Global Warming to Cut Snow Water Storage

[Anne]

Snow is great for storing water at altitude, but if H₂O falls instead as rain, some other means of storing it needs to be devised to avoid flooding and drought.

This paper¹ deals specifically with the McKenzie River watershed of the Oregon Cascade Range but the principle applies elsewhere. 

The findings by scientists at Oregon State University, which are based on a projected 3.6 degree Fahrenheit temperature increase, highlight the special risks facing many low-elevation, mountainous regions where snow often falls near the freezing point. In such areas, changing from snow to rain only requires a very modest rise in temperature.
As in Oregon, which depends on Cascade Range winter snowpack for much of the water in the populous Willamette Valley, there may be significant impacts on ecosystems, agriculture, hydropower, industry, municipalities and recreation, especially in summer when water demands peak.

Among the findings of the study:
* The average date of peak snowpack in the spring on this watershed will be about 12 days earlier by the middle of this century.
* The elevation zone from 1,000 to 1,500 meters will lose the greatest volume of stored water, and some locations at that elevation could lose more than 80 days of snow cover in an average year.
* Changes in dam operations in the McKenzie River watershed will be needed, but will not be able to make up for the vast capability of water storage in snow.
* Summer water flows will be going down even as Oregon's population surges by about 400,000 people from 2010 to 2020.
* Globally, maritime snow comprises about 10 percent of Earth's seasonal snow cover.
* Snowmelt is a source of water for more than one billion people.
* Precipitation is highly sensitive to temperature and can fall as rain, snow, or a rain-snow mix.

From Science Daily
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¹E. Sproles, A. Nolin, K. Rittger, T. Painter. Climate change impacts on maritime mountain snowpack in the Oregon Cascades. Hydrology and Earth System Sciences Discussions, 2012; 9 (11): 13037 DOI: 10.5194/hessd-9-13037-2012

[JimD]

An additional effect of rain instead of snow in the medium high country is that it can have a significant effect on soil moisture in the spring (and who knows what the timing of spring is anymore).

Rain which falls during a time when it should snow adds much less to the soil moisture levels than snow does.  Since snow melt is almost always relatively gradual the water soaks into the ground vice running off as a winter rain usually does.  So you might get a short rise in soil moisture from a winter rain and then the soil dries out quicker due to the higher than usual temperatures for winter and the ground being exposed to sun and wind due to lack of snow cover.  Then later in the spring when you need the high soil moisture for growing it is not there.

Here in Prescott AZ at 4500-6500 ft elevation there was not enough soil moisture after a winter with little moisture for the grass to green up and grow in the spring.  Now that the July monsoons have arrived the grass started to green up and grow about 10 days ago.  There is going to be a lot less pasture grass this year.

[ritter]

This is expected to be a significant problem in California. Our Sierra snowpack serves as the state's biggest reservoir. As it melts, it is stored in on-stream reservoirs and released as needed for agriculture and municipal demands. Several issues that will occur: 1) decreased snowpack means less water in the "reservoir"; 2) rainfall events on top of snowpack can result in massive melt leading to flooding (existing reservoirs and levees are sized for historical norms); and, 3) dried mountain conditions leading to increased wildfires.

I suppose on the bright side, there will be fewer mosquitoes during backpacking season. 

[JimD]

New study: Dust, warming portend dry future for the Colorado River

Excellent description of the dust issue.

Reducing the amount of desert dust swept onto snowy Rocky Mountain peaks could help Western water managers deal with the challenges of a warmer future, according to a new study led by researchers at NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder. ...

The article talks about reducing dust by changing land use management and land restoration.  That does not sound at all like a practical idea to me as the prime human activities increasing the dust are livestock grazing - cattle - and over development drawing down the ground water supplies and drying out the springs and lowering the water table so that it does not feed surface plants.  Big political problems with either of those possible fixes.

...During recent years, desert dust has been settling thick and dark on the snowpack in the northern Rocky Mountain headwaters of the Colorado River, and snowpack is melting out as many as six weeks earlier than it did in the 1800s, ....

...The river’s flow falls by more than 20 percent by 2100 in some of the future climate scenarios ...

...That earlier work was based on the moderately dusty years of 2005­–2008, with about five times as much dust than in the 1800s. But during 2009, 2010 and 2013, unprecedented amounts of desert dust fell on Colorado snowpacks, about five times more than observed from 2005–2008...

http://www.colorado.edu/news/releases/2013/11/14/new-study-dust-warming-portend-dry-future-colorado-river

[morganism]

blocking high causes CA droughts....

http://m.phys.org/news/2014-09-california-drought-linked-climate.html

[GeoffBeacon]

JimD

the prime human activities increasing the dust are livestock grazing - cattle ...  Big political problems ...

Yes but cattle are terrible for global warming. See NoBeef.org.uk
http://www.nobeef.org.uk/

Reducing cattle farming is a sine qua non for tackling global warming.