Chapter 13
The action of water in arid regions
1. Introduction
In the proceeding chapters we have discussed the importance of water in weathering
processes, its role in the generation of patterned ground, varnishes, and crusts. In these
cases the activity of water occurs in a thin film and its movement is clearly local. We now
turn our attention to the action of water activity connected to surface flow, organized in
fluvial systems. These are characterized (Schumm, 1977) by a headwater source area,
where water and sediments are derived, a transfer zone and, finally, a lower area or
sedimentation zone. This fluvial system has a group of independent and dependent
variables (Schumm and Lichty, 1965) that have different influences. Time, initial relief,
geology (lithological and structural) and climate are the independent variables most
significant in influencing the hydrology and erosive activity. The vegetation depends on
climate and lithology. The rest of the dependent variables are variation in relief, hydrology
(run-off, discharge, sediment production) and the morphology of the drainage network
and slopes. In arid regions the first three independent variables vary substantially between
different deserts, unlike climate that depends on precipitation and which is, in turn, the
cause of run-off and erosion that are conditioned by vegetation cover.
2. Precipitation, vegetation, and evapotranspiration
In the majority of the arid regions of the world precipitation data are rare as there are
few meteorological stations and generally, where they do exist, they are very isolated. As
a consequence, extrapolations under these conditions are likely to be incorrect. Precipitation is a consequence of different atmospheric processes: storm fronts, tropical storms,
orographic effects, and convective cells (Graf, 1988). As most deserts are located in anticyclonic belts within the global atmospheric circulation system, precipitation is scarce
and limited to the sporadic penetration of storm fronts that are rare and normally produce
little rainfall. Tropical storms (cyclones, hurricanes, and typhoons) are rarer than storm
fronts, but occasionally they can penetrate the interior of arid regions, such as in the
Sonora Desert, between Mexico and Arizona. Orographic effects occur due to a reduction
in pressure and a cooling of the rising air, conditions that favour condensation and
precipitation. When the air descends on the other side of a mountain range, it warms and,
because it has lost the majority of its humidity while rising, it creates a rain-shadow zone.
The final type of process is the convective storm that is a few kilometres in diameter and
results from warming of the air. When they affect small basins they can produce important
The action of water in arid regions
1. Introduction
In the proceeding chapters we have discussed the importance of water in weathering
processes, its role in the generation of patterned ground, varnishes, and crusts. In these
cases the activity of water occurs in a thin film and its movement is clearly local. We now
turn our attention to the action of water activity connected to surface flow, organized in
fluvial systems. These are characterized (Schumm, 1977) by a headwater source area,
where water and sediments are derived, a transfer zone and, finally, a lower area or
sedimentation zone. This fluvial system has a group of independent and dependent
variables (Schumm and Lichty, 1965) that have different influences. Time, initial relief,
geology (lithological and structural) and climate are the independent variables most
significant in influencing the hydrology and erosive activity. The vegetation depends on
climate and lithology. The rest of the dependent variables are variation in relief, hydrology
(run-off, discharge, sediment production) and the morphology of the drainage network
and slopes. In arid regions the first three independent variables vary substantially between
different deserts, unlike climate that depends on precipitation and which is, in turn, the
cause of run-off and erosion that are conditioned by vegetation cover.
2. Precipitation, vegetation, and evapotranspiration
In the majority of the arid regions of the world precipitation data are rare as there are
few meteorological stations and generally, where they do exist, they are very isolated. As
a consequence, extrapolations under these conditions are likely to be incorrect. Precipitation is a consequence of different atmospheric processes: storm fronts, tropical storms,
orographic effects, and convective cells (Graf, 1988). As most deserts are located in anticyclonic belts within the global atmospheric circulation system, precipitation is scarce
and limited to the sporadic penetration of storm fronts that are rare and normally produce
little rainfall. Tropical storms (cyclones, hurricanes, and typhoons) are rarer than storm
fronts, but occasionally they can penetrate the interior of arid regions, such as in the
Sonora Desert, between Mexico and Arizona. Orographic effects occur due to a reduction
in pressure and a cooling of the rising air, conditions that favour condensation and
precipitation. When the air descends on the other side of a mountain range, it warms and,
because it has lost the majority of its humidity while rising, it creates a rain-shadow zone.
The final type of process is the convective storm that is a few kilometres in diameter and
results from warming of the air. When they affect small basins they can produce important