Why are Lake Waco and the Bosque Watershed Being Modeled?
In 2001, the City of Waco, Baylor University and ENSR International were awarded a grant from the US-EPA to assess the state of water quality in Lake Waco and develop a best management practices plan to ensure good water quality for the residents of central Texas that depend on Lake Waco as a water supply. Recognizing that a "one-size-fits-all" approach was not necessarily the best approach to assessing and protecting water quality, the City of Waco, Baylor and ENSR are working together to develop watershed-specific models provided that could be to (a) assess the current state of health of the Bosque Watershed and Lake Waco, and (b) develop water quality management plans for the watershed and reservoir.
Along with a comprehensive field data monitoring program, new modeling tools help to answer many questions including:
What is a Water Quality Model?
A water quality model is a set of mathematical equations that represent the physical, chemical, and biological characteristics and processes of a water body in a way that approximates reality.
Water quality models can be used to simulate water quality changes that could be expected to result from changes in pollutant loads and different water quality management strategies. These simulations, called "scenarios," allow us to predict positive or negative changes on lake water quality.
What makes up the modeling system used in the Lake Waco Project?
The Lake Waco Modeling Project consists of three main modeling components:
SWAT is comprehensive, watershed-scale hydrologic model developed to predict the impact of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions over long periods of time. Some of the parameters predicted by SWAT include weather, surface runoff, return flow, percolation, ET, transmission losses, pond & reservoir storage, crop growth & irrigation, groundwater flow, reach routing, nutrient & pesticide loading, and water transfer. The SWAT model provides a planning tool that can be used to better understand how the watershed works and how to meet management goals within that context.
Data base construction, model development, and model calibration are important activities that must be performed. Acquisition and compilation of the of the spatial data (soils, topography, land use) into complete GIS data coverages for the Bosque watershed as well as acquisition of information on land values and soil productivity will enable us to model the development of the integrated assessment of the alternative future scenarios.
SWAT has been calibrated with data from the years 1992 through 1994, and validated with data from the period 1996 through 1998.
For impoundments such as Lake Waco, the period of time that water is held within the lake (the retention time) and how water mixes and moves within the lake affect water quality conditions and eutrophication. Therefore, to accurately model pollutant transport and transformation, the flow of water into, within, and out of the lake- the "hydrodynamics"- must be simulated. Some of the key physical factors that affect the health of the lake include the quantity and quality of water entering from the watershed, the climate, and the shape and bathymetery of the lake. In addition to the water coming into the lake, in-lake processes need to be examined and modeled. For example, during periods of high inflow, bottom sediments are resuspended bringing nutrients up off the bottom into the water column. During periods of extended low flow, such as during the summer, flushing rates decrease thereby promoting increased growth of algae.
The CE-Qual-W2 model is used to develop a 3-dimensional simulation of the complex flow patterns within the lake. The model simulates both water movement and water quality within the reservoir. CE-Qual-W2 predicts the processing of nutrient loads within the lake, the generation of algal biomass, and the impact on transparency, oxygen content, and other related parameters. A key factor driving the lake model is the nutrient inputs from the lake's tributaries, the majority of which are monitored.
The CE-Qual-W2 model has been calibrated with data from the years 1992 through 1994, and validated with data from the period 1996 through 1998. The model is considered to accurately represent lake hydrodynamics.
Wetlands are among the most biologically productive and diverse ecosystems in the world and are often compared to tropical rain forests or coral reefs. As part of the project to raise the level of Lake Waco seven feet, the US Army Corps of Engineers, US Fish and Wildlife Service and the City of Waco have built a 174 acre wetland at the upper end of Lake Waco along the North Bosque River. The Lake Waco Wetlands are being constructed in order to accommodate wildlife that will become displaced due to the plans to raise the level of Lake Waco. The wetlands have an added benefit for citizens in that they can be used to naturally purify our water supply.
To assess the positive effects the wetlands is having on water quality, a screening-level, analytical model is used for estimating the amount of water quality improvement provided by the Waco wetlands. Given basic characteristics about the wetland, pollutant removal efficiency can be computed for total suspended solids, total coliform bacteria, biochemical oxygen demand, total nitrogen, total phosphorus, and contaminants (e.g., organic chemicals and trace metals). These removal rates depend on a number of processes, such as microbial metabolism, adsorption, volatilization, denitrification, settling, etc., and ambient conditions, such as water temperature.
How Accurate are the Models?
The tools used in the modeling of Lake Waco and the Bosque Watershed produce estimates of water quality and quantity within the watershed and reservoir and predictions of water quality response in the lake. While they can provide good insight into water quality, they do not provide us with perfect forecasts. However, used properly, and with an understanding of the model limitations, they can help us to develop nutrient management strategies that protect Lake Waco for the long-term.
Each model used in the project has been "calibrated" and "verified" with real-world monitoring data for the watershed and reservoir. Based on the verification process, the model predictions are considered to be close to actual field observations. However, the appropriate use of the model and results is for making relative comparisons of water quality changes, rather than absolute predictions.
Therefore, our modeling results (a) acknowledge the limitations of the models, and (b) be reasonably adaptable as our level of information and understanding of watershed and reservoir processes and predicted response improves.