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000589 marked trend toward simulation of streamflow data based on the statistical characteristics of flow during the period of record such as the mean, stan- dard deviation, and skewness coefficient. A long record of streamflow is the best basis for defining statistical characteristics. Although it is not feasible to collect a long continuous record at every site where it will be needed, a number of such stations are required to provide information which can be transferred to ungaged sites, or to sites where a small amount of streamflow data is available., For natural streams the transfer of streamflow information Is done by regression methods, either by relating flow characteristics to basin char- acteristics such as drainage area, topography, and climate; by relating a short record to a long one; or by interpolating between gaged points on a stream channel. These methods are not applicable to streams where the flow is affected by regulation or diversion. Because different techniques are required to provide information on natural streams and regulated streams, the two are considered separately in program design. For regulated streams a systems approach is necessary if meaningful information is to be provided. Operation of continuous gaging stations defines the flow only at the sites of the stations but provides no basis for prediction of future flows in time or at other points on the stream. The systems approach req~ires an analytical model and detailed information on the characteristics of the water management scheme 'that has been imposed on the stream. These characteristics may include stage-capacity curves of reservoirs, operating-rule curves for release of water, diversions of water, and the channel geometry of the stream. The required analytical models are simple in concept and will usually consist of water-budget equations and flow-storage equations. However, in many cases the use of the digital computer will be required for complex computations, or to handle efficiently large volumes of data. A computer program tailored to the individual system can be prepared. Information on flow at some point or points on the stream is also necessary. Fortunately, many of the stations operated for the purpose of current data are on regulated streams and the records provide a base for the systems study. i For the purpose of setting accuracy goals, streams are further classi- fied as minor streams ~nd as principal streams. The intent is to use size of drainage area as an index of value of data. More costly developments on larger streams justify a higher accuracy goal for principal streams than, for minor streams. The point of division between the two classes is a drainage area of 500 sq mi, except in the western third of the State, where streams draining more than 1,000 sq mi are termed principal streams. The western third of the State includes al I the counties west of the tier of counties from Phillips County south (figs. I and 2). This flexible defini- II