Search

Hillslope topographic change in response to climate and climate change is a key aspect of landscape evolution. The impact of short‐duration rainstorms on hillslope evolution in arid regions is persistently questioned but often not directly examined in landscape evolution studies, which are commonly based on mean climate proxies. This study focuses on hillslope surface processes responding to rainstorms in the driest regions of Earth. We present a numerical model for arid, rocky hillslopes with lithology of a softer rock layer capped by a cliff‐forming resistant layer. By representing the combined action of bedrock and clast weathering, cliff‐debris ravel, and runoff‐driven erosion, the model can reproduce commonly observed cliff‐profile morphology. Numerical experiments with a fixed base level were used to test hillslope response to cliff‐debris grain size, rainstorm intensities, and alternation between rainstorm patterns. The persistence of vertical cliffs and the pattern of sediment sorting depend on rainstorm intensities and the size of cliff debris. Numerical experiments confirm that these two variables could have driven the landscape in the Negev Desert (Israel) toward an observed spatial contrast in topographic form over the past 105–106 years. For a given total storm rain depth, short‐duration higher‐intensity rainstorms are more erosive, resulting in greater cliff retreat distances relative to longer, low‐intensity storms. Temporal alternation between rainstorm regimes produces hillslope profiles similar to those previously attributed to Quaternary oscillations in the mean climate. We suggest that arid hillslopes may undergo considerable geomorphic transitions solely by alternating intra‐storm patterns regardless of rainfall amounts. Plain Language Summary: Cliffs and escarpments in deserts are often shaped by rare and brief, but very heavy, rainstorms. However, over geologic time, it is difficult to evaluate topographic changes caused by such short‐duration storms. Here we present a new model that simulates how a cliff forms and changes over time, through cumulative weathering and erosion. We conduct experiments to explore how the topography changes over the course of many rainstorms. The model results indicate that hillslope erosion and form are sensitive to magnitude of short rain bursts and to the size of cliff‐derived rock fragments. A comparison of the model simulations with observations confirms that these two factors could explain spatial differences in cliff's form in the Negev Desert (Israel). Short‐duration and high‐intensity storms are associated with the persistence of cliffs and greater lateral retreat relative to longer, low‐intensity storms. We conclude that hillslopes in arid areas may undergo significant topographic changes due to changes in the intensity and duration of rainstorms, even when the total rainfall amount remains unaltered. Key Points: We developed a numerical model with explicit hydrology to study the evolution of arid cliffs in response to rainstormsCliff height increases with decreasing cliff‐debris grain size and increasing intensity of severe rainstormsAlterations in intra‐storm intensity patterns, without a change in total rainfall, trigger hillslope geomorphic changes [ABSTRACT FROM AUTHOR]

Establishing accurate palaeo‐hydroclimatic reconstructions from lacustrine and marine archives is a long‐standing challenge in palaeoenvironment studies. Closed‐basin evaporites, and especially halite, record episodes of extremely arid conditions during rapid climate change. However, the complex limnological behaviour of deep hypersaline water bodies and the stochastic nature of the hydroclimatic regime and its variations limit detailed palaeo‐hydroclimatic interpretations from such records. Therefore, a mass‐balance model was developed to explore hydrology–limnology–sedimentology relationships in hypersaline environments under both deterministic and stochastic approaches that generates synthetic halite–mud sequences. Applying the model to the Holocene Dead Sea halites yields novel insights into palaeoenvironmental conditions in the Levant. The deterministic framework indicates that: (i) under a series of similar hydroclimatic cycles, the thickness of each subsequent halite interval decreases, due to the depletion of dissolved‐ions storage in the brine; (ii) halite deposition requires lake levels to drop below the minimal lake level of the preceding cycle; (iii) the time interval between halite deposition and the hydrological minimum is increasingly longer in subsequent cycles. Thus, counter‐intuitively, halite deposition mostly takes place as water discharge increases, providing that the water balance is still negative. The stochastic approach produced random sequences comparable to the observed Dead Sea sedimentary record. It demonstrates that some hydrological minima are not represented by halite deposition at all. Furthermore, the thickness and number of halite beds at each hydrological cycle vary substantially, depending on the specific hydrological conditions realized. Finally, these results imply that the major Dead Sea level drop at the pre‐Holocene deglaciation (ca 14 ka bp), previously assumed to be a record minimum, could not have been as pronounced as suggested, and must have been milder than the subsequent drop at the early Holocene (ca 11–10 ka bp). [ABSTRACT FROM AUTHOR]

