Roy R. Gu

Impacts of climate change on streamflow in the Upper Mississippi River Basin: A regional climate model perspective

[1] Impact of climate change on streamflow in the Upper Mississippi River Basin is evaluated by use of a regional climate model (RCM) coupled with a hydrologic model, Soil and Water Assessment Tool (SWAT). The RCM we used resolves, at least partially, some fine-scale dynamical processes that are important contributors to precipitation in this region and that are not well simulated by global models. The SWAT model was calibrated and validated against measured streamflow data using observed weather data and inputs from the U.S. Environmental Protection Agency Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) geographic information systems/ database system. Combined performance of SWAT and RCM was examined using observed weather data as lateral boundary conditions in the RCM. The SWAT and RCM performed well, especially on an annual basis. Potential impacts of climate change on water yield and other hydrologic budget components were then quantified by driving SWAT with current and future scenario climates. Twenty-one percent increase in future precipitation simulated by the RCM produced 18% increase in snowfall, 51% increase in surface runoff, and 43% increase in groundwater recharge, resulting in 50% net increase in total water yield in the Upper Mississippi River Basin on an annual basis. Uncertainty analysis showed that the simulated change in streamflow substantially exceeded model biases of the combined modeling system (with largest bias of 18%). While this does not necessarily give us high confidence in the actual climate change that will occur, it does demonstrate that the climate change ‘‘signal’’stands out from the climate modeling (global plus regional) and impact assessment modeling (SWAT) ‘‘noise.’’ INDEX TERMS: 1655 Global Change: Water cycles (1836); 1860 Hydrology: Runoff and streamflow; 1866 Hydrology: Soil moisture; KEYWORDS: climate change, streamflow, SWAT Citation: Jha, M., Z. Pan, E. S. Takle, and R. Gu (2004), Impacts of climate change on streamflow in the Upper Mississippi River Basin: A regional climate model perspective, J. Geophys. Res., 109, D09105, doi:10.1029/2003JD003686.

Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater

Constructed wetlands (CWs) are considered to be important sources of nitrous oxide (N(2)O). In order to investigate the effect of influent COD/N ratio on N(2)O emission and control excess emission from nitrogen removal, free water surface microcosm wetlands were used and fed with different influent. In addition, the transformation of nitrogen was examined for better understanding of the mechanism of N(2)O production under different operating COD/N ratios. It was found that N(2)O emission and the performance of microcosm wetlands were significantly affected by COD/N ratio of wastewater influent. Strong relationships exist between N(2)O production rate and nitrite (r=0.421, p<0.01). During denitrification process, DO concentration crucially influences N(2)O production rate. An optimal influent COD/N ratio was obtained by adjusting external carbon sources for most effective N(2)O emission control and best performance of the CWs in nitrogen removal from wastewater. It is concluded that under the operating condition of COD/N ratio=5, total N(2)O emission is minimum and the microcosm wetland is most effective in wastewater nitrogen removal.

Biosorption of Cd(II) by live and dead cells of Bacillus cereus RC-1 isolated from cadmium-contaminated soil

The present study investigated the biosorption capacity of live and dead cells of Bacillus cereus RC-1 for Cd(II). The biosorption characteristics were investigated as a function of initial pH, contact time, and initial cadmium concentration. Equilibrium biosorption was modeled using Langmuir, Freundlich and Redlich-Peterson isotherm equations. It was found that the maximum biosorption capacities calculated from Langmuir isotherm were 31.95 mg/g and 24.01 mg/g for dead cells and live cells, respectively. The kinetics of the biosorption was better described by pseudo-second order kinetic model. Desorption efficiency of biosorbents was investigated at various pH values. These results indicated that dead cells have higher Cd(II) biosorption capacity than live cells. Furthermore, zeta potential, transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX), and Fourier transform infrared spectroscopy (FTIR) studies were carried out to understand the differences in the Cd(II) biosorption behavior for the both biosorbents. The bioaccumulation of Cd(II) by B. cereus RC-1 was found to depend largely on extracellular biosorption rather than intracellular accumulation. Based on the above studies, dead biomass appears to be a more efficient biosorbent for the removal of Cd(II) from aqueous solution.

Characterization and micellization of rhamnolipidic fractions and crude extracts produced by Pseudomonas aeruginosa mutant MIG-N146.

Two representative rhamnolipidic fractions, RL-F1 and RL-F2, produced by the P. aeruginosa mutant strain MIG-N146, were separated and chemically characterized by TLC, HPLC-MS, and FTIR. The RL-F1 fraction is predominantly mono-rhamnolipid homologues with a high content of one or two fatty acid moieties. The RL-F2 fraction is mainly composed of di-rhamnosyl moieties with two hydrophobic tails. Micellization behavior was investigated to assess the physicochemical properties of the surfactants, RL-F1, RL-F2, and crude rhamnolipidic extracts. The variations in morphology of micelle formation and growth were examined by dynamic light scattering measurements as a function of surfactant concentration. Critical micelle concentration (CMC), average minimal surface tension (gamma(CMC)), saturated surface excess (Gamma(m)), mean surface area per molecule (S), and adsorption efficiency (pC(20)) were determined from the surface tension profiles and compared for the three surfactant systems. It was found that micelle growth was significantly enhanced by increasing rhamnolipid bulk concentration, which was most probably accompanied with an aggregate shape transition. Well-separated multi- or bi-modal distributions of particle size were observed in RL-F2 and the crude extracts solutions. The results of this study demonstrate that molecular architecture of different surfactant compositions profoundly influences the performance of rhamnolipidic surfactants.

