Applied geomorphology and arid regions
471
annual rainfall of 400-500 mm) for the prevention of serious erosion. Where plant cover
is below 70 per cent, the amount of soil vulnerable to erosion increases rapidly. Francis
and Thornes (1990) lowered this threshold for the conditions of the semi-arid
Mediterranean, although they did differentiate between low and high density rainfall.
4.1. Water erosion
The erosion of slopes by water involves the detachment of particles from the soil and their
consequent transport. The energy required for this is supplied by the impact and the
splashing of raindrops, by surface flow, and by interactions between these. The size
distribution of soil particles, structural stability and crusting are the main properties of the
soil, determining its vulnerability to water erosion in arid environments. Land use can also
influence this (Lal, 1990). About nine per cent of the world's dry regions are affected by
water erosion induced by human activity (Thomas and Middleton, 1994). The countries of
the Mediterranean have some of the highest human activity-induced erosion rates in the
world (Woodward, 1995).
The techniques for calculating water erosion rates are numerous and include the use of
experimental stations, measurement of the sediment load in rivers, the use of
radioisotopes, satellite imagery and map analysis, and the revised universal soil loss
equation (RUSLE), and so forth. The results obtained with these techniques can be used in
soil conservation, the prediction of reservoir longevity, and to decide on the best places to
store waste, and other factors.
The influence of human activity on water erosion was studied by Iverson (1980) in the
Mojave Desert. The intense use of four-wheel drive vehicles has caused much surface
disturbance (Fig. 19.10) that has led to greatly increased runoff and erosion. The responses
to this activation were studied with a rainfall simulator, and it was found that damaged
areas had a reduced infiltration capacity, that there was more production of sediment (Fig.
19.11), and that runoff became channelled into wheel ruts.
Evenari et al. (197 l a,b), who worked in the Negev, reported another case related to the
water erosion of slopes. Some 2500 years ago, water was collected on slopes in this desert
and channelled towards the valleys for irrigation purposes. The runoff increased
considerably during this time, stones accumulated on the surface, and the underlying
loess became encrusted through the action of the rain, reducing its infiltration capacity.
Human activity was also involved in the rapid reduction of the suspended load carried
by the Colorado River (in its Grand Canyon stretch) between 1940 and 1946. This
reduction (of around 50-100 • 106 metric t/yr), was attributed to conservation practices,
flood control, a reduction in stock raising, and drought (Schumm, 1977).
Badlands are areas of intense water erosion that can affect man-made structures. Rilling
and gullying destroyed a large part of the town of Pastici in the Province of Materna, Italy,
in 1688, and a new town had to be built (Zachar, 1982). On the Yellow River in China,
intensive gully erosion produced a rapid gully head retreat affecting houses in its basin
(Zachar, 1982). In a 5-year study, Crouch (1983, 1990) used a microtopographic profiler to
obtain gully head retreat values of 0.5-1 m/yr and of 2.5 m/yr in areas with intense
subsurface flow.
471
annual rainfall of 400-500 mm) for the prevention of serious erosion. Where plant cover
is below 70 per cent, the amount of soil vulnerable to erosion increases rapidly. Francis
and Thornes (1990) lowered this threshold for the conditions of the semi-arid
Mediterranean, although they did differentiate between low and high density rainfall.
4.1. Water erosion
The erosion of slopes by water involves the detachment of particles from the soil and their
consequent transport. The energy required for this is supplied by the impact and the
splashing of raindrops, by surface flow, and by interactions between these. The size
distribution of soil particles, structural stability and crusting are the main properties of the
soil, determining its vulnerability to water erosion in arid environments. Land use can also
influence this (Lal, 1990). About nine per cent of the world's dry regions are affected by
water erosion induced by human activity (Thomas and Middleton, 1994). The countries of
the Mediterranean have some of the highest human activity-induced erosion rates in the
world (Woodward, 1995).
The techniques for calculating water erosion rates are numerous and include the use of
experimental stations, measurement of the sediment load in rivers, the use of
radioisotopes, satellite imagery and map analysis, and the revised universal soil loss
equation (RUSLE), and so forth. The results obtained with these techniques can be used in
soil conservation, the prediction of reservoir longevity, and to decide on the best places to
store waste, and other factors.
The influence of human activity on water erosion was studied by Iverson (1980) in the
Mojave Desert. The intense use of four-wheel drive vehicles has caused much surface
disturbance (Fig. 19.10) that has led to greatly increased runoff and erosion. The responses
to this activation were studied with a rainfall simulator, and it was found that damaged
areas had a reduced infiltration capacity, that there was more production of sediment (Fig.
19.11), and that runoff became channelled into wheel ruts.
Evenari et al. (197 l a,b), who worked in the Negev, reported another case related to the
water erosion of slopes. Some 2500 years ago, water was collected on slopes in this desert
and channelled towards the valleys for irrigation purposes. The runoff increased
considerably during this time, stones accumulated on the surface, and the underlying
loess became encrusted through the action of the rain, reducing its infiltration capacity.
Human activity was also involved in the rapid reduction of the suspended load carried
by the Colorado River (in its Grand Canyon stretch) between 1940 and 1946. This
reduction (of around 50-100 • 106 metric t/yr), was attributed to conservation practices,
flood control, a reduction in stock raising, and drought (Schumm, 1977).
Badlands are areas of intense water erosion that can affect man-made structures. Rilling
and gullying destroyed a large part of the town of Pastici in the Province of Materna, Italy,
in 1688, and a new town had to be built (Zachar, 1982). On the Yellow River in China,
intensive gully erosion produced a rapid gully head retreat affecting houses in its basin
(Zachar, 1982). In a 5-year study, Crouch (1983, 1990) used a microtopographic profiler to
obtain gully head retreat values of 0.5-1 m/yr and of 2.5 m/yr in areas with intense
subsurface flow.