Transmissivity and groundwater flow exert a strong influence on drainage density

The extent to which groundwater flow affects drainage density and erosion has long been debated but is still uncertain. Here, I present a new hybrid analytical and numerical model that simulates groundwater flow, overland flow, hillslope erosion and stream incision. The model is used to explore the relation between groundwater flow and the incision and persistence of streams for a set of parameters that represent average humid climate conditions. The results show that transmissivity and groundwater flow exert a strong control on drainage density. High transmissivity results in low drainage density and high incision rates (and vice versa), with drainage density varying roughly linearly with transmissivity. The model evolves by a process that is defined here as groundwater capture, whereby streams with a higher rate of incision draw the water table below neighbouring streams, which subsequently run dry and stop incising. This process is less efficient in models with low transmissivity due to the association between low transmissivity and high water table gradients. A comparison of different parameters shows that drainage density is most sensitive to transmissivity, followed by parameters that govern the initial slope and base level. The results agree with field data that show a negative correlation between transmissivity and drainage density. These results imply that permeability and transmissivity exert a strong control on drainage density, stream incision and landscape evolution. Thus, models of landscape evolution may need to explicitly include groundwater flow.

Geomorphological patterns of remotely sensed methane hot spots in the Mackenzie Delta, Canada

We studied geomorphological controls on methane (CH4) hotspots in the Mackenzie Delta region in northern Canada using airborne imaging spectroscopy collected as part of the Arctic Boreal Vulnerability Experiment (ABoVE). Methane emissions hotspots were retrieved at ~25 m2 spatial resolution from a ~10,000 km2 AVIRIS-NG survey of the Mackenzie Delta acquired 31 July – 3 August 2017. Separating the region into the permafrost plateau and the lowland delta, we refined the domain wide power law of CH4 enhancements detected as a function of distance to standing water in different ecoregions. We further studied the spatial decay of the distance to water relationship as a function of land cover across the Delta. We show that geomorphology exerts a strong control on the spatial patterns of emissions at regional to sub-regional scales: compared to methane hotspots detected in the upland, we find that methane hotspots detected in the lowland have a more gradual power law curve indicating a weaker spatial decay with respect to distance from water. Spatial decay of CH4 hotspots in uplands is more than 2.5 times stronger than in lowlands, which is due to differences in topography and geomorphological influence on hydrology. We demonstrate that while the observed spatial distributions of CH4 follow expected trends in lowlands and uplands, these quantitatively complement knowledge from conventional wetland and freshwater CH4 mapping and modelling.

Components and Tidal Modulation of the Wave Field in a Semi-Enclosed Shallow Bay

The wave field in coastal bays is comprised of waves generated by far-off storms and waves generated locally by winds inside the bay and regionally outside the bay. The resultant wave field varies spatially and temporally and is expected to control morphologic features, such as beaches in estuaries and bays (BEBs). However, neither the wave field nor the role of waves in shaping BEBs have been well-studied, limiting the efficacy of coastal protection and restoration projects. Here we present observations of the wave field in Tomales Bay, a 20 km long, narrow, semi-enclosed embayment on the wave-dominated coast of Northern California (USA) with a tidal range of 2.5 m. We deployed pressure sensors in front of several beaches along the linear axis of the bay. Low-frequency waves (4 * 10^-2 * 2.5 * 10*^-1 Hz or 4 - 25 s period) dissipated within 4 km of the mouth, delineating the "outer bay" region, where remotely-generated swell and regionally-generated wind waves can dominate. The "inner bay" spectrum, further landward, is dominated by fetch-limited waves generated within the bay with frequency >= 2.5 < 10*-1 Hz. The energy of both ocean waves and locally-generated wind waves across all sites were modulated by the tide, owing to tidal changes in water depth and currents. Wave energies were typically low at low tide and high at high tide. Thus, in addition to fluctuations in winds and the presence of ocean waves, tides exert a strong control on the wave energy spectra at BEBs in mesotidal regions. In general, it is expected that events that can reshape beaches occur during high wind or swell events that occur at high-tide, when waves can reach the beaches with less attenuation. However, no such events were observed during our study and questions remain as to how rarely such wind-tide concurrences occur across the bay.

