Ben R. Hodges

Impacts of hydrological changes on phytoplankton succession in the Swan River, Western Australia

The Swan River estuary, Western Australia, has undergone substantial hydrological modifications since pre-European settlement. Land clearing has increased discharge from some major tributaries roughly 5-fold, while weirs and reservoirs for water supply have mitigated this increase and reduced the duration of discharge to the estuary. Nutrient loads have increased disproportionately with flow and are now approximately 20-times higher than pre-European levels. We explore the individual and collective impacts of these hydrological changes on the Swan River estuary using a coupled hydrodynamic-ecological numerical model. The simulation results indicate that despite increased hydraulic flushing and reduced residence times, increases in nutrient loads are the dominant perturbation producing increases in the incidence and peak biomass of blooms of both estuarine and freshwater phytoplankton. Changes in salinity associated with altered seasonal freshwater discharge have a limited impact on phytoplankton dynamics.

Thin-Layer Gravity Current with Implications for Desalination Brine Disposal

Measurements of stratification and dissolved oxygen (DO) illustrate a hypersaline gravity current with salt loads similar to a desalination plant brine discharge. Over a 48-h sampling period in August 2005, alternating cycles of high- and low-temperature hypersaline water were observed along the bottom of Corpus Christi Bay in Texas, coincident with low benthic DO and tidal flushing from an adjacent smaller bay. The gravity current underflow was typically less than 10% of the overall water depth. Strong salinity gradients prevented wind-mixing of the entire water column. Hypoxic and near-hypoxic conditions were associated with limited DO replenishment from the ambient water. High DO levels in the underflow source water did not deter the development of offshore benthic hypoxia. A quasi-Lagrangian analysis is used to evaluate the relationship between ambient mixing and lateral mixing within the underflow. The analysis is further applied to estimating DO demand rates in the hypersaline plume. Mixing between ...

Short communication: Challenges in Continental River Dynamics

Continental River Dynamics (CRD) is herein defined as modelling the flow dynamics in all channels of a continental-scale river basin using the physics-based Saint-Venant equations. At the boundary of hydraulics and hydrology, CRD requires significant collaborative efforts to make new progress. Six constraints and seven challenges are identified in the areas of dynamics, dimensionality, resolution, uncertainty, model coupling, and data availability. Three key short-term needs for CRD are identified as (1) scaling up Saint-Venant river models to continental scales, (2) standards for integrating river and hydrology models, and (3) methods for effective use of lidar data and synthetic methods for approximating geometry for 1D dynamic models. An over-arching need for comprehensive data collection programs for river geometry is discussed.

Semi-implicit two-level predictor-corrector methods for non-linearly coupled, hydrostatic, barotropic/baroclinic flows

The unsteady shallow-water equations for barotropic/baroclinic (free-surface/density-stratified) flows with non-linear coupling of density transport and momentum are solved using a family of two-time-level, semi-implicit predictor–corrector methods (PC2). The PC2 methods are a general family that includes the popular TRIM method for hydrostatic flows. PC2 is characterised by four ‘θ’ parameters controlling the time ‘n’ and ‘n + 1’ weighting of (1) free surface gradient, (2) predictor step, (3) baroclinic gradient and (4) density temporal interpolation. Stability of the non-linear coupling between momentum and density transport for PC2 is examined in the inviscid limit. Central difference and quadratic (QUICK) spatial interpolation for density are compared. Second-order temporal accuracy for both barotropic and baroclinic flows is simultaneously obtained with appropriate θ parameters, which has previously been shown to be impractical for TRIM. The 2nd-order PC2 method has near-neutral non-linear stability (slightly positive amplification factor) where linear theory predicts exactly neutral stability. QUICK is shown to be preferable to central difference spatial discretisation to reduce the amplification factor. Adjusting the baroclinic weighting or adding small artificial viscosities can stabilise the model for non-linear internal wave simulations.

Uncertainty quantification and reliability assessment in operational oil spill forecast modeling system

As oil transport increasing in the Texas bays, greater risks of ship collisions will become a challenge, yielding oil spill accidents as a consequence. To minimize the ecological damage and optimize rapid response, emergency managers need to be informed with how fast and where oil will spread as soon as possible after a spill. The state-of-the-art operational oil spill forecast modeling system improves the oil spill response into a new stage. However uncertainty due to predicted data inputs often elicits compromise on the reliability of the forecast result, leading to misdirection in contingency planning. Thus understanding the forecast uncertainty and reliability become significant. In this paper, Monte Carlo simulation is implemented to provide parameters to generate forecast probability maps. The oil spill forecast uncertainty is thus quantified by comparing the forecast probability map and the associated hindcast simulation. A HyosPy-based simple statistic model is developed to assess the reliability of an oil spill forecast in term of belief degree. The technologies developed in this study create a prototype for uncertainty and reliability analysis in numerical oil spill forecast modeling system, providing emergency managers to improve the capability of real time operational oil spill response and impact assessment.

