Watershed-lake water quality modeling: Verification and application

ABSTRACT:

Water quality degradation in many Central Plains reservoirs results from a combination of lake morphology and watershed characteristics. This leads to high rates of eutrophication, sedimentation and related problems. This study coupled two models—Agricultural Nonpoint Source (AGNPS) watershed and BATHTUB lake—to simulate actual lake water quality conditions in three contrasting subbasins and subwatersheds of Clinton Lake, Kansas, and demonstrated the use of the coupled model for simulating take response to changes in watershed land use and management. Watershed event-based yields were used to determine monthly watershed loads of water, sediment, nitrogen (N), and phosphorus (P) to Clinton Lake. The lake was initially surveyed for depth and sampled monthly for various water-quality parameters with depth at 12 sites from May through December 1997 and April through November 1998. Watershed loads for 1997 conditions (26.5% cropland, 56.0% grassland) accurately simulated lake levels of total N, total P, chlorophyll a (chl a), and Secchi depth for each subbasin. The calibrated model simulated in-lake reductions as much as 35% for N, 38% for P, and 49% for chl a from the native-grass scenario, and increases as much as 6% for N, 12% for P and 9% for chl a from removing all existing terraces (17.8% of watershed). The calibrated current-conditions model also demonstrated a decrease in N:P ratio from 9.3 to 8.7 as lake depth decreased from sedimentation over the next century. The coupled model shows promise for load allocation and lake water-quality response determinations to support the total maximum daily load (TMDL) process.