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0.9) for all equations except Turk. The results showed that Blaney-Criddle, Modified Jensen-Haise and Hargreaves had better accuracy compared to other equations so that their root mean square errors (RMSE) were 0.572, 0.721 and 0.945 mm.d-1, respectively. In addition, saffron water requirement determined with FAO56-PM was equal to 2350 m3.ha-1.y-1. Hargreaves equation, with differences about -161.23 m3.ha-1.y-1 compared with FAO56-PM, was determined as a good equation. Measured irrigation water by saffron farmers was about 1184.17 m3.ha-1.y-1. According to the results it found that deficit irrigation was applied by saffron farmers. Saffron grow in arid regions, however, drought stress have a negative effect on its plant development and yield (Khorramdel et al., 2014; Khashei Siuki et al., 2015). Despite the lack of doing no test about the effect of deficit irrigation on saffron yield during 2014 in this region, it seems that deficit irrigation may be a reason of low saffron yield (SYASK, 2010) compared to average saffron yield in this region.
Results of gamma test revealed that temperature is a key parameter to develop a single model. In addition, results showed that the single parameter model based on temperature had better accuracy compared to above-twelve mentioned equation.
Conclusion
Hargreaves equation was only used the temperature (T) and extraterrestrial radiation (Ra) for estimating evapotranspiration. In addition, single parameter equation based on the mean temperature was only used one parameter (T) to estimate the evapotranspiration. Since both of the equations had an acceptable accuracy to use in this region, it is recommended to use them to determine saffron irrigation demand. It is due to lack of meteorological equipment for estimating all parameters in the plain.
Acknowledgement
The authors would like to thank the anonymous reviewers whose valuable comments have helped us to clarify some parts of this paper.]]>
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