Using paleo-climate data to assess management options related to shortage risk on the Colorado River

Over the years, the large storage capacity of the Colorado River system has played a major role in ensuring water demand be consistently met. The current drought, however, has depleted system storage to its lowest level since the major reservoirs were filled. For water managers, current storage levels have two important implications: 1) future shortage declarations are very likely, and 2) climate variability is likely to have a greater effect on water supply reliability than previously when storage capacity was high. In this study, we examine possible interactions between climate and policy as they affect shortage declarations. Recently developed management guidelines identify three threshold elevations in Lake Mead that signal the implementation of progressively greater levels of involuntary shortage. For water managers and stakeholders, it is essential to know the timing and probability of breaching these thresholds. While climate is an overarching regulator of Lake Mead water levels, the rate of change in water elevation is also influenced by water demands in the Lower Basin. The recently implemented Intentionally Created Surplus (ICS) policy, a mechanism developed to encourage the Lower Basin States of Arizona, Nevada and California to conserve water in Lake Mead affects the level of these demands. ICS projects can reduce system waste, boost inflows, or reduce outflows. Under ICS, there are specific rules governing the annual and total amounts of water that the Lower Basin States can store in Lake Mead. Initial analyses show that ICS can delay the onset of involuntary shortages. Here, we explore the effect of climate-based ICS decisions on Lake Mead water levels, where the amount of water stored in Lake Mead depends on annual streamflow input. In other words, is ICS more efficient when practiced during wet years? Or, during dry years? Or, does it not matter? We use paleoclimatic data to characterize the risk of shortage declaration under different climate scenarios. Specifically, we use data from a tree-ring based reconstruction of streamflow to drive the Bureau of Reclamation's long-term planning model of the Colorado River, the Colorado River Simulation System (CRSS).