The University of Arizona

2011 Water Year in Review

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Summary: 
The 2011 Water Year in Review is a summary of the information presented in the Southwest Climate Outlook between October 1, 2010, and September 30, 2011. The water year is a standard period of measurement used in hydrology because the natural seasonal ground recharge and discharge cycles are more aligned with the October-September period than the calendar year due to precipitation and evaporation. This review highlights precipitation, temperature, reservoir levels, drought, wildfire, and El Niño-Southern Oscillation (ENSO) conditions.

The 2011 Water Year in Review is a summary of the information presented in the Southwest Climate Outlook between October 1, 2010, and September 30, 2011. The water year is a standard period of measurement used in hydrology because the natural seasonal ground recharge and discharge cycles are more aligned with the October–September period than the calendar year due to precipitation and evaporation. This review highlights precipitation, temperature, reservoir levels, drought, wildfire, and El Niño–Southern Oscillation (ENSO) conditions.

The 2011 water year may be most remembered for extreme events, fueled in part by the presence of a moderate to strong La Niña event. At the beginning of the water year, Arizona and New Mexico were largely drought-free. By the end of the water year, however, about 25 percent of Arizona was classified with extreme drought and 36 percent of New Mexico, was classified with exceptional drought. Extreme and exceptional droughts occur, on average, once in every 20 and 50 years, respectively. These dry conditions set the stage for widespread and intense fires, which burned at record levels in both states. The fires, in turn, facilitated flash floods in the summer as copious rains streamed off denuded landscapes. The dry conditions continued in the summer with a generally lackluster monsoon, particularly in southeastern New Mexico where a persistent high-pressure zone inhibited monsoon storms. Temperatures also soared during the monsoon season—August was the hottest on record for Arizona and New Mexico. To top it off, enormous dust storms, or haboobs, plowed through central Arizona throughout the monsoon with greater-than-average frequency.

But not all the news was grim. Copious winter snows in the Upper Colorado River Basin helped fill the region’s most important reservoirs, likely postponing water conservation measures that seemed imminent just one year ago.

Top 5 headlines of the water year

  1. Scant Winter Rain and Snow Scant rain and snow fell in Arizona and New Mexico between November 2010 and March 2011. The five-month total for Arizona ranked as the 28th driest winter in the last 117 years, while New Mexico had the 5th driest, receiving an average of only 1.36 inches of rain and snow. The driest conditions blanketed the southern tier of both states. The lack of winter precipitation set the stage for expansive fires and widespread drought.

  2. Record-Setting Fire Season Until the monsoon began in early July, parts of Arizona and New Mexico had not received a drop of rain for more than 90 days. The dry weather combined with hot temperatures and wind to create the perfect firestorm. More than 1 million acres burned in each state between January 1 and September 30. More than 538,000 acres burned alone in Arizona’s largest wildfire, the Wallow Fire.

  3. Storage Jumps in Lakes Powell and Mead While Arizona and New Mexico experienced a dry winter, the mountains in the Upper Colorado River Basin were blanketed in snow. As a result, combined water storage in Lakes Mead and Powell increased by about 5.2 million acre-feet (maf) during the water year and currently stands at about 61 percent of capacity. Unregulated flow into Lake Powell for the water year, which is used to gauge water delivery in the Upper Colorado River Basin,  will likely be around 16.8 maf, or about 140 percent of average.

  4. Drought Expands and Intensifies Almost all of Arizona and New Mexico experienced below-average precipitation during the water year. A dry winter followed by a generally drier-than-average monsoon has caused drought conditions to expand and intensify. In Arizona, moderate drought or a more severe drought category expanded by about 50 percent during the water year, while New Mexico experienced a jump of about 96 percent.—more than 36 percent of the state went from no drought to exceptional drought.

  5. La Niña Sticks Around A moderate to strong La Niña event developed in August 2010 and played a major role in delivering dry conditions to the region during the winter. After a brief hiatus this summer, La Niña returned in September and will likely bring continued dry weather to the region. Moderate and strong La Niña events often persist during consecutive winters.