The impact of climate on topography, which is a theme in landscape evolution studies, has been demonstrated, mostly, at mountain range scales and across climate zones. However, in drylands, spatiotemporal discontinuities of rainfall and the crucial role of extreme rainstorms raise questions and challenges in identifying climate properties that govern surface processes. Here, we combine methods to examine hyperarid escarpment sensitivity to storm‐scale forcing. Using a high‐resolution DEM and field measurements, we analyzed the topography of a 40‐km‐long escarpment in the Negev desert (Israel). We also used rainfall intensity data from a convection‐permitting numerical weather model for storm‐scale statistical analysis. We conducted hydrological simulations of synthetic rainstorms, revealing the frequency of sediment mobilization along the sub‐cliff slopes. Results show that cliff gradients along the hyperarid escarpment increase systematically from the wetter (90 mm yr−1) southwestern to the drier (45 mm yr−1) northeastern sides. Also, sub‐cliff slopes at the southwestern study site are longer and associated with milder gradients and coarser sediments. Storm‐scale statistical analysis reveals a trend of increasing extreme (>10 years return‐period) intensities toward the northeast site, opposite to the trend in mean annual rainfall. Hydrological simulations based on these statistics indicate a higher frequency of sediment mobilization in the northeast, which can explain the pronounced topographic differences between the sites. The variations in landscape and rainstorm properties across a relatively short distance highlight the sensitivity of arid landforms to extreme events. Plain Language Summary: Identifying the link between climatic properties and topography helps to elucidate key controlling processes affecting Earth's landscape evolution. In drylands, however, it is harder to associate topography with average climate as surface processes are significantly affected by spatially disconnected, discrete and short‐duration rainstorms. In this study, we investigate whether variations in rainstorm properties can account for topographic differences along a 40‐km‐long hyperarid escarpment. Results indicate that in the drier parts of the escarpment, cliffs are steeper, and slopes are covered by finer sediment. Rainstorm statistical analysis indicates that the drier part of the escarpment is associated with extreme rainstorms characterized by higher rainfall intensities. We demonstrate that these storms mobilize sediment more frequently in the drier part of the escarpment, which can explain the pronounced topographic differences along the escarpment. This study presents a rarely‐recognized link between small‐scale variations in arid rainstorm properties and topography and highlights the susceptibility of arid landscapes to extreme rainstorms. Key Points: The morphology of cliffs and sub‐cliff slopes varies significantly with rainfall along a 40‐km hyperarid escarpmentThe observed morphologic variability is triggered primarily by differences in sediment mobilization frequencies on sub‐cliff slopesRainstorm properties in arid landscapes could change over relatively short distances and leave a significant geomorphic imprint [ABSTRACT FROM AUTHOR]

