Soil pipe flow tracer experiments: 1. Connectivity and transport characteristics

Much debate has occurred in catchment hydrology regarding the connectivity of flow paths from upslope areas to catchment outlets. This study was conducted in two catchments, one with three upper branches, in a loess soil with a fragipan that fosters lateral flow and exhibits an extensive distribution of soil pipe collapse features. The study aimed to determine the connectivity of multiple soil pipe networks as well as determine pipe flow velocities during storm events. Fluorescein dye was injected directly into soil pipes at the upper most pipe collapse feature of four different hillslopes. Breakthrough curves (BTC) were determined by sampling multiple pipe collapse features downslope. The BTCs were used to determine the ‘average’ (centre of mass) and ‘maximum’ (first arrival) flow velocities. This study confirmed that these catchments contain individual continuous soil pipe networks that extend over 190 m and connect the upper most hillslopes areas with the catchment outlet. While the flow paths are continuous, the individual pipe networks consist of alternating reaches of subsurface flow through soil pipes and reaches of surface flow through gullies formed by pipe collapses. In addition, flow can be occurring both through the subsurface soil pipes simultaneous with surface flow generated by artesian flow from the soil pipes. The pipe flow velocities were as high as 0.3 m/s, which was in the range of streamflow velocities. These pipe flow velocities were also in the range of velocities observed in pinhole erosion tests suggesting that these large, mature soil pipes are still actively eroding. Copyright © 2015 John Wiley & Sons, Ltd.     

Biodiversity and studies of marine symbiotic siphonostomatoids off South Africa

Current knowledge of the biodiversity of the symbiotic marine siphonostomatoids from South African waters (136 species) is sparse compared to that globally (1 388 species). The difference is especially apparent when taking into account the diversity of fish (more than 2 000 species) and invertebrates (approximately 12 914 species) found in South African waters, all of which are all potential hosts of symbiotic siphonostomatoids. Most reports of symbiotic siphonostomatoids from South African waters are from Chondrichthyes and represent 80% of the currently known families and about 66% of the known genera. Taxonomic studies on the siphonostomatoids also provide information about their morphology, biology, habitat preference and host association, as well as the distribution of species and their associated host taxa. Molecular analysis, especially sequencing, provides additional information on the existence of cryptic species and population structure, but has high financial costs and a very low success rate in DNA isolation and PCR amplification. Reported declines in abundance of piscivorous fish species, for example, as a result of fishing pressure, with possible synergistic effects of environmental changes, and also the reported potential effects of anthropogenic and climate change on the broader coastal biota, may result in the loss of siphonostomatoid species before they can be described. Furthermore, in South Africa, there is a lack of professional taxonomists to study poorly known groups and this situation will not improve while research emphasis remains on commercially important groups and while taxonomic studies are not recognised for their value in research programmes.     

Enhancing RUSLE to include runoff‐driven phenomena

RUSLE2 (Revised Universal Soil Loss Equation) is the most recent in the family of Universal Soil Loss Equation (USLE)/RUSLE/RUSLE2 models proven to provide robust estimates of average annual sheet and rill erosion from a wide range of land use, soil, and climatic conditions. RUSLE2's capabilities have been expanded over earlier versions using methods of estimating time‐varying runoff and process‐based sediment transport routines so that it can estimate sediment transport/deposition/delivery on complex hillslopes. In this report we propose and evaluate a method of predicting a series of representative runoff events whose sizes, durations, and timings are estimated from information already in the RUSLE2 database. The methods were derived from analysis of 30‐year simulations using a widely accepted climate generator and runoff model and were validated against additional independent simulations not used in developing the index events, as well as against long‐term measured monthly rainfall/runoff sets. Comparison of measured and RUSLE2‐predicted monthly runoff suggested that the procedures outlined may underestimate plot‐scale runoff during periods of the year with greater than average rainfall intensity, and a modification to improve predictions was developed. In order to illustrate the potential of coupling RUSLE2 with a process‐based channel erosion model, the resulting set of representative storms was used as an input to the channel routines used in Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) to calculate ephemeral gully erosion. The method was applied to a hypothetical 5‐ha field cropped to cotton in Marshall County, MS, bisected by a potential ephemeral gully having channel slopes ranging from 0·5 to 5% and with hillslopes on both sides of the channel with 5% steepness and 22·1 m length. Results showed the representative storm sequence produced reasonable results in CREAMS indicating that ephemeral gully erosion may be of the same order of magnitude as sheet and rill erosion. Copyright © 2010 John Wiley & Sons, Ltd.     

Streambank dewatering for increased stability

Streambank erosion is often the dominant source of sediment leaving modified watersheds. Mass failure of high, steep banks is one of the most serious forms of streambank erosion. The risk of a given bank experiencing mass failure is a function of bank height, angle, and soil strength, which is governed by soil moisture. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low‐cost pump system and subsurface horizontal drains. Pore water pressures (both positive and negative pressures, or matric suction) were continuously monitored for 2 years at the pumped site, at an adjacent untreated control section, and for 1 year at the site stabilized with horizontal drains. Resulting data were used to calculate a time series of the factor of safety using a computer model. Over the course of two wet seasons, average bank retreats for the control and pumped plots were 0·43 and 0·21 m, respectively. More limited monitoring revealed that the site with passive drains retreated about 0·23 m. At the pumped site pore water pressure was 3–4 kPa lower than at the control site during the most critical periods. Accordingly, computed factors of safety were above the failure threshold at the pumped site, but fell below unity at the control site on 11 occasions over the period of observation. Similarly, the drained site displayed generally lower pore water pressure and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. These results suggest, but do not prove, that bank dewatering promoted lower rates of bank retreat and higher levels of stability since the three sites had slight differences in soils, geometry and boundary conditions. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater. Published in 2009 by John Wiley & Sons, Ltd.     

