Role of aeolian sediment accretion in the formation of heuweltjie earth mounds,western South Africa

The action of organisms in shaping landforms is increasingly recognized; the field of biogeomorphology and the conceptual framework of ecosystem engineering have arisen in response to the need for integrated studies of the interactions between biotic and abiotic components of landscapes. Pathways by which organisms influence landscape development may be complex. For example, primary change initiated by one biotic element may initiate a cascade of other changes that eventually produce a significant landscape modification. Mound‐like landforms in North America and southern Africa are widely cited examples of biogenic structures, yet there is considerable controversy regarding the processes responsible for their formation. Heuweltjies (Afrikaans for little hills) are circular mounds ranging from 10–30 m diameter and 0.5–2 m height and are widespread in western South Africa. Colonies of the termite (Microhodotermes viator) are typically associated with heuweltjies and some investigators have attributed heuweltjie formation to the direct action of termites in redistributing earthen materials. However, rather than being directly responsible in this way, termites simply create nutrient‐rich islands, which support denser vegetation, thereby inducing the localized accretion of aeolian sediments and upward growth of mounds. Contrasting soil features in heuweltjies in one locale indicate these processes have occurred throughout the late Quaternary. Geographic variation in sizes of mounds is explained in part by the local availability of sediments that can be mobilized and redistributed by the wind. Recognition of the operation of aeolian processes in the formation of heuweltjies has important implications for conservation. Any land use that diminishes the sediment‐trapping effect of vegetation on heuweltjies truncates the very process by which new aeolian materials can accrue and may promote irreversible erosion and landscape degradation. Copyright © 2014 John Wiley & Sons, Ltd.     

Developing a high-resolution climatology for the Central California coastal region

This work presents a procedure for developing a high-resolution, regional climatology estimate, named RClimo, off the coast of central California. This high-resolution climatology may provide an alternative way to initialize numerical nowcast/forecast exercises in coastal regions. The methodology includes two primary steps: (1) averaging available data on a high-resolution grid and (2) objective interpolating the resulting average profiles onto a regular grid. The first step involves the computation of averages over density layers in the vertical and allowing for data gaps in the horizontal if data are unavailable at a high resolution. The OA in the second step uses anisotropic correlation length scales derived from the data themselves and an averaging radius to preserve the scales and variability of the synoptic fields.     

Relationships between water availability and Eucalyptus camaldulensis growth in a riparian forest

The effects of short-term flooding on soil water content and subsequent tree response were examined in a riparian Eucalyptus camaldulensis forest which was dissected by a series of shallow ephemeral channels, locally known as runners. Twelve isolated plots, each approximately 0.8 ha, were established in three blocks of four treatments. One of the blocks was underlain by a moist, sandy aquifer 2–4 m below the surface. The four treatments were (1) flooding each spring; (2) flooding each summer; (3) flooding each spring plus each summer; (4) control (zero flooding). Depth of water percolation after a summer flooding varied from 1.3 to over 6 m below the surface. Horizontal movement away from the edge of the floodwater ranged from almost zero on some plots to at least 38 m. The extensive horizontal movement was confined within narrow aquifers which occurred under some plots. Trees in plots underlain by a shallow aquifer always had higher xylem pressure potential (XPP, MPa) than other trees, and flooding these plots increased XPP by a non-significant quantity (−0.14 MPa to −0.12 MPa). However, on the other plots, flooding resulted in a statistically significant increase in XPP from −0.45 to −0.10 MPa. The effect of flooding on XPP was evident for between 22.5 and 37.5 m from the floodwater. This was ascribed to root interception and some horizontal movement of water. Increased flood frequency from zero to one to two per year resulted in mean leaf areas of 11.0 cm², 12.2 cm² and 13.2 cm², respectively. Trees in the runner, at 8 or at 38 m from the channels, had mean leaf areas of 12.9 cm², 13.6 cm² and 9.9 cm², respectively. The presence of shallow aquifers increased mean leaf area from 11.5 to 13.3 cm². Increased flood frequency significantly increased relative growth rate of trees up to 22.5 m from the edge of the floodwater. We conclude that short-term flooding of channels that occupied 15–20% of the forest floor temporarily improved tree moisture status and this increased tree growth rate in up to 70% of the forest.     

Cross-shelf transport into nearshore waters due to shoaling internal tides in San Pedro Bay, CA

In the summer of 2001, a coastal ocean measurement program in the southeastern portion of San Pedro Bay, CA, was designed and carried out. One aim of the program was to determine the strength and effectiveness of local cross-shelf transport processes. A particular objective was to assess the ability of semidiurnal internal tidal currents to move suspended material a net distance across the shelf. Hence, a dense array of moorings was deployed across the shelf to monitor the transport patterns associated with fluctuations in currents, temperature and salinity. An associated hydrographic program periodically monitored synoptic changes in the spatial patterns of temperature, salinity, nutrients and bacteria. This set of measurements show that a series of energetic internal tides can, but do not always, transport subthermocline water, dissolved and suspended material from the middle of the shelf into the surfzone. Effective cross-shelf transport occurs only when (1) internal tides at the shelf break are strong and (2) subtidal currents flow strongly downcoast. The subtidal downcoast flow causes isotherms to tilt upward toward the coast, which allows energetic, nonlinear internal tidal currents to carry subthermocline waters into the surfzone. During these events, which may last for several days, the transported water remains in the surfzone until the internal tidal current pulses and/or the downcoast subtidal currents disappear. This nonlinear internal tide cross-shelf transport process was capable of carrying water and the associated suspended or dissolved material from the mid-shelf into the surfzone, but there were no observation of transport from the shelf break into the surfzone. Dissolved nutrients and suspended particulates (such as phytoplankton) transported from the mid-shelf into the nearshore region by nonlinear internal tides may contribute to nearshore algal blooms, including harmful algal blooms that occur off local beaches.     

