TIME-SCALE AND GEOMORPHIC PROCESSES OF GULLY EROSION ON ABANDONED ALLUVIAL SURFACES IN THE HYPER ARID NEGEV DESERT, SOUTHERN ISRAEL

Abandoned alluvial surfaces are ubiquitous landscapes that are often considered to be effective recorders of past climatic fluctuations, tectonic activity and landscape evolution processes over Quaternary timescales. In this study we characterized the primary drivers for erosion and destruction of these landscapes along the Dead Sea Transform in the hyper-arid Negev desert in southern Israel. Our findings indicate that, as in other deserts worldwide, local dendritic gully networks that develop over the abandoned alluvial surfaces are primary erosion processes that continuously modify and dissect these landscapes. Gully evolution is typically controlled by overland flow during rain events and is therofre inversely proportional to surface infiltration rates. Here, we conducted a suite of simulated rainfall experiments for small-scale plots (several m²) over a chronosequence of four late Pleistocene – Holocene alluvial terraces that display poorly- to well-developed desert pavement characteristics. Luminescence dating of the sediemnts was applied to constrain abandonment ages of the surfaces and Terrestrial Laser Scanning (TLS) was preformed to determine their microtopographic (cm to decimeter) roughness. Surface ages ranged between ~8 and 140 ka and compare with documented Reg soil development elsewhere in this region. Roughness, a surface property that promotes infiltration and impedes runoff generation, was found inversely proportional to surface age. Infiltration rates, ranging between ~60 to 5 mm/hr, decrease with increasing surface age as did the thresholds for runoff initiation due to soil development. Our results suggest that 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 occurs on these landscapes and effective incision through gully erosion is initiated.