| Land evapotranspiration is a central process in hydrology and a nexus of the water, energy and carbon cycles. Research on influence of climatic variables to potential evapotranspiration is helpful to understand the response of hydrological process to climatic change. Sensitivity analysis of potential evaportranspiration to climatic variables is important to further understand the influence of climatic change to hydrological resource. At present, more and more observational evidence from all continents that many natural systems are being affected by climate changes, particularly temperature increase. Tibetan Plateau (TP) is a typical area in the global climate system and regional climatic change of TP poses a more sensitive and stronger effect to global climatic change. Therefore it is very meaningful to investigate the climatic change and evaportranspiration of the TP. Based on data (1970-2010) of maximum temperature, minimum temperature, average air temperature(Ta), precipitation, wind speed(u2), solar radiation, relative humidity and pan evaporation collected at75meteorological stations, we analysed the time series and spatial variations of climate on the TP. In order to get more accurate radiation input parameter when calculating reference evapotranspiration, we calibrated the Angstrom coefficient. Time series (1970-2010) and temporal trends of Penman-Monteith potential evapotranspiration(ETref) estimates for75stations on the TP were explored in this research and three different evapotranspiration equations results were compared. We conducted the detrended analysis to evaluate the contributions of different climatic variables to the temporal change of ETref, which are air temperature, wind speed, relative humidity and net solar radiation, and tested the response of the ETref to the change of the key climatic variables. The temporal trends of pan coefficient Kp (the ratio of P-M evapotranspiration and pan evaporation) were analysed further to understand the potential evapotranspiration on the TP. Then we estimated the actual evapotranspiration by using NCEP and MODIS data, mapped the spatial distribution of surface drought condition on the TP in year2000and2010. Besides, the evaporative drought index (EDI), based on the estimated actual evapotranspiration and potential evapotranspiration described. Based on the analysis mentioned above, the following conclusions can be drawn from the study:1. Almost95%of stations show warming trends in all seasons, especially the majority stations in the north of the TP show significantly increasing trends; however, the increase of maximum temperature is not so obvious compare to that of the minimum temperature and DTR(durnal air temperature) showed a decreasing tendency. About90%or more stations showed significantly increasing trend in minimum temperature during the three seasons. Seasonal warming trends regarding maximum and minimum temperature were statistically obvious in winter, followed by summer and spring. The annual precipitation had a modest increasing trend in compare to the temperature and the highest increase of precipitation was found in winter, followed by summer and spring. The annual averaged vapor pressure deficit data demonstrated an increasing trend in all seasons. Particularly, the northern and central and extreme eastern parts of the TP showed the strong increasing tendency. The increase of VPD and the decrease of relative humidity means the decrease of humid conditions and the decrease in runoff of Yellow headwater areas further give the evidence of that over the TP. The pan evaporation trends were significantly negative in all seasons on the plateau. The drying trend and the declined tendency of pan evaporation combine to indicate that there probably exists a water limitation over the TP. The climatic trends suggest a general warming and drying trends over the plateau, particularly the northern and central part of the TP. The complicated climatic trends over the Tibetan Plateau reveal that climatic factors have nonlinear relationships.2. The vegetation response to ENSO phases was detected using AVHRR NDVI and SST information. NDVI varied between ENSO phases. Cold phase resulted in above average vegetation response in growing season(Apr.-Sep.), that cold phase provide optimal climate conditions for vegetation on the TP, but resulted in the lowest NDVI in winter(Oct.-Dec). Warm phase resulted in lower vegetation condition in both growing season and winter, indicating a negative effect on vegetation. The neutral ENSO phase resulted in below average vegetation response in growing season and best vegetation response in winter.3. Both seasonal and annual ETref showed a decreasing trend on the TP. The averaged annual ETref trend was-0.6909mm/year from1970to2010. The maximum and minimum trend values were found in Jan.-Mar. and Jul.-Sep.. There was an obvious decreasing trend in wind speed and net radiation and a slightly decreasing trend in relative humidity. In order to explore the underlying cause of ETref variation, an attribution analysis was performed using Penman formulation to quantify the contribution of Ta, u2, Rn and ea. The results showed that the changes of u2, Rn and ea produced the negative effect whilst Ta produced the positive effect in ETref rates. The changes of u2(-0.7mm) were found to produce the largest decrease in ETref rates, followed by ea(-0.4mm)and Rn(-0.1mm). Although the significant increase of Ta(0.51mm) had large positive effect on ETref rates, changes in other three variables each reduced rates, resulting in an overall negative trend in ETref. Sensitivity analyses revealed that the most sensitive variable for ETref on the TP was net solar radiation followed by relative humidity, wind speed and mean air temperature.4. There was a statistical decrease trend in pan evaporation. Annual trend was-3.763mm/year and seasonal trends were0.4067mm/year in Apr.-Jun.,0.3209mm/year in Jul.-Sep.,0.3029mm/year in Oct-Mar. There was a high correlation between annual pan evaporation and annual reference evapotranspiration. Annual pan coefficient Kp showed an increase trend. There was an obvious negative trend in Apr.-Jun. and Jul.-Sep while a positive trend in Jan.-Mar. and Oct.-Dec. Apr.-Jun. showed the maximum decrease with significant value P0.0153and Jan.-Mar. showed the maximum increase with significant value P0.084. The stations showed Kp increase mainly located on northeast and southeast edge as well as central part along river of the TP. This distribution and variation mainly depends on the spatial distribution and variation of wind speed, solar radiation and relative humidity.5. The actual evapotranspiration of of the TP in the growing season (Apr.-Sep.) in year2000and2010estimated by remote sensing data and NCEP data. The results showed90%area exhibited the increase from April to June in year2000and then started decreasing from July to Sep. In year2010, the persistent increase of actual evapotranspiration started from May end on July and decreased from July to September. Based on the results of potential evapotranspiration and actual evapotranspiration, the evaporative drought index (EDI) was calculated of growing season in year2000and2010. The EDI described the spatial distribution energy fluxes in response to soil moisture and vegetation condition and the results indicated EDI index perform well in assessing drought at continental scales.
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