Efficient utilization of available water resources in arid area of Egypt (The New Valley area) is crucial. Toward having a good and high efficient water management the determination of crop water requirement is appreciable. Many methods has been adopted to be used and required to be validated. The objectives of this study were to calibrate and validate some ETo models using lysimeter under the New Valley conditions. Estimation of accurate ETo under these climatic conditions was also considered. Nine non-weighing lysimeters were installed, planted with alfalfa as a reference crop. Daily and monthly values of twelve ETo models were compared with Lys-ETo during February 1st 2013 to January 31st 2015. The comparison was first made using original constant values in each ETo model, then the selected models were
calibrated as second step using data of first year through modified constant values involved in each model. In the last step, calibrated models were validated using both measured and estimated ETo data of second year. The comparative study indicated that the original FAO-24 Radiation and Blaney- Criddle models gave the lowest values of RMSE and RRMSE as compared with Lys-ETo, they were 0.90 mm day-1 and 10.24% for FAO-24 Radiation and 1.37mm day-1 and 15.58% for Blaney-Criddle, respectively. Also, locally calibrated FAO-24 pan model was the best model and gave the excellent coinciding as compare with the Lys-ETo observations under the New Valley conditions.