Synthetic Flows for Engineered Systems with Nonstationary Parameters: Study of Maui’s Wailoa Ditch

Abstract

Water flow through engineered channels is important for decision making given its close ties to availability for allocation. However, planners often rely on estimates for natural streamflow, then use stream-by-stream assumptions and aggregation to estimate allocatable flows rather than directly assessing flows through engineered channels. Further, synthetic flows based on historical records can be unreliable when parameter nonstationarity due to effects like climate change is likely. This case study of the Wailoa Ditch, a major engineered surface water supply system on Maui, Hawaii, uses a natural experiment based on Maui’s declining rainfall to demonstrate and validate that both problems can be addressed. For Wailoa, synthetic and actual flow characteristics differ by less than 5% when historical records are adjusted to reflect changing rainfall. Direct simulation of Wailoa’s flows reproduces modern conditions more accurately than stream-by-stream approximations. Precipitation-based scenario analysis suggests that under the influence of both decadal oscillations and climate change, Maui is far more likely to experience water supply shortages on its main engineered system in the future than in the past.