Estimating time-variable transformation rate of atrazine in a reservoir

Abstract The persistence of atrazine, one of the most applied herbicides in corn cropping areas, in an aquatic environment is dependent upon environmental conditions, i.e. temperature, sunlight, and presence of microorganisms. As these conditions vary seasonally, accurate determination of a time-variable degradation rate is important for the prediction of its fate and transport in surface water. A mass balance was performed to estimate the time-variable transformation rate (or half-life) of atrazine in the Saylorville Reservoir, Iowa. Calculated atrazine concentrations were compared with field data to verify the estimated half-life, which agreed reasonably well with the trends of observed values. A significant inverse relationship between the half-life and the hours of sunlight was obtained, showing the effectiveness of photodegradation. Estimated annual atrazine budget showed that 60% of the atrazine transported into the reservoir exited unchanged via outflow releases, while 40% was by kinetic processes within the reservoir. The half-life obtained in this study can be used as a preliminary value of degradation rate in the fate modeling of atrazine in reservoirs under similar environmental conditions.

A Multi-Objective Optimization Model for Coordinated Regulation of Flow and Sediment in Cascade Reservoirs

This paper presents a multi-objective optimization model for the coordinated regulation of flow and sediment in cascade reservoirs. The model was developed to address two contradicting issues: sediment trapping and flow regulation. The benefits of flood control, hydropower generation and navigation, and sedimentation in cascade reservoirs were considered as the target functions; then the corresponding submodels for reservoir operation and sediment computation were established. The model was implemented by reducing it to a single objective nonlinear model using the constraint method. Non-inferior solutions were obtained by solving the model with catfish effect particle swarm optimization algorithm. The model was applied to the cascade system of Xiluodu and Xiangjiaba reservoirs in the lower Jinsha River in the flood recession period. Under the safety of flood control and navigation, a non-inferior set for impounding time, power generation, and siltation-loss rate of capacity was obtained and optimal solutions with different weights were derived. The results demonstrate that the model is a useful tool in coordinated operations of cascade reservoirs.

Risk analysis of seasonal stream water quality management

Seasonal discharge programs, which take advantage of temporal variation of stream assimilative capacity, are cost effective. However, these seasonal discharge control programs should not increase the risk of water quality violations. A method is presented to estimate the allowable pollutant loads under both seasonal and non-seasonal discharge control programs for a single discharger that maintains the same level of risk of water quality violation. An enhanced in-stream water quality model QUAL2E-UNCAS was applied to a 39-km river reach of the Des Moines River below Des Moines Sewage Treatment Plant (DMSTP) in Iowa. The model was calibrated for dissolved oxygen (DO), biological oxygen demand (BOD), and ammonia as nitrogen with standard errors of 10, 17, and 23% by comparing with the observed water quality data. Monte-Carlo simulation technique was then implemented for seasonal and non-seasonal discharge program to assess the water quality violation risk and the allowable pollutant load. The results indicated that the four-seasonal program offers about 136% increase in BOD loading and 61% increase in ammonia loading when compared with the non-seasonal program without any increase in the violation probabilities, whereas the two-seasonal program only offers 13% decrease in BOD loading and 56% increase in ammonia loading. It is found that the multi-discharge program was beneficial for both water quality indicators, and thus provides a way of reducing the overall cost of waste treatment.

IMPACT OF INFLOW AND AMBIENT CONDITIONS ON DENSITY-INDUCED CURRENT IN A STRATIFIED RESERVOIR

The effects of inflow boundary conditions, ambient stratification, and geometry on the behavior of a density-induced current in a stratified reservoir are quantified through numerical experiments with a validated two-dimensional (2D) simulation model. Model results are analyzed to evaluate the sensitivity of reservoir flow variables to changes in the parameters characterizing inflow and ambient water. Quantitative evidence is presented to show that reservoir flow is more sensitive to Froude and Reynolds numbers than to other parameters. A critical value for each parameter is identified, which divides significant and insignificant effects on flow and dilution.

2D numerical simulation of flow characteristics in a channel narrowed by a cofferdam

Contracted flows caused by short hydraulic structures (length L/depth H=2.5–20) were extensively analysed by previous investigators. However, flows narrowed by long cofferdams (L/H>20) have neither been thoroughly examined nor well understood. In this study, a series of simulations were conducted by using a plane two-dimensional numerical model to analyse the influence of a cofferdam on flows in a narrowed channel and to evaluate the effects of the contraction ratio, longitudinal cofferdam length and cofferdam angle on the flow field. The model was developed with the finite-volume method and was validated by using laboratory flume measurements. Simulation results and analyses show that the cofferdam significantly affects the flow field and that the contraction ratio has a greater effect on flow characteristics in the narrowed channel than the longitudinal cofferdam length and cofferdam angle. This study assists in the better understanding of the impacts of cofferdam on the flow field and provides useful information in cofferdam design for multi-staged diversion during the construction of hydraulic projects.

MODELING THE FATE AND TRANSPORT OF TOXIC CONTAMINANTS IN RESERVOIR OVERFLOW AND INTERFLOW

A two-dimensional model for toxic contaminants was developed and incorporated into a laterally integrated hydrodynamics and transport model to investigate the effect of reservoir flow regime on contamination level in a reservoir after a toxic spill. The model describes the physical, chemical, and biological processes and predicts unsteady vertical and longitudinal distributions of a toxic chemical. Simulation results suggested that the persistence of a contaminant was significantly influenced by different flow regimes. It was found that the toxicant plume was more persistent in an interflow than in an overflow which moved more slowly and experienced greater volatilization and dissipation. This analysis can assist in spill control and reservoir management.