September in the Sonoran Desert

September is one of the more quiet months in the desert.  With heat dropping, things feel a little more relaxing.  Their aren't a lot of really exciting things going on in the desert, but to the person who looks closely there are all kinds of subtleties happening.

September in the Sonoran Desert means the end of monsoon season.  Usually, the month begins with the waning of monsoons and higher humidity.  Later in the month however, humidity drops and the rains end.  Humidity has dropped from dew points in the 50's to the 30's this past weekend.  Temperatures are also dipping into the 90's for highs which is very good news.  Effects from the scant rainfall continue to linger in the desert.  We will have left over mosquitoes and gnats that hatched during the rain we received earlier in September for the next few weeks or so.  There typically are more than the usual butterflies out starting in September also.  Fall bird migration is also just beginning in the desert.  I have already seen a few hawks migrating through the area.  Cacti are also slightly more plump from the rainfall their roots absorbed and stored in their stem tissue.  Other plants such as Occotillos still have their leafs, which will likely only last a few weeks or so.

Monsoon season is also one of the times of year that dry washes run with short lived flash floods.  Unfortunately, this year the rains were far too scant so only washes with an abundance of bedrock in their channels ran.  Fortunately, their was enough rain in most locations to refill some of the water holes in the mountains, which desert wildlife is extremely grateful for.  These waterholes should get a lot of desert animals through the next several months or so.  September does mark the beginning of the Sonoran Deserts second annual seasonal drought.  The most intense annual drought occurs from late April through early July.  After monsoon season a much less intense and less consistent annual drought occurs from late September through late November.  Temperatures continue to drop during this time, humidity is usually slightly higher (but not nearly as high as monsoon season), and rain does happen on occasion during this second drought.  This all makes desert life much more bearable.

Desert Water Part 2

A Sonoran Desert ephemeral drainage the morning after a flash flood.

This post is second in a series of two about desert hydrology.  Very timely, being he have finally had at least a little rain in the past week.  Here is Part 1 if you missed it.

Flash Flood
A few lucky times a year, storms bringing an inch or more of rain move across the landscape.  Storms like these, depending on how hard and fast the rain comes, will quickly saturate soil surfaces.  In these cases, instead of absorbing into the ground, water runs off into small drainage ways, slowly building volume and force along the way.  Small drainage ways join other small drainages and water flow strengthens.  Continued intense rain feeds volumes of water into larger washes creating a flash flood.  All this can take place in a matter of minutes during intense storms.  The fastest I ever encountered was a flood after only 15 minutes of extremely intense rainfall.  Flash floods are a ground breaking and ground building desert phenomena.  Desert landforms are both built and destroyed by flash floods.  Bajadas, the smooth slopes flanking mountain sides, are nearly entirely built by flash floods carrying sediments from the mountain.  Then once established, bajadas will also be destroyed by the powerful erosive action of flash floods.

Flash floods however are few and far between, and as quickly and violently as they appear they disappear.  However, they do not leave however without lasting effect.  For months, and in some cases almost a year, water from the flash flood is held in wash sediments.  Deep roots penetrate deep into wash sediments harvesting this stable reservoir of water.  This is why often, the only location you will find thick vegetation and trees in the desert is along washes.  Often, because of the erosive nature of flash floods plant roots are exposed in washes due to the soils once concealing the roots being washed away.  Many of these trees have obvious rooting patters that make little effort penetrating into the uplands outside of the wash.  Instead, nearly all trees plunge thick roots directly into wash sediments where moisture is stored.  Palo Verde, Ironwood, Mesquite, and Acacia trees of washes all show this rooting pattern.  If you were to dig a hole in one of these washes, a foot or so under the surface you would find a thick network of fine roots in place, ready for water whenever it come down the wash.  But even wash sediments become dry after rainless periods.  If drought persists, some trees will shed leafs to preserve water.  Under continued drought even branches will die to reduce water use.  But these trees stand ready with roots in position.

Animal Life
Animal life responds also to these rain events but in much less dramatic ways.  Some animals, such as Kangaroo Rats, never require drinking water, instead obtaining water from the food they eat.  Other animals such as Mule Deer require permanent drinking water sources.  During drought these animals are constrained to live only short distances from waterholes.   With rainfall however, water holes become more abundant and deer can range far from these more permanent holes.  The greater effect of rain on animals is seen later on as it aides the hydration of young, growing animal populations in good rainfall years.  The boom and bust of animal and bird populations goes right along with the boom and bust of annual rainfall.

A small waterhole in a Sonoran Desert mountain canyon.

During the winter, cooler temperatures allow for the desert to stay wet for months on end some years.  Typically one average winter rainfall of half an inch every month is sufficient for this.  Winter rainfall typically ends the begining of April and the desert quickly heats up and dries out until monsoon season starting in July.  May and June are the driest times of the year and typically no rain falls what-so-ever during these months.  During this time the soil and plants dry out.  Waterholes also dry out leaving only the largest most reliable waterholes wet.  Summer rainfall is a lot different from winter rain.  Summer heat quickly dries out the moisture left after a July or August thunderstorm.  Occasionally moisture can evaporate within hours of a rainfall event.  More often it takes days for the land to desiccate.  In good summer rainfall years though rain every few days can keep the landscape wet for weeks on end.  But as is inevitable, the desert always returns to dry.  Dryness is what defines a desert and dryness is what orchestrates the composition of plants and animals here.  And as the desert dries, desert life clings to water, often thriving in its absence.  But desert life always waits for that next rainfall, longing for the life it gives. 

