The Characteristics Of Energy Balance Closure Under Wet/Dry Conditions Over Native Grassland In The Semi-arid Loess Plateau

In the context, the Bowen ratio and precipitation is used to define the wet and dry conditions by eddy covariance data from Semi-Arid Climate Observatory and Laboratory Observatory (SACOL) in Lanzhou University, which was being chosen from May to October in years of2007-2009. Then analyze the effects of energy balance closure in different wet and dry conditions over Native Grassland on the Loess Plateau semi-arid by the OLSs method. There are two cases of wet and dry conditions. The first one is that divided the moisture of surface into four levels, as:1) extreme dry condition,2<β<10;2) dry condition,1.2<β<2;3) wet condition,0.6<β <0.8;4) extreme wet condition,0.3<β<0.6, which were used to analyze the characteristics of energy balance closure when the surface changing from dry to wet. The second one is that using precipitation to distinguish wet and dry, and then analyzed the characteristics of energy balance closure in the same month in different years in different wet and dry conditions. Select the months of7,8,10in each year for analysis. From the year2007to2009, the precipitation in months of8was111.6mm-.84.2mm-.160.9mm respectively, which were quite different in each month; the precipitation in months of10was65.1mm、34.8mm、7.6mm respectively, which were quite different in three months, and the precipitation in2007was32.1mm, which was very high; the precipitation in months of7was49.5mm、38.7mm、42.1mm respectively.The following are the main conclusions:(1) Dividing the wet and dry conditions with Bowen radio, three typical cases have been study and got the unanimous conclusion. In the daytime (09:00-18:00), the energy balance closure rate is gradually increased in the arid to humid transition process. In detail, for the PlateCal method, the energy balance closure rate is69.6%、72.6%、76.3%、79.5%respectively changing from dry to wet. For the TDEC method,the energy balance closure rate is72.5%、75.7%、77.8、80.9%respectively changing from dry to wet. Both of the methods were consistent with the conclusion that the more humid surface conditions, the higher energy balance closure rate. Compared with PlateCal method, from the extreme dry to the extreme wet, the energy balance closure rates were increased by2.9%,3.1%,1.5%and1.4%with the TDEC method. There were differences between the two methods of calculating surface soil heat flux, but not the conclusion of energy balance closure rate.(2) Use precipitation to distinguish wet and dry, and then analyzed the characteristics of energy balance closure in the same month in different years in different wet and dry conditions. The conclusion is that in the daytime (09:00-18:00) when the surface layer is unstable, the energy balance closure rate will be high if the surface is wet.(3) For the night (from22:00to05:00) when the stratification of surface is stable, the correlation between energy closure rate and wet and dry conditions of surface is uncertainty. Specifically:at night, the energy balance closure rate is generally low, the coefficient of determination is also very small; either use PlateCal law or TDEC method of surface heat flux, the near surface energy balance closure rate did not show the rule that it was relatively large in wet condition and small in dry conditions.In summary, using different methods to divide wet and dry conditions, and then analyzed the energy balance closure under different conditions in semi-arid Loess Plateau near land surface over Native Grassland. We found that the moister the land surface, the greater the energy balance closure rate during the daytime (09:00to18:00) when the underlying surface stratification is unstable. It did not show the rule when the underlying surface stratification was stable at night (22:00-05:00).