The Colorado River basin has experienced several prolonged droughts in the past, which have also become more frequent and extended since the early twenty-first century. Some of these prolonged drought events were unexpectedly mitigated by a significantly large amount of spring precipitation falling within a month; this is generally called a Miracle Spring (like miracle March or May). However, a generally accepted definition or specific quantitative criteria for characterizing “miracle spring” events does not exist in current scientific literature, even though this term is fairly popular among water managers and news media, reflecting the importance of such events for water management. In order to understand the drivers of such events, how well they are represented in Earth system models, and how they might change in response to larger scale climate change, a tractable definition that reflects real-world management relevance must first be developed.

Under the joint effort of the HyperFACETS Project funded by US DOE, we analyze the different drought indices from historical observational data including precipitation, Palmer Drought Severity Index and Standardized Precipitation Index for both upper and lower Colorado River Basins. Since a common definition of miracle does not exist and no specific criteria has been developed, we test different metrics i.e. different drought indices to find the miracle years. In addition, we also used focus group discussions with regional water managers to better understand the metrics and miracle events of management relevance. We further analyze the climate model data and evaluate the miracle years.

Our preliminary results show that the identification of miracle years is sensitive to the chosen drought index, reflecting the different conditions measured by the indices. Climate models also produces miracle years although there is a large discrepancy in miracle years from observation. Both CMIP5 and CMIP6 produce the 2015 Miracle event that is the most recent case from observation. To further this initial work, we will engage with regional water managers to refine these results and better define the characteristics of a miracle event. In this presentation, we will discuss current and future miracle cases in warming climate and their connection to larger scale climatic processes.