Everyone is talking about snowpack this year. The news boasts epic-sounding statistics for snowpack that is \u201c649 percent of normal,\u201d \u201c128 percent of average,\u201d \u201c440 percent of median,\u201d or, my personal favorite, \u201c1,776 percent of last year.\u201d One need not speak math to know that big snowpack is generally a good thing. But unpacking the statistical lingo can help in understanding what those numbers actually indicate.<\/p>\n
People generally understand snowpack to refer to snow depth. But in the context of water supply, snow depth doesn\u2019t matter as much as snow-water equivalent. SWE is the amount of liquid water released when snow is melted instantaneously. SWE data mainly comes from automated sites across Colorado that capture, record and report daily data.<\/p>\n
SWE is a more useful measurement for understanding water supply than snow depth, because water content in snow layers can vary \u2014 consider the difference between two inches of wet, heavy snow and dry powder. And when discussing snowpack, what most people care about is not cubic inches of snow in the mountains; it\u2019s how much water will flow through our rivers, ditches and reservoirs.<\/p>\n
This year, peak SWE for the Upper Colorado Basin was 20 inches. To add context, last year, which was dry, the SWE was 16 inches, while in the 2011 flood year it was 25 inches.<\/p>\n
Snowpack varies throughout the year. Imagine a mountain profile where the left side is an easy, long slope that steps up to a series of peaks, and the right side is a steep, jagged descent. This is what the actual SWE data<\/span> looks like plotted for each day as snowpack starts to build in October, slowly accumulates through the winter, peaks in April, then starts to melt (with some intermittent, spring-storm gains) though June or July when it\u2019s fully depleted.<\/p>\n When most people bandy snowpack statistics around, they are referring to a point on that graph and \u201caverage\u201d is probably the most confused term that gets used. We typically understand \u201caverage\u201d to mean the arithmetic \u201cmean,\u201d which is the sum of numbers in a set divided by the number of numbers used in the set. (For example, the mean of 4, 6 and 20 is 10). But when discussing snowpack, the \u201caverage\u201d typically refers to the median, which is the number that falls in the middle of the set. (So, the median of 4, 6 and 20 is 6.) In other words, there are different ways of describing what is \u201cnormal.\u201d<\/p>\n Snowpack averages most often compare a relevant date of the current year to the median value on that same date within the study period (for western snowpack, the dataset is from 1981 to 2010). The median is more appropriate because it is not affected by outlier years.<\/p>\n You can see on an SWE graph<\/span> that the median snowpack is less than the mean snowpack. This means there have been more low-snow years than high-snow years, but the high-snow years were really snowy, thus skewing the mean higher and the median lower.<\/p>\n Anyway, math schmath. \u2026 What most people really care about isn\u2019t the difference between median and mean. People care about how much water there is. We want to know when the rivers will really start to flow, whether they\u2019ll flood, and when runoff will peter out. And the relationship between snowpack and runoff has almost as much to do with the timing and intensity of spring and summer weather as the timing of peak SWE.<\/p>\n For example, because of spring-weather timing, runoff this year is late and expected to be sustained for longer. Peak snowpack also occurred later, and temperatures are staying cooler, so runoff is expected to peak in late June. Compare that to 2011, another heavy snow year, where warmer weather and rain caused runoff to peak in May, with more flooding.<\/p>\n This year\u2019s snowpack-generated runoff is good news for water users now, but how good is it for the future? It will recharge the soil and aquifers, refill reservoirs, make for long recreation and irrigation seasons, and help mitigate Colorado\u2019s long-term drought.<\/p>\n However, this region has been in a 16-year drought and needs several more years of increased snowpack and cool temperatures to alleviate the pressures on regional water supplies. According to Colorado River District engineer John Currier, we will need at least seven consecutive 2019s to fully fill lakes Powell and Mead, which are currently at historic lows.<\/p>\n Understanding snowpack, in sum, has more than some depth to it.<\/p>\n Erika Gibson is a contributor to this monthly column for the Middle Colorado Watershed Council, which works to evaluate, protect and enhance the health of the middle Colorado River watershed through the cooperative effort of watershed stakeholders: anyone standing in the watershed. To learn more about the MCWC, visit https:\/\/www.midcowatershed.org<\/a><\/span>. You can also find the Council on Facebook at https:\/\/www.facebook.com\/midcowatershed<\/a><\/span>.<\/p>\n