In their seminal paper in 1979, Bull andSchick proposed a conceptual model for the geomorphic response to Pleistocene to Holocene climate change, based on the hyperarid Nahal Yael watershed in the southern Negev Desert. In this model, the change from semiarid late Pleistocene to hyperarid early Holo cene climates reduced vegetation cover, increased the yield of sediment from slopes, and accelerated aggradation of terraces and alluvial fans. The model is now over 30 yr old, and during this time, chronologic, paleoenvironmental, and hydrogeomorphic research has advanced. Here, we reevaluate the model using data acquired in Nahal Yael over the 30 yr since the original model was proposed. Recent studies indicate that the late Pleistocene climate was hyperarid, and a transition from semiarid to hyperarid climates did not occur. The revised chronology reveals a major 35-20 ka episode of accelerated late Pleistocene sediment production on slopes (with lower rates probably already at ca. 50 ka) due to increased frequency of wetting-drying cycles caused by frequent extreme storms and fl oods between 35 and 27 ka. Without lag time, these sediments were transported and aggraded in depositional landscape components (fl uvial terraces and alluvial fans). This intensifi ed sediment production and delivery phase is unrelated to the Pleistocene-Holocene transition. The depositional landforms were rapidly incised between 20 and 18 ka. Since and/or soon after this Last Glacial Maximum (LGM) incision, most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock.Using these new data, we propose a revision to the Bull and Schick model in this hyperarid environment. Our revision suggests that the model should include the frequent storms and fl oods responsible for a late Pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fl uvial terraces and alluvial fans. We also discuss the common use and pitfalls of using the Bull and Schick conceptual model to explain observations in diverse arid environments, usually without suffi cient data on basin-specifi c stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.

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We use 10 Be and 26 Al to determine long-term sediment generation rates, identify significant sediment sources, and test for landscape steady state in Nahal Yael, an extensively studied, hyperarid drainage basin in southern Israel. Comparing a 33 yr sediment budget with 33 paired 10 Be and 26 Al analyses indicates that short-term sediment yield (113-138 t• km-2 •yr-1) exceeds long-term sediment production (74 ± 16 t• km-2 •yr-1) by 53%-86%. The difference suggests that the basin is not in steady state, but is currently evacuating sediment accumulated during periods of more rapid sediment generation and lower sediment yield. Nuclide data indicate that (1) sediment leaving the basin is derived primarily from hillslope colluvium, (2) bedrock weathers more rapidly beneath a cover of colluvium than when exposed, and (3) long-term erosion rates of granite, schist, and amphibolite are similar.

The method used to derive a crude ten-year sediment budget for Nahal Yael-a small hyper-arid catchment in the Southern Negev Desert-is applied to a later ten-year period, for which a much more accurate sediment budget is available. Results indicate that the budget of the first decade has overestimated the suspended sediment output by a factor of four, and underestimated the bed material output by a factor of two. Evaluated against the background of similar sediment budget studies, it is concluded that the main source of potential errors in sediment budgets, especially in those involving considerable amounts of bed material, lies not so much in the volumetric before-after comparisons, based on surveying. Rather, errors are caused mainly by the inadequate procedures presently available for the estimation of sediment input into, and output from, the studied reach. [