Evaluation of Two Hourly Combination Models for Estimation of Grass Reference Evapotranspiration (Case study: Climate of Kerman)

In this study, the Penman-Monteith methods proposed by the Food and Agriculture Organization (FAO-56) and American Society of Civil Engineers (ASCE) were used for hourly ETo estimation under the semiarid climate of Kerman, Iran. Hourly ETo estimations obtained from the proposed methods were compared with measured ETo values by using a large weighing electronic lysimeter during April to September 2005 (totally 3352 hourly ETo data cases). Simple linear regression and statistical factors such as root mean square error and index of agreement were used for estimated and observed value comparison. The average of measured and estimated hourly ETo values using these methods for integrated data were 0.28 and 0.23 mm hr-1, respectively, which means that average estimated ETo values were approximately 21 percent less than the measured ETo values. This analysis was also performed for hourly data of each month during the study period. The results showed that FAO-56 Penman-Monteith underestimated ETo values by 18.4, 19.3, 26.3, 20.4, 21.4 and 22.1 percent for April to September, respectively, when compared with the measured values. Similarly, the ASCE Penman-Monteith underestimated ETo values by 17, 19.6, 18.4, 18.2, 19.7 and 20.9 percent for the same period, respectively, when compared with the lysimetric data. Finally, a set of regression equation for transformation of the estimated hourly data into the measured hourly ETo values has been presented for each month.