Estimating seasonal reference evapotranspiration using limited weather data MAHESH CHAND SINGH 1* , SHIVAM POONIA 1 , SANJAY SATPUTE 1 , VISHNU PRASAD 2 and SOMPAL SINGH 1 1 Dept of Soil and Water Engineering, PAU, Ludhiana, India 2 ICAR-Water Technology Centre, New Delhi, India 3 Dept of Climate Change and Agril Meteorology, PAU, Ludhiana, India * Corresponding author email : msrawat@pau.edu A reliable estimation of reference evapotranspiration (ET o ) is highly essential for managing agricultural water in water scarce regions. ET o can be computed directly using micro-meteorological techniques based on energy balance and water vapour mass flux transfer approaches or indirectly using the empirical methods. However, using direct methods may involve high cost and time, whereas the indirect approaches are based on site specific weather data (Prasad and Kumar 2013; Jadhav et al 2015; Sibale et al 2016). Among the numerous available empirical methods of estimating ET o , the FAO-56 PM equation has been globally recognized as a standard method (Allen et al 1998; Tabari et al 2011). However, it involves a maximum number of parameters for estimation of ET o . The FAO-PM equation based ET o calculator helps to reduce the number of input parameters for ET o estimation. Thus, the FAO-ET o calculator can also be recognized as a standard method for accurate ET o estimation. The present study was undertaken to estimate long-term (1995-2019) daily average ET o using seventeen methods including FAO-ET o calculator, compare the performance of sixteen methods with FAO-ET o calculator and identify the appropriate alternative (s) to FAO-ET o calculator for seasonal ET o estimation. The present study was undertaken at the department of Soil and Water Engineering, Punjab Agricultural University (PAU), Ludhiana to investigate appropriate substitute (s), requiring limited weather data for estimating seasonal ET o as accurately as FAO-ET o calculator. The daily weather data for twenty-five years (1995- 2019) was obtained from the weather observatory of PAU, Ludhiana (located between 30°54’ N latitude and 75°48’ E longitude with an altitude of 247 m above mean sea level). The climatic data included daily air temperature (minimum and maximum), relative humidity (minimum and maximum), wind speed, sunshine hours, rainfall and evaporation. Ludhiana district is bounded between latitude 30°33’’ to 31°01’’N and longitude 75°25’’ to 76°27’’E, having geographical area of 3767 km 2 . In this region, the summer temperature exceeds 38°C and touches 47°C with dry spells. Winter experiences frost during December and January with minimum temperature below 4°C, dominated by North-Eastern winds during winter season. The average annual rainfall in the region is 680 mm, 75-80% of which is received during monsoon period (June to September). There are mainly four seasons viz. Spring (March-May), Summer (June- August), Autumn (September-November) and Winter (December- February). The FAO-ET o calculator was used as standard for performance evaluation of the other sixteen methods of ET o estimation. Finally, nine most appropriate methods (Tabari (2011a), Tabari (2011b), Irmak (2003), Hargreaves-Samani (1985), Pan- Evaporation (1998), Priestly-Taylor (1972), Caprio (1974), Jensen and Haise (1963) and Penman (1948)) were screened for further evaluation based on their closeness with FAO-ET o calculator in terms of estimated ET o . The analysis was carried out to identify the appropriate alternative (s) to FAO-ET o calculator, requiring limited weather data (2-3 parameters only) for seasonal ET o estimation. The performance analysis included computation of different statistical parameters viz. mean absolute error (MAE), root mean square error (RMSE), standard deviation (SD) and Willmott index of agreement (d). Identifying best suitable alternative (s) to FAO-ET o calculator for ET o estimation Spring season: The ET o computed by using FAO-ET o calculator, Jensen and Haise (1963), Hargreaves-Samani (1985) and Pan- Article info - DOI: https://doi.org/10.54386/jam.v24i1.786 Received: 15 April 2021; Accepted: 29 December 2021; Published online: 11 February 2022 This work is licenced under a Creative Common Attribution 4.0 International licence @ Author(s), Publishing right @ Association of Agrometeorologists Short Communication Journal of Agrometeorology ISSN : 0972-1665 Vol. No. 24 (1) : 99-102 (March 2022) https://journal.agrimetassociation.org/index.php/jam