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In order to investigate the importance of the uncertainty associated with ETo methods, it is compared with GCM uncertainty which is considered as the main source of uncertainty in climate change studies (Wilby & Harris 2006; Kay et al. 2009; Chen et al. 2011; Exbrayat et al. 2014) and also that of future greenhouse gas scenarios (RCP). For the monthly ETo change and also taking the full range of daily ETo change into consideration, the uncertainty in GCMs is larger than that in the ETo methods and RCPs (Table 2). For both mentioned cases, the ETo uncertainty is of comparable magnitude to the GCM uncertainty. As for the changes in high daily ETo percentiles, the uncertainty related to the ETo methods is the dominant source of uncertainty for both summer and winter seasons, while RCP has the lowest contribution to the total uncertainty in ETo changes. Generally, the uncertainty analysis shows that the uncertainty in ETo methods is noticeable for all the cases considered. This shows the importance of addressing the uncertainty associated with ETo methods next to that in GCMs and future greenhouse gas scenarios as is traditionally done.

Table 2

Comparison of the total variance decomposition related to the uncertainty in GCMs, RCPs, and ETo methods

UncertaintyMonthly EToDaily ETo percentile changes (full range ofDaily ETo percentile changes (>90%)
sourcechangespercentiles)DJFJJA
GCMs 44 39 25 36 
RCPs 28 27 21 23 
ETo methods 28 34 54 42 
UncertaintyMonthly EToDaily ETo percentile changes (full range ofDaily ETo percentile changes (>90%)
sourcechangespercentiles)DJFJJA
GCMs 44 39 25 36 
RCPs 28 27 21 23 
ETo methods 28 34 54 42 

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