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Browsing by Author "West, Deborah E."

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    Modeling of Watershed Input and Potential Climate Change Effects on Water Quality in Agricultural Reservoirs in the Little Washita River Watershed
    (St. Anthony Falls Laboratory, 1999-06) West, Deborah E.; Stefan, Heinz G.
    In this study, the water quality of three agricultural impoundments in Oklahoma is simulated. A deterministic, one-dimensional water quality model (MINLAKE98) simulating physical, chemical, and biological processes in a lake or impoundment was used. The model operates on a time step of one day and makes year-round simulations of temperature, dissolved oxygen, phosphorus, and chlorophyll a. The model is designed for simulating the effect of different weather conditions including global warming for individual lakes. Input requirements include daily weather data, lake morphometry, initial Secchi depth, initial nutrient and chlorophyll concentrations and inflow quantity and quality from the watershed. A process oriented model, the Soil and Water Assessment Tool (SWAT), calibrated for the Little Washita River watershed by Hanaratty and Stefan (1997) was used to obtain estimates of the runoff into the impoundments, including flow rate, nutrient load, temperature, dissolved oxygen, and biochemical oxygen demand. The effect of climate warming on the individual impoundments was simulated. Previous research on the effect of a 2XC02 climate assumed steady trophic state and lake volumes. The new simulations use both a dynamic trophic state (determined by the simulated time-variable phytoplankton popUlation) and variable lake volume (as a result of inflows and outflows). Simulations of three conditions were made and compared: (l) past climate conditions (lxC02); (2) 2xC02 weather impact on lake/impoundment waterquality (with past watershed input); and (3) 2xC02 weather impact on both the watershed input and the resulting lake/impoundment water-quality. The results indicate that atmospheric conditions have a stronger impact on water temperature than watershed inputs. Nutrient levels and phytoplankton populations in the impoundments are strongly impacted by the watershed. However, the uncertainties in projecting the effects of land use on water quality are larger than the projected potential changes due to climate warming. Land use decisions are likely to have a larger impact on these small agricultrual impoundments than projected climate warming.
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    Simulation of Lake Water Quality Using a One-Dimensional Model with Watershed Input: Model Description and Application to Lake Riley and Lake Elmo
    (St. Anthony Falls Laboratory, 1998-12) West, Deborah E.; Stefan, Heinz G.
    This report Covers development of a model called MINLAKE98, which combines Riley's (1988) lake water quality model with a year-round model of temperature and dissolved oxygen (Fang and Stefan, 1994). Once validated this combined model will be used to simulate· the effect of runoff from two rural watersheds on the water quality of several ponds in the watershed. For many lakes in the US, point source inputs (such as municipal and industrial effluents) have been monitored, modified, diverted, and modeled. Today, non-point sources are the concern. Current problems include changes in land use, runoff quantity and quality, and population density. The results are seen e.g. in Lake Sammamish, Washington; In 1968 wastewater effluent was diverted out of the lake decreasing the annual mean total phosphorus concentration by the late 1970's. However, since the early 1980's the total phosphorus concentrati0l1 has begun to increase due to land use changes (Perkins, et al. 1997). An added dimension, is the effect of climate change has on runoff quality and the subsequent impact on lake water quality. Lake water, quality models which simulate year-round temperature and concentrations of phytoplankton, dissolved oxygen, and nutrients (phosphorus, nitrogen, and silica) can be used to study changes in trophic status of a lake. Examples include predicting changes resulting from global, warming or different ,landuse management, practices. For many lakes, noticable changes in the trophic state due to changes in management practices or changes due to global warming take more than a single openwater season to be observed. A model which includes simulation tlu'ough the winter icecover period can provide a prediction oftl~e following spring as opposed to an open-water model which requires re-initialization in the spring. Year-round simulation is also necessary for the prediction of long-term changes to a lake in response to changes in the watershed or due to climate change. Additionally, year-round models can predict anoxic periods during the winter which may be potential winter fish-kill events.