Eddy-Covariance Evaporation Measurements In Mountain Forests

In the past two decades,eddy-covariance system which can meet all of these demands was more and more applied to measure energy flux,especially for carbon and water exchange.Every coin has two sides,there are many application examples of eddy covariance system for evaporation measurement,but most experimental sites are flat,uniform and have low vegetation.In these sites farmland takes up a big percent,and the technical means for farmlands are mature.On the contrary,eddy covariance measurement for mountain forests is seldom because the prerequisites of eddy system application are plat and uniform underlying surface and relative stable atmospheric conditions.In mountain forests,these conditions can't be fit.Even if there are some researches,they only use single-height eddy system.We picked Xitaizi small watershed which belongs to Miyun reservoir watershed as the experimental site,and use 15 m and 30 m eddy covariance system to monitor the evaporation in order to know about the hydrological cycle more clearly.At the same time,we also use four component net radiometer and heat flux plate to measure the solar net radiation and geothermal to take the energy closure analysis.In most cases,the underlying surface conditions don't meet the requirements,so the raw data needs to be corrected.Furthermore,we must do the quality evaluation to make sure the flux data we obtain is reasonable and useful.Data correcting concludes coordinate rotation,water vapor concentration correction,WPL correction and frequency response correction.The quality assessment involves the consistency test and fully turbulence test.This paper use Eddypro software to correct the raw data derived from flux tower,results show that the evaporation of two heights are similar and the corrected data is 1.02 times as raw data.On this basis,we develop a complete set of mechanism for quality grade assessment of corrected data so that we can screen out the useful data from the entire population.And the results show that the higher level of data,the higher closure ratio of energy.And every time optimization can improve the correlation of data by about 7%,and the energy closure is 0.60 at 15 m,0.63 at 30 m where the data of 0 and 1 level is included.Observation results show that the annual evaporation of this watershed is 324 mm,the diffrerence between two heights is within 5%,the annual runoff in flow weir is 76 mm and average annual rainfall is 530.5mm.These results is reasonable by being compared with other similar experiment.Then we take the analysis of wind direction and find that the prevailing wind in the area from May to October is southwest wind.Source area of 90% is about 250 m away from the flux tower,and the peak point is 100 m.Finally,we conclude that the higher of the vegetation,the smaller of the source area and the contribution is more concentrated;however,the higher of the measurement height,the larger of the source area and the contribution is more sparse.