Landscapes of Nahal Yael, Southern Negev Desert

This chapter describes the landscape of a small watershed called Nahal Yael in southernmost Israel as the outcome of its climatic, geologic, hydrologic and geomorphologic conditions. The Precambrian crystalline rocks of the watershed and the hyper-arid climate of today and of the late Pleistocene create the unique conditions for weathering processes, mass movement on slopes and sediment transportSediment transport along the channels by flash foods. Understanding the evolution of landscape by the slow-motion operating processes in an arid environment like the watershed of Nahal Yael requires long-term measurements of rainfall, flow, sediment transported on slopes and in channels, otherwise, one might get the impression that the landscape is stagnant and that nothing happens in such a dry climate. Over fifty years of rainfall measurements reveal its high annual and spatial variability. Due to the high intensities, only few millimeters of rain are needed to initiate runoff on slopes and in channels. The flash floodsFlash flood created carry mostly bedload (60–70%), and rather small amounts (30–40%) of suspended load originated from slopes and channel. The source of bedload transported by nowadays floods is remnants of Quaternary landforms–talus slopes and terraces created probably during a wetter episode (35–27 Ka) within the hyper-arid climate of the Quaternary in the Southern Negev. Such conditions create today a state of dynamic equilibriumDynamic equilibrium where the amount of sediment supplied to the channel is also the amount transported out of the catchment. Evidence of this equilibrium is the fluvio-pedogenic layer (FPUFluvio-pedogenic units) within the alluvial channel. The outcome is a watershed with unique landscape that has become a perfect field laboratory for studying and understanding the processes operating today in desert environments. As well, it enables to reveal wetter episodes within the general arid climate, with non-typical landforms in desert environments.