Post-wildfire sediment cascades: A modeling framework linking debris flow generation and network-scale sediment routing

Wildfires represent one of the largest disturbances in watersheds of the Intermountain West. Yet, we lack models capable of predicting post-wildfire impacts on downstream ecosystems and infrastructure. Here we present a novel modeling framework that links new and existing models to simulate the post-wildfire sediment cascade, including spatially explicit predictions of debris flows, storage of debris flow sediment within valleys, delivery of debris flow sediment to active channels, and the downstream routing of sediment through river networks. We apply the model to sediment dynamics in Clear Creek watershed following the 2010 Twitchell Canyon Fire in the Tushar Mountains of southern Utah. The debris flow generation model performed well, correctly predicting 19 out of 20 debris flows from the largest catchments, with only four false positives and two false negatives at observed rainfall intensities. In total, the model predicts the occurrence of 160 post-wildfire debris flows across the Clear Creek watershed, generating more than 650 000 m³ of sediment. Our new storage and delivery model predicts the vast majority of this sediment is stored within valleys, and only 13% is delivered to the river network. The sediment routing model identifies numerous sediment bottlenecks within the network, which alter transport dynamics and may be hotspots for aggradation and aquatic habitat alteration. The volume of sediment exported from the watershed after seven years of simulation totals 17% of that delivered, or 2% of the total generated debris flow sediment. In the case of the Twitchell Canyon Fire, this highlights that significant post-wildfire sediment volumes can be stored in valleys (87%) and within the stream network (11%). Finally, we discuss useful insights that can be gleaned from the model framework, as well as the limitations and need for more monitoring and theory development in order to better constrain essential inputs, process rates, and morphodynamics. © 2019 John Wiley & Sons, Ltd.     

An Analysis Platform for Multiscale Hydrogeologic Modeling with Emphasis on Hybrid Multiscale Methods

One of the most significant challenges faced by hydrogeologic modelers is the disparity between the spatial and temporal scales at which fundamental flow, transport, and reaction processes can best be understood and quantified (e.g., microscopic to pore scales and seconds to days) and at which practical model predictions are needed (e.g., plume to aquifer scales and years to centuries). While the multiscale nature of hydrogeologic problems is widely recognized, technological limitations in computation and characterization restrict most practical modeling efforts to fairly coarse representations of heterogeneous properties and processes. For some modern problems, the necessary level of simplification is such that model parameters may lose physical meaning and model predictive ability is questionable for any conditions other than those to which the model was calibrated. Recently, there has been broad interest across a wide range of scientific and engineering disciplines in simulation approaches that more rigorously account for the multiscale nature of systems of interest. In this article, we review a number of such approaches and propose a classification scheme for defining different types of multiscale simulation methods and those classes of problems to which they are most applicable. Our classification scheme is presented in terms of a flowchart (Multiscale Analysis Platform), and defines several different motifs of multiscale simulation. Within each motif, the member methods are reviewed and example applications are discussed. We focus attention on hybrid multiscale methods, in which two or more models with different physics described at fundamentally different scales are directly coupled within a single simulation. Very recently these methods have begun to be applied to groundwater flow and transport simulations, and we discuss these applications in the context of our classification scheme. As computational and characterization capabilities continue to improve, we envision that hybrid multiscale modeling will become more common and also a viable alternative to conventional single‐scale models in the near future.     

Reproductive characteristics and steroid levels in the scleractinian coral Oculina patagonica inhabiting contaminated sites along the Israeli Mediterranean coast

In this study we compared reproductive characteristics and steroid hormone levels in the non-indigenous scleractinian coral, Oculina patagonica, inhabiting contaminated vs. uncontaminated reference sites along the Israeli Mediterranean coast. Our results indicate significantly higher steroid levels in both seawater and coral tissue samples from contaminated sites as compared to reference sites, suggesting that corals tend to accumulate steroids from the surrounding waters. Despite their higher steroid levels, corals from the contaminated sites showed reproductive potential comparable to those of the reference sites. Interestingly, a clear distinction could be seen between corals exposed to pollution for long vs. short durations, with the latter showing a failure to complete gametogenesis. This suggests that reproduction in O. patagonica is susceptible to acute rather than chronic stress. The involvement of adjustment/adaptation processes may explain this species tolerance, and may reflect the ability of O. patagonica to successfully invade new areas in the Mediterranean Sea.     

