Definitions of Reliability Terms from the 2018 OC Water Reliability Study
For those who want a deeper dive, Water “Supply” and “System” Reliability with respect to water systems are often referenced by Karl Seckel as they have formed an essential aspect in what Karl has pursued during his career. These terms are defined in detail below from the 2018 OC Study which can be accessed via the link provided.
The continuing ability of a local water agency to meet customer water demands when there are unplanned emergency outages of key water facilities (e.g., treatment plants, conveyance, and distribution pipelines), caused by seismic events, facility failures, or other catastrophic events (typically for the duration of weeks or months). Water demand during these emergency events would be at reduced levels, most likely through mandated water use restrictions and requests for conservation of supplies. System reliability is needed to ensure public health and safety of residents and business continuity in the area affected by the outage.
Having enough water supplies to meet water demands under different hydrologic conditions, measured in terms of frequency (probability of occurrence), duration (length of occurrence), and magnitude (size) of water shortages. These water shortages can be caused by hydrologic droughts and/or limitations in supply availability due to regulatory constraints (e.g., Endangered Species Act). Water supply availability is driven by water contracts and appropriations, temperature, precipitation, and storage conditions; whereas water demand is driven by demographic/economic growth, levels of water use efficiency, and customer behavior. During severe and longer-lasting droughts, water agencies can help to manage available supplies by requesting water customers to reduce water usage (i.e., water rationing or demand curtailment). However, studies in California have shown that frequent use of mandatory water use restrictions can lead to significant economic and quality of life impacts. Continuing supply reliability is needed to ensure long-term economic vitality and quality of life.
Water Rationing/Demand Curtailment
Water Reliability does not mean the absolute elimination of water rationing or demand curtailment. In fact, during major system outages it is expected that nonessential water uses (e.g., landscape irrigation) would be minimized for the duration of the outage to ensure enough water for public health and safety. As for the use of demand curtailment during droughts, there are generally two schools of thought: (1) it is acceptable that water agencies ask water customers to cut back on water use during severe droughts; or (2) because of significant investments in permanent/structural water efficiency (e.g., plumbing codes, landscape ordinances, and utility conservation rebates for ultra-efficient devices and landscape transformation), water agencies should strive to develop sufficient supplies so that demand curtailments are minimized. In discussions with Orange County water agencies during both the 2016 OC Study and 2018 OC Study, most agencies believed it was appropriate to count on demand curtailments roughly 1 in 20 years (5 percent of the time), with expected curtailments to result in a 10 percent reduction in water use into the future. While it is true that during the most recent drought water demands were reduced by roughly 20 percent statewide, “demand hardening” will make it more difficult for water customers to achieve such high reductions in the future. Demand hardening occurs because as water efficiency (from plumbing codes, landscape ordinance and utility rebates) increases there will be less ability to conserve additional water. In the end, however, it is up to local and regional water agencies and their customers to determine how frequent demand curtailment will be used and at what target reduction in water use.