ABSTRACT
The word ‘arid’ is usually taken to mean very dry. Areas are arid, however, not simply because they receive very low rainfall, but also because the moisture balance between precipitation and evapotranspiration is negative: that is, annual evapotranspiration exceeds annual precipitation. This chapter aims to examine arid and semiarid landscapes, it must be remembered that fundamentally similar processes operate but that their relative importance is different, since water is not abundant. Very few processes in arid regions involve no water whatsoever, and contrasts between humid and arid landscapes tend to arise because of the irregularity of water availability and the general lack of surface vegetation in arid areas. Because evaporation actually exceeds precipitation, permanent rivers do not originate within arid areas. On a world scale, several notable examples exist of rivers, such as the Nile or the Colorado, which rise in humid zones and flow through arid regions to the sea.
