Water quality modeling of the effects of macrophytes on dissolved oxygen in a shallow tailwater reservoir

Lake Ogallala, a 260-hectare tailwater reservoir, is subject to wide fluctuations of inflow water quality and quantity. These fluctuations impact lake temperatures, water stage, dissolved oxygen (DO), and nutrients. A two-dimensional, continuous simulation hydrodynamic and water quality model, CE-QUAL-W2, was used to simulate dissolved oxygen levels and to quantify DO sources and sinks in the lake. The elements modeled include surges of low DO and temperature; high chemical oxygen demands and dissolved nutrients; response of in-lake algae, macrophytes, and epiphytes; temperature and bathymetric induced circulation patterns; and weather impacts. The model's epiphyte routine was used to simulate macrophytes. The model was able to replicate the diurnal DO fluctuations, which ranged from 2 to 12 mg/L, as well as other water quality parameters. The study found that the chemical oxygen demand depressed daily DO minima by approximately 1.5 mg/L. This study also found that macrophyte respiration was an important factor in the low daily DO minima, depressing daily DO minima approximately 2 mg/L overnight.