Stream temperature and dissolved oxygen modeling in the Lower Flint River Basin, Ga
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The tributaries of the Lower Flint River, southwest Georgia, are incised into the upper Floridan semi-confined limestone aquifer, and thus seepage of relatively old groundwater sustains baseflows and provides some influence over temperature and dissolved oxygen (DO) fluctuations. This hydrologic and geologic setting creates unique aquatic habitats. Groundwater withdrawals for center-pivot irrigation and proposed water supply municipal reservoirs threaten to exacerbate low flow conditions during summer droughts, which may negatively alter stream temperature and dissolved oxygen conditions. To evaluate possible effects of human modifications to stream habitat, we developed a one-dimensional Dynamic stream Dissolved Oxygen and Temperature (DDOT) model. DDOT was constructed with both Continuously Stirred Tank Reactor (CSTR) based and the one-dimensional Advection-Dispersion-Reaction Equation (ADRE) based formulations, and integrates the effects of dynamic streamflow and groundwater inputs, riparian shading, channel geometry, and channel hydraulics on the spatial and temporal dissolved oxygen and temperature dynamics. The major contributions of model DDOT to existing models include the integration of an easy-to-use SHADE module and a BED module. The SHADE module generates accurate estimation of riparian vegetation shading to direct solar radiation on stream water surface, while the BED module calculates the streambed layer vertical temperature and DO profiles that are necessary to account for groundwater input effect on surface water quality. The model was calibrated with field data collected in 2002 and evaluated with data from 2003, years in which flow and water quality behavior were very different. The two formulations provided nearly equivalent simulations. The model performed well and allowed robust exploration of system sensitivities and responses to management actions. With DDOT, we conducted sensitivity analysis of stream temperature and DO to the upstreamflow input, groundwater discharge, stream riparian vegetation shading, and stream width. It indicated that 1)reduced instreamflow rate leads to increased stream temperature and decreases stream DO in summer, 2)reduced groundwater input exacerbates stream temperature problems,especially during drought seasons, 3) reduced groundwater input does not exacerbate stream DO problems due to the fact that ground water itself has a DO concentration as low as 5 mg/L, 4)problematic DO levels occur only at very low flows, and 5)stream width and riparian vegetation have strong effects on stream temperature and DO levels. The model was then used to predict time series stream temperature and DO with long-term time series (1950 - 2003) streamflow data simulated by Hydrological Simulation Program - FORTRAN (HSPF) model and groundwater discharge data simulated by MODular Finite-Element (MODFE) model under three different agricultural pumping scenarios for Ichawaynochaway Creek and Spring Creek watersheds in the Lower Flint River Basin. The simulation indicated that the spatial patterns of water quality dynamics in the two watersheds were associated with groundwater input, stream aspect, and stream width.