Changing climate conditions and growing demand for fresh water have raised concerns about the reliability of traditional water supplies. Assessment of resultant water-related risks requires consideration of the interactions between the natural, engineered and human systems. Due to complex policies and significant data needs, appropriately accounting for water management protocols (e.g., reservoir operations, water rights, instream flows) in such assessments is often problematic. This raises the question of just how important are these factors to understanding the risks to water delivery? To explore this question two multi-model systems with differing purposes (short-term operations vs. long-term planning), and associated levels of complexity in their representation of water operations and water rights, are compared. The San Juan River Basin in the Four Corners Region of the Southwestern U.S. was used as the test case. Both multi-model systems link a surface hydrology model, the Variable Infiltration Capacity (VIC) model, with a river/reservoir routing model, RiverWare, and in one case additional models that simulate priority administration of water rights. Analyses were forced by five different climate projections (all using RCP 8.5) and two different water use futures. Comparisons were drawn on the basis of five metrics: storage in Navajo Reservoir, local water deliveries, instream flows, transmountain exports to the Rio Grande basin, and discharge to the Colorado River. Both multi-model systems indicated generally negative impacts from climate change and increased water use on all five metrics. However, the multi-model system that lacked a full accounting of the rules governing water rights administration and water operations consistently overestimated water-related risks across all five metrics. This makes sense, as the purpose of these water policies is to manage operations in times of drought and emphasizes their importance in evaluating system performance.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.