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<br /> <br />-6- <br /> <br />COMMON ERRORS AND LIMITATIONS IN HYDROLOGIC RECORl)S . <br />GENERAL CONSIDERATIONS <br /> <br />The preceding part of this manual has considered limitations that <br />are irihe~ent to hydrologic records a~d data, even when those records <br />comprise accurate measurements or determinations. Here considered are <br />the more COMMon sources of error in t~e records--error in the sense that <br />the quantities recorded differ from those th/ilt actually pertained. <br /> <br />Errors have been, and always will be present in hydrologic records, <br />largely because the instrumentation and procedures of measurement are of <br />necessi ty a compromise between the ideal and the practical. Over the <br />'years, instruments and procedures have been improved so that certain <br />sources o.f error have diminished; further improvement will ensue but <br />error-less records probably never will be realized. <br /> <br />Certain errors are random in time and in magnitude, and may compen- <br />sate within a period shorter than the term of study. If so, conclusions <br />derived from the study may not be inf;luenced substantially. Other erro~s <br />may be systematically plus or systematically minus, or, even if random <br />in magnitude and algebraiC sign, they maybe much more prevalent in a <br />particular part of the record. In th,is event, conclusions may be dis- <br />torted if the errors are not identified. <br /> <br />Further, most hydrologic records have been tabulated, totaled or <br />averaged, typed, and proofread by human beings--of whom all are prone <br />to error now and then. Misplaced decimal points, transposed or extra <br />digits, and typographic errors occasionally survive meticulous checking <br />and proofreading, and so become published. Some such errors can be <br />discovered readily and the cause may be deduced by a diligent user of <br />the records--for example, in a series that defines a relatively steady <br />hydrologic state, an erroneous value commonly can be discriminated with <br />assurance. On the other hand, a similar error in a series that spans an <br />unsteady state could be isolated only by going back to the original data. <br /> <br />In a presumably continuous record it is obvious that "the maximum <br />today must be at least as great as the minimum of yesterday, today, and <br />tomorrow; and the minimum today must be at least as small as the maxim1llll <br />of yesterday, today, and tomorrow." Philosophically that principle is <br />inviolate, yet some violations that have been published are too numerous, <br />to be considered inadvertent or typographic. The philosophically <br />impossible Tecord must be verified o~ discarded. <br /> <br />In every hydrologic study, therefore, ,m early step should be to test <br />the pertinent data for internal consrstency and, as has been stated, to <br />discriminate data that appear abnormal and may be susceptible to adjust- <br />ment. Tests must be devised to suit 'objectives'of the partioular study. <br />COMMon procedures are double-mass plotting of records for a given variable <br />.from various stations, pairing mass diagrams of unlike but related <br />variables, "routing" streamflows thr~ugh successive stations, and arrqing <br />data by magnitude and examining the enviroIDnental aspects of enrme <br />Vlllues. Other procedure,s will be suggested by the kind at stud;r. <br />