MANAGEMENT AND SUBSURFACE EFFECTS ON RUNOFF AND SEDIMENT YIELD FROM SMALL WATERSHEDS

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ASD field hyperspectral measurements for discrimination of the ferruginous rocks and the iron ore types at El Gedida-Ghorabi area,Bahariya Oasis,Western Desert,Egypt

The ironstone succession at El Gedida-Ghorabi-Naser area of El Bahariya depression is subdivided into lagoonal manganiferous mud and fossiliferous ironstone consisting mainly of hematite and goethite-hydrogoethite. The application of the ASD field spectroradiometer measurements (spectral range) in the ASTER data led to the interpretation of the presence of ferruginous units as quartzitic sandstone, gluconitic sandy clay, and pink marly limestone. The existing iron ore minerals in the iron ore localities were also classified into high Mn hematite, low Mn hematite, goethite, hydrogoethite as well as low- and high-grade Hematite and Barite. Quartz, feldspars, rutile, and clay minerals (e.g., kaolinite and illite) are mainly associated with the iron ore. Accessory minerals of manganese, e.g., psilomelane and pyrolusite, were also present. The Barite mineral is recorded as a common mineral association with the iron ore deposits at El Gedida and Ghorabi localities. The stratigraphical units investigated in the study area include the oldest gravely clayey sandstones of the Bahariya Formation overlain by the fossiliferous and oolitic limestones of the El-Hamra, Qazzun, and Naqb Formations. Quartztic sandstones and clayey sandstones of the Radwan Formation and youngest Quaternary sediments of sandy-clayey materials were often found as intermittent cover and overburden in unconformity surfaces over the iron ore bands.     

Soil pipe flow tracer experiments: 2. Application of a streamflow transient storage zone model

Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three branches (west, middle, and east) of a main catchment and a back catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS‐P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross‐sectional area (A), soil storage zone cross‐sectional area (A_s), and exchange rate between the soil pipe and the soil storage zone (α_s). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely, estimated soil pipe flow velocities were higher with increased tortuosity, which led to a smaller cross‐sectional areas predicted for the soil pipe flowpaths, while other parameters were not sensitive to tortuosity. In general, application of One‐Dimensional Transport with Inflow and Storage‐P to this unique soil pipe condition suggested larger transient storage (A_s and α_s) compared with most stream systems. This was hypothesized to be because of relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Variation in curve numbers derived from plot runoff data for New South Wales (Australia)

The curve number method is a simple one parameter (the curve number) rainfall runoff model. While its theoretical underpinning has been questioned it remains a powerful hydrological tool in the absence of detailed data and is therefore used extensively in hydrological models. This study aims to characterize the variation in maximum retention values (S), which underlie curve numbers, for a range of agricultural treatments across a large spatial area in New South Wales (NSW), Australia. The data used for the analysis spans several decades of rainfall runoff observations. A range of different derivation methods result in variation in mean and variance of S. In particular, methods that emphasize the larger storms result in greater S and thus lower runoff. For larger spatial scales, emphasis on larger storms gives more reliable estimates of S. Systematic variation in S arises from variations in treatment, pre‐runoff soil moisture, rainfall depth, and variations in cover. On the basis of the analysis, a table of curve number values for different land uses found in NSW is presented. The resulting distributions of S and curve numbers provide guidance for rainfall runoff modelling studies in the agricultural important areas of NSW. Copyright © 2011 John Wiley & Sons, Ltd.     

Soil properties controlling seepage erosion contributions to streambank failure

The majority of sediment leaving catchments may be from streambank failure. Seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks. The objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure. Seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern Mississippi. Using field observations as a guide, the soil profile conditions of a shallow (45 cm) streambank, consisting of 30 cm of topsoil, a 10 cm conductive layer, and a 5 cm restrictive layer, were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head. The time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer. Seepage erosion began within minutes of flow initiation and resulted in substantial (3 to 34 cm) undercutting of the bank. Sediment concentrations of seeps were as high as 660 g l^?1 in situ and 4500 g l^?1 in the lysimeters. Sediment concentrations were related to the layer slope, thereby indicating the importance of detailed site characterization. The USDA‐ARS Streambank Stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion, but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks. Published in 2006 by John Wiley & Sons, Ltd.     

Siphonostomatoid copepods infecting Squalus acutipinnis Regan, 1908 off South Africa

Squalus acutipinnis (previously included in S. megalops) individuals caught off the east, south and west coasts of South Africa were examined for siphonostomatoid copepod infection. Collected siphonostomatoids were fixed and preserved in 70% ethanol and studied under microscopes using both transmitted and incidental light. The copepods were identified and the host-parasite associations estimated by calculating their prevalence, mean intensity and mean abundance. The siphonostomatoids represented four families and six species, and included new host records for Nemesis sp., Lernaeopoda sp. and Eudactylina acanthii, and a new geographical record for E. acanthii. Squalus acutipinnis individuals were infected by relatively low loads of siphonostomatoids. The most commonly found species was Achtheinus pinguis, the only species found off all three coasts. There was no difference in the prevalence of A. pinguis in sharks sampled off the three coasts but the highest mean intensity was found on the East Coast followed by the West Coast. The overall prevalence of A. pinguis was 33.6%, and was an order of magnitude higher than that of the other siphonostomatoids recorded. At such low levels of infection it will thus be necessary to examine a large number of hosts to determine the biodiversity of siphonostomatoids and geographical differences in prevalence in South African waters.