A field study was carried out to investigate the seasonal variations in alfalfa (Medicago sativa L.) water use efficiency (WUE) using Eddy Covariance (EC) measured CO2 and H2O fluxes, aiming at optimizing the use of irrigation water under hyper arid conditions of Saudi Arabia. The EC system used for this study was installed on a center pivot irrigated 50 ha alfalfa field in a farm located in the Eastern Province of Saudi Arabia. Results of the study showed EC estimated WUE values of 1.57 and 1.07 kg m-3 for winter (November 2013 to March 2014) and summer (June 2013 to October 2013 and April 2014) periods, respectively. However, the actual WUE values calculated for the harvested alfalfa crop were 0.70 and 0.71 kg m-3 for winter and summer periods, respectively. Attaining an actual crop WUE of 30-55% lower than the estimated one emphasizes the need for the application of precision irrigation systems to enhance water conservation practices.
