Rakesh K. Gelda

Calibration, Verification, and an Application of a Two-Dimensional Hydrothermal Model [CE-QUAL-W2(t)] for Cannonsville Reservoir

ABSTRACT The successful testing of a two-dimensional hydrothermal/hydrodynamic model, CE-QUAL-W2(t), for Cannonsville Reservoir is documented. The model is calibrated to the detailed temperature data collected in the reservoir (depth-profiles at six locations) over the April-November interval (weekly) of 1995, using comprehensive hydrologic and on-site meteorological forcing data. Further, the frequency of current oscillations predicted for the lower layers matched results of independent determinations made from thermistor chain deployments (two locations). The model is verified through the successful continuous simulation of the observed thermal stratification regime of the reservoir for the 1988–1994 interval, a period in which wide interannual differences were observed related to variations in meteorology and operations. The model performs well in simulating: 1) the timing of stratification and turnover, 2) the duration of stratification, 3) the dimensions of the epilimnion and hypolimnion, 4) the temp...

Development and Testing of a Dissolved Oxygen Model for a Hypereutrophic Lake

ABSTRACT A mechanistic, two-layer mass balance model for dissolved oxygen (DO) in Onondaga Lake, a hypereutrophic, urban system, has been developed and tested. The model accommodates the processes of reaeration, algal photosynthesis and respiration, carbonaceous biochemical oxygen demand (CBOD), nitrogenous biochemical oxygen demand (NBOD), sediment oxygen demand (SOD) and water column oxidation of reduced species. Most model inputs, and many of the kinetic coefficients, were developed through a program of field monitoring and field and laboratory experimentation. The model was calibrated and verified using data collected in 1989 and 1990, respectively. Severe dissolved oxygen depletion is observed in the hypolimnion (lower mixed layer; LML) of Onondaga Lake during summer stratification. SOD and water column oxidation of reduced species diffusing from the sediment are together responsible for >70% of the summer depletion; CBOD, NBOD and algal respiration account for the balance. DO depletion occurs in the...

Tripton, transparency and light penetration in seven New York reservoirs ∗

Temporal patterns and inter-system differences in the attenuation coefficient for scalar irradiance (Ks), Secchi disc transparency (SD), several measures of tripton, and chlorophyll a (Chl) are documented for the lacustrine zones of seven reservoirs (nine distinct basins) in New York (U.S.A.), based on a single year of comprehensive measurements. Analyses of these data and historic (12 years) observations of SD and Chl, including application of empirical and deterministic modeling frameworks, demonstrate that inorganic tripton is the primary attenuating constituent responsible for the substantial differences in Ks and SD among these basins, and the major temporal variations observed in these optical characteristics in most of the study basins. These inorganic particles, of terrigenous origins, are supplied directly to the water column of these basins in inflows, particularly during runoff events, and through the sediment resuspension process. Comparison of the measures of tripton indicates electron-microscopy-based measurements performed somewhat better than gravimetric analyses in explaining the variations in Ks and SD in the lower concentration systems. Increases in average SD values by factors of 2–5, compared to prevailing values, are predicted for the study basins with the deterministic model for the case of no tripton.

Turbidity Model for Ashokan Reservoir, New York: Case Study

Terrigenous inorganic particles delivered during runoff events cause problems of high turbidity in many lakes and reservoirs. A turbidity model, composed of a two-dimensional hydrothermal/transport submodel and a turbidity submodel, is developed and tested for Ashokan Reservoir, New York, that experiences elevated turbidity levels following runoff events. A robotic monitoring network, rapid profiling instrumentation, and individual particle analyses are used to support the modeling, by specifying turbidity loads and in-reservoir patterns and features of the particles that guided representation of settling. The turbidity-causing particles are clay minerals, 1–10 μm in diameter. The hydrothermal/transport submodel that serves as the physical framework for the overall model, was separately validated for a 13-year period. The turbidity submodel considered three particle-size/settling velocity classes of turbidity, consistent with the independent individual particle characterizations. Robust performance is dem...

Hydrologic Analysis and Model Development for Cannonsville Reservoir

ABSTRACT The hydrology of Cannonsville Reservoir, a water supply and flow augmentation reservoir for the City of New York, is analyzed. Measurements of components of the hydrologic budget of the reservoir over the entire 30 years of operation are reviewed. The largest tributary is gauged near its discharge to the reservoir; all others are ungauged. Water surface elevation and outflow, in the form of spillway flow, direct releases for flow augmentation, and drinking water withdrawal are monitored. Multiple uses of the reservoir and variations in runoff common to the region result in strong seasonal and interannual variations in hydrology. Spillway flow typically occurs in late winter and spring, and is rare in summer and early fall. Releases, to meet minimum flow requirements in the Delaware River downstream, generally occur only in summer and early fall. Drinking water withdrawal is relatively uniform throughout the year. On average, spill, dam releases, and withdrawal for the water supply have represente...

Estimating oxygen exchange across the air-water interface of a hypereutrophic lake

Patterns of estimates of oxygen flux (J) across the air–water interface of hypereutrophic Onondaga Lake, NY, U.S.A., are characterized for time scales ranging from diel to seasonal for an 8-month period. The analysis is supported by a high frequency (most often hourly) monitoring program, conducted with a robotic buoy, that included measurements of dissolved oxygen (DO), temperature, and fluorometric chlorophyll a in the lakes surface waters, vertical profiles of DO through the epilimnion, and wind speed and solar radiance. The magnitude and direction of J is demonstrated to vary dramatically at diel, day-to-day, and seasonal time scales. Thus, large errors in estimates of J may result from extrapolating flux calculations made from short-term data to longer time periods. The variations in J were driven by variations in metabolic activity and meteorology, and were mediated by departures from equilibrium DO concentrations and wind-driven turbulence. Extended periods of high J values are shown to coincide with intervals of large departures from equilibrium DO concentrations, but day-to-day differences are driven mostly by variations in wind. A distinct diel pattern of J estimates is manifested for average conditions, with substantially higher J values during daylight hours. This pattern reflects the common diel patterns of the drivers of both higher DO oversaturation and wind speed over those hours. It is demonstrated that the magnitude of J is substantial relative to net changes in the epilimnetic DO pool, and thus must be accommodated accurately in estimates of primary production and community respiration that are to be based on diel monitoring of DO in the water columns of productive lakes.

