Water resources data tables (hereafter referred to as DATA tables) contain the environmental field and laboratory results, with each record representing an individual observation or test result. As such, these tables will normally contain as many as several hundred thousand records, depending on the number of stations data were recorded at, the number of parameters tested, the frequency of data collection, and the length of the period of record. As the core of the entire database, these tables were carefully designed to have efficient data storage. The most important fields in the database tables are:

 Station Identifiera user-defined code, linked to the STATIONS table, that defines where a particular sample was collected

 Parameter Codea user-defined code, linked to the PCODES table, that defines what type of data was collected (for example, rainfall, discharge, stage) or what type of field or laboratory test was performed on the sample (for example, conductivity, dissolved oxygen, five-day BOD)

 Compositing Codea user-defined code, linked to the CCODES table, that describes the type of compositing that may have occurred for the data value (for example, grab sample, 24-hour composite, daily average, monthly total, etc.); this can also be used in advanced applications to track biological data by, for example, fish species

 Result Codea one-character code to further characterize the result value. Use it to indicate, for example, if the test result was below or above specified detection limits

 Resultthe actual numeric data value (field measurement, test result, estimate, water quality model input or output, etc.).

More than one data table will be used for a given Project. When a batch of data are first entered into the computer, they are placed into a named Working file. Personnel are free to review, manipulate, edit, and delete records in the Working file. Each record must be validated, which indicates that it has been checked for accurate typing, and a responsible person has reviewed equipment calibration records and data collection techniques, and perhaps examined the data for reasonableness. Only when a Working file is complete and every record is validated may the Supervisor choose to transfer the data into a Master file.