The March 1 forecast for the Rio Grande in New Mexico suggests we are heading into another dry year on the Rio Grande in New Mexico, with a median forecast of 80 percent of the 1981-2010 average flowing into Elephant Butte Reservoir. (source pdf)
That is close enough to average that there is a lot of room on the wet side of the probability distribution to yet have a wet year. But when I wrote that last sentence I chose “average” rather than “normal” with some care, because “normal” for the years since 2000 has been dry. The graph to the right is what is called the “Otowi Index Flow”, a measure of native Rio Grande water flowing past Otowi, the key measurement point in northern New Mexico for Rio Grande Compact compliance calculation. The horizontal line is the long term mean, going back to 1940. You can see that just two years since 2000 have been above that mean. If the March forecast holds, 2016 will be the eighth consecutive year below that mean. One expects high variability on this system, with a big gap between wet years and dry years.*
But eight consecutive dry years would be extraordinary. The previous longest runs were four consecutive dry years. Since 2000 inclusive, flows on the Rio Grande at Otowi have average 30 percent less than the 1940-1999 average.
* In statspeak, the “coefficient of variation”, the size of the standard deviation relative to the mean, is 54 percent, which is big. By comparison the Colorado River at Lee’s Ferry has a coefficient of variation of 29 percent. The Mississippi at St. Louis is 34 percent.