Abstract:
Evapotranspiration (ET) is of great importance in many disciplines, including irrigation
system design, irrigation scheduling and hydrologic and drainage studies. A large number of
more or less empirical methods have been developed to estimate the evapotranspiration from
different climatic variables. The Food and Agriculture Organization (FAO) rates the Penman-
Monteith equation as the major model for estimation of reference (grass) evapotranspiration
(ET0) because of the fact that it gives more accurate and consistent results as compared to the
other empirical models. However, the main disadvantage of this method is that it cannot be used
when the sufficient data are not available. The FAO-56 PM equation requires quite a few
independent variables such as solar radiation, air temperature, wind speed, and relative humidity
in predicting ET0. Worldwide, the weather stations measuring all these variables are few as the
majority measure air temperature only. Therefore, for regions which may not be measuring all
these meteorological variables, the temperature based models like Ritchie, Hargreaves-Samani
and Thornthwaite equations is necessarily used instead of the FAO-56 PM equation. In this
study, the Ritchie equation is applied on the measured data recorded at 158 stations at the
Coastal are of Turkey (Mediterranean, Aegean, Marmara and Black Sea regions of Anatolia),
and the monthly ET0 values computed by it are observed to be smaller than those given by the
Penman-Monteith equation. Next, average values for the coefficients of the Ritchie equation,
which are constants originally developed in [6], are recomputed using the ET0 values given by
the FAO-56 PM equation at all weather stations in coastal regions of Anatolia (Turkey). The
Ritchie equation modified in such manner is observed to yield greater determination coefficients
(R2), smaller root mean square errors (MSE), and smaller mean absolute relative errors (MARE)
as compared to the original versions of Ritchie equation suggested by [6]. It is concluded that for
estimation of reference evapotranspiration at coastal regions of Anatolia where the
meteorological measurements are scarce, the modified Ritchie equation can be easily used for
estimating the ET0 values.
