An Observational And Numerical Study On The Effects Of Meteorological Factors On Pan Evaporation In Arid Region In China

Designed the "Microclimate and Evaporation Experiment" in the northwest arid area of China, three pans evaporation were observedin the same environment,comparedthe characteristics of the three pansand the micro meteorological data, compared and analyzed the relationshipbetween four principalmeteorological factors(wind speed, air temperature, air humidity and solar radiation) and pans evaporation in process of the hourly variation, and estimated one of these factors contribute to pans evaporation in number. In addition, based on the energy conservation principle, sensible heat fluxes of the evaporation pan were calculated by eddy-covariance technique. Meanwhile, the heat transport of the lateral wall and bottom of20cm and Class A was parameterized, three evaporation pan models were built in this paper. The main conclusions summarized as follows:(1) The results of potential evaporation were reflected by three pans were different, but the trend was similar, the daily evaporation results:20cm was the largest, Class A was middle, E601B was the smallest.In addition, in the process of daily variation (hourly evaporation) were different in the different time, the response time of three pans for the external environment were different,20cm was the fastest, E601B was the slowest, Class A was middle. Hence, if we choose different pans reflect evaporation potential of the environment, the resultswill be different.(2) Assuming that other meteorological factors remain constant, daily average number of wind speed increasea unit, the daily evaporation of20cmpan will add1.87mm, Class A pan evaporation will add1.93mm, E601B pan evaporation will add1.17mm.Daily average number of air temperatureincreasea unit, the daily evaporation of20cmpan will add0.32mm, Class A pan evaporation will add0.30mm, E601B pan evaporation will add0.16mm.Daily average number of air humidity increasea unit, the daily evaporation of20cmpan will reduce0.11mm, Class A pan evaporation will reduce0.10mm, E601B pan evaporation will reduce0.13mm.Daily average number of solar radiationincreasea unit, the daily evaporation of20cmpan will add0.022mm, Class A pan evaporation will add0.014mm, E601B pan evaporation will add0.012mm.(3) The affect intensity of20cm pan evaporation from wind speed, air temperature, air humidity and solar radiation were0.54,0.58,-0.42and0.49in respectively. The affect intensity of Class A pan evaporation from wind speed, air temperature, air humidity and solar radiation were0.67,0.65,-0.49and0.38in respectively. The affect intensity of E601B pan evaporation from wind speed, air temperature, air humidity and solar radiation were0.55,0.48,-0.84and0.46in respectively. Air humidity on evaporation was negative, the other three meteorological factorswas positive.(4) With the principle of conservation of energy and boundary layer theory of gradient transport, eddy covariance observation system data are used to calculate the sensible heat flux of water evaporation pan, the side wall and bottom heat transfer energy of20cm and Class A were parameterized, three kinds of pan evaporation model were built to three kinds of commonlyused evaporation pans. Energy transfer of E601B side wall and bottom was just contraryfrom20cm and Class A in daily variation, and the direction was alternative, around the0,20cm and Class A werepositive.Daily amount of20cm was biggest, Class A was second, E601B is third.In addition, analysis the reason of these results in the physical view.(5) The simulation results of evaporation model show that the model successfully simulates the three kinds of variation of the pan evaporation process, the square error (RMSE) and the average relative error (MRER) of20cm was0.09(mm/h) and27.1%, R2was0.88. The square error (RMSE) and the average relative error (MRER) of Class A was0.02(mm/h) and25.8%, R2was0.88. The square error (RMSE) and the average relative error (MRER) of E601B was0.019(mm/h) and27.4%, R2was0.82. Three models were all pass confidence level of inspection by0.01.The correlation coefficients of simulation and observation of three panswere all above0.95, the square error (RMSE) and the average relative error (MRER) of20cm was1.42(mm/d) and10.0%. The square error (RMSE) and the average relative error (MRER) of Class A was1.37(mm/d) and13.1%. The square error (RMSE) and the average relative error (MRER) of E601B was1.22(mm/d) and16.6%.The three kinds of model can reflect the different weather conditions well, on the three pan evaporation, under different weather conditions have good applicability.(6) Under dry underlying surface, the heteropical degree between water surface of pan and surrounding environment was much greater than the humid underlying surface, the heteropical degree was more greater, the energy transfer between water surface of pan and surrounding environment was more stronger, which affected directly the size of the pan evaporation. Under dry underlying surface, the pan evaporation was much bigger than the pan evaporation under humid underlying surface,the reason ofwhich was the difference of evaporation the heteropical degree.