Water Supply in the Lower Colorado River Basin: Effectiveness of the 2019 Drought Contingency Plan

The Colorado River supplies water to seven southwestern states and Mexico. It is one of the most stressed river systems in the US. To address sustainability in the Lower Colorado River Basin (LCRB), the US Bureau of Reclamation (USBR) organized discussion of river operations and diversions among the LCRB states, eventually leading to the 2007 Interim Guidelines (2007-IG) and then to a revised river management structure, the 2019 Drought Contingency Plan (DCP). Among other features, the DCP increased the severity of reductions, or shortages, in annual water deliveries to the LCRB states (California, Arizona, and Nevada) and Mexico that are triggered by low year-end water levels in Lake Mead. Shortage measures were designed to mitigate the effects of long-term southwestern drought on reservoir levels, perhaps avoiding the worst outcomes for regional water supply while maintaining the energy-generating capability of the Colorado River system. The objective of this study was to evaluate water supply reliability in the LCRB with and without DCP measures and with and without anticipated climate effects. Four combinations of hydrological conditions and management strategies were analyzed. The results show that both 2007-IG and DCP measures will reduce the most severe shortages in Colorado River Water (CRW) supply to the LCRB states and Mexico under historical hydrologic conditions. Neither set of regulating measures insulates the LCRB against drought with anticipated climate change. However, the most distressing low-water projections are predicted to be less frequent and severe with DCP measures in place. The methods used to project the impacts attributable to climate change and the DCP on LCRB water deliveries can be used to anticipate the effects of alternative river management provisions now under consideration for drought mitigation.