Friday, May 3, 2013

LET IT MELT


Spring has sprung with the days getting longer, the sun is feeling warmer and the snow pack is ripening for runoff.  April 1 marks the official hydrological start of runoff season when our snow pack typically reaches its peak depth and volume and starts to melt.  Runoff can often start as early as March 1 down south and at lower elevations and as late as May 1 up north and at the highest elevations.  With a cold and wet April this year we are running a little late.  If this weather persists thru the month of May we may have a runoff issue, even from our anemic snowpack.  With normal or hot weather the snow could be gone in a matter of weeks.

Surprisingly enough, nature and thermodynamics typically evacuate our mountain snow pack in an orderly and controllable fashion no matter how big or small the snowpack.  As long as the weather cooperates and allows the snow to melt over an extended period, from April to July, there usually isn’t any catastrophic flooding.  If the lower snowpack and the south facing snow are allowed to melt early, followed by the east and west faces and finally the higher, north facing snow, we hardly notice the majority of our surface water supply passing by in streams, filling our reservoirs and percolating to recharge our dwindling groundwater resources.  It is only when the weather stays inclement and the snowpack continues to build on all aspects and elevations, that we run the risk of springtime flooding.  This flooding can result from a large or a small snowpack as we have now or as we had in 2010.  Conversely the large snowpack of 2011 can came off quietly due to ideal weather conditions.

 In 1983 our healthy snowpack continued to build thru April and May and on Memorial Day weekend the skies opened up and temperatures soared unseasonably to 90 degrees.  The combined runoff from all aspects and elevations flooded many canyon streams, kicked off the Thistle landslide and forced Salt Lake officials to divert City Creek Canyon down State Street because the pipes under North Temple, where they
usually hide that stream, were full to capacity.  That was an anomaly, or the mythical 100 year runoff event.  In 2010 we had a normal snowpack but it held on later than normal with the May/June Monsoons we have been experiencing lately.  When the weather turned sunny, the runoff from all aspects and elevations surprised even our best forecast professionals and caused moderate flooding in Big and Little Cottonwood canyons. Water managers can sometimes evacuate some water behind their dams proactively to make room for the surface runoff and mitigate some of the flooding potential by storing it and taking the peak runoff the floods.

A ripening snowPack means that the snow density approaches 50% water and achieves the same temperature throughout the entire gradient of snow.  This means that it becomes isothermal and homogeneous so that energy added to the snow by the sun creates melt water that actually exits the snowpack.  The second ingredient necessary to trigger runoff is 3 to 5 days of non freezing temperatures that allow heat to permeate the snow pack and let real melting progress uninterrupted.  The third ingredient is the soil moisture condition that, if it is saturated by a wet autumn or excessive snow melt, can prevent further infiltration and promote more surface runoff.  Finally, late additions of faster melting spring snow can exacerbate the runoff rate.  Luckily the amount of surface water produced by a square mile of snow in Utah is almost always thermodynamically limited to 32 cubic feet per second or 14,000 gallons per minute.  If that runoff is spread around the different aspects and elevations of a watershed the effects will be minimal.

Much of the water held in the snowpack can be lost by evaporation or sublimation - passing form a solid state directly to gas.  The amount of water lost off the surface of the snowpack on a sunny and windy day can approach 3-4 inches of water per day.  The Native Americans call the warm southern Chinook winds the ‘snow eater’ because winds can decimate a healthy snowpack with no visible runoff, as they did in 2007.  This is natural but unfortunate because this is water that is lost from our collection systems.  Slow snowmelt recharges groundwater better and rapid snowmelt produces more surface runoff.  Water suppliers pray for a slow melt to recharge the groundwater aquifers but also hope for an early summer so they can sell water and recharge their financial coffers.  Thankfully there is usually a balance of losses to the sky, the ground and to runoff that allows a huge amount of water to drain without flooding. 

It seems that with a below normal snow pack, as we have this year, we are complacent, less vigilant and proactive.  I don't think that we have anything to worry about this year but you never know, random spring weather is the most important variable in the runoff equation and April and May are our wettest months.  It is with the normal or below normal snow pack that last well into the spring that we get surprised, as we did in 2010.  Unfortunately, it is only when we experience those brush back floods that we learn to respect and appreciate our watersheds, dams, rivers and natural floodplains as well as the power, patience and persistence of snow and water.