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What relevance do these statistics have on the study period selection? Significant trends and long-term <br />persistence can indicate that the data are not time-homogeneous; in other words, that the mean isn't <br />stable over time. If the selected study period is too short, it may not be representative of long-term <br />conditions. The longer the study period selected, the less important trend and persistence factors <br />become. Of the two factors, persistence appears to be the least problematic for RGDSS; if there is <br />persistence, it is short-term. On the other hand, the increasing trend in streamflows observed since the <br />1950s could be the upswing side of a longer-term climatic cycle. Long-term climatic conditions are <br />discussed later in this memo. <br />With respect to the spatial distribution of streamflow data, most of the stations with long periods of <br />record are located in the central and southern part of the basin, on the Rio Grande main stem or on the <br />Conejos River. <br />Key Stations <br />Surface water modeling of the Rio Grande and its tributaries requires the identification of "key" stations <br />on which to base development and use of hydrologic and climatic data. Key gages are those for which <br />historical data will be used to develop model baseflows, and in this sense are "included" in the model. <br />streamflow gages that should be considered as key are indicated as such in Table 1. In general, <br />streamflow gages that should be considered key are those having a substantial period of record and are <br />located on streams where surface water rights are represented in the model. Key climate stations in the <br />San Luis Valley are all stations shown in Table 3. <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 4 of 18 <br />