Evaluation and Calibration of Christiansen Method for Estimating Daily Evaporation from Class-A Pan under the Conditions of Van, Turkey

Evaporation (Epan) measured from Class-A pan evaporimeter is widely used in many studies within the scope of hydrology. Due to various reasons, it may be necessary to complete the unmeasured evaporation data using empirical estimation methods. The reliability of these methods varies depending on climatic and environmental conditions. Therefore, they need to be tested under the local conditions and calibrated if necessary. This study aims to test the usability of Christiansen evaporation estimation method under the conditions of Van, and to calibrate it in compatible with local conditions. Firstly, the original equation of this method was tested using nine years of daily climate data measured between 2012 and 2020. Then, the original equation was calibrated using the same data and its modified equation was created. The validity of evaporation values estimated using both the original and modified equations was tested with climate data from the period of 2021–2022. The performance of Christiansen method, calibrated using the linear regression approach, in estimating daily evaporation was evaluated using the determination coefficient (R2), mean absolute percentage error (MAPE), and Nash–Sutcliffe Efficiency (NSE) statistical metrics. While the original Christiansen equation estimated evaporation values with 74.90% accuracy (R2= 0.79, MAPE= 25.10%, NSE= 0.48), the accuracy improved to 86.58% (R2= 0.79, MAPE= 13.42%, NSE= 0.77) using the modified equation. The differences between the means of the data groups consisting of the measured evaporation values and those estimated using the modified Christiansen equation were not statistically significant (p > 0.05). It has been concluded that, the daily evaporation values estimated by the modified Christiansen equation can be used instead of the measured values.