By the end of the last ice age, approximately 18,000 to 15,000 years ago, thick loess sediments covered vast areas in the Southern Levant. At the beginning of the Holocene, increased rainfall and flood events in the newly established hot climate promoted the formation of gullies that gradually dissected the loessic cover along stream valleys. The deep gullies became the primary channels for floodwater, serving as the main natural irrigation source in arid and hyper-arid environments. Consequently, the once well-irrigated loessic valleys, covered by dense vegetation of annuals and perennials, were cut off from their irrigation supply, leading to a dramatic reduction in their relatively rich biomass. This ongoing natural process of gully expansion, caused by runoff and incision, has promoted a desertification process that is gradually transforming relatively rich ecosystems into much more barren ones.
However, the gullying process has imposed another fundamental change in arid environments – the penetration of floodwater into deep layers of the alluvial deposits exposed along the gullies. This occurs because a thick layer of small rock fragments forms a blanket of highly penetrable hydraulic cover along most of the gullies. This mechanism promotes the germination of desert trees and large shrubs, such as Pistacia atlantica, Colutea istria, and Amygdalus ramonensis, along the banks of the gullies. By controlling the retreat of the headcuts, which developed at the transition from the valleys to the gullies, we can maintain both ecosystems – well-irrigated loessic valleys and fixed gullies that support rich tree communities.
