Abstract:A system has been installed to automatically monitor rainfall, streamflow, bedload discharge and suspended sediment concentration in the arid to hyper-arid setting of Nahal Rahaf, Southern Judean Desert in Israel. The Rahaf gauging station is located in a relatively steep, wide channel with an unsteady bed driven by flash floods. It is an attempt to deploy modern automatic equipment for continuous sediment transport monitoring in harsh, arid fluvial environments.Unit bedload discharges are the highest recorded hitherto, suggesting they may represent an upper end member in the worldwide climate-bedload discharge relationship. Suspended sediment concentration is much higher than is typical of perennial fluvial humid environments. There is high correlation between suspended sediment concentration and water discharge on an event scale, with diverse intra-event relations. The sediment yield of individual events is large, but the small number of floods limits the mean annual sediment yield to low values in this arid environment. This also has environmental implications, as large-scale quarrying requires a long period of self-restoration in such an arid fluvial setting.

[1] A 15-year record for the semiarid upland Eshtemoa catchment provides a basis for estimating the efflux of water, sediment, and solutes in a type of environment for which there is a dearth of information. Average annual runoff was 5.4 mm, giving a runoff coefficient of 2.1%. Average annual yields were 275 Mg km À2 for suspended sediment, 15.3 Mg km À2 for bed load, and 0.6 Mg km À2 for dissolved load, though distinct seasonal differences are identified and related to the prevalence of cellular, convectively enhanced storms in autumn and spring and frontal storms in winter. The majority of the average annual total sediment and solute yield of 291 Mg km À2 is generated by as few as 8 of the 74 flow events of the 15-year record. The relative importance of bed load and the consequent ranking of the load components as suspended load ) bed load ) dissolved load distinguishes this dryland environment from its humid counterparts.Citation: Alexandrov, Y., H. Cohen, J. B. Laronne, and I. Reid (2009), Suspended sediment load, bed load, and dissolved load yields from a semiarid drainage basin: A 15-year study, Water Resour. Res., 45, W08408,

[1] A decade of data for the Nahal Eshtemoa, an unarmored, gravel bed, ephemeral river, reveals that bed load flux during rain-fed flash floods is a simple function of channel average boundary shear stress. However, the relation is inadequately described by a power function of the type commonly used in predictive bed load equations, and a linear function gives a much better fit. The success of a linear function is related to the unarmored nature of the bed material, typical of ephemeral gravel bed rivers, and the ready availability of sediment at all levels of transport stage. Bed load response to changing shear stress is largely undifferentiated as flash floods wax or wane. However, while most data conform to a simple relation between bed load and hydraulic stress, some for individual floods or flood segments do not, reflecting variability in sediment supply and texture. Three type behaviors are identified that assist in explaining both the general, simple pattern and the limited scatter of the bed load-shear stress plot. Type 1 provides the majority of the data, giving the general linear relation between bed load flux and boundary shear stress. Type 2 is rare, given the flashy nature of the hydrograph, but represents comparatively steady flows well above the entrainment threshold, during which bed load sheets or the breakup of grain clusters lead to spasmodic changes in bed load flux. Type 3 shows a weak relation between shear stress and bed load flux, representing flow conditions just above the entrainment threshold.

Transport processes that lead to the initiation of bedload motion in gravel‐bed rivers have not yet been clarified. We report patch‐ and grain‐scale processes involved in the initiation of bedload motion in a natural gravel‐bed stream as observed through a series of video experiments. With increasing flow strength, the phases of initiation of motion that have been identified are (1) within‐patch grain instability (grain vibration, pivoting, and grain‐scale rolling), (2) within‐patch gyratory step‐and‐rest motion, and (3) general sediment motion involving downstream transport from an individual patch and the throughput of grains inherited from upstream.

The prediction of arid region flash floods (magnitude and frequency) is essential to ensure the safety of human life and infrastructures and is commonly based on hydrological models. Traditionally, catchment characteristics are extracted using point-based measurements. A considerable improvement of point-based observations is offered by remote sensing technologies, which enables the determination of continuous spatial hydrological parameters and variables, such as surface roughness, which significantly influence runoff velocity and depth. Hydrological models commonly express the surface roughness using Manning’s roughness coefficient (n) as a key variable. The objectives were thus to determine surface roughness by exploiting a new high spatial resolution spaceborne synthetic aperture radar (SAR) technology and to examine the correlation between radar backscatter and Manning’s roughness coefficient in an arid environment. A very strong correlation (R2 = 0.97) was found between the constellation of small satellites for Mediterranean basin observation (COSMO)-SkyMed SAR backscatter and surface roughness. The results of this research demonstrate the feasibility of using an X-band spaceborne sensor with high spatial resolution for the evaluation of surface roughness in flat arid environments. The innovative method proposed to evaluate Manning’s n roughness coefficient in arid environments with sparse vegetation cover using radar backscatter may lead to improvements in the performance of hydrological models.