Understanding the surface hydrology of the Lake Eyre Basin: Part 1—Rainfall

This is the first of two papers that describe the surface hydrology of the Lake Eyre Basin (LEB) (1,140,000 km²) in central Australia and compares some key characteristics with those observed from arid regions globally. This paper concentrates on annual rainfall, whereas the second paper is devoted to streamflow. The first part describes the LEB's climate (arid to semi-arid), which is dominated by a subtropical high pressure ridge stretching latitudinally across central Australia. Then follow major analyses that include the characteristics of rainfall, wet and dry spell lengths and cumulative surpluses and deficiencies, rainfall trends and intra- and inter-decadal fluctuations, and the relationship between rainfall and El Niño-Southern Oscillation (ENSO). The paper concludes with six conclusions, the key ones being: the variability of the annual rainfall (based on the coefficient of variation) in the LEB is approximately 60% greater than that found for stations located in arid regions in the rest of the world; there is a bias towards longer lengths of dry years than observed in the rest of Australia; and, there is a significant lag correlation between rainfall and ENSO, particularly in the east and in the latter part of a year.     

The size, mass, and composition of plastic debris in the western North Atlantic Ocean

This study reports the first inventory of physical properties of individual plastic debris in the North Atlantic. We analyzed 748 samples for size, mass, and material composition collected from surface net tows on 11 expeditions from Cape Cod, Massachusetts to the Caribbean Sea between 1991 and 2007. Particles were mostly fragments less than 10 mm in size with nearly all lighter than 0.05 g. Material densities ranged from 0.808 to 1.24 g ml⁻¹, with about half between 0.97 and 1.04 g ml⁻¹, a range not typically found in virgin plastics. Elemental analysis suggests that samples in this density range are consistent with polypropylene and polyethylene whose densities have increased, likely due to biofouling. Pelagic densities varied considerably from that of beach plastic debris, suggesting that plastic particles are modified during their residence at sea. These analyses provide clues in understanding particle fate and potential debris sources, and address ecological implications of pelagic plastic debris.     

Review of self-potential methods in hydrogeophysics

The self-potential (SP) method is a passive geophysical method based on the natural occurrence of electrical fields on the Earth's surface. Combined with other geophysical methods, SP surveys are especially useful for localizing and quantifying groundwater flows and pollutant plume spreading, and estimating pertinent hydraulic properties of aquifers (water table, hydraulic conductivity). Laboratory experiments have shown that the involved coupling coefficients mainly depend on the fluid chemistry, conductivity and pH, and on the soil or rock properties. The interpretation of SP observations can be done qualitatively, for instance, by correlation of SP gradients with water fluxes (through electrokinetics) or salt fluxes (through electro-diffusion). In recent years, the interpretation has been improved with the help of modelling or/and inversion of the Poisson equation and endeavours to estimate hydraulic parameters by means of the intensity of electric current sources caused by underground flows.     

Simulation of Soil Erosion and Sedimentation in Small River Basins

Potential erosion and sedimentation for three small stream basins near Dodoma, Tan-zania are simulated using SOILCART, a computer program recently developed to assist in the investi-gation of soil erosion problems. Using only widely available generalized data, the predicted values of soil erosion and sedimentation derived from SOILCART compare favorably to measured values which required a 2-year field investigation. Potential areas of high erosion are identified that could be amelio-rated with proper management. The potential use of SOILCART in preliminary evaluation of erosion and sedimentation problems promises useful results, especially in areas where small dams are proposed.     

Earthquake Rupture at Focal Depth,Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault,TauTona Mine,South Africa

