Drought management in the Lower Colorado River basin: combining paleo-streamflow ensembles, CRSS, and decision trees
The risk of shortage (drought) of different magnitudes and frequencies is essential information for water managers planning for future supply reliability. Using the paleo record it is possible to drill down to individual traces to investigate how the actual sequence of dry, normal, and wet years, combined with initial conditions, matters to system management. Using individual traces we can ask what set of conditions push the system into shortage, or deeper into shortage? What sequence of years will likely maintain shortage at a certain level? Or enable system recovery? Answers to these questions may provide water managers with an indication of the likely future water supply path they are on and therefore the suite of demand management and adaptation options they are likely to need to deploy. In time, the actual sequence may become more useful to water managers as annual and decadal forecasts of streamflow improve with enhanced inter-annual and decadal climate prediction skill.
To investigate the policy implications of climate change scenarios, we use the projections of shortage risk from CRSS under various flow scenarios as input into a water manager's decision tree: to do nothing; utilize alternative supplies; invest in alternative water sources/system efficiency; invest in demand management strategies; or combinations of these strategies. Using ensemble analysis we identify options for Arizonan water managers given the severity and frequency matrix of drought. Specifically, we investigate the decision sets of the Central Arizona Water Conservation District and generalized decision sets for tribal, and city water resource managers. We find that water management strategies evolve both over time and with the profile of future droughts. We also find that there are decision windows in which: proactive investment in demand management and alternative water sources; and cooperative action, are robust management options.