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<br />011244 <br /> <br />-5- <br /> <br />Even after all reasonable adjustments have been applied, many long- <br />term streamflow records may retain a progressive, man~caused bias. As a <br />result, the later part of the record may, in, comparison with the earlier' <br />part and with the natural stream regimen, be abnormal in daily, seasonal, <br />or yearly runoff; in extremes of flow; or in any combination of these. <br />In, some instances, magnitude of the abnormality will be suggested by <br />double-mass plotting against records from adjacent stations presumably <br />not so biased. In other instances, there is no ready means for demon- <br />strating magnitude. As a consequence, long-term correlations between <br />streamflow and other hydrologic variables must be interpreted with <br />caution. <br /> <br />Long-term records from climatologic stations in and near metropolitan <br />areas well may embody substantial man-caused bias. In such areas, large <br />and generally increasing amounts of waste heat have been and are dis- <br />charged into the atmosphere; industrial smoke and dusts inject abundant <br />nuclei that may induce precipitation, and that probably modify the natural <br />regimen of thermal radiation; also, ever-changing structures interpose <br />variable barriers to the movement of low-level winds and may affect the <br />"exposure" of clima'!;ologic instruments. Currently, atmospheric physics <br />is not understood sufficiently well to appraise all the potential <br />influences on climatic records. In some instances, the amount of' <br />influence will be suggested by double-mass plottings against rec,ords from' <br />outlying stations. <br /> <br />Such bias is not,restricted to metropolitan areas--witness the <br />familiar change in humidity regimen, and probably in air-temperature <br />regimen, that has been induced in areas of extensive irrigation. <br />Exposure of climatologic stations may have been changed by the growth <br />or felling of trees, .construction or demolition of buildings, and the <br />like. The hydrologist must ever be alert to the possibility of such <br />bias in any of the longer records with which he deals. <br /> <br />SHORT-TERM ABNORMALITIES AND "STEPS" IN RECORDS <br /> <br />Both man's activities and natural events may cause short-term <br />abnormalities or lasting "steps" in hydrologic records--more commonly in <br />streamflow records. Among numerous examples are the effects of temporar,y <br />cofferdams and by-pass channels, initial filling of reservoirs, and <br />emergency sluicing; the perennially repeated abnormal flow regimen down- <br />stream from a "peaking" hydroelectric plant is conunon. The larger of <br />such events may be well known and may be documented in the "remarks" <br />section of the station records in Water-8upply Papers. However, mallT <br />will not be documented in the usual sources of information. If of <br />consequence in a particular hydrologic study, the possibility of such <br />an event can be t.ested by plotting records against one another, and <br />searching out an explanation for data that deviate markedly from the <br />general regimen. <br /> <br />",,; ~ <br /> <br />