Hector R. Bravo

State space model for river temperature prediction

A stochastic state space model for the estimation of fiver temperature is presented, setting the stage for its implementation in optimal control algorithms. Physical processes modeled include advection, diffusion, and environmental heat exchange; the mathematical formulation is one dimensional. The deterministic formulation uses a hybrid characteristics-finite differences numerical scheme for the solution of the governing equations. The stochastic formulation accounts for uncertainty due to model assumptions, errors in the model inputs and parameters, and fiver temperature measurements. Model use is demonstrated by the estimation of temperatures in a 5-mile (8 kin) reach of the Des Plaines River, Illinois, below the Joliet power station. River temperature measurements and hydrological and meteorological data are used to estimate the model parameters; model formulation and limitations are assessed based on the model application results. The stochastic formulation improves river temperature estimation, as measured by the mean, variance-covariance, correlation, and range of the residuals.

Development of a commercial code-based two-fluid model for bubble plumes

Abstract Bubble plume models are applied to study the de-stratification of lake water, aeration of reservoirs, wastewater treatment, and gas injection into liquid metals. Several existing models exemplify numerical modeling as problem-specific art, solve a mixture momentum equation, and have limitations in the availability of documentation, definition of boundary conditions, and post- and pre-processing capabilities. The transfer of problem-specific models to a client or to a multidisciplinary research and development team is a difficult process. The questions addressed in this study were as follows: (a) can one use a commercial code as a basis to develop a user-friendly, efficient model that simulates two-phase flow in bubble plumes? (b) what are the capabilities and limitations of such a model? The two-fluid model developed has flexibility in the definition of the multiphase and viscous models, easily understood definition of boundary conditions, simple definition of spatial dimensionality and time dependency, efficient numerical solution, clear documentation and user-friendly pre- and post-processing capabilities. Water and air phase velocities, water turbulent kinetic energy, and air volume fraction are predicted with accuracy similar to that of existing problem-specific models. A strategy to overcome some under-dispersion and include air–water mass transfer effects through user-defined functions is discussed.

Trend and oscillations in the ice-cover duration of Lake Mendota, Wisconsin, USA

Abstract The 150-year time series of ice duration in Lake Mendota, Wisconsin was analysed to investigate possible effects of climate variability, and a non-parametric test confirmed a significant, decreasing trend. Singular-spectrum analysis (SSA) and the multi-taper method (MTM) of spectral analysis revealed a nonlinear trend and quasi-periodic oscillations. Analysis of the linearly detrended series showed significant frequencies ranging from quasi biennial to >100 years. The interannual and interdecadal signals detected are within the range of periodicity of the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) modes of variability. A smoothing procedure was used to separate low frequencies, or trend, and higher frequencies. Four reconstructed components of the low-pass-filtered time series were sufficient to explain 95% of the filtered series variance. The reconstruction showed signals with periods longer than 20 years that could be associated with the PDO. The series showed a relatively constant trend between approximately 1895 and 1965, and a steep declining trend in the last 40 years. Twenty four reconstructed components of the high-frequency residuals were required to explain 90% of the variance in the residual series. The reconstruction showed interdecadal signals that could be associated with the PDO and signals with periods between two and seven years that could be associated with the Southern Oscillation Index (SOI). Results of a coherence analysis showed that ice-cover duration is significantly correlated with the PDO for periods longer than about 20 years, and with the SOI for periods between two and seven years. The results suggest a relationship between the decreasing ice duration since about 1970 and the accompanying increase in Northern Hemisphere mean annual surface temperature. This interpretation agrees with previous descriptions of observed historic changes that appear to be related to anthropogenic activity.

Assessing the effects of thermal and hydro energy production on water systems

Energy production systems can be ranked in order according to the amount of energy generated as follows: petroleum, natural gas, coal, nuclear, biomass, hydro, wind/solar, and geothermal. However, regardless of the type of system, most energy production systems use water, with a direct effect on local water systems. This study reviews the effects on water systems of thermal and hydro energy production. Thermoelectric power is reviewed because it uses the largest water withdrawals, and hydropower is analyzed because it is the clearest example of the interconnection between energy production and water systems. The thermoelectric part of the study describes water use by thermal plants, types of cooling systems, a solution to the effects of once-through cooling systems on river and lake systems, and the potential effects of climate change on water systems and power production. The hydroelectric part of the study elaborates on the effects of large hydropower on the environment, reduction of those effects provided by small-scale hydropower, retrofitting dams for hydropower, and the potential effects of climate change on hydropower production. Society will benefit from stakeholder collaborations that optimize the freshwater efficiency of energy production, promote responsible energy operations with respect to water quality and the ecosystem, and better use of the synergies between water and energy systems. Thermal power and hydropower are, and will continue to be, important and necessary forms of energy production. Scientific analyses of energy and water systems under conditions of climate change will continue to enhance the reliability and resilience of both systems.

Coherence among climate signals, precipitation, and groundwater.

Climate signals may affect groundwater level at different time scales in different geographical regions, and those patterns or time scales can be estimated using coherence analysis. This study shows that the synthesis effort required to search for patterns at the physical geography scale is possible, and this approach should be applicable in other regions of the world. The relations between climate signals, Southern Oscillation Index, Pacific Decadal Oscillation, North Atlantic Oscillation, North Pacific Pattern (SOI, PDO, NAO, and NP), precipitation, and groundwater level in three geographical areas of Wisconsin are examined using a three-tiered coherence analysis. In the high frequency band (<4(-1) cycles/year), there is a significant coherence between four climate signals and groundwater level in all three areas. In the low frequency band (>8(-1) to ≤23(-1) cycles/year), we found significant coherence between the SOI and NP signals and groundwater level in the forested area, characterized by shallow wells constructed in sand and gravel aquifers. In the high frequency band, there is significant coherence between the four climate signals and precipitation in all three areas. In the low frequency band, the four climate signals have effect on precipitation in the agricultural area, and SOI and NP have effect on precipitation in the forested and driftless areas. Precipitation affects groundwater level in all three areas, and in high, low and intermediate frequency bands. In the agricultural area, deeper aquifers and a more complex hydrostratigraphy and land use dilute the effect of precipitation on groundwater level for interdecadal frequencies.

Modeling DO conditions in streams using Lagrangian advection method

Advection, dispersion, and kinetics govern the transport of biochemical oxygen demand and dissolved oxygen in streams. Analytical solutions of the governing equations are scarce; numerical methods are the tool commonly used to solve problems having arbitrary geometries and flows. However, the numerical solution of advection may produce numerical damping or oscillation. Some existing methods are therefore applicable only for advection-dominated flows, others have narrow discretization limits, or are computationally expensive. The simple method developed in this study is intended to relax these limitations. The method solves advection with a Lagrangian method and solves the diffusive and kinetics terms using an Eulerian method. The Lagrangian method uses a simple cubic spline interpolation that does not require the solution of additional transport equations. Analytical solutions and a convolution approach verified the accuracy of the method; the method permitted to obtain some new solutions for unsteady inp...