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001353 5. Hydrology Data 5.1 General The CRSS hydrology data consist of natural flow and salt data for 29 inflow stations in the Colorado River Basin. With the exception of Lower Basin actual flow stations, all data in the hydrology data base are natural data. Natural data are defined as historical data adjusted to remove the effects of human development. Consumptive use, reservoir regulation, exports, and imports have been factored into historically recorded data to obtain "natural" conditions. The 29 stations are located on key reaches of the mainstem, most of the major tributaries, and below major structures. Fourteen of the stations are actual flow stations. In the Upper Basin, actual flow stations are the most upstream points modeled in each tributary. In the Lower Basin, actual flow stations represent the entire tributary. The remaining 15 stations are intervening flow stations. Intervening flow stations contain gains or losses in flow and salt between a station and the last upstream station. All 29 stations are located on the CRSS Basin Map. Actual flow stations are designated AF and intervening flow stations are designated IF. 5.2 Hydrology Data Base The hydrology data base currently contains monthly values of flow and salt load for each station from water year 1906 through.water year 1980, plus provisional data for water years 1981 through 1983. The hydrology data base is updated every 5 years. The data base will be updated in 1987 to include 1981 through 1985 values. Values of flow are stored in units of 1,000 acre-feet. Values of salt load are stored in units of 1,000 tons. The hydrology data base is not input directly to the model but is input to the Hydrology Input Data Generation Program (MHYDRO). With the hydrology data base as input, MHYDRO generates the hydrology input data fil e for CRSM. Hydrology data bases containing stochastic flow and salt data can also be used by the CRSM. These are described in section 5.5. 5.3 Sources and Derivation of Hydrology Data The major source of data for the hydrology data base was USGS streamflow records. Twenty-nine streamflow gauging stations were selected for use in the CRSS. To produce a homogeneous data set for modeling purposes, it was necessary to adjust the data at each station to reflect the same conditions, i.e., a common level of depletion and a common period of record. The common level of depletion chosen is "natural," sometimes called virgin or undepleted/unregulated. All of the data in the hydrol~ ogy data base have been adj usted to thi s "natural" level except the Lower Basin actual flow stations which are described in section 5.3.2. 18