Potential evapotranspiration ( ET0
) is a key variable of hydrological cycle and has important applications to
hydrological modeling and crop irrigation scheduling． Under the background of global warming，analyzing the spatiotemporal
characteristics of the present and future ET0 can provide important information for regional water resource management and
sustainable agricultural development． To study the impacts of ecological construction and climate change in the past 50 years
on the hydrology in the Jinghe watershed，some researches have assessed the changes of ET0 for the present period;
however，almost no study has focused on the potential changes of ET0 ． The objective of this study is to project the change of
ET0 during 21st century on the basis of the current ET0 analysis and further discuss its impacts on climate in the Jinghe
watershed． The data used in this study included daily weather data from 15 meteorological stations，NCEP reanalysis data
which reflects the quasi-observed climate condition and GCM grid outputs from HadCM3 under A2 and B2 scenarios．
Penman-Monteith ( PM) method and statistical downscaling model ( SDSM) were used to calculate ET0 for the present
period of 1961—2005 and project ET0 for the future period of 2011—2099，respectively． The determination coefficient and
Nash – Sutcliffe model efficiency coefficient were used to assess the performance of SDSM． The inverse distance weighted
interpolation and nonparametric Mann-Kendall test was used to analyze the spatial distribution and temporal change of ET0
，respectively． Ｒesults showed that ET0 calculated by PM formula correlated to the pan evaporation well，which means that the
estimated ET0 is reliable． With annual average of 934．6 mm for the present period，ET0 had a spatial difference decreasing
from the east to the southwestern part of the Jinghe watershed． An insignificant upward trend of annual ET0 was detected for
the present period and an abrupt change occurred in 1977． The monthly ET0 distributed as a single peak curve with the
greatest and smallest value in June and December，respectively． During 21st century，ET0 would greatly increase by 1．9 mm/
a and 0．9 mm/a under A2 and B2 emission scenarios，respectively． ET0 appeared to increase dramatically since 2049 and
have an upward abrupt change in 2061． Future ET0 also varied among seasons with the greatest increase in summer and the
least in winter． The spatial distribution of future ET0 was similar to the current period，but the regional difference would
increase over time． The aridity index calculated by the ratio of ET0 to precipitation revealed that the Jinghe watershed would
be threatened by more severe drought during 21st century，and most regions would have semiarid climate at the end of 21st
century though now it is a transition region with semiarid and subhumid climate． The uncertainties in this study are mainly
from GCM and downscaling method; however，as the GCM from HadCM3 used in this study has the best performances in
simulating Chinese climate and SDSM has good performances in downscaling the present ET0
，the results should be reliable
and will be further validated with more GCM data and downscaling methods． The continuous increase in ET0 and drought
would aggravate the water shortage and further influence the ecological construction and regional development in the Jinghe
watershed; therefore，some countermeasures should be adapted in advance to minimize the adverse impacts．
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