The Desert Blooms in Arizona

About once every 5 to 20 years the fall and winter rains soak the desert and bring forth spectacular blooms in this hot and dry land. Normally dry streams flow with water to keep the riparian corridor and its habitat healthy.*

These pictures were taken by a family member about a week ago. The pictures do not do the landscape full of blooms justice. Also, the bloom is the early stages as many trees, shrubs, and cacti bloom later in the season.


Close up of Golden Poppies


Golden Poppies on Hillslope


Flow in Normally Dry Stream




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* Following is a brief discussion of deserts of the world and their hydrology for those who would like a description of these arid and semiarid areas we call deserts.

The term arid is used in many ways. In each usage it carries the connotation of words such as dry, desert, barren, and empty. Of these, only the first, dry, is generally true of arid zones. Arid areas receive too little rainfall to support dryland agricultural or domestic livestock grazing. In contrast, in semiarid areas adequate moisture is usually available at some time during the year to produce forage for livestock, and there are some years when dryland crop production may be successful. However, both climates are characterized by extreme variability with commonly occurring droughts and infrequent periods of flooding.

A first order classification of arid zones can be made using long term, mean annual precipitation as the criterion. More than a third of the world's land surface is either arid, generally receiving less than 250 mm of annual precipitation, or semiarid with between 250 mm and 500 mm of annual precipitation. More precise definitions of arid and semiarid areas are based on climatology and are given in climatic classifications based on precipitation, temperature and their seasonal distributions. In the remainder of this discussion, we use arid zones to mean those areas of the world, excluding the polar deserts, which are arid and semiarid by the above first order classification.

Most arid zones of the world are found along two wide belts at approximately 30 degrees latitude north and south of the equator. In these subtropical belts, the air is generally descending (high pressure) and dry much of the time. At this latitude in the northern hemisphere we find the major deserts of north Africa, the Middle East, Pakistan and India in Asia and northern Mexico and the American southwest in North America. In the southern hemisphere at about this latitude south of the equator we find the deserts of southern Africa, Australia, and South America. On a more localized scale, a combination of terrain and prevailing wind direction can cause rain shadow effects, resulting in arid zones downwind of major mountain features.

Geology, describing the form, properties, and dimensions of the earth's rocks greatly influences development of landscapes. The landscapes currently seen are also the result of interactions between the atmosphere and the earth's surface as soil erosion and sedimentation processes dissect the earth's surface with stream channels, alluvial fans, and sometimes, sand dunes.

The term watershed (or catchment) means an area above a specified point on a stream channel enclosed by a perimeter of higher ground. The watershed perimeter defines an area where surface runoff will move into the stream or tributaries above the specified point. In arid zones, sparse vegetation and normally clear, dry air make these features obvious.

Arid zone hydrology, literally the study of water in these areas, is conveniently split into surface water hydrology dealing with the atmosphere, the land surface, and their interactions and ground water hydrology dealing with waters below the surface of the earth. Surface water and ground water are linked in nature as infiltrated water seeping through the soil can recharge ground water and ground water can return to the surface as springs, seeps, and streamflow.

A convenient way to visualize arid zone hydrology is through the concept of an annual water balance on watersheds. Precipitation that falls on the land surface can take several paths on its way through the hydrologic cycle back to the streams, lakes, rivers, oceans and atmosphere. An annual water balance is a way of accounting for all the precipitation that falls on a watershed in a year.

If the rate of rainfall is sufficiently high and for a sufficient duration, surface runoff can occur resulting in overland flow, streamflow, and in the extreme, flooding. Rainfall that stays on the surface of rocks, soil, vegetation, litter, etc. can be evaporated directly back into the atmosphere. Rainfall that infiltrates beneath the surface of the soil recharges soil moisture and can then move deeper and recharge ground water, or can move back into the atmosphere by two routes. Soil moisture can be directly evaporated into the atmosphere. Soil moisture can also enter plant roots and through biological processes be released back into the atmosphere as transpiration. Movement of rainfall back into the atmosphere by these two processes is termed evapotranspiration and simply means the combined processes of evaporation and transpiration. On an annual basis, one accounts for all of the precipitation falling on a watershed by stating that the annual change in soil moisture storage in the watershed is equal to the precipitation minus the surface runoff, minus the evapotranspiration, and minus the recharge to groundwater. Precipitation falling as snow complicates the above water balance, but the principles are the same.

Arid zone hydrology is thus defined as the study and quantification of components of the water balance. Time scales can be longer (from decades to centuries) or shorter (from seconds to seasons) than a year but the basic principles remain the same. Important features of arid zone hydrology that differ from hydrology in more humid zones include the key fact that on an annual time scale, the maximum possible, or potential evapotranspiration, far exceeds the mean annual precipitation resulting in the dryness associated with arid zones. This dryness in turn results in ephemeral stream channels and rivers that are dry most of the time. Locally, these dry streams are called by a variety of names, for example, wadis (Middle East), washes and arroyos (North America).

Because of the dryness, one might consider hydrology to be less important in arid zones than in more humid areas. Nothing could be further from the truth. Because water is so scarce, its occurrence and abundance is of primary concern in arid zones, because here, the links between water and life are most evident.