Micro-meteorological Observations Of Pan Evaporation Process And Its Simulation Study

Despite the global average temperature was increasing, observations in most regions of the world showed that the rate of pan evaporation has been steadily decreasing over the past50years. This was known as the pan evaporation paradox. Order to explain the phenomenon of evaporation paradox, in this paper, a pan evaporation experiment at field was set up to reveal the physical mechanism of pan evaporation process, with the micrometeorological observation methods near the land surface layer. Using the data which obtained from the field experiment, the interaction between pan evaporation and the surrounding environment was analyzed, and the impact of non-uniformity environment on pan evaporation was discussed. The physical and mathematical model of pan evaporation was summarized with the observations. The pan evaporation characteristics were studied by numerical method, with the model which was summarized from the observation data. The main conclusions as following:(1) Comparison of the measurements between three types of pans (were E601B, Class A, and20cm pan, following was same) showed that the20cm pan have the largest rate of the daily evaporation, followed by the Class A pan and the E601B pan. The proportional coefficients of20cm pan and Class A pan to E601B pan were1.659and1.351, respectively, which were determined by statistical the daily evaporation rate of three types of pans.(2) Measurements of the pan water temperature at different depths showed that the E601B pan have a clear thermal stratification between10and20cm depths during the water heated period, and the maximum temperature difference between two layer was more than2℃. However, Class A pan did not have a significance temperature difference.(3) The albedo of water surface in Class A pan had a "W" shape variation at daytime which was measured with Four-component net radiometer, and the daily mean albedo was0.087.(4) The daily equilibrium evaporation (P-T), potential evaporation (LEp), reference evapotranspiration(ET0) were estimated by the observed meteorological data in pan experiment. At the same time, the actual evapotranspiration (LE) was measured by eddy correlation system, and the pan evaporation rates (LEpan) was measured by three type of pans. Finally, the differences between these evaporations have been compared, and the results showed that these evaporations have a magnitude relationship as LE<ETo<P-T<LEp<LEpanB<LEpanA<LEpanD.(5) Based on the principle of energy balance and mutual feedback mechanism, the complementary behavior has been revealed between actual evapotranspiration and pan evaporation which were measured by Eddy correlation system and three types of pans, respectively. With the non-uniformity intensity between pans and surrounding environment successively increasing from E601B pan to Class A pan, and to20cm pan, the proportionality coefficient b in asymmetric complementary relationship which was corresponding to three type of pans were increasing. Moreover, from the perspective of the evaporation rate, the intensity of pan evaporation rate (Ipan) was defined, so we can see the proportionality coefficient b was linear increasing with the pan evaporation intensity Ipan. Energy analysis showed that the stronger the non-uniformity between the evaporation surface and the surrounding environment, the evaporation intensity of evaporation surface (Ipan) was larger, and the proportionality coefficient b was greater.(6) According to the general feedback mechanism, a complementary relationship between actual evaporation and pan evaporation was established, which without involve complex soil, vegetation and near surface atmospheric processes and interrelation. This complementary relationship could be used to estimate actual evaporation with data of pan evaporation, net radiation, soil heat flux, and air temperature. Meanwhile, it was provided a new approach to estimate the soil actual evapotranspiration.(7) Based on the energy conservation principle and the boundary layer gradient transport theory, a single layer20cm pan evaporation model has been built. In this model, both sensible and latent heat fluxes of the evaporation pan were calculated by Monin-Obukhov similarity function. Meanwhile, the heat transport of the lateral wall was parameterized. The results showed that the model could reflect the non-uniformity of net radiation and sensible heat flux between water surface and land surface, and the model could also simulate the20cm pan water temperature and evaporation diurnal fluctuation successfully. During the whole experiment, the simulated root mean square error (RMSE) and the mean relative error ratio (MRER) of hourly water temperature were1.84’C and9.8%. The simulated RMSE and MAER of hourly pan evaporation rate were0.018mm/h and39.8%, and for water weight were26(g) and4.2%, respectively. Furthermore, the model performance was tested in desert and farmland. It was found that the RMSE and MRER of hourly water temperature by simulating were2.24℃癈and9.2%, and the RMSE and MRER of hourly pan evaporation rate were0.013mm·h-1and33.8%in desert, while the RMSE and MRER of daily pan evaporation were0.44(mm-d-1) and3.7%. In farmland the RMSE and MRER of daily pan evaporation by simulating were0.57(mm·d-1) and6.8%. This indicated that the model could simulate pan evaporate process well in both arid and humid environment. In addition, the model was also able to simulate the water temperature and pan evaporation when it was tested with the observations of Class A pan and E601B pan.