The Last Water Year

The Water Year has drawn to a close and the report is good, odd, but good.  First to the basics, like what is a water year.  A water year runs from October 1 to September 30.  CoCoRaHS (the Community Collaborative Rain, Hail & Snow Network) describes the water year as:

The water year is the best consecutive twelve months that span the "water storage/water usage" hydrologic cycle. The water year cycle is particularly obvious in the Rocky Mountains and western U.S. where snow begins to accumulate at high elevations in October and doesn't melt and run off until next spring and summer. But this same important annual cycle takes different forms across the entire country.


Another way to think of the Water Year is the resting/replenishing season followed by the water consuming season where vegetation grows, crops are cultivated and then harvested. For much of the country, the months of October through March are months where precipitation from the sky exceeds evaporation from the ground. This means that soil moisture and ground water can recharge. When next spring arrives, temperatures will warm again, plants will come back from dormancy and once again evapotranspiration will surge.

CoCoRaHS is a great “citizen science” organization, gathering data from throughout the nation in a structured thoughtful manner.  NOAA even uses CoCoRaHS data to validate their models.  

There are several CoCoRaHS reporting stations in our area including three in Hillsboro, one in Kingston, and several in the Mimbres.  Anyone living in this area for a rain or two knows the precip can be very spotty in the Black Range.  The CoCoRaHS information from Hillsboro, for instance, has allowed the quantification of that impression.  The three reporting stations are roughly east Hillsboro, central Hillsboro, west Hillsboro - spread out over less than a mile.  Even in such a short distance there are distinct differences in reporting, with practically every rain showing a definite cline - almost always supported by the Kingston station.

The other basic concept is “average”.  I am not talking about simple averages like mean, median, and mode - nor of the statistical averages - but rather the end point and start point for the range which is used to compute the average.  The November 3, 2015 post on the original Black Range website had a description of how that average is computed (currently a 30 year average based on the years 1981 to 2010). This information is maintained by the Oregon State University PRISM program.  It computes the average over the 30 year period as 13.35 inches.

On August 18, 2013 the website posted an analysis of the 116 years of rain data available at that time.  Over the years the original website had many postings about rainfall and flood events - use the search box on the home page.  Snowfall is currently converted to rainfall based on water content - it is not clear how snowfall was rolled into the calculation in the early reporting years.

The August 2013 analysis found an average (simple mean) of 12 inches per year.  That report goes into quite a bit of detail.  The following paragraph addresses the point at hand:

For the annual rainfall amount, there are data for 96 years.  (See notes in the charts below for information about when and why years are left in or out.  In 1931 July was missing data, but I left the annual figure in the analysis.)  The maximum annual rainfall during this period was 21.59 inches (1905).  The minimum rainfall in any year was 3.35 inches (1956).  The average (simple mean) was 12 inches.  During this period there were three years (3% of all years) where more than 20 inches of rain fell: 1905, 1941, and 1986.  There were 26 years (27% of all years) where more than 15 inches of rain fell and 30 years (31% of all years) where less than 10 inches fell.

So how does that compare with the last two years?  The total rainfall in Hillsboro (the NM-SR-46 reporting station) for the water year ending in September 2016 was 14.19 inches and for the last year it was 15.89 inches.  Two good years which will skew the 30 year average (whichever you use) upward a bit when the new (1991 to 2020) average is computed.  That is the “good” part.  What about the “odd" part of my opening comment?

The August 2013 analysis pays particular attention to the monsoon period, when we receive most of our rain - generally July through September.  However, the beginning and end dates are variable and often imprecise.  In the year ending September 30, 2016 we had 1.1 inches in July, 5.7 in August, and 2.53 in September.  9.33 inches of the 14.19 inches fell during those three months.  Last year only 5.6 inches fell during those three months, even though the annual total was greater.  What was the difference?  For starts, it was a wet winter - 1.69 inches fell on January 15 for instance (more rain than we received in August) - and we had a bit more than that the first week of November.  The shape of the rainfall this year was different, skewed away from a monsoonal dominance that we usually see.

My informal (and generally uninformed) assessment is that this was not a good wildflower year, or more precisely it was a decent spring but the rest of the year - not so good.  Performing a more structured assessment of the relationship between rainfall (shape and distribution [temporal and geographic]) and flora production might be an interesting study.  If only there was…….

© Robert Barnes 2018