Use of Fuzzy Logic to Determine the Evapotranspiration in Gonabad City

Evapotranspiration are the main component of hydrologic cycle and estimation of the amount of evapotranspiration is important to study the water balance, design the irrigation systems and ultimately plan and manage the water resources to achieve the stable development.  Evapotranspiration are nonlinear and complicated phenomenon due to requirement for different factors and their interactions. One important step in non-linear system modeling is pre- investigation of inputs to achieve suitable combination of them. Pre-investigation of input data prevents several trial and error steps and helps to understand the most important parameters which affect the phenomenon to be able to modeling the system in an intelligent way. Thereby, this study, has aimed to implement the ability of Fuzzy logic system to estimate Evapotranspiration by using the data from Gonabad weather station in a 21-year period (1993-2014).

To reach this aim, after investigation of available models and different combination of weather information, the final model to estimate Evapotranspiration has been designed. In this model with 20 surface as input and one surface as output or evaporation and transpiration, 50 rules were determined in Mamdani Inference System and the estimated value of Evapotranspiration from the Fuzzy Inference were compared with the results from Fao Penman Monteith (F-P-M).

Statistical parameters including Root Mean Square Error (RMSE), Main Bias Error (MBE) Coefficient of Determination, Jacovides factor (t) and Sabbagh et al. factor (R2/t) have been used to investigate the efficiency of this model. Comparison of the results from the Fuzzy Model and the results from F-P-M shows a high level of correlation between these two methods indicating the highest level of Evapotranspiration in July (RMSE: 0.08, R2: 0.98, MBE: 0.05, t:0.25 and R2/t: 3.92).