Temporal And Spatial Change Of Pan Evaporation And Its Impact On Water Balance In The Qinghai-tibet Plateau

The Qinghai-Tibet Plateau, its unique global weather and climate system has aunique response and feedback to global climate change under the background ofglobal warming as the main feature and frequent extreme weather events; Thehydrothermal environment changes will lead to huge changes in water production, theenergy balance change caused by changes in the hydrothermal will affect the climateof the whole of Asia and even the world. The spatial and temporal characteristics ofpan evaporation (Epan) to reflect changes in energy and water, which has become a hotin the field of hydrology and water resources. Especially, under the global panevaporation showed a generally downward trend, the research of temporal and spatialvariation of pan evaporation can not only a deeper understanding of climate changeand hydrothermal conditions, but also provide the theoretical basis and scientificguidance for rational development, utilization and management of hydrology andresources, in the Qinghai-Tibet Plateau.Based on meteorological data of42years (1970-2011) of77surface weatherstations in the Qinghai-Tibet Plateau, it include monthly temperature (average,minimum, maximum, extreme minimum and extreme maximum), precipitation, windspeed, actual vapor pressure, relative humidity, sunshine hours, pan evaporation andyearly runoff major basins of the Qinghai-Tibet Plateau. Trends and significantchanges of climate variables and hydrological factors are examined by theMann-Kendall test, adopt Penpan model and Budyko’s coupled water and energybalance hypothesis simulate and calculate pan evaporation (Epan) and actualevapotranspiration (E) in the Qinghai-Tibet Plateau, and analysis the temporal trendsof between the two. At last, analysis the reasons of pan evaporation and actualevapotranspiration from a qualitative trend changes, and analysis their relationshipbetween the two in brief. The main conclusions as follow:1By Penpan model, the simulation accuracy of pan evaporation (Epan) in theQinghai-Tibet Plateau is high, especially monthly simulation results, thedecisive factor (R2) is up to0.87; Relative error(MAE/mean) is4.1%, the standard mean square root of variance(RMSE/Mean) is25%, that canthrough Penpan model to simulate the evaporation pan evaporation ofthe qinghai-tibet plateau region. And the dryness index (Epan/Pr)showed that the qinghai-tibet plateau climate to the direction of wet andwarm.2The distribution of Epanin the Qinghai-Tibet Plateau have remarkable verticalzonality differentiation. Low altitude Basin-valley is evaporation highvalue area (>2500mm), high altitude-complex terrain area isevaporation of low-value area (<1000mm). The distribution of theradiation term (Erad) and aerodynamic-item (Eaero) evaporation areconsistent with the distribution law of pan evaporation, but trends ofbetween the two are very different, the radiation term evaporation showa significant upward trend while aerodynamic-item evaporation has asignificant downward trend, the trend of aerodynamic-item evaporationis very consistent with the changes in the wind speed.The Qinghai-TibetPlateau Epanwas a no significant downward trend with the rate of24mm/10a, generally. But there is a periodic change in the time sequence, ithas a trend of first down then up with2001as turning point. Qualitativetrend analysis found that Epandecline is mainly caused by the decline inwind speed and total solar radiation from1970to2001; from2002to2011, Epanrise was mainly caused by increased total solar radiation andtemperature. So, total solar radiation is dominant factor causefluctuations of Epan.3Using Budyko’s coupled water and energy balance hypothesis estimateannual actual evapotranspiration in main basins of Qinghai-TibetPlateau,the results show that the simulation precision is high. Especially,Yarlung Zangbo River basin and the Hengduan Mountains, both thedecisive coefficient (R2)reached0.99, relative deviation between2.5~8.7mm, the standard mean square root of variance between3.1~10.6%.Budyko’s coupled water and energy balance hypothesis is suitablefor estimate actual evapotranspiration in the Tibetan Plateau.Most of theQinghai-Tibet Plateau basins, actual evapotranspiration under climatewarm and wet trend had no significantly trend, in other ward, actualevapotranspiration is less sensitive to temperature while strong sensitivity to precipitation. Analysis from the qualitative trends changeand correlation found that actual evapotranspiration is negativecorrelation with pan evaporation, this result can be explained throughthat the complementarity principle of actual evapotranspiration and panevaporation was negative correlation of Budyko’s coupled water andenergy balance hypothesis in the water restriction zone.