Abstract :
ABSTRACT: The current study was performed with the aim of investigating the error of pan evaporation data of Gorgan synoptic station in Golestan province, Iran, the calculation of pan evaporation coefficient (Kp) by pan data, its estimation by 7 empirical methods including Cuenca (1989), Snyder (1992), modified Snyder (1992), Pereira et al. (1995), Orang (1998), Allen et al. (1998) and Abdel-Wahed and Snyder (2008) and the comparison of these. The results showed that the correlation between the average air temperature and pan evaporation (Ep) was less than the correlation between average air temperature and ETo. On the other hand, the scattering of Ep data related to the average air temperature was more than the scattering of ETo to average air temperature. Thus, it can be said that the relation between Ep and air temperature in addition to the effective factors on evaporation is affected by other factors that is human error in reading and recording of pan evaporation data. Also, due to that, in considerable percent of days Kp is more than 1 while this coefficient is reported between 0.35 to 0.85 for various conditions. Kp was obtained more than 1 in rainy months. The investigations showed that in most of the cases, the station watchman didn’t add rainfall amount as reducing pan water level due to evaporation. The results showed that using empirical equations to estimate Kp and calculation of ETo led into underestimation of this parameter compared to FAO Penman Monteith (1998) method.
