Identification of Suitable Method for Estimation of Reference Evapotranspiration for Paleru Sub Basin of River Krishna Soumya Kiran P. 1 , Chandra Bose A.S. 2 and Giridhar M.V.S.S. 3 1 Assistant Professor, CMR Technical Campus, Medchal, Hyderabad 2 Senior Lecturer in Civil Engineering, Government Polytechnic, Station Ghanpur, Janagaon District, Telangana state. 3 Assistant Professor and Head, Centre for water Resources, IST, JNTUH, Hyderabad ABSTRACT Grass Reference evapotranspiration as ‘ the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height of 0.12 m , a fixed surface resistance of 70 sec/m and an albedo of 0.23 closely resembling the evapotranspiration from an extensive surface of green grass of uniform height actively growing completely shading the ground and with adequate water’. Evapotranspiration is one of the major components of the hydrologic cycle. Accurate estimation of RET is essential in many environmental studies such as hydrologic water balance, irrigation scheduling, and efficient water resource planning and management. Different meteorological parameters such as Maximum and minimum temperature, Maximum and minimum relative humidity, wind speed, sunshine hours shall be collected in order to estimate RET. Various methods are available in the literature to determine Reference evapotranspiration based on temperature methods, radiation methods and combination methods. However, all the methods are not suitable for accurate estimation of RET for all the study areas, as RET methods are site specific. Keeping this in view, it is proposed to study and estimate daily, monthly and yearly reference evapotranspiration for the study area using the above methods. Paleru sub basin (K-11) of river Krishna, located in the Andhra Pradesh has been considered for the study area. Out of all the available methods, a suitable method for estimation of RET shall be identified pertaining to the study area. Keywords: RET, Paleru sub basin, Evapotranspiration. INTRODUCTION Evaporation is one of the major processes in the hydrological cycle and it is a vital consideration in water resources planning and for effective management of water. Evapotranspiration constitutes the loss of water to the atmosphere by the combined process of evaporation from soil and plant surfaces and transpiration from plants. The accurate estimation of evapotranspiration is the requirement for many studies such as hydrologic water balance, irrigation system design and management, crop yield simulation, and water resources planning and management. Doorenbous and Pruitt (1977) defined the reference evapotranspiration rate as “the rate of evapotranspiration from an extensive surface of 8cm to 15cm tall green grass cover of uniform height, actively growing, completely shading the ground and not short of water”. Later Allen et al (1994) defined the grass reference evapotranspiration as “the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height of 0.12m a fixed surface resistance of 70s m -1 and an albedo of 0.23 closely resembling the evapotranspiration from an extensive surface of green grass of uniform height actively growing completely shading the ground and with adequate water”. The widely used reference equations for accurate estimation of ETO are of the combination type and the one recommended by FAO-56 is the Penman Monteith (PM) equation. Further, ET O estimation methods other than PM method will continue to remain in practice owing to simplified data requirements. Evapotranspiration is a complex phenomenon as it depends on several climatological factors, such as temperature, humidity, wind speed, radiation and type and growth stage of the crop. ET can be either directly measured using Lysimeter or water balance approaches or estimated indirectly using climatological data. Several methods have been developed to assess RET based on temperature, radiation and their combination. There are three general approaches to estimate reference crop evapotranspiration viz., temperature methods, radiation methods, and combination methods. However, in the present study, the ET o program is used to calculate ET o values by using various methods based on temperature radiation and their combination. Keeping these points in view, identification of suitable method of estimation of Reference Evapotranspiration is found essential and it is set as the objective of the present study. The objectivities of the present study are as follows 1. To study and estimate RET using the existing equations in the Palleru sub basin of Krishna basin. Proceedings of 4 th National Conference on Water, Environment & Society (NCWES-2017) : ISBN : 978-93-5230-182-9 56 Copyright © 2017, Published by BS Publications.