Colorado River Sustainability in a Changing Climate: Development and Application of Sustainability Metrics for the Lower Basin States Under Drought Condition

Abstract

The Colorado River is the main water source in the southern United States serving seven states and Mexico. Overallocation of the Colorado river in the original compact governing Colorado River water allocation and drought conditions over the past two decades have left the river’s reservoirs (primarily Lakes Powell and Mead) at historically low water levels, requiring the need for new administrative measures. To help maintain higher Lake Mead elevations, an initial set of strategies was established under the 2007 Interim Guidelines (2007-IG) outlined a set of penalties to Arizona and Nevada once Lake Mead levels reached 1075 ft amsl. Mexico later joined the conservation effort under Minute 319 also accepting reductions in deliveries when Lake Mead reached1075ft amsl. Rapidly decreasing lake elevations and drought conditions after the adoption of the 2007-IG motivated new legislation even before Lake Mead hit the 1075ft critical mark. The new agreement, the Drought Contingency Plan (DCP) of 2019, was developed and adopted by all seven Colorado River basin states and Mexico. The DCP increased penalties beyond those established in the 2007-IG. The DCP also added new penalty tiers beginning with reductions at Lake Mead elevations of 1090ft amsl. New penalties included reductions to California which was exempt from reductions under the 2007-IG; it also added measures relative to conservation and allowed state to recover reductions specific to the DCP in later years if lake levels recovered. To assess the effectiveness of the DCP in comparison to the 2007-IG. sustainability, resilience, and robustness (SRR) metrics applied under conditions specific to either the 2007-IG and the DCP. Four metrics are used here as measures of DCP performance. These include functionality, sustainability, mean time to repair (MTTR) and mean time between failures (MTBF). Functionality is calculated annually and measures fractional demand delivered over the period analyzed. Similarly, sustainability is a long-term parameter measuring cumulative supply delivered as a fraction of allocation over multi-year periods of interest (simulation period). Resilience metrics including MTTR and MTBF are used to determine the ability of a system to recover from failures or remain in satisfactory states. To compare the effectiveness of the alternative management tools (2007-IG v. DCP), a MATLAB model was developed, applied, and verified using USBR results for lake elevations, losses, and power generation for both Lakes Powell and Mead. Results compared within 0.1% of the Unites States Bureau of Reclamation’s Colorado River Simulation System (CRSS) coded in RiverWare. Two hydrological scenarios were used. These include the direct natural flows (DNF), a tool for predicting streamflow characterizing based on historical natural flows patterns, and the Climate Model Intercomparison Project phase 3 (CMIP3), a hydrology based on global circulation modes coupled with variable infiltration models and correction to calculate a naturalized flows within the Colorado River Basin (CRB). The later methodology incorporates anticipated climate effects. Model results suggest that described management techniques could stabilize SRR metrics in CRB water supply where hydrology was based on the historic record (DNF). However, neither management alternative provided that stability in the presence of climate change (CMIP3). Both management techniques limit deliveries and increase lake storage. This directly improves power generations (economic gain) and evaporation (economic losses). These differences were compared with and without DCP in place based on present value for both hydrologic conditions. Both conditions showing positive net gains.