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I I I I I I I I I I I I I I I I I I I Hvdroloev Historical streamflows are commonly used in water supply studies as indicators of the streamflows that can reasonably be expected in the future. This approach was used for the hydrologic analysis conducted for this study. In the San Miguel study area, natural variations in precipitation and runoff produce streamflows which differ greatly from year to year. The historical period selected for analysis should represent the typical behavior of the streams, but also contain periods of wet and dry years, The wide year-to-year variations in streamflows in the study area are illustrated by the graph of annual flows for Beaver Creek near Norwood shown on Figure III- 1. Beaver Creek is used as an example because it is a major stream and the main source of water supply in the study area at the present time. On the graph, the heights of the bars represent the total volume of flow in Beaver Creek which would have passed Norwood in the years 1951 through 1983 under natural conditions (without diversions). Examination of the graph shows that there have been some very wet years such as 1958 and 1983 and some very dry years such as 1953 through 1956 and 1977 during this period. The entire period of 1951 through 1983 was selected as the study period for the hydrologic analysis because of the relatively good availability of required data and because it includes both very wet and very dry periods such as the extended drought of 1953 through 1956. Inclusion of critically low runoff periods is very important for accurately estimating water supplies. The data for these years were the basis for all of the analyses dealing with the water supplied by alternative projects. The primary sources of data were streamflow records from the U.S. Geological Survey (USGS) and ditch diversion records from the State Engineer's Office. Streamflow in the study area not only varies widely from year to year, it varies widely from month to month. Because snowmelt runoff is the major contributor to streamflow in the area, streamflows are distributed very unevenly throughout the year, The montWy pattern of streamflow typical for the study area is illustrated by the graph of average montWy flows for Beaver Creek near Norwood shown on Figure III- 1. Streamflows jump dramatically in April, peak in June, drop-off significantly in July and remain low for the rest of the year. The average montWy values shown represent streamflow under natural conditions for the years 1951 through 1983. Because of the uneven distribution of streamflow throughout the year, all calculations in the hydrologic analysis were performed on a monthly basis. III.3