Availability of an irrigation system is defined as the ratio of time of successful system operation to total desired operation time. Operation is successful when the flow rate of water supplied exceeds a given percentage of the lesser of crop water requirements or design flow rate. Thus, availability provides adequacy and dependability information. Furthermore, this ratio can be decomposed into unavailabilities indicating the percentage of time low flow was caused by various problems. This is valuable information for improving water delivery performance. Over 12 seasons availability varied from 0.0 to 0.90 in two Nepal irrigation systems. Canal capacity less than crop water requirements was the greatest reason for unavailability during the monsoon season. Lack of water in the source was the greatest reason during other seasons.