Development and Testing of a Nutrient-Phytoplankto Model for Cannonsville Reservoir

ABSTRACT A dynamic multi-layer one-dimensional mass balance nutrient-phytoplankton (eutrophication) model is developed and tested for the lacustrine zone of eutrophic Cannonsville Reservoir. The model simulates concentrations of chlorophyll (Chl), zooplankton biomass, various forms of phosphorus (P) and nitrogen (N), and dissolved oxygen (DO). Model development was integrated with, and supported by, limnological analysis of detailed monitoring data and findings of various system-specific process/kinetic studies. Model testing is supported by comprehensive monitoring data of in-reservoir concentrations and important environmental and operational forcing conditions. Model credibility is enhanced by the independent determination of a number of important model coefficients from the process/kinetic studies, which greatly constrains the role of calibration. The model performed well in simulating observations of seasonal average Chl concentration, the progressive depletions of nitrate plus nitrite from the epili...

Sediment Resuspension in Cannonsville Reservoir

ABSTRACT The operation of the sediment resuspension process in Cannonsville Reservoir, NY, a eutrophic flow-augmentation and water supply impoundment for New York City, during the major drawdown year of 1995 is documented. Methodologies used in the assessment included transmissometer profiling; electron microscopy-based physical and chemical characterization of individual particles from the water column, sediments and the principal tributary; analysis of sediment trap collections; and mass balance calculations for total suspended solids (TSS). The resuspension process had several pronounced manifestations. First was the development of a conspicuous benthic nepheloid layer (BNL) which, by mid-summer, extended nearly 10 m above the bottom at one location. Second was the increase in the concentration of inorganic particles in the upper waters as the reservoir was drawn down. These particles, ultimately derived from the watershed, caused increases in turbidity. Third was the measurement of higher downward flu...

A Strategy for Reservoir Model Forecasting Based on Historic Meteorological Conditions

ABSTRACT A strategy for the application of linked watershed and reservoir models in the analysis of water quality management proposals for a water supply reservoir is presented. This strategy is based on the use of a long-term historical record of meteorological data, so that die predicted changes in water quality may be evaluated by considering the variations in streamflow, material loading, reservoir heat transfer and mixing associated with natural variations in meteorological conditions. Model simulations for a baseline condition and for several management proposals involving point source nutrient control, nonpoint source nutrient reduction, and reservoir operations are presented. The predictions are presented as distributions of the frequency of occurrence of selected annual statistics of nutrient loading, reservoir stratification, and reservoir water quality. Simulations for Cannonsville Reservoir indicate that reductions in phosphorus loading from wastewater treatment plants in the watershed would h...

Patterns and modeling of the long-term optics record of Onondaga Lake, New York

A retrospective analysis of long-term data sets of Secchi disc depth (SD, 40 years), the diffuse light attenuation coefficient for downwelling irradiance for photosynthetically available radiation (k d (PAR), 23 years), and chlorophyll-a ([Chl], 28 years), is presented for culturally eutrophic and industrially polluted Onondaga Lake, New York. The effects of changes in multiple drivers on SD and k d (PAR) are resolved in the record, including: (1) salinity decreases from closure of an industry, (2) changes in the grazing by exotic and native Daphnia, (3) long-term variations in populations of planktivorous fish, (4) runoff events, and (5) progressive decreases in phosphorus (P) loading. Four time intervals, or regimes, are identified for the record that had unifying features with respect to drivers and SD observations. The most dramatic changes in clarity conditions were the abrupt transition to annual occurrences of a clear-water phase (CWP) for 16 years following a coupled decrease in salinity and return of native Daphnia, and the subsequent loss of the CWP (for 5 years) from the loss of these cladocerans. The CWP was characterized by coincident dramatic increases in SD (annual maxima ≥ 4.55 m) and decreases in [Chl] and high densities of Daphnia. The benefits of P management were most clearly manifested by a 3-fold decrease in average [Chl] and a 2-fold increase in SD between two of the regimes, separated by 25 years, which both lacked CWPs. A probabilistic mechanistic model for SD and k d (PAR), that represents the effects of multiple constituents on the regulating processes of absorption and scattering, is developed, tested for the long-term record. The model is applied to partition the scattering coefficient (b) according to contributions of phytoplankton and tripton (b NAP ). Good model performance is demonstrated through: (1) closure of estimates of b with independent measurements, (2) consistencies of b NAP predictions with independent estimates based on an individual particle analysis technique, and the timing of runoff events, and (3) closure of predictions of k d (PAR) with observations. Broadly applicable nomographs are formed with the model that represent the dependence of SD and k d (PAR) on phytoplankton ([Chl]) and inorganic tripton (b NAP ) levels, that can be used to predict responses of these optical metrics to changes in the levels of these attenuating constituents. Inorganic tripton in the size range 1 to 10 μm, has played a critical role in influencing important features of the lakes optical regime by: (1) contributing importantly to b throughout the record (30 to 40 %, on average), (2) regulating optical signatures from runoff and CWP events, and (3) limiting increases in SD that can be achieved from decreases in phytoplankton biomass.