evaluation the post processed outputs of dynamic models in estimation potential evapotranspiration changes under rcp scenarios (case study: mashhad plain)

as a direct consequence of warmer temperatures, the hydrologic cycle will undergo significant impact with accompanying changes in the rates of precipitation and evaporation. climate change will cause changes in climate variable such as precipitation, temperature, sunshine hours, wind speed and etc. so as a result of climate variable change, the related variable such as potential evapotranspiration will change too. as the soft computing skills increased in recent decades, more number of climate models has been developed for weather and climate predictions which have significantly improved the quality and quantity of projections. this notable increase in number of climate models has enabled the scientists to estimate a wide range of main climate variables such as precipitation and temperature in fine temporal and spatial resolutions. although the uncertainty in model outputs still remains a main challenge. upon the release of new scenarios based on radiative forcing which are known as representative concentration pathway scenarios (rcp scenarios), by intergovernmental panel on climate change (ipcc) in fifth assessment report (ar5), a new set of 42 global climate models (gcms) have been proposed for future climate projections. apart from increased number of available models, three main sources of uncertainty including: measurement error, variability, and model structure, that have been explained and studied in ar5.the aim of the current study is to investigate of changes of potential evapotranspiration (et) over mashhad plain, northeast of iran in future period 2021-2070 under two rcp scenarios i.e. rcp4.5 and rcp8.5. the main synoptic station in the region is mashhad station located at 59◦ 38ﹶe, 36◦ 18ﹶn, with elevation of 990 m. above m.s.l. the required meteorological data including maximum and minimum temperature, sunshine hours,wind speed for period of 1991 to 2005 were obtained from iran meteorological organization for et calculation using fao penman-monteith (hereafter, fao-pm) equation. besides, the downscaled historical data of potential evapotranspiration provided by swedish meteorological and hydrological institute (smhi) have been retrieved for the baseline period of 1991-2005.then these historical estimated data were compared with those estimated using fao-pm equation. the historical et values were post-processed using a statistical proposed method for more accuracy. by completion of this part, the accuracy of historical dataset provided by smhi was confirmed and used for further comparisons. in the second section the et variations for future period of 2021 to 2070 under two rcp scenarios of 4.5 and 8.5 was studied. the results showed better estimation of et during warm months. statistical comparisons using t-test revealed significant differences between historical and estimated values of et in months of february, march and december. the correlation coefficient between post processed and observed data showed similar results as in t-test. since the historical dataset of potential evapotranspiration provided by smhi was acceptable, it was used for the analysis during future period (2021-2070) under rcp4.5 and rcp8.5 scenarios compared to baseline observed data. the result of this part showed that the highest increase of potential evapotranspiration would be for january by 15.4% and 16.4% under rcp4.5 and rcp8.5 scenarios respectively and october would experience lowest decrease by -12.5% and -10.0% decrease, respectively. in general et increase will be more under rcp 8.5 scenario comparing to rcp 4.5