Daily and mean weekly output from seven evapotranspiration models with original constants (FAO-24 Blaney-Criddle, Hamon, Pristiey taylor , Hargreaves, Jenson-Haise, Romanenko and Turc), have been tested against reference evapotranspiration data computed by FAO Penman-Monteith model to assess the accuracy of each model in estimating gross reference evapotranspiration in Tamil Nadu.  Models were compared at fourteen stations of the India Meteorological Department observatories using data from 1981 to 1992.  From the seven models, five models were selected for evaluation. The criteria for selection was, the values of cross correlation (R2), intercept ‘a’ and slope ‘b’ of regression line.  The constants of each selected model were recalibrated using three different procedures.  In the first approach, it is assumed that the mean annual ETo estimated by each selected model is equal to the mean annual ETo estimated by the (PM) model. Therefore the percentage error between annual ETo estimated by each selected model and that of PM model was determined.  ETo Value, equal in magnitude to the rate of percentage error was applied uniformly to mean daily ETo for any one model considered.  If the error is positive, correction is negative and vice versa.  Then the adjusted ETo was equated to the expression of the respective models by using the corresponding daily meteorological variable required for the models and the constant was calculated on daily basis.  Yearly average and the averages for the period were determined, model wise, for all locations.  The constant values thus obtained were averaged, considering the frequently occurred values and also the values near to it, to obtain a unique value of the constant for each model.  Assumption in the second approach was that the percentage error in ETo of any one model and the PM model is zero.  The procedure described in the first approach was followed in determining the constant, model wise for the region.  

Keywords: Evapotranspiration, Penman-Monteith model, Estimating gross reference, Crop water requirement