Harmful Algal Bloom Modeling Framework for the Highland Lakes, Colorado River, Texas, 1984–2018

The Highland Lakes are a chain of five reservoirs (Lake Buchanan, Inks Lake, Lake Lyndon B. Johnson, Lake Marble Falls, and Lake Travis) on the Colorado River of central Texas that are collectively managed by the Lower Colorado River Authority (LCRA). Since 2013 the LCRA has used a combination of Soil and Water Assessment Tool (SWAT, version 2005 [Neitsch and others, 2005] or 2009 [Neitsch and others, 2011]) watershed models and CE-QUAL-W2 (version 3.5 or 3.6; Portland State University, 2021) reservoir models to simulate and manage water-quality concerns such as harmful algal blooms in the Highland Lakes. Five CE-QUAL-W2 models (one for each reservoir) were produced for LCRA between 2009-2013 and simulated hydrodynamics, water temperature, and water quality in the reservoirs for calendar years 1984-2006 for Lake Travis; 1984-2008 for Inks Lake, Lake Lyndon B. Johnson, and Lake Marble Falls; and 1984-2011 for Lake Buchanan (LCRA, 2025). In co-production with the CE-QUAL-W2 models, the SWAT watershed models were used to simulate the constituent loads arising from the watershed and accounting for the various land uses and associated activities using the same time periods (LCRA, 2025). Output from the SWAT model for each reservoir watershed was used as input into the CE-QUAL-W2 models, thereby connecting the CE-QUAL-W2 models to the upstream watershed conditions. 
  
The U.S. Geological Survey, in cooperation with the LCRA, converted the LCRA (LCRA, 2025a) CE-QUAL-W2 (version 3.5 or 3.6) models to CE-QUAL-W2 version 4.5 (Portland State University, 2021) models; the converted models include improvements in model performance and added complexity for simulating up to three algal groups. Additionally, the following was completed: (1) the five CE-QUAL-W2 models were connected into a single framework, insofar that output from the upstream model flows into the downstream model as input; (2) lengthened the model time frame as uniform for all five models through calendar year 2018. As part of this effort, the SWAT models were extended through December 31, 2018, as documented in a LCRA addendum report (LCRA, 2025), in order to provide the necessary SWAT output to generate updated CE-QUAL-W2 input files; however, these models only provide output to create input files and are not explicitly connected to the CE-QUAL-W2 models. The modeling framework published in this data release also includes new R language scripts to create CE-QUAL-W2 formatted model inputs derived from SWAT watershed model outputs. The R language scripts of the workflow are provided to assist in the calibration process, although no calibration was performed on the converted models published in this data release. Each reservoir model in this modeling framework is tied directly to the next downstream reservoir model. However, the upstream model must be run to completion in downstream order (Lake Buchanan, Inks Lake, Lake Lyndon B. Johnson, Lake Marble Falls, and Lake Travis) because output files from the upstream model are used as input files for the next downstream model. with the exception of performing a water balance calibration.