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Browsing by Author "Stefan, H. G."

Now showing 1 - 7 of 7
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    Comparisons of Actual Fish Observations with Simulated Suitable Fish Habitat in Minnesota Lakes
    (St. Anthony Falls Hydraulic Laboratory, 1993-09) Stefan, H. G.; Hondzo, M.; Eaton, J. G.; McCormick, J. H.
    The purpose of this study is to relate simulated water quality with fish presence observations. The typical seasonal patterns of water temperatures and dissolved oxygen concentrations in twenty-seven classes of Minnesota lakes have been simulated by calibrated models and related to observations of three fish guilds i.e, coldwater, coolwater, and warmwater fishes. Data from 3002 lakes were available in the Minnesota Department of Natural Resources lake database. Water temperature and dissolved oxygen criteria derived from a very large USEP A fish-temperature data base and dissolved oxygen observations were used to define and link simulated water temperatures and dissolved oxygen conditions to suitability of habitats for various species of fish. One-dimensional, dynamic models driven by 25 years of observed weather data were used to model daily water temperature and dissolved oxygen as a function of depth. The lakes are categorized according to surface area, maximum depth, and Secchi depth as a measure of trophic state. Good agreement between fish observations and numerical simulations of fish habitat defined by water temperatures and dissolved oxygen concentrations was found.
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    Experimental Analysis of Sedimentary Oxygen Demand in Lakes; Dependence on Near-Bottom Flow Velocities and Implications for Aerator Design
    (St. Anthony Falls Hydraulic Laboratory, 1993-06) Mackenthun, A. A.; Stefan, H. G.
    Aeration technology is applied in hundreds of Minnesota lakes and reservoirs for at least three purposes: (a) to prevent winterkill of fish in shallow lakes under ice cover, (b) to reduce nutrient release rates from the sediments and (c) in aquaculture to provide aerated water to high-density fish populations. A major uncertainty in the design, selection and application of aeration systems is the often observed increase in oxygen demand after aeration systems are installed and operated. As a result, the improvement in dissolved oxygen is often less than anticipated, even zero. In this study we have investigated this problem through a series of carefully designed experiments. We have shown that sedimentary oxygen demand (SOD), frequently the major oxygen consumer in lakes, increases proportionally to the velocity with which the water above the sediments moves. Aeration devices often and intentionally increase water velocity above the sediments and thereby increase oxygen consumption in the lake. The results given in this report allow a more realistic estimation of oxygen demand in lakes for aerator selection. Recommendations for aerator placement are also given.
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    Experimental Study of Sedimentary Oxygen Demand in Lakes: Dependance on Near-Bottom Flow Velocities And Sediment Properties
    (St. Anthony Falls Hydraulic Laboratory, 1994-12) Mackenthun, A. A.; Stefan, H. G.
    Sedimentary oxygen demand, SOD, is the uptake of dissolved oxygen, DO, by chemical and biological processes in the uppermost portion of lake sediments. The oxygen is removed from the water column by chemical oxidation processes and by respiration of the microbial population in the sediments. Low DO kills fish in lakes and ponds, especially under ice cover, and increases nutrient release rates from the sediments. To develop DO models and to effectively counteract oxygen depletion an improved understanding of SOD is required. In this study we have investigated SOD through a series of laboratory experiments. We have shown that SOD, frequently the major oxygen consumer in lakes, increases proportionally to the velocity with which the water above the sediments moves. This velocity dependent relationship has, however, an upper bound which depends on the sediment material. The results given in this report allow a more realistic estimation of oxygen demand in lake oxygen models (budgets).
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    Groundwater Interactions with Holland Lake, MN
    (St. Anthony Falls Laboratory, 2001-07) Mohseni, O.; Stefan, H. G.
    Holland Lake, a small but deep mesotrophic lake in the Twin Cities Metropolitan Area, has been considered by the Minnesota Department of Natural Resources, Division of Fisheries, for stocking with brown trout. Holland Lake, with a surface area of 0.14 km:2 (35 acres) and a maximum depth of about 18.8 m (61 ft) consists of two shallow bays covered with rooted macrophytes and a deep main basin. The deep basin is thermally suitable for brown trout. However, due to a high oxygen depletion rate in summer, the lake becomes anoxic below the surface mixed layer. The field study conducted in the summer of 1999 by the authors concluded that several mechanisms, all regarding some sort of horizontal advection process, could explain the observed high dissolved oxygen (DO) depletion rates: transport of detrital material from the shallow bays, density currents combined with sediment oxygen demand in the shallow bays and flushing effect by groundwater flow through the lake. Density currents from the shallow bays were attributed to the temperature regimes of the shallow bays. To aid in the design of an aeration system for the lake, a new field study was conducted in the summer of 2000 to quantify the potential groundwater flow through the lake, especially through the shallow bays. The field study included the measurement of groundwater piezometric heads underneath the lake bed using a potentiomanometer and DO concentrations and temperatures of groundwater. In addition, the water temperature profiles were measured at several locations in the shallow bays to investigate the potential for density currents.
