ABSTRACT
16 Climatic Variants Semi-Arid Environments Deserts have low rainfalls - less than potential evaporation - and may be hot or cold. With the lack of surface water, wind erosion and transport become effective but periodic water erosion is still the dominant process, except in rare totally arid deserts. Wadis, or arroyos, are desert valleys, normally dry but subject to flash floods from isolated rainstorms. Flood flows decrease and sediment loads increase downstream. Roads across wadi floors which only rarely flood can be designed to be overflooded; built on gabions to stop downstream scour and undercutting. Selective erosion is by slow weathering, wind transport and isolated flood events; leaves residual inselberg mountains, flat-topped mesas and pillar buttes in layered rocks, and natural arches where weathering breaches thin rock ribs.