Local meteoric water lines describe extratropical precipitation

Stable water isotopes δ¹⁸O and δ²H are used to investigate precipitation trends and storm dynamics to advance knowledge of precipitation patterns in a warming world. Herein, δ¹⁸O and δ²H were used to determine the relationship between extratropical cyclonic precipitation and local meteoric water lines (LMWLs) in the eastern Ohio Valley and the eastern United States. Precipitation volume weighted and unweighted central Ohio LMWLs, created with samples collected during 2012–2018, showed that temperature had the greatest effect on precipitation isotopic composition. HYSPLIT back trajectory modelling showed that precipitation was primarily derived from a mid-continental moisture source. Remnants of major hurricanes were collected as extratropical precipitation during the 2012–2018 sampling period in central Ohio. Extratropical precipitation samples were not significantly different from the samples that created the central Ohio LMWL. Six additional LMWLs were derived from United States Geological Survey (USGS) Atmospheric Integrated Research Monitoring Network (AIRMoN) samples collected in Pennsylvania, Delaware, Tennessee, Vermont, New Hampshire, and Oxford, Ohio. Meteoric water lines describing published samples from Superstorm Sandy, plotted with these AIRMoN LMWLs, showed isotopic composition of Superstorm Sandy precipitation was commonly more depleted than the average isotopic composition at the mid-latitude locations. Meteoric water lines describing the Superstorm Sandy precipitation were not significantly different in slope from LMWLs generated within 300 km of the USGS AIRMoN site. This finding, which was observed across the eastern Ohio Valley and eastern United States, demonstrated a consistent precipitation δ²H–δ¹⁸O relationship for extratropical cyclonic and non-cyclonic events. This work also facilitates the analysis of storm development based on the relationship between extratropical event signature and the LMWL. Analysis of extratropical precipitation in relation to LMWLs along storm tracks allows for stronger development of precipitation models and understanding of which climatic and atmospheric factors determine the isotopic composition of precipitation.     

Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the upper Waipaoa River,New Zealand

Although much is known about overall sediment delivery ratios for catchments as components of sediment production and sediment yield, little is known about the component of temporary sediment storage. Sediment delivery ratios focused on the influence of storm-related sediment storage are measured at Matakonekone and Oil Springs tributaries of the Waipaoa River basin, east coast of New Zealand. The terrace deposits of both tributaries show abundant evidence of storm-related sedimentation, especially sediment delivered from Cyclone Bola, a 50 year return rainfall event which occurred in 1988. The sediment delivery ratio is calculated by dividing the volume of sediment transported from a tributary to the main stream by the volume of sediment generated at erosion sites in the tributary catchment. Because the sediment delivery volume is unknown, it can be calculated as the difference between sediment generation volume and sediment storage volume in the channel reach of the tributary. The volume of sediment generated from erosion sites in each tributary catchment was calculated from measurements made on aerial photographs dating from 1960 (1:44 000) and 1988 (1:27 000). The volume of sediment stored in the tributary can be calculated from measurements of cross-sections located along the tributary channel, which are accompanied by terrace deposits dated by counting annual growth rings of trees on terrace surfaces. Sediment delivery ratios are 0·93 for both Matakonekone catchment and Oil Springs catchment. Results indicate that Oil Springs catchment has contributed more than twice the volume of sediment to the Waipaoa River than the Matakonekone catchment (2·75 × 10⁶ m³ vs 1·22 × 10⁶ m³). Although large volumes of sediment are initially deposited during floods, subsequent smaller flows scour away much of these deposits. The sediment scouring rate from storage is 1·25 × 10⁴ m³ a⁻¹ for Matakonekone stream and 0·83 × 10⁴ m³ a⁻¹ for Oil Springs stream. Matakonekone and Oil Springs channels respond to extreme storms by instantaneously aggrading, then gradually excavating the temporarily stored sediment. Results from Matakonekone and Oil Springs streams suggest a mechanism by which event recurrence interval can strongly influence the magnitude of a geomorphic change. Matakonekone stream with its higher stream power is expected to excavate sediment deposits more rapidly and allow more rapid re-establishment of storage capacity. Copyright © 1999 John Wiley & Sons, Ltd.     

The effects of vertical shear and stratification on stationary rossby waves

Abstract

The generation of stationary Rossby waves by sources of potential vorticity in a westerly flow is examined here in the context of a two-layer, quasi-geostrophic, β-plane model. The response in each layer consists of a combination of a barotropic Rossby wave disturbance that extends far downstream of the source, and a baroclinic disturbance which is evanescent or wave-like in character, depending on the shear and degree of stratification. Contributions from each of these modes in each layer are strongly dependent on the basic flows in each layer; the degree of stratification; and the depths of the two layers. The lower layer response is dominated by an evanescent baroclinic mode when the upper layer westerlies are much larger than those in the lower layer. In this case, weak stationary Rossby waves of large wavelengths are confined to the upper layer and the disturbance in the lower layer is confined to the source region.

Increasing the upper layer flow (with the lower layer flow fixed) increases the Rossby wavelength and decreases the amplitude. Decreasing the lower layer flow (with the upper layer flow fixed) decreases the wavelength and increases the amplitude. Stratification increases the contribution from the barotropic wave-like mode and causes the response to be confined to the lower layer.

The finite amplitude response to westerly flow over two sources of potential vorticity is also considered. In this case stationary Rossby waves induced by both sources interact to reinforce or diminish the downstream wave pattern depending on the separation distance of the sources relative to the Rossby wavelength. For fixed separation distance, enhancement of the downstreatm Rossby waves will only occur for a narrow range of flow variables and stratification.