30 Tex. Admin. Code § 298.510
For the Rio Grande, and its associated tributaries located within Texas, the commission finds that the environmental flow standards in this subchapter are appropriate environmental flow standards that are adequate to support a sound ecological environment at the locations specified in this subchapter to the maximum extent reasonable considering other public interests and other relevant factors. The commission finds that the sound ecological environment can best be maintained by a set of flow standards consisting of a schedule of flow quantities that contain subsistence flow, base flows, and high flow pulses at defined measurement points. Minimum flow levels for these components will vary by season and by year since the amount of precipitation and, therefore, whether a system is in subsistence, dry, average, or wet hydrologic conditions, will vary from year to year and within a year from season to season, and the number of pulses will also vary with the amount of precipitation.
30 Tex. Admin. Code § 298.510