Desert Water Part 1

Sonoran Desert after a rainfall.  Within days the leafless plants in this photo will sprout leafs.

Well, we had a decent rain finally this past weekend in the Sonoran Desert.  So in honor of that, this post is the first of a two part series on desert hydrology.  Part 2 will be posted this coming Friday.

All desert life clings to water.  As odd as it might sound all desert life is intensely shaped by water.  When passing clouds condense and rain across the landscape a desert raindrop scarcely finds any impediments between it and the ground.  That is, of course if the raindrop doesn't evaporate before hitting the ground, a common desert frustration.  Sparse vegetation allows most rain direct access to the ground, its most logical destination.  However, being most precipitation events are quite small, once a raindrop falls soil moisture is typically short lived, and rapidly evaporates back into the atmosphere, renewing the scarcity of moisture.  This scarcity is exactly what created the desert in the first place and creates the often dusty atmosphere or the semi-frequent dust storm.  Combining together, rain and dust can turn into another uniquely desert phenomena, muddy rain.  All rain, muddy or not, is coveted by desert life.  And all desert rain brings with it cleansing and life.  The once dusty atmosphere is washed crystal clear after rain subsides and plant and animal life is renewed.

Trace Rain
Even trace amounts of rain have an effect on the desert.  If a few raindrops are fortunate enough to fall on black soil crust moisture is readily received by the ground.  Cryptobiotic soil crusts thinly coat desert soils in undisturbed areas.  Most of the year these crusts lay shriveled and dry on the soil surface, with no obvious signs of life.  But even minute trace amounts of rain spring to life microscopic cyanobacteria and fungi composing these crusts.  Once awaken, brittle crusts transform into slightly swollen, softer, and biologically active versions of what they were before.  Cyanobacteria rapidly absorbs sunlight in order to photosynthesize and harvest nitrogen from the air.  Nitrogen is then deposited into the soil, serving nutrients to associated fungi as well as nearby plants.  These crusts cling to water, holding it in the top centimeter of soil much longer then if these crusts were absent.  But hours to days after rainfall, once the soil crusts dry out, they return to their original dry brittle form.

These spine lined ribs of the Saguaro expand apart as the cactus stores water in the stem and shrink together as water is used up.

More Rain
If a fortunate larger rainfall event occurs, say one-half inch, raindrops will penetrate much deeper into the soil bringing to life dormant plants.  As this moisture soaks is it pools above impenetrable caliche soil horizons. Rarely does rain ever penetrate below the caliche line which is typically around two feet below the surface. Shallow rooted plants such as cacti and Triangle-Leaf Bursage are the first to take advantage of this new moisture pooling above the caliche.  Desiccated plant roots rapidly respond by growing new rootlets, exploiting as much of the shallowly penetrating moisture as possible.  New rootlets transport water to the stems of plants which in the case of cacti begin to swell.  After long months of drought, the accordion shaped exterior of a Saguaro cacti appears compressed due to water loss.  Water loss during rainless periods decreases the diameter significantly, leaving a thin anemic looking cactus.  However, as water is rapidly absorbed the accordion shaped exterior expands as the cactus swells and becomes plump.  The cacti chug as much water as fast as they can, storing it in their stem tissue for later use.  This drinking strategy categorizes cacti with other succulents as water storers.  Storage of water within expandable stems serves to supply cacti with water for continuous slow growth and metabolic function months after soil moisture has been depleted and other plants appear shriveled and dry.  Once soil moisture is depleted smaller cacti roots die and are shed while large roots remain intact but enter a state of dormancy.

Ocotillo sprouting new leafs and flowering after spring rains.

Other shallow rooted plants such as Triangle-Leaf Bursage or Brittlebrush also absorb water but at a slower pace.  These plants, known as intensive exploiters, utilize very shallow roots to exploit only shallow soil moisture.  Rather than storing water, these plants instead quickly resurrect seemingly dead crunchy leaves, turning them green again.  Available water is also incorporated into new stems and leafs, growing the plant as fast as possible.  When moisture is depleted new roots are shed, growth stops, and leaves slowly shrivel and are shed.  Plants can become so shriveled and leafless after long rainless periods that they can appear all-together dead.  Some plants such as the Ocotillo only sprout leafs after rainfall, then shedding them as the soil drys.  Other plants prefer deeper moisture and for this reason avoid soils with caliche.  Caliche often prevents rainwater from penetrating deep into the soil, therefore preventing deep rooted plants from establishing themselves.  Younger soils without caliche therefore become the primary home for the deep rooted Creosote Bush and Mesquites.

Friday I will post the rest of this article about water in the desert.  See you then!