Alternating Extremes: The 2011 Flood and 2012 Drought
While the flood of 2011 was not predicted, there was a moment as the snowmelt accelerated and anomalously large rainfall fell in the upper part of the basin that it became clear the Corps could not evacuate water in the Upper Basin dams fast enough. This led Nebraska Life Magazine (Bartels and Spencer 2012) to characterize the flood as a hungry lion waiting to devour large areas of the basin. The conditions that led to the 2011 flood were a confluence of events that were difficult to predict. The basin had experienced wet conditions in the previous 4 years, and 2010-2011 continued this trend. The cold and wet winter resulted in above-average snowpack above the Fort Peck and Garrison dams and across the Great Plains. In particular, the cold conditions decreased loss of the snowpack in the plains from evaporation and delayed the snowmelt. The final component that contributed to the flood was record late spring rainfall in Montana, Wyoming, and the Dakotas (Hoerling et al. 2013; NWS 2012). This resulted in runoff of more than 48 million acre-feet above Sioux City, Iowa, from March through July, which is 20 percent more than the design runoff for the system (USACE 2012a). January-May 2011 was the wettest period on record (since 1895) for the Missouri Basin (Hoerling et al. 2013).
In late 2011, as the floodwaters were still receding, NOAA issued a La Nina advisory for the coming winter (2011-2012). The 2010-2011 winter had also been a La Nina winter, and many were concerned that a second, or "double dip," La Nina winter could cause another high runoff year. What occurred instead was a rapidly developing drought in late spring 2012, which intensified in the summer. Figure 15.3 illustrates how rapidly the drought emerged, peaking on October 2, 2012, with approximately 92 percent of the basin in some level of drought. The 2012 water year (beginning in October 2011)
Percentage of the Missouri River Basin in drought based on the US Drought Monitor (USDM) classification scheme from 2003 to present. The dates on the X-axis correspond to the weekly publication of the USDM. DO is equivalent to abnormally dry conditions and is not considered a drought category. D1 is considered "moderate drought" (20th percentile); D2 is "severe drought" (10th percentile); D3 is "extreme drought" (5th percentile); and D4 is considered "exceptional drought" (2nd percentile).
started poorly. Fall through spring (October-May) temperatures were the second warmest on record, and the March-May 2012 period was the warmest on record (since 1895) for the Northern Rockies/Great Plains region (Montana, Wyoming, North Dakota, South Dakota, and Nebraska). In May, the spring rains for the region largely failed, causing the May-August period to be the third driest on record (since 1895). Hoerling et al. (2014) assessed the causes of the drought and found it was due largely to a reduction in atmospheric moisture transport into the Great Plains from the Gulf of Mexico and was difficult to predict. Based on climate simulations, the authors also concluded the Great Plains may have shifted toward warmer and drier summertime conditions compared to the 1980s and 1990s, and that this shift may have increased the risk of drought in the of summer 2012. Impacts from the drought were extreme. For example, Nebraska hay production decreased by 28 percent, corn and soybean production decreased by 16 and 21 percent, respectively, and indemnity payments to producers totaled $1.49 billion (NDMC 2014). In South Dakota, the number of pheasant roosters harvested was the lowest since 2002, and by the end of the 2012 growing season, 80 percent of the pasture and range conditions were rated as poor or very poor statewide, which caused feed shortages and increases in feed costs. Like the flood of 2011, the drought of 2012 caught many by surprise. Because of its rapidly evolving nature and extreme impacts to the agricultural sector, many were referring to this as a "flash drought." Flash droughts are characterized by rapidly deteriorating crop and range conditions and are caused by anomalously warm surface temperatures coupled with short-term dryness (Svoboda et al. 2002). Flash droughts were a relatively uncommon phenomenon, but given the magnitude and intensity of the 2012 drought event, many are using it as a case study to develop drought indicators to improve how we monitor these rapidly emerging drought events (Hobbins et al. 2016; Otkin et al. 2015, 2016).