Runoff generation, rill erosion and time-scales for hyper-arid abandoned alluvial surfaces, the Negev desert, Israel

Abstract

Sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to Quaternary-timescale climatic fluctuations and tectonic activity. The post-depositional evolution surficial properties of these alluvial surfaces change with time: (a) surface clast breakdown and development of desert pavement, (b) dust and salts accumulation (c) Reg (gravelly) desert soil evolution.In the present study, we focus on the hydrological response and the resulting erosional processes, to these changes in surface properties over a sequence of late-Pleistocene-Holocene, abandoned alluvial surfaces in Nahal Evrona catchment at the hyperarid southern Arava Valley north of the towns of Eilat (Israel) and Aqaba (Jordan).The methods used in the study are: (a) simulated rainfall experiments to determine hydrological characteristics, (b) Terrestrial Laser Scanning (TLS) to determine surface roughness, (c) OSL dating for the chronology, (d) surface and soil descriptions and measurements and grain-size analyses to determine texture of the uppermost Av horizon of the Reg soil (e) GIS analysis of the new drainage network developing over the older abandoned alluvial surfaces.The results show that with time, roughness decreases, infiltration decreases to about 8–9% relative to present rates, point runoff generation increases >5 fold, and as a result, rill and gully erosion initiated, finally leading to the destruction of the alluvial surface. Under the current hyper-arid climate in the southern Negev desert, between ~30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and >100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces. This local-source runoff and gully erosion essentially destroys the alluvial surfaces in the southern Arava valley within a period of 400–500 ka. Roughness, extracted from spaceborne Lidar scanning, which is inversely related to surface age, directly reduces friction and resistance to runoff, can provide an estimate for the hydrological characteristics of the surfaces. This can support applications such as rainfall-runoff models, risk assessment, etc.