The effect of irrigation water amount is a very important issue for the growth and yield of potato. A field experiment was conducted during the late winter planting seasons of 2004:2005 in a clayey loam soil in Aga. Dakahlia Governorate. This research aimed to study two Irrigation systems (subsurface and surface drip) and four different treatments of irrigation water amount (0.6Ep. 0.SEp. 1.0Ep, and 1.2Ep). The surface drip irrigation system gave higher values for most of the yield parameters that were measured in this experiment. Even though the application of 120% of the class A pan evaporation resulted in the highest fresh tuber yield. 873.3 glplant. the highest water use efficiency was associated with the application of 80% of class A pan evaporation. Thus. the optimum c value was the 1,2, which was equivalent to a potato water requirement of 598-mm. It could be concluded that weather service class A pan was a Successful tool in determination of the water reduirements of potato crop < /p> under Egyptian conditions.