One such metric called the Successful Delivery Index (SDI) was developed by Rubicon Water in conjunction with Southern Rural Water in Australia and has proven effective as a measure of supply reliability. Improving the reliability of supply to farmers is a key objective of Southern Rural so it made sense to define what that meant and then measure.
The SDI metric gives a score to each irrigation delivery according to the percentage of time that the delivered flow rate was within ±10% or 0.5 megalitres/day (0.2 cfs) of the ordered flow rate (whichever is greater).
The graph below shows the flow rate variance over time for two orders with the limits shown as dashed lines. The green order achieved a 99% SDI and the blue order a 94% SDI value. SRW also chose to define “success” as a value of 95% or more.
The graph below shows the SDI values for all orders for every month of the 2007-2008 irrigation season at Southern Rural Water. There were a total of 764 irrigation deliveries, of which 84% were classified as successful by the Successful Delivery Index.
SRW uses this data to conduct root cause analysis of unsuccessful irrigations (below 95%) to provide pointers to improving system-wide performance. In the case cited here and shown in the next figure, it was found that the majority of unsuccessful irrigations were caused by just three problems: problems at the demand scheduling level, poor customer infrastructure (generally high ground on-farm or undersized on-farm infrastructure) and incorrect manual intervention during automated operation.
The Successful Delivery Index is now a key management concept used by SRW to manage their water resources and serve their customers better. Having a reliable supply helps farmers to adopt easier-to-manage irrigation systems on-farm and reduces on-farm water losses and nutrient losses into drainage water.