2
Temporary Water Bodies and Lakes

However paradoxical and surprising it may seem, a veritable "desert hydrology" (Joly 1968) does exist. A satellite image or a flight over a desert is enough to be convinced of the truth of this. One may observe temporary water bodies and flows created in recent rains, which leave hardly any trace in the soil, as well as permanent lakes and ancient hydrographic networks, etched into the topography during one of the "pluvials" in the distant (or recent!) past. These forms are testimonies of a hydrography that is largely inherited from the past and whose chief characteristic is the absence of any perennial flow at present and the consequent accentuated degradation in the existing networks (Dubief 1953; Joly 1962), as well as an almost-complete generalization of closed drainage systems or endorrheic systems. Only the large formations made up of dunes (ergs) and certain hyper-arid spaces are truly arheic, i.e. totally lacking any flow. On the other hand, at a regional level, there are also large allogenous rivers, which arise outside of the truly desertic region, but travel through the desert and sustain the life around their course.

2.1. Water bodies

The presence of stagnant water bodies is the result of a combination of favorable conditions that often occur in a desert environment. The chief of these, at a topographical level, is the existence of a number of closed, endorrheic depressions (Martonne and Aufrère 1928), that is, depressions bordered by relief features whose altitude is larger than the highest point reached by the water body. Some of these features can be born out of general geological conditions: craters caused by tectonic events, meteors or volcanic activity. From this point of view, the relief features are not very different from those found in other regions of the world. Other features are more directly dependent on the geomorphological evolution of a region itself, whether present or inherited, i.e. the climate and its variations over time. Finally, other features can be born out of the disappearance of an organized hydrographic network, following the capture, depletion of flow, or drying out of the climate.

The climate of a region plays a principal role in the specific profiles of their water bodies, which make up so many individual hydrological units. The climate governs the quantity of precipitation received, which is the primary source of water coming in, and the rate of evaporation, which counteracts the precipitation. Climate also determines pluviometric contrasts between basins and surrounding heights, as well as the pattern of alternating humid and dry periods across all timescales. However, the hydrological balance for water bodies that must survive in these harsh conditions is generally more complex: in addition to the water coming in from rain, there is water brought in through inflows, diffuse run-off along the slopes and floods in the rivers (when they occur), upsurging and resurging of groundwater, and snow and ice in some cases. However, these are balanced by losses through infiltration, seepage and human activity.

The water bodies that are present in arid regions today can be broadly categorized into two types. The first are those that only have a precarious existence, where humans play a primordial role given their dispersion. The second are more localized, permanent (or at least multi-year) features of the landscape.

2.1.1. Elementary water bodies

The desert has a wide variety of water bodies: stand-alone, scattered, poorly supplied, but all highly important and much-sought after by those who live in deserts. These water bodies are most often small and intermittent, containing freshwater or saltwater, and are known to the nomads and travelers (albeit, under a variety of names) whom they sustain. Every downpour leaves a number of pools of water in hollows or in sheltered spots:

After the rains, not all the water escapes the mountains and get absorbed into the sands; almost everywhere, some water remains in the holes. Small pools will dry out by evening or the next day, mid-size ones will last for a week, others for a month, four months, six months or even longer, and the large pools, that is, the deep pools, will endure as long as needed till the next rains, for years. Th. Monod, Méharées

"Water! Water lay everywhere; in pools both great and small; in depressions and ancient sand channels. Fading in memory was that dusty-grey-brown half-dead dehydrated land; [...] for here and now was the real world of sparkling waters, and [...] the vivid green of grass and herbage". A. Upfield, The Torn Branch

Most water bodies only last as long as it takes for the water to get absorbed into the soil. They last long enough, however, to allow the germination of seeds that have fallen which, at times, allow the desert to be covered in flowers or get greened. These form the acheb that Saharan pastoralists use, a true windfall for their herds.

Deep hollows or large depressions hold larger ponds for a longer time and these are also more valuable. In the Sahara, a residual and semipermanent pond that occupies a hollow in a rock, or at the bottom of a wadi in a valley, is called a guelta (or glat). These are a sort of natural reservoir, sometimes surrounded by some palm trees, whose positions are fixed and known. They also serve as drinking pools for wild, as well as domesticated, animals. Some of these ponds are simply fed by the rains, are residual waters from floods, or are temporary upsurges from underflows. These dry out easily. Other ponds are enduring and are maintained by seepage or springs (gravity or artesian springs). Some ponds are very old and may have preserved some unique relict fauna like shrimps, fish or dwarf crocodiles (the Archei gueltas in Ennedi, or the Matmata guelta in Tagant).

Larger ponds (from some ten meters to a few hundred meters in diameter) temporarily occupy closed depressions that are more or less circular and have a flat bottom. In the septentrional Sahara region, these depressions are called dayas. These are either true dolines (in limestones) or simple deflation basins (blowouts). The water bodies that form in these in the rainy season, or after a very heavy shower, are quite shallow (1-2 m deep) and normally contain freshwater or water with low salinity. The presence of these water bodies facilitates the chemical alteration of the floor of the depression, especially the decarbonatation of limestone. At the same time, the water bodies collect fine debris swept down the slopes by diffuse run-off streams. When dry, the bottom of these depressions is clayey and cracked, or clayey-silty and capable of supporting bushy vegetation that is more or less halophytic. Dayas are quite frequent in the septentrional Sahara (the plateau of Dayas, in eastern Algeria) and are also found in similar forms in South Africa (called vleis in Afrikaans), Australia (called pans) and Central Asia (tsaidams).

The groundwaters held within the large ergs sometimes flow out into hollows, either temporarily or permanently, in the form of small interdune ponds. The dried out bottoms of these ponds are often marked by a slight vitrification of the sand, or by a thin, hardened layer of clay or gypsum. Around the periphery of the ergs, at the interface between the dunes and their rocky substrates, the deepest water tables feed many springs, which are often quite large and sometimes spread out into proper small lakes that attract plant and human life.

2.1.2. Playas

In the deserts of Mexico and the Western United States, the alluvial fan floodplains that extend the debris-covered lower slopes of mountains (bajada, or bahada) at the center of a vast endorrheic basin (bolson), are called playas. Stretching over a kilometer or longer, these are almost equivalent to the North African chotts or the Mongol gobis. The playa is the topographical and hydrological base of the bolson. It is the terminal point of all the slopes and surface flows, especially tributaries. It also captures waters from groundwater tables in the slopes. On the other hand, however, a playa is also the site of significant water loss, not only through evaporation (which is very active on stagnant and shallow water), but also through infiltration, depending on the soil's infiltrability and permeability.

The magnitude of the floods depends, of course, on the size of the watershed, the nature of the substrate and climatic conditions. The smaller playas are the most inconstant. In the absence of an adequate catchment area, they are more often filled by local showers, rather than by regional rains. They lose more water through evaporation than through infiltration; they dry out faster than larger playas and most of them are ephemeral. Larger playas are more dependent on seasonal patterns and ground water discharge. Some playas have significant salinity, depending on the drainage from the slopes, the material brought in through inflows and by the wind and, above all, on concentration by evaporation. Playas with a high salt content are called sabkhas in the Sahara, kavir in Iran, takyrs in Turkestan, nors in Mongolia, salt pans in the United States, South Africa and Australia, and salinas or salares in South America and Mexico.

Most playas are subject to an alternation between short, wet periods and longer dry...