Friday, September 16, 2011

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. 

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