The action of water in arid regions
287
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1.84
Figure 13.2. The distribution of rainfall from thunderstorm cells over Walnut Gulch, Arizona: Above,
a single cell event. Dots represent rain gauges. (after Renard (1970), in Graf (1988), Figure 3.4).
Vegetation in deserts is sparse or even nonexistent. In environments below 100 mm of
precipitation the vegetation may only be developed in wadis (Figure 13.3) or closed
depressions. If precipitation varies between 100 and 500 mm, the natural vegetation is
arboreal. Generally, this has been degraded by human activities to a more open landscape
of shrubs and grasses that may also have been affected by overgrazing (Francis, 1994).
The scarcity of water and the elevated temperatures mean that plants must adapt to these
inhospitable environments by means of physiological or morphological adaptations and
procedures that can cope with the stress associated with an increase in aridity (Gfinster
et al., 1993).
We have already seen the role that flora plays in the biological alteration of rocks and
the influence that sparse organic material has on the processes of hydrological erosion.
There exists, in general, a good correlation between the percentage of vegetation cover and
the mean annual precipitation. Interception is a significant factor in hydrological erosion.
Nfivar and Bryan (1990), in a study on the influence of vegetation in a semi-arid region of
northeast Mexico, found that after precipitation of 230 mm, divided between 17 storms,
287
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1.0 0.8
o.~ 0.4 o.6 o.8~~
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9
.
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,
..
,
9 ,~~~,~~~~
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..
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~
z,-.,,..,~M,~
0.2 0.4,1:0. ,.,-i", 1.8 1,8
0.4
0~~_~;;~
~ ~o . 4 ~~_,,~
7/.~j
( 7 o:8
o.6 ~
"J,C'{'.//, #.2'f~Je' ~ .o
~ o~~. ~ ~-.~~ o.8
0.6
1.84
Figure 13.2. The distribution of rainfall from thunderstorm cells over Walnut Gulch, Arizona: Above,
a single cell event. Dots represent rain gauges. (after Renard (1970), in Graf (1988), Figure 3.4).
Vegetation in deserts is sparse or even nonexistent. In environments below 100 mm of
precipitation the vegetation may only be developed in wadis (Figure 13.3) or closed
depressions. If precipitation varies between 100 and 500 mm, the natural vegetation is
arboreal. Generally, this has been degraded by human activities to a more open landscape
of shrubs and grasses that may also have been affected by overgrazing (Francis, 1994).
The scarcity of water and the elevated temperatures mean that plants must adapt to these
inhospitable environments by means of physiological or morphological adaptations and
procedures that can cope with the stress associated with an increase in aridity (Gfinster
et al., 1993).
We have already seen the role that flora plays in the biological alteration of rocks and
the influence that sparse organic material has on the processes of hydrological erosion.
There exists, in general, a good correlation between the percentage of vegetation cover and
the mean annual precipitation. Interception is a significant factor in hydrological erosion.
Nfivar and Bryan (1990), in a study on the influence of vegetation in a semi-arid region of
northeast Mexico, found that after precipitation of 230 mm, divided between 17 storms,