Abstract. Catchment-scale hydrological studies on drylands are lacking because of the scarcity of consistent data: observations are often available at the plot scale, but their relevance for the catchment scale remains unclear. A database of 24 years of stream gauge discharge and homogeneous high-resolution radar data over the eastern Mediterranean allows us to describe the properties of floods over catchments spanning from desert to Mediterranean climates, and we note that the data set is mostly of moderate intensity floods. Comparing two climatic regions, desert and Mediterranean, we can better identify specific rainfall-runoff properties. Despite the large differences in rainfall forcing between the two regions, the resulting unit peak discharges and runoff coefficients are comparable. Rain depth and antecedent conditions are the most important properties to shape flood response in Mediterranean areas. In deserts, instead, storm core properties display a strong correlation with unit peak discharge and, to a lesser extent, with runoff coefficient. In this region, an inverse correlation with mean catchment annual precipitation suggests also a strong influence of local surface properties. Preliminary analyses suggest that floods in catchments with wet headwater and dry lower section are more similar to desert catchments, with a strong influence of storm core properties on runoff generation.

Documenting hillslope response to hydroclimatic forcing is crucial to our understanding of landscape evolution. The evolution of talus‐pediment sequences (talus flatirons) in arid areas was often linked to climatic cycles, although the physical processes that may account for such a link remain obscure. Our approach is to integrate field measurements, remote sensing of rainfall and modeling to link between storm frequency, runoff, erosion and sediment transport. We present a quantitative hydrometeorological analysis of rainstorms, their geomorphic impact and their potential role in the evolution of hyperarid talus‐pediment slopes in the Negev desert, Israel. Rainstorm properties were defined based on intensity–duration–frequency curves and using a rainfall simulator, artificial rainstorms were executed in the field. Then, the obtained measured experimental results were up‐scaled to the entire slope length using a fully distributed hydrological model. In addition, natural storms and their hydro‐geomorphic impacts were monitored using X‐band radar and time‐lapse cameras. These integrated analyses constrain the rainfall threshold for local runoff generation at rain intensity of 14 to 22 mm h‐1 for a duration of five minutes and provide a high‐resolution characterization of small‐scale runoff‐generating rain cells. The current frequency of such runoff‐producing rainstorms is ~1–3 per year. However, extending this local value into the full extent of hillslope runoff indicates that it occurs only under rainstorms with ≥ 100‐years return interval, or 1% annual exceedance probability. Sheetwash efficiency rises with downslope distance; beyond a threshold distance of ~100 m, runoff during rainstorms with such annual exceedance probability are capable of transporting surface clasts. The erosion efficiency of these discrete rare events highlights their potential importance in shaping the landscape of arid regions. Our results support the hypothesis that a shift in the properties and frequency of extreme events can trigger significant geomorphic transitions in areas that remained hyperarid during the entire Quaternary. © 2020 John Wiley & Sons, Ltd.

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Abstract. Flash floods are among the most devastating and lethal natural hazards. In 2018, three flash-flood episodes resulted in 46 casualties in the deserts of Israel and Jordan alone. This paper presents the hydrometeorological analysis and forecasting of a substantial storm (25–27 April 2018) that hit an arid desert basin (Zin, ∼1400 km2, southern Israel) claiming 12 human lives. This paper aims to (a) spatially assess the severity of the storm, (b) quantify the timescale of the hydrological response, and (c) evaluate the available operational precipitation forecasting. Return periods of the storm's maximal rain intensities were derived locally at 1 km2 resolution using weather radar data and a novel statistical methodology. A high-resolution grid-based hydrological model was used to study the intra-basin flash-flood magnitudes which were consistent with direct information from witnesses. The model was further used to examine the hydrological response to different forecast scenarios. A small portion of the basin (1 %–20 %) experienced extreme precipitation intensities (75- to 100-year return period), resulting in a local hydrological response of a high magnitude (10- to 50-year return period). Hillslope runoff, initiated minutes after the intense rainfall occurred, reached the streams and resulted in peak discharge within tens of minutes. Available deterministic operational precipitation forecasts poorly predicted the hydrological response in the studied basins (tens to hundreds of square kilometers) mostly due to location inaccuracy. There was no gain from assimilating radar estimates in the numerical weather prediction model. Therefore, we suggest using deterministic forecasts with caution as it might lead to fatal decision making. To cope with such errors, a novel cost-effective methodology is applied by spatially shifting the forecasted precipitation fields. In this way, flash-flood occurrences were captured in most of the subbasins, resulting in few false alarms.

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