The weathering of continental surfaces and the transport of sediments via rivers into the oceans is an integral part of the dynamic processes that shape the Earth's surface. To understand how tectonic and climatic forcings control regional rates of weathering, we must be able to identify their effects on sedimentary archives over geologic timescales. Cosmogenic nuclides are a valuable tool to study rates of surface processes and have long been applied in fluvial systems to quantify basin‐wide erosion rates. However, in large rivers, continual processes of erosion and deposition during sediment transport make it difficult to constrain how long sediments spend within the fluvial system. In this study, we examine the role of rivers in buffering erosional signals by constraining the timescales of fluvial transport in large rivers across the world. We apply a stochastic numerical model based on measurements of cosmogenic nuclides concentrations and calculate sediment residence times of 104–105 years in large rivers. These timescales are equal to or longer than climatic cycles, implying that changes to rates of erosion brought on by climatic variations are buffered during transport in large rivers and may not be recognizable in the sedimentary record. Plain Language Summary: Large rivers are the most effective agent for transporting sediment from the weathering continents into the oceans, with the world's biggest rivers draining nearly half of the continental surface. In this work, we calculate the time sediment spends in large rivers between weathering and deposition in four large rivers across the world. We do this by simulating the processes of sediment erosion and deposition in rivers and applying this model to new and existing data. The results of this model show that the time it takes for sand to be eroded from the source rock and transported down the river is tens to hundreds of thousands of years. These extended timescales mean that sediment transport in large rivers buffers the effect of climatic fluctuations on weathering rates. This finding can explain how seemingly contradictory evidence of climatic variations impact erosion rates, while products of erosion measured at the oceans show no significant changes during these times. Key Points: We constructed a numerical model simulating sediment transport dynamics in large‐scale fluvial systems constrained by cosmogenic nuclidesExamining data from four large rivers across the world, we constrain sediment residence time in large riversEstimated residence times of sediments in four large rivers across the world range 104–105 yr [ABSTRACT FROM AUTHOR]

Heavy precipitation events (HPEs) can lead to deadly and costly natural disasters and are critical to the hydrological budget in regions where rainfall variability is high and water resources depend on individual storms. Thus, reliable projections of such events in the future are needed. To provide high‐resolution projections under the RCP8.5 scenario for HPEs at the end of the 21st century, and to understand the changes in sub‐hourly to daily rainfall patterns, weather research and forecasting (WRF) model simulations of 41 historic HPEs in the eastern Mediterranean are compared with "pseudo global warming" simulations of the same events. This paper presents the changes in rainfall patterns in future storms, decomposed into storms' mean conditional rain rate, duration, and area. A major decrease in rainfall accumulation (−30% averaged across events) is found throughout future HPEs. This decrease results from a substantial reduction of the rain area of storms (−40%) and occurs despite an increase in the mean conditional rain intensity (+15%). The duration of the HPEs decreases (−9%) in future simulations. Regionally maximal 10‐min rain rates increase (+22%), whereas over most of the region, long‐duration rain rates decrease. The consistency of results across events, driven by varying synoptic conditions, suggests that these changes have low sensitivity to the specific synoptic evolution during the events. Future HPEs in the eastern Mediterranean will therefore likely be drier and more spatiotemporally concentrated, with substantial implications on hydrological outcomes of storms. Plain Language Summary: Heavy precipitation events are large storms that can recharge freshwater reservoirs, but can also lead to hazardous outcomes such as flash floods. Therefore, understanding the impacts of climate change on such storms is critical. Here, a weather model similar to those used in weather forecasts is used to simulate heavy precipitation events in the eastern Mediterranean. A large collection of storms is simulated in pairs: (a) historic storms, selected for their high impact, and (b) the same storms placed in a global warming scenario projected for the end of the 21st century. Using these simulations we ask how present‐day storms would look like were they to occur at the warmer end of the 21st century. The future storms are found to produce much less rainfall compared to their historic counterparts. This decrease in rainfall is attributed mainly to the reduction in the area covered by storms' rainfall, and happens despite increasing rainfall intensities. These results suggest that the region will be drier in the future with larger dry areas during storms; however, over short durations, it would rain more intensely over contracted areas–increasing local hazards associated with heavy precipitation events. Key Points: End of 21st century heavy precipitation events in the eastern Mediterranean are projected to have substantially reduced rainfall yieldThis reduction results mainly from a major decrease in rain area during future events, despite increased conditional rain rateChanges in rain yield, rate, and area are consistent across many events, suggesting significant hydrological implications for the region [ABSTRACT FROM AUTHOR]