Effects of diurnal vertical mixing and stratification on phytoplankton productivity in geothermal Lake Rotowhero, New Zealand

Abstract Mixing processes in lakes are key factors controlling light availability for phytoplankton growth, but understanding the contribution of mixing is often confounded by other factors such as nutrient availability and species dynamics. Our study examined this problem in a low pH, geothermally heated lake dominated by one phytoplankton genus and lacking the complexity of nutrient limitation, phytoplankton species interactions, or grazing pressure. We hypothesized that the continuous strong convectively driven circulation resulting from atmospheric instability and sediment heating would negate any tendency of thermal stratification, entraining phytoplankton and transporting them away from high surface irradiance that could induce photoinhibition. During our study, water temperatures were considerably warmer than air temperatures, with a diurnal maximum surface temperature of 37.5 °C and minimum of 35.5 °C. Surface heating induced stratification, with a temperature difference of 1–2 °C evident during the day, but there was sufficient heat loss and mixing during the night to erode the stratification and create isothermal conditions. The vertical entrainment velocity driven by convective circulation was on the order of 0.1 mm s−1, but when there was strong solar heating, phytoplankton within the top 0.5 m of the water column still showed depressed photosynthetic quantum efficiencies, as determined with a Pulse Amplitude Modulated fluorometer (PHYTOPAM); however, this depression was less than for phytoplankton cells maintained throughout the day in surface waters with bottle incubations. At other times mixing generated by continuous heating and atmospheric instability meant that phytoplankton did not show photoinhibition; therefore, despite the geothermally driven mixing in Rotowhero, the intensity of solar radiation is still the key mechanism determining the stratification response and resultant photoinhibition of the phytoplankton. Lake Rotowhero provides an excellent natural laboratory to examine the relative time scales of mixing and phytoplankton photoinhibition responses because small changes in solar radiation have such marked impacts on the diurnal stratification and radiation experienced by cells located above the diurnal thermocline.

Dynamic river network simulation at large scale

Fully dynamic modeling of large scale river networks is still a challenge. In this paper we describe SPRINT, an interdisciplinary collaborative effort between computer engineering and hydroscience to address the computational aspect of this challenge. Although algorithmic details differ, SPRINT draws many design considerations from SPICE, one of the most fundamental EDA tools. Experimental results demonstrate that SPRINT is capable of simulating large river basins at over 100× faster than real time.

An environmental information system for hypoxia in Corpus Christi Bay: A WATERS network testbed

This project is creating and demonstrating a prototype Environmental Information System (EIS) that couples sensor measurements with end-to-end cyberinfrastructure to improve understanding of hypoxia in Corpus Christi Bay (CC Bay), Texas. Hypoxia is a common estuarine phenomenon that occurs when dissolved oxygen concentrations fall below 2 mg/L, and has resulted in about a ten-fold reduction in benthic standing stock and diversity in CC Bay. The hypoxia in CC Bay is correlated with salinityinduced stratification of the bay, but the stratification forcing and the spatial and temporal patterns of the hypoxia remain uncertain. In this project, an interdisciplinary team of hydrologists, environmental engineers, biologists, and computer scientists are collaborating to improve understanding of hypoxia by: (1) creating an Environmental Data Access System for CC Bay data archives, leveraging CUAHSI Hydrologic Information System (HIS) Web service developments to create data services that automatically ingest observed data in both national and local remote data archives; (2) developing an Environmental Modeling System for CC Bay hypoxia, leveraging NCSA Environmental Cyberinfrastructure Demonstrator (ECID) CyberIntegrator technology to combine numerical hydrodynamic, dissolved oxygen, and oxygen demand models with data mining using hierarchical machine learning algorithms; and (3) demonstrating the effectiveness of the EIS for supporting adaptive hypoxia sampling and collaborative research using ECID’s CyberCollaboratory. This paper will give initial results and future plans for the project.

Pre-impoundment assessment of the limnological processes and eutrophication in a reservoir using three-dimensional modeling: Abolabbas reservoir, Iran

Eutrophication in lakes and reservoirs plays a key role in aquatic environments and water quality management by affecting oxygen and nutrient cycles, especially in deep and large impoundments. Prior to reservoir construction, understanding the potential for eutrophication would provide valuable information to water resources planners on the critical factors affecting eutrophication and how reservoir operations might need to be limited or controlled. The pre-impoundment problem, by definition, can only be studied by numerical models, and herein a methodology for applying three-dimensional (3D) hydrodynamic and ecosystem models is proposed. In this paper, the 3D hydrodynamic Estuary, Lake, and Costal Ocean Model is coupled with the Computational Aquatic Ecosystem Dynamic Model to simulate the oxygen and nutrient cycles (eutrophication processes) in a planned deep reservoir in southwest Iran (Abolabbas reservoir). The effects of three scenarios (one normal and two drought conditions) on the reservoir’s eutrophication are investigated. Simulated are the life, metabolism, and the settling/resuspension cycles for two groups of phytoplankton (cyanobacteria and chlorophytes), as well as the nitrogen–phosphorous–carbon and dissolved oxygen cycles within the water column. To evaluate the eutrophication, the Trophic State Index and Vollenweider’s model are applied to the model output for the total phosphorus. The results show that under normal conditions, the reservoir should be oligotrophic, whereas the drought scenarios cause a general lowering of the water quality indices and the development of a mesotrophic–eutrophic or even a fully eutrophic state. Under drought conditions, the reservoir might suffer from severe oxygen depletion, especially in the hypolimnion. The analyses indicate further that phosphorus in the reservoir’s inflow is the eutrophication-limiting factor for all the scenarios. The sensitivity analysis indicates that the wind drag coefficient, the light intensity, and the sediment-oxygen exchange exhibit the strongest influence on the modeled eutrophication state in the planned reservoir.

Rivers and Electric Networks: Crossing Disciplines in Modeling and Simulation

Electric circuits and river networks share similarities in both their network structure and derivation from conservation principals. However, the disciplines have evolved separately and developed methods and models. This paper presents the foundations for network analysis for both disciplines and shows how numerical methods developed for circuit simulations can significantly improve river network models. The equations, models, and jargon are described, providing a reference for future studies to transfer knowledge across disciplinary boundaries. B. R. Hodges and F. Liu. Rivers and Electric Networks: Crossing Disciplines in Modeling and Simulation. Foundations and Trends