Tuesday, December 01, 2009


There's a touch of snow falling this morning, so this seems like a good time to talk about the snowpack graph posted by the Natural Resources Conservation Service.  Click this link for the updated version.  This graph is for the headwaters of the Rio Grande, and tells how much water is being stored in the snowpack.  Living on the bank of the Rio Grande as I do, I'm vitally interested in how much water I can expect to see flowing past my front window next summer.  This snowpack graph is my first clue about what to expect. 

The red line is the average snowpack, and is a smooth curve, since the year-to-year variations smooth themselves out.  The snowpack starts accumulating in early October, peaks in mid-April, and then rapidly decreases as it melts.  The curves for each year have a zigzag shape.  Winter storms cause the snowpack to increase rapidly.  Between major storms, the snowpack increases slowly or not at all.

Turning to the brown line at the top, that's how the snowpack shaped up for water year 2008, during the winter of 2007-08.  This was our heaviest snowpack for a long while, but even so, provided only two year's worth of irrigation water.  The snowpack started out slowly, but in early December some major storms hit and the curve went almost vertical.  In less than 2 weeks the snowpack was already well above average, and remained that way for the rest of the winter.  Even this huge snowpack melted very rapidly; this is one effect we can expect from global warming:  warm spring temperatures that quickly melt the snowpack.  I remember reading about flooding in the Rio Grande tributaries that spring as the watercourses couldn't handle the snowmelt.

The next line down, the green line, is last winter's snowpack, water year 2009, the winter of 08-09.  This one started out very promising.  By late December/early January, it was on track to eclipse even the 2008 snowpack.  (Back-to-back heavy snowpacks are the only way the lakes along the Rio Grande get filled up.  Without heavy input every spring, the lakes quickly lose their water to irrigation and evaporation.)  But alas (from an irrigation point of view), the rest of the winter was only a little better than average, and look at how quickly it dwindled starting in late April!  Those hot spring winds really wreak havoc on a snowpack.  In fact, too much hot air will cause the snow to evaporate rather than melt.  When this starts to happen every year, which it probably will, the entire agricultural equation will have to be recalculated.

The bottom line, the blue one, is the snowpack for water year 2007, the winter of 06-07.  This one started out average, but by March 1 the snowpack simply stopped accumulating.  The result was a below-average snowpack.  (Some snowpacks are even worse.)

So far this year, the black line, for water year 2010, is closely tracking the 2007 line.  The graph hasn't been updated yet to include the recent storm.  I would expect to see a strong vertical jag in the black line within a day or two.  There's a lot to be learned from reading these graphs.  As winter wears on, I enjoy checking this website several times a week to see how the snowpack is doing.  (This is what I do for entertainment around here.)