Modeling of Watershed Input and Potential Climate Change Effects on Water Quality in Agricultural Reservoirs in the Little Washita River Watershed

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.