NATURAL ONGOING DESERTIFICATION IN THE ARID AND SEMI-ARID REGIONS OF THE MIDDLE EAST

In the arid and semiarid regions of the Middle East, gully incision erodes alluvial sediments and loess soils deposited during the Upper Pleistocene along the valleys. This phenomenon becomes critical in the arid region of the Negev Highlands of southern Israel where the land carrying most of the natural biomass and which has agricultural value is limited to narrow valleys. These fields, cultivated during the Iron Age (3000 years BP) and the Roman-Byzantine Age (1800-1400 BP), were irrigated by runoff harvesting techniques. The semi-nomadic population in the region continues to cultivate parts of these fields today. During flood events, the runoff penetrates the alluvial cover of the valleys, forming vertical headcuts, which gradually retreat up the valleys. The formation of deep and narrow gullies within the valleys concentrates the runoff into narrow channels, preventing the floodwater from irrigating the whole width of the valleys. The sharp change in irrigation efficiency is reflected in a sharp drop in biomass, up to 80%. This phenomenon continues downstream along the gullies, almost without recovery. The annual migration rate of the headcuts in the study area exceeds several meters up to 100 m. The ongoing stripping of the valleys floor has severely reduced the agricultural potential of the region and the vegetative biomass available for pastoral herds and other grazing animals. This phenomenon causes ongoing degradation of soil and biomass, leading to the increasing desertification of the region. OSL dating of alluvial units, as well as the position of archeological sites relative to the gullies, indicates that gully incisions initiated during the Early Holocene. It is concluded that desertification, caused by gully incision and headcut migration, is an ongoing process, which has been active in the Negev Highlands for the last few millenniums. The loess sediments, deposited during the late Pleistocene within the drainage basins, are being eroded and removed during the Holocene. This process is related to the long-term re-adjacent of the geomorphologic system to the Holocene climate. Similar processes of gully incision and ongoing desertification are active in wide regions of the Middle East, as well as in other semiarid and arid regions worldwide, such as southern Africa, central Asia and northern China.