We analyze here the rupture mechanics of the 2004, M2.2 earthquake based on our observations and measurements at focal depth (Part I). This event ruptured the Archean Pretorius fault that has been inactive for at least 2 Ga, and was reactivated due to mining operations down to a depth of 3.6 km depth. Thus, it was expected that the Pretorius fault zone will fail similarly to an intact rock body independently of its ancient healed structure. Our analysis reveals a few puzzling features of the M2.2 rupture-zone: (1) the earthquake ruptured four, non-parallel, cataclasite bearing segments of the ancient Pretorius fault-zone; (2) slip occurred almost exclusively along the cataclasite-host rock contacts of the slipping segments; (3) the local in-situ stress field is not favorable to slip along any of these four segments; and (4) the Archean cataclasite is pervasively sintered and cemented to become brittle and strong. To resolve these observations, we conducted rock mechanics experiments on the fault-rocks and host-rocks and found a strong mechanical contrast between the quartzitic cataclasite zones, with elastic-brittle rheology, and the host quartzites, with damage, elastic–plastic rheology. The finite-element modeling of a heterogeneous fault-zone with the measured mechanical contrast indicates that the slip is likely to reactivate the ancient cataclasite-bearing segments, as observed, due to the strong mechanical contrast between the cataclasite and the host quartzitic rock.     

A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations

We discuss the potential geodynamic connections between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean. Paleozoic arcs in the Iapetus and Rheic oceanic realms are preserved in the Appalachian–Caledonide and Variscan orogens, and in the Paleopacific Ocean realm they are preserved in the Terra Australis Orogen. Potential geodynamic connections are suggested by paleocontinental reconstructions showing Cambrian–Early Ordovician contraction of the exterior ocean as the interior oceans expanded, and subsequent Paleozoic expansion of the exterior oceans while the interior oceans contracted. Subduction initiated in the eastern segment of Iapetus at ca. 515 Ma and Early to Middle Ordovician orogenesis along the flanks of this ocean is highlighted by arc–continent collisions and ophiolite obductions. Over a similar time interval, subduction and orogenesis took place in the exterior ocean and included formation of the Macquarie arc in the Tasmanides of Eastern Australia and the Famatina arc and correlatives in the periphery of the proto-Andean margin of Gondwana. Major changes in the style of subduction (from retreating to advancing) in interior oceans occurred during the Silurian, following accretion of the peri-Gondwanan terranes and Baltica, and closure of the northeastern segment of Iapetus. During the same time interval, subduction in the Paleopacific Ocean was predominantly in a retreating mode, although intermittent episodes of contraction closed major marginal basins. In addition, however, there were major disturbances in the Earth tectonic systems during the Ordovician, including an unprecedented rise in marine life diversity, as well as significant fluctuations in sea level, atmospheric CO₂, and ⁸⁷Sr/⁸⁶Sr and ¹³C in marine strata carbonates. Stable and radiogenic isotopic data provide evidence for the addition of abundant mantle-derived magma, fluids and large mineral deposits that have a significant mantle-derived component. When considered together, the coeval, profound changes in the style of tectonic activity and the disturbances recorded in Earth Systems are consistent with the emergence of a superplume during the Ordovician. We speculate that the emergence of a superplume triggered by slab avalanche events within the Iapetus and Paleopacific oceans was associated with the establishment of a new geoid high within the Paleopacific regime, the closure of the interior Rheic Ocean and the amalgamation of Laurussia and Gondwana, which was a key event in the Late Carboniferous amalgamation of Pangea.     

FeS-Induced Radical Formation and Its Effect on Plasmid DNA

Plasmid DNA was incubated at 25°C with aqueous solutions of dissolved Fe(II), S(-II), and nanoparticulate FeS with a mackinawite structure, FeS_m. At ??0.1 mM total dissolved Fe(II) and S(-II), an increase in the proportion of the relaxed plasmid DNA occurs, through scission of the DNA backbone. In solutions where FeS_m was precipitated, nanoparticulate FeS_m binds to the DNA molecules. In solutions with concentrations below the FeS_m solubility product, nicking of supercoiled pDNA occurs. Plasmid DNA appears to be a sensitive proxy for radical reactions. The reactant is proposed to be a sulfur-based radical produced from the iron-catalyzed decomposition of bisulfide, in a manner analogous to the Fenton reaction. This is further supported by experiments that suggest that sulfide free radicals are produced during the photolysis of aqueous solutions of polysulfides. Supercoiling of DNA affects nearly all DNA?Cprotein transactions so the observation of relaxation of supercoiled forms through reaction with FeS solutions has direct implications to biochemistry. The results of this experimentation suggest that genotoxicity in FeS-rich systems is a further contributory factor to the limited survival of organisms in sulfidic environments. Mutations resulting from the interactions of organisms and mobile elements, such as plasmids, in sediments will also be affected in sulfide-rich environments.