Orographic impact on extreme subdaily precipitation is critical for risk management but remains insufficiently understood due to complicated atmosphere‐orography interactions and large uncertainties. We investigate the problem adopting a framework able to reduce uncertainties and isolate the systematic interaction of Mediterranean cyclones with a regular orographic barrier. The average decrease with elevation reported for hourly extremes is found enhanced at subhourly durations. Tail heaviness of 10‐min intensities is negligibly affected by orography, suggesting self‐similarity of the distributions at the convective scale. Orography decreases the tail heaviness at longer durations, with a maximum impact around hourly scales. These observations are explained by an orographically induced redistribution of precipitation toward stratiform‐like processes, and by the succession of convective cores in multihour extremes. Our results imply a breaking of scale‐invariance at subhourly durations, with important implications for natural hazards management in mountainous areas. Plain Language Summary: Preparedness to natural hazards in mountainous areas strongly relies on the knowledge of extreme rainfall probability. The presence of mountains influences the motion of air masses, thereby modifying the storms characteristics. Here, we use a novel approach to quantify the impact of mountains on the probability of occurrence of extreme rainfall of short duration. We find that mountains tend to decrease the mean annual maximum intensities at subhourly scales and tend to decrease the extremely high intensities, such as the events occurring on average once in 100 years. The second effect is however nonmonotonic, in that it grows between 10 min and 1 h and diminishes between 1 and 6 h. This means that subhourly extremes could be higher than what we can estimate from hourly data, and implies that statistical methods typically adopted for risk assessment may systematically underestimate the risk of short‐duration extremes. Key Points: The "reverse orographic effect" previously observed for hourly extremes is found to be more marked at subhourly durationsOrography negligibly affects tail heaviness of 10‐min intensity and decreases it for longer durations with a minimum around 1 hBreaking of scale‐invariance at subhourly durations in mountainous areas bares important implications for risk management [ABSTRACT FROM AUTHOR]

Projections of extreme precipitation based on modern climate models suffer from large uncertainties. Specifically, unresolved physics and natural variability limit the ability of climate models to provide actionable information on impacts and risks at the regional, watershed and city scales relevant for practical applications. Here, we show that the interaction of precipitating systems with local features can constrain the statistical description of extreme precipitation. These observational constraints can be used to project local extremes of low yearly exceedance probability (e.g., 100‐year events) using synoptic‐scale information from climate models, which is generally represented more accurately than the local scales, and without requiring climate models to explicitly resolve extremes. The novel approach, demonstrated here over the south‐eastern Mediterranean, offers a path for improving the predictability of local statistics of extremes in a changing climate, independent of pending improvements in climate models at regional and local scales. Plain Language Summary: Climate change impact studies are currently restrained by the limited accuracy of climate models in resolving precipitation extremes, by the computational power required to run high‐resolution models, and by the uncertainties characterizing their analysis. Here, we present a novel approach which permits to project extreme precipitation for future climatic scenarios based on the combination of coarse‐scale information from climate models with local observations. Focusing on the south‐eastern Mediterranean, we provide projections of precipitation extremes which could not yet be derived using traditional methods, such as the events occurring on average once in 100 years. The combined effect of changes in intensity and occurrence of two dominant synoptic systems is projected to increase the intensity of the 100‐year events in the coast and in the desert areas of the region and to decrease it elsewhere. The novel approach offers a path for improving the predictability of extremes in a changing climate, independent of pending improvements in climate models. Key Points: Local observations can be used to constrain the intensity distribution of precipitation events associated with different synoptic systemsThe constraints allow projecting extreme return levels at scales relevant for impact studies from synoptic information from climate modelsThe approach improves the predictability of local extremes, independent of improvements in climate models at regional and local scales [ABSTRACT FROM AUTHOR]