Comparison of Penman and Thornthwaite methods of estimating potential evapotranspiration for Indian conditions

Abstract

Thornthwaite"s (1948) empirical method of estimating potential evapotranspiration (PE) has been preferred by several scientists in India to Penman"s (1948) theoretical combination approach, because of the former"s simplicity. However, in view of the doubts expressed in various quarters regarding the applicability of Thornthwaite"s method for monsoon climates, a comparison is made of the performance of these two methods over different parts of India, using about 26 years of data at 15 stations spread over the country. Various aspects of the manifestations and their differences are presented. It is found that Thornthwaite"s method gives considerably higher estimates ofPE and shows lower inter-annual variability than Penman"s method during the southwest monsoon season. A systematic annual variation of the difference between the two methods is also noticed which is found to be mainly due to the actual vapor pressure and sunshine duration included in Penman"s method.