Evaluation of Alfalfa Reference Crop Evapotranspiration (ETr)Ratio to Grass Reference Crop Evapotranspiration (ETo) Ratio in Shahrekord Plain

It is necessary that ETr (Alfalfa-reference evapotranspiration) values be converted to ETo (Grass-reference evapotranspiration) or vice versa. The main objective of this study was to develop ETr to ETo ratios (Kr values) for a growing season in Shahrekord plain، Shahrekord، Iran. Mean monthly and total (growing season) values of Kr were calculated based on 185 daily ET data set in Chaharthakhteh Agricultural Resaerch Station of Shahrekord. The ETr and ETo values were calculated using six models for developing Kr values. The models included the Standardized American Society of civil Engineers Penman-Monteith (ASCE-stPM)، American Society of Civil Engineers Penman-Monteith (ASCE-PM)، 1982 Kimberly-Penman (KP)، and modified Jensen-Haise (JH). Kr values as a function of some of the climatic variables in FAO56 Irrigation and Drainage Paper were compared with lysimeter ETr values. For the growing season، Kr values based on lysimeter study was 1. 12، which was comparable to 1. 12، and 1. 16 for ASCE-PM، and JH، respectively، but was far from 1. 27 for 1982 KP models. ETr values in each method compared to ASCE-stPM-ETo were not comparable to 1. 40 derived from Kr value based on FAO56 method.