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 and 2005 in a clay loam soil in Aga, Dakahlia Governorate, Egypt. The amount of water used was based on free surface evaporation from a class-A pan of the U.S. Weather Service. This research aimed to study two irrigation systems (subsurface and surface drip) under four treatments of plant-pan coefficients, Kcp (0.6, 0.8, 1.0 and 1.2). Therefore water applications were 0.6, 0.8, 1.0 and 1.2 times the cumulative pan evaporation measured within the irrigation interval of 7 days. Fresh and dry tuber yield, tuber starch and protein contents and above ground biomass were measured. The surface drip irrigation system gave better 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, 43665 kg ha-1, the highest water use efficiency (73 kg ha-1 mm-1) was associated with the application of 80% of class A pan evaporation. Fresh tuber yield increased by about 3080 kg ha-1 (~8%) as irrigation increased from 0.8 to 1.2 of the evaporated from the class A pan (i.e. 50%increase). Therefore, a Kcp value of 1.2 was the best of those studied, which was equivalent to a potato water requirement of 598 mm. However, when cost of water exceeds the profit gained from the higher yield associated with the 1.2 Kcp value, the 0.8 Kcp value should be considered. It could be concluded that weather service class A pan was a successful tool in determining water requirements of potato crop under Egyptian conditions.