potential evaporating rates of different sands in large-size column,there sorting is: sand loam>aeolian sand> weathered sandstone,while they have little difference in value.These three sands are all belong to “fine sand”,either the daily average or the value in no-rain days are smaller than evaporation rates in semiarid semi-humidity region,that's because the air temperature in arid semi-arid region is too small.Analysing the diurnal potential evaporation rates' of three sands,there is little difference between sand loam and aeolian sand in daily variation,while the value of weathered sandstone is smaller,especially in night,this phenomenon is due to different sands has different thermodynamic parameters caused by saturated moisture content.Comparing the diurnal potential evaporating rates in small-size column,the fluctuation of open water and weathered sandstone is more drastic than aeolian sand.The values of these two sands and water are almost same,but the rules of different periods are different,in daylight : aeolian sand? weathered sandstone > water,in night : water > aeolian sand?weathered sandstone,this result is the same as the result in semi-humid region,which proves that the evaporation of water at night is bigger than saturated bare soil.