Colorado River Water Availability Assessment Under Climate Variability

The Colorado River is ruled by a complex system of agreements and regulations that fully allocate its resources. During the drought from 2000 to 2008, decision makers and stakeholders began to question the abilities of existing operating procedures to handle such events; and whether dry events were historically common. A recent study found that a scenario of 20% reduction in the mean annual Colorado River flow due to climate change by 2057 resulted in a nearly 10-fold increase in the probability of annual reservoir depletion by 2057. That study also suggested that flexibility in current management practices could mitigate some of the increased risk from climate change-induced reductions in flows. The general approach of past studies has been to model and generate streamflow scenarios under climate change that are then driven through decision and management models of water resources systems to quantify the risk and reliability of various aspects of the system. Management alternatives are considered to improve the reliability to a desired level under streamflow variability. The methodology used by previous studies assumed that the projected demand growth is a given and that it has to be met in the future. We are of the opinion that this need not be the case, which motivates the present study. To this end an assessment of water availability in the basin was performed using a simple heuristic model under different climate scenarios for the available storage capacity and minimum storage requirements. From the simulations, using the predicted future levels of demand of 13.5 and 14.4 MAF as per current agreements in the basin and the current consumptive use of 12.7 MAF, the key findings are that (i) The reliability of meeting a demand of 13.5 MAF, decreases from 99% to 94% for natural climate variability and a 20% reduction in mean streamflow due to climate change, respectively; and, (ii) For a demand of 14.4 MAF, the reliability decreases from 97% to 70%. However, for the current consumptive use of 12.7 MAF, the reliability remains steady at 99% under both natural climate variability and 20% mean streamflow reduction. As expected, the reliability decreases substantially for increased levels of minimum storage. These findings indicate that policy makers will have to confront and institute plans to mitigate reductions in water supply reliability with any growth plans.