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    Lakes/Reservoir Destratification Induced by Bubble Plumes
    (St. Anthony Falls Hydraulic Laboratory, 1990-12) Zic, K.; Stefan, H. G.
    A numerical model that simulates the destratification of a lake/reservoir by an air bubble column/point diffuser is presented. In the numerical model, the lakeLreservoir is divided into a nearfield and a farfield. The nearfield model includes the bubble plume and the flow in its vicinity. The farfield model treats the rest of the lake and considers the flow from the plume toward the lake and from the lake toward the plume. The mixing model is combined with multilayer dynamic stratification models WESTEX and CE-QUAL-R1 by the U.S. Army Corps of Engineers Waterways Experiment Station, to account for heat transfer through the water surface and other processes in the lake/reservoir during the mixing induced by the release of the air bubbles.
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    Predicted Effects of Global Climate Change on Fishes of Minnesota Lakes
    (St. Anthony Falls Hydraulic Laboratory, 1992-09) Stefan, H. G.; Hondzo, M.
    According to global climate change models, e,g. that from the Columbia University Goddard Institute for Space Studies (GISS), Minnesota's mean air temperature will increase by an annual average of approximately 4.0 'C if atmospheric C02 doubles. This is likely to have many environmental consequences, including changes in lake water temperatures and dissolved oxygen concentrations which in turn are likely to affect fish populations. This interaction between climate parameters, lake water quality parameters and fish populations has been investigated through model simulations of Minnesota lakes. Previous results of this study were summarized in September 1991 (Stefan et al., 1991, 1992). Specifically the description of fish habitat is extended herein to include lake benthic area, in addition to lake volume used in the previous report. The findings are as follows: After the projected climate change, good growth habitat bottom area (GGHA) and good growth habitat volume (GGHV) will be reduced for coldwater fish. In contrast, GGHA and GGRV will be increased for coolwater and warmwater fish. Coldwater, coolwater and warmwater fish habitat will change approximately by the same percentage in terms of GGHA or GGHV. The reduction in good growth habitat area or volume for coldwater fishes will be about twice as high for southern Minnesota as it will be for northern Minnesota lakes. The increases for cool water and wa.rmwater fishes will be three times greater for northern Minnesota lakes than for southern Minnesota lakes. The models I and assessment techniques employed to derive these conclusions can serve as templates for analysis of projected climate change impacts in other regions. i
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    Uncertainties in Projecting Stream flows in Two Watersheds Under 2xC02 Climate Conditions
    (St. Anthony Falls Laboratory, 1998-03) Mohseni, O.; Hanratty, M. P.; Stefan, H. G.
    Two surface water runoff models (SWAT and MINRUN96) were utilized to project changes in streamflows in two watersheds under a 2xC02 climate scenario, One is the Baptism River watershed (363 km2), in northern Minnesota, mainly forested, with a cold and humid climate, and the other is the Little Washita River watershed, in Oklahoma (538 km\ an agricultural watershed, with a warm and seasonally dry climate, 2xC02 climate conditions were obtained from the Canadian Climate Center General Circulation Model. No agreement was evident in the streamflows projected by the two watershed models for the Baptism River. Snow accumulation and snowmelt were the main differences between the two models. MINRUN96 simulated the past streamflow more accurately than SWAT. SWAT includes a biomass submodel which incorporates the effects of vegetation changes under 2xC02 climate conditions. SWAT also takes into account the effects of carbon dioxide in the estimation of evapotranspiration, while MINRUN96 does not contain any algorithm projecting the changes in vegetation or evapotranspiration due to doubling of atmospheric carbon dioxide. Both models projected more runoff in the Baptism River watershed in winter due to reduced snowfall under the 2xC02 climate scenario; however, the magnitudes of the projected increases were an order of magnitude different. SWAT was developed for agricultural watersheds and is probably not ,suited for application to forested watersheds at this time. For the Little Washita River watershed, the streamflows projected by the two models were in agreement. Both models showed an increase in fall and spring runoff and a decrease in summer runoff. The two models also proj ected a significant increase in the mmual streamflow of the Little Washita River under a 2xC02 climate scenario. The magnitUdes of the projected increases were comparable.