Abstract: Based on a predigested but equivalent canal system for the Zhanghe Irrigation Distric,tusing an empirical formula,the irrigation efficiency(IE) in the system is calcu latedatd ifferent scales under different conditions.The result shows IEs are found to be higher at the primary and trunk canals than that of at the submain and branch canals.These IE differences are mainly due to the differences in the canal cross section and length,as well as the lining ratio.We calculate the maxmium irrigation efficiency(MIE) and its corresponding cost,as well as the water saving potential and ratio underd ifferent conditions.An input output analysis of the relationships between the cost and MIE aswell as the water sav ing ratio is conducted.It shows that bothM IE and the water saving ratio are proportional to the cost,but comply the rule of decreasing rewards during the course of the input output process.The deceleration rate of the water saving ratio would be greater than that of MIE.