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San Antonio River Basin Report Card 2021

2021

Report Card Summary

2021
San Antonio River Basin Report Card Summary
The overall grade for the 2021 San Antonio River Basin Report Card is B. This grade is the average of twelve individual indicator grades, which are explained in greater detail below.
67.0
Instream Inflows
Instream Inflows
How is this being measured?
Explanation of the grade
Key findings

AInstream Inflows

While all the metrics in the San Antonio River Basin Report Card have overlapping correlation to the safe, clean, enjoyable creeks and rivers aspects of the River Authority’s mission, the Instream Flows grade is primarily related to the clean aspect.

Freshwater Inflows

In 2001, the Texas Legislature enacted Senate Bill 2 establishing the Texas Instream Flows Program. The Texas Parks and Wildlife Department, Texas Commission on Environmental Quality (TCEQ), and the Texas Water Development Board (TWDB) were tasked to work with stakeholders to develop instream flow recommendations that will maintain a sound ecological environment for both the instream and riverine habitats (as well as freshwater inflows into the bay and estuary system) while balancing human needs through the water-rights permitting process. According to the TWDB, “instream flows are the amounts of water running in a river, usually measured by the volume, moving down the channel in a specified amount of time.”

Available water within river systems is finite and there is often competition for those limited natural resources between human and environmental needs. Humans require drinking water as well as water for manufacturing, farming, power, and recreation. Aquatic and riparian habitats (as well as bays and estuaries) also require fresh water for fish, plants, and other wildlife species. Establishing instream flows are an important way to strike a balance between all these competing interests.

In coordination with the above-mentioned state agencies, the River Authority conducted instream flow studies which provided the fundamental science to advance collective knowledge of the interconnected ecosystems within the San Antonio River Basin. In September 2011, a regional, stakeholder-driven, scientific-based process chaired by the River Authority’s General Manager, submitted to the TCEQ the Environmental Flows Standards and Strategies Recommendations Report from the Guadalupe, San Antonio, Mission, and Aransas Rivers and Mission, Copano, Aransas, and San Antonio Bays Basin and Bay Area Stakeholders Committee. The instream flow recommendations contained with that report became the basis for the regulatory standards for the San Antonio River adopted in 2012 by the TCEQ.

How is this being measured?

A healthy river requires a variety of instream flow regimens, including low, medium, and high volumes, spread throughout different seasons of a year and lasting for different durations of time. In the TCEQ regulations, these different instream flow regimens are known as Subsistence Flows, Base Flows, Small Seasonal Pulses, and Large Pulses.

The TCEQ instream flow regulations utilize six different United States Geological Survey (USGS) gages spread throughout the San Antonio River Basin. Each gage has an identified target flow, measured in cubic feet per second (cfs), per season and hydrologic condition (i.e. dry, average, and wet) for subsistence, base, and small seasonal pulses. Four of these six gages also include a separate regulatory standard for large pulse flows that includes the time of year, frequency, target volume (cfs), and duration. The instream flow regulatory standards for all six USGS gages in the San Antonio River Basin are found in the TCEQ Environmental Flows for Surface Water.

Freshwater Inflows

Regulatory issues for water are complicated, to say the least. We have developed a simplified approach to grade instream flows for this basin report card that can be easily understood by the layman public and maintain sufficient scientific validity. Therefore, for simplicity, this basin report card indicator grade will only use the TCEQ regulatory seasonal dry hydrologic condition (DRH) base flow data from the River Authority’s Fiscal Year 2020-2021 (July 1, 2020 to June 30, 2021) as recorded from only the USGS gage 08188500 on the San Antonio River at Goliad. According to the TWDB, “base flow is the “normal” flow conditions found in a river in between storms, and base flows provide adequate habitat for the support of diverse, native aquatic communities and maintain groundwater levels to support riparian vegetation.” Since base flows in all hydrological conditions support these critical ecological components, base flow in seasonal DRH was chosen as the comparison point.

TCEQ seasonal average base flow standards for the USGS gage on the San Antonio River at Goliad.

Season Hydrologic Condition Base Flow Standard
Summer (Jul - Sept) Dry 139 cfs
Fall (Oct - Dec) Dry 167 cfs
Winter (Jan - Mar) Dry 200 cfs
Spring (Apr - Jun) Dry 174 cfs

The full list of TCEQ regulatory information specific to the USGS gage on the San Antonio River at Goliad are listed on pages 23 and 24 of the TCEQ Environmental Flows for Surface Water.

The first step in calculating a grade for this river basin report card metric is to determine separate scores for each seasonal DRH base flow. The seasonal DRH base flow scores are determined by:

Number of days in the season at base flow or greater

                Number of days in the season   x 100 = Score seasonal base flow

The final grade for this river basin report card metric is the average of the four seasonal base flow scores.

We recognize this report card grade is a simplification of the TCEQ regulatory standards. This San Antonio River Basin Report Card metric is not intended to replace the important TCEQ instream flows regulations, which the scientific staff of the River Authority helped to create and fully supports.

For the purpose of this river basin report card metric, the USGS gage located on the San Antonio River at Goliad was selected to serve as a surrogate for the larger, more complicated instream flows regulatory process for the entire San Antonio River Basin. The River Authority selected this gage for this report card metric because, of the six USGS gages in the San Antonio River Basin, it is the furthest downstream location with a drainage area of 3,921 square miles.

Certainly, the various low, medium, and high flow regimens as identified in the TCEQ standards are important for the health of the San Antonio River Basin and grading only the seasonal DRH base flow does not capture the full complexity of the various instream flow regimens. However, we believe that grading the seasonal DRH base flow does provide a sufficient, albeit simplified, way to determine how well instream flows are faring throughout the basin. In a particularly dry year (or a period of multi-year drought), if the data shows the seasonal average base flow is not being met, we will review the data against the subsistence flow standard and make note of this in the “key findings” section of this indicator. In a particularly wet year (or a year that includes a major flood event), we will review the data against the small seasonal pulses and large pulses standards and make note of this in the “key findings” section of this indicator.

Explanation of the grade

Season Days in Season Days at Base Flow
or Greater
Score for Days at Base Flow
or Greater
Summer
(Jul – Sept 2020)
92 63 68.5
Fall
(Oct – Dec 2020)
92 92 100
Winter
(Jan – Mar 2021)
91 91 100
Spring
(Apr – June 2021)
91 91 100

Grade: A
(2020-2021 Average at Base Flow or Greater: 92.1)

Key findings

In Fiscal Year 2020-2021 (July 1, 2020 to June 30, 2021) 336 of the 365 days had an average flow rate at or above the seasonal DRH "base flow" condition for the four seasons  at the USGS gage on the San Antonio River at Goliad. All 29 days that were below the “base flow” conditions occurred in the summer season, July-September 2020. Base flow for that particular season is 139 cfs. The overall  grade is a good result given that much of the San Antonio River Basin was identified by the National Integrated Drought Information System as being in abnormally dry, moderate, severe, or extreme drought conditions throughout several months of the fiscal year.

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