The Construction of Territories in the Qin Empire

This essay revisits the territoriality of the Qin empire by examining the spatial division underneath its commandery-county system. With the universal implementation of centralized administration, scholars usually believe that the Qin empire exerted strong control across its territories. But new Qin sources suggest otherwise. It is evident that the Qin regime devised multiple schemes to structure its empire into three concentric zones with asymmetrical political relations. The respective features and functions of these zones were consonant with those of the center, semiperiphery, and periphery in the “core-periphery” model. The regime’s spatial strategy can be understood as a compromise made to accommodate the diverse landscape in different parts of its vast empire, especially in the newly conquered regions. This reminds us that despite having installed the unitary commandery-county system, the territorial control wielded by the Qin regime in its new territories was tenuous at best.

Strong control of effective radiative forcing by the spatial pattern of absorbing aerosol

Over the coming decades it is expected that the spatial pattern of anthropogenic aerosol will change dramatically and that the global composition of aerosols will become relatively more absorbing. However, despite this the climatic impact of the evolving spatial pattern of absorbing aerosol has received relatively little attention, in particular the impact of this pattern on global-mean effective radiative forcing. Here we use novel climate model experiments to show that the effective radiative forcing from absorbing aerosol varies strongly depending on their location, driven by rapid adjustments of clouds and circulation. Our experiments generate positive effective radiative forcing in response to aerosol absorption throughout the midlatitudes and most of the tropical regions and a strong ‘hot spot’ of negative effective radiative forcing in response to aerosol absorption over the Western tropical Pacific. We show that these diverse responses can be robustly attributed to changes in atmospheric dynamics and highlight the importance of this previously unknown ‘aerosol pattern effect’ for transient forcing from regional biomass-burning aerosol.

Texture and Trace Element Composition of Rutile in Orogenic Gold Deposits

Rutile from a wide range of orogenic gold deposits and districts, including representative world-class deposits, was investigated for its texture and trace element composition using scanning electron microscopy, electron probe microanalysis, and laser ablation-inductively coupled plasma-mass spectrometry. Deposits are hosted in various country rocks including felsic to ultramafic igneous rocks and sedimentary rocks, which were metamorphosed from lower greenschist to middle amphibolite facies and with ages of mineralization that range from Archean to Phanerozoic. Rutile presents a wide range of size, texture, and chemical zoning. Rutile is the dominant TiO2 polymorph in orogenic gold mineralization. Elemental plots and partial least square-discriminant analysis suggest that the composition of the country rocks exerts a strong control on concentrations of V, Nb, Ta, and Cr in rutile, whereas the metamorphic facies of the country rocks controls concentrations of V, Zr, Sc, U, rare earth elements, Y, Ca, Th, and Ba in rutile. The trace element composition of rutile in orogenic gold deposits can be distinguished from rutile in other deposit types and geologic settings. Elemental ratios Nb/V, Nb/Sb, and Sn/V differentiate the rutile trace element composition of orogenic gold deposits compared with those from other geologic settings and environments. A binary plot of Nb/V vs. W enables distinction of rutile in metamorphic-hydrothermal and hydrothermal deposits from rutile in magmatic-hydrothermal deposits and magmatic environments. The binary plot Nb/Sb vs. Sn/V distinguishes rutile in orogenic gold deposits from other geologic settings and environments. Results are used to establish geochemical criteria to constrain the source of rutile for indicator mineral surveys and potentially guide mineral exploration.

Application of a New Generalized Fractional Derivative and Rank of Control Measures on Cholera Transmission Dynamics

In this study, the mathematical model of the cholera epidemic is formulated and analyzed to show the impact of Vibrio cholerae in reserved freshwater. Moreover, the results obtained from applying the new fractional derivative method show that, as the order of the fractional derivative increases, cholera-preventing behaviors also increase. Also, the finding of our study shows that the dynamics of Vibrio cholerae can be controlled if continuous treatment is applied in reserved freshwater used for drinking purposes so that the intrinsic growth rate of Vibrio cholerae in water is less than the natural death of Vibrio cholerae. We have applied the stability theory of differential equations and proved that the disease-free equilibrium is asymptotically stable if R 0 < 1 , and the intrinsic growth rate of the Vibrio cholerae bacterium population is less than its natural death rate. The center manifold theory is applied to show the existence of forward bifurcation at the point R 0 = 1 and the local stability of endemic equilibrium if R 0 > 1 . Furthermore, the performed numerical simulation results show that, as the rank of control measures applied increases from no control, weak control, and strong control measures, the recovered individuals are 55.02, 67.47, and 674.7, respectively. Numerical simulations are plotted using MATLAB software package.