Data Selection

H-GAC staff assembled the dataset to be analyzed from data in the TCEQ SWQMIS database. Quality-assured data collected by regional monitoring entities as part of their participation in the Clean Rivers Program, as well as by TCEQ Region 12 staff, between June of 1995 and May of 2010, representing a 15-year period, were downloaded. Comprehensive flow data for 61 monitoring stations was downloaded from the USGS website.

H-GAC staff further limited the data included for analyses to the following:

  • Data collected during routine monitoring (data from special and targeted studies was deleted)

  • Data collected at the surface of the water body (profile data was deleted)

No more than one data point for each parameter for a given monitoring station during a month

  • Stations and parameters where the most recent data was reported no later than 2006 and for a minimum of four distinct years

  • Data where field records indicate there was at least some flow (for example, flow severity results of "no flow" were deleted)

  • Parameters were limited to a subset of those available in SWQMIS. The parameters selected for analysis were alkalinity, ammonia, chloride, chlorophyll-a, dissolved oxygen, E. coli, enterococci, flow severity, instantaneous flow, nitrate/nitrite, orthophosphate, total phosphate, salinity, Secchi transparency, specific conductance, sulfate, temperature, total dissolved solids, total hardness, total organic carbon, total suspended solids, volatile suspended solids and pH.

Because the focus of this review is the watersheds in the H-GAC region, data from all unclassified tributaries were combined with the associated classified segment for some analyses. In addition, H-GAC staff created a value of ½ the numerical portion of censored results ("<") for analytical purposes, calculated natural logarithms of all data except pH, added the 2010 water quality standards and nutrient screening levels, combined equivalent parameter codes into a meaningful parameter name, created dummy variables to be used in regression analysis and created a variety of other variables to be used as labels in graphs and tables.

Report Contents