(a) For new water right authorizations in the Rio Grande Basin which increase the amount of water authorized to be stored, taken, or diverted as described in §298.10 of this title (relating to Applicability), the determination of the hydrologic condition for a particular season shall be determined once per season. The conditions present on the last day of the month of the preceding season will determine the hydrologic condition for the following season for the applicable measurement point. For each measurement point, cumulative streamflow for the previous 12 months will determine the hydrologic condition.     
      (b) For purposes of permit special conditions related to hydrologic conditions, for water right applications in the Rio Grande Basin, which increase the amount of water to be stored, taken, or diverted, the hydrologic condition shall be calculated using the full period of record for the United States Geological Survey (USGS) gage or the International Boundary and Water Commission (IBWC) gage, as applicable, at each measurement point such that subsistence conditions occur approximately 10% of the time, dry conditions occur approximately 15% of the time, average conditions occur approximately 50% of the time, and wet conditions occur approximately 25% of the time.     
      (c) For purposes of water availability determinations, for water right permit applications in the Rio Grande Basin, which increase the amount of water to be stored, taken, or diverted, hydrologic conditions used in the commission's water availability model shall be calculated such that subsistence conditions occur approximately 10% of the time, dry conditions occur approximately 15% of the time, average conditions occur approximately 50% of the time, and wet conditions occur approximately 25% of the time, based on the period of record and simulated flows of the water availability model.     
     Source Note:        The provisions of this §298.520 adopted to be effective March 6, 2014, 39 TexReg 1416