Two field experiments were conducted at Sakha Agricultural Research Station, Kafr El-Sheikh governorate during the two successive seasons 2005/2006 and 2006/2007 to study the effect of irrigation after 40%, 60% and 80% depletion of available soil moisture (ASMD), and three potassium rates i.e. 0, 24 and 48 kg K₂O/fed. on sugar beet yield and water use efficiencies. A split plot design with four replications was used. Irrigation treatments occupied the main plots, while potassium rates arranged in sub-plots. Results showed that increasing soil moisture depletion from 40% to 80% significantly decreased root diameter by 2.9%, root weight/plant by 6.8%, top yield/fed. by 4.6%, root yield/fed. by 4.1% and sugar yield/fed. by 10.0% . On the other hand, root length, total soluble solids and sucrose percentage were increased by 16.1%, 1.2% and 1.20%%, respectively. Increasing potassium application up to 48 kg K₂O/fed. significantly increased root length, root diameter , root weight/plant, fresh top and root yields/fed. by 2.2%, 3.0%, 1.7%, 4.5% and 6.3%, respectively, compared to the control treatment. Seasonal water consumptive use values were 61.0 cm, 56.19 cm and 46.38 cm for irrigation after depletion of 40%, 60% and 80% of available water, respectively. In addition increasing K-rates up to 48 kg K₂O/fed. slightly increased seasonal water use. Seasonal irrigation water applied values were 68.28 cm (2867.8 m³/fed.), distributed on eight irrigations, 62.08 cm (2607.4 m³/fed.), distributed on seven irrigations and 55.07 cm (2312.9 m³/fed.), distributed on six irrigations, for irrigation after 40%, 60%, and 80% of available soil moisture depletion, respectively. Water use efficiencies values for both root or sugar yields increased as soil moisture depletion increased. While water use efficiencies for both root or sugar yields significantly increased as potassium rate increased up to 48 kg K₂O/fed. The mean percentage values of water extracted from the upper 30 cm soil layer were 76.36, 71.78 and 65.18% when sugar beet plants irrigated at 40%, 60% and 80% of ASMD, respectively, A linear slop indicated that each one cm of water applied increased the productivity of root and sugar yields by 74 and 15.7 kg/fed. In addition irrigation water applied is strongly positively correlated with roots yield and negatively to water use efficiencies. Therefore, when water is becoming a limited factor, irrigation at 80% of ASMD could be applied for saving 17.8% of irrigation water against 4.1% and 3.4% reduction in the root and sugar compared to irrigation at 40% of ASMD.