Winter in the Sonoran Desert

A winter rain shower in the Sonoran Desert.

Seasons in the Sonoran Desert don’t follow the typical patterns found further to the north.  At the very beginning of December though we make a rather predictable transition from dry and warm fall weather to cooler and wetter winter weather.  The start of the Sonoran Desert winter is almost like clockwork with temperature and rainfall.  Within the last few days of November through the second week of December low temperatures begin to dip close to the freezing mark and a very predictable winter rain shower waters the desert.  The cooler and wetter conditions will continue through February before the desert begins to warm-up again.  

Usually the first indicator that winter has arrived is the arrival of freezing temperatures.  Of course you can identify freezing temperatures by looking at your thermometer, but there are also some biological indicators you can look out for.  Some desert plants such as Ironwood cannot tolerate temperatures very far below zero.  Freezing temperatures will cause Ironwood leaves to yellow and fall off.  This is not spectacular in any sense compared to how deciduous trees in the east change color.  To me the tree just looks like a sickly yellow.  Other plants may also show evidence of freezing with similar yellowing.  Another indicator of cold temps is the lack of reptile activity with many in hibernation.  Most “cold-blooded” organisms require the desert heat to stay active but come the end of November temperatures are just too cold.  

Winter rain shower soaking the ground.

One of the most exciting signs of the start of winter is the very predictable rain shower that happens anywhere from the last few days of November through the second week of December.  If my memory is correct we have received this rain shower six out of the past seven years.  The year we didn’t receive it we had not even a trace of rain from October through March.  The rest of those years we typically received about one-half to two inches over a one or two day period.  This is the one time of year that you can expect the dry washes to flow for a few hours during the peak of the rain and this year has been no exception.  About one inch of rain fell on December 12th and 13th causing most of the washes to run.  Water running through these washes quickly is absorbed into the sediments so washes only run for a very short period of time.  The water is not lost however, but is rather stored in the sediments for many months.  The larger the flow the larger the volume of water is that is stored in the sediments.  This water can persist through many months of drought helping deep rooted plants survive high temperatures and bone dry conditions on the surface.  The denser vegetation around dry washes are typically there because of this water stored deep in the sediments.  

Rain this time of year will also saturate the soils and the cooler temperatures will prevent the soils from drying out too quickly.  If about one-half inch or more of rain is received a massive number of seeds that lay dormant in the soil will germinate.  These seeds will often wait years and even decades, simply waiting for the right conditions to germinate.  Seeds from different plant species require specific temperatures and amounts of moisture in order to germinate.  Amazingly, these seeds are not tricked into germinating with rain coming at the wrong time of year.  Winter ephemerals, as they are called, wait for cooler temperatures and a significant rainfall in order to germinate.  With one inch falling across most of the desert we should see a profusion of tiny green plants within a few weeks.  These tiny plants could possibly result in a major spring wildflower bloom.  The trick however is that rain must continue to fall at regular intervals through March so these new sprouts survive.  Without rain the sprouts will shrivel-up and die.  A typical winter has at least one rainstorm to germinate a large number of wildflower seeds but does not have enough rain in the following months to allow them to survive until spring when they can produce spectacular displays.  Desert ephemeral wildflowers are highly adapted to this, having the ability to produce tiny flowers very quickly to made seed under dry conditions.  And as the name ephemeral suggests, these flowers are here today with wet conditions and gone tomorrow once dry conditions return.  It has been six years since we had a good spring wildflower bloom.  Right now conditions are good for another spring bloom but only if we receive rain every few weeks for the next several months.

A hedgehog cactus, common to the Sonoran Desert.

The cooler temperatures makes large animals and birds more active during the day when it is warmest.  Wetter conditions also provide more water across the landscape for larger animals.  With dry conditions in most Octobers and Novembers most mule deer will stay close to water holes in the mountains.  However, with washes flowing and forming new waterholes further away from the mountains the deer will venture further out making them easier to find.  Some of the best places to look for deer and javelina are along dry washes being these are sort of natural trails for these animals.  Rain will also be good for helping the survival of young gambles quail and if rain persists the overall population can grow significantly, simply because there is more water and more food as a result of the increased moisture.  

So winter in the Sonoran Desert is a little like Paradise.  We have rain and beautiful temperatures which make a day in the desert very enjoyable.  Quite different from what northerners are experiencing this time of year.  However, northerners, don’t complain too much when winter and spring end in the desert the nearly intolerable heat and dryness of summer will return to the desert.  But of course, the grass is always greener somewhere else...  So enjoy your weather no matter what it is.  I’ll be enjoying the deserts winter while I can.