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<br /> <br />-2- <br /> <br />INADEQUACY OF' SAMPLING <br /> <br />The natural range of most hydrologic variables is large snd some <br />classes of records sample the range most inadequately. Commonly this <br />inadequacy of sampling handicaps the hydrologist much more seriously <br />than errors of measurement at the points sampled. <br /> <br />, <br />For example, there is ample evidence that instantaneous rates of <br />precipitation at a given station vary considersbly from moment to moment.' <br />At ,the particular station, this variability probably compensates in some , <br />degree over the term of a single storm. and more so over a season or yesr. <br />Similarly, among the stations of a, network. substantial compensation <br />occurs within the geographic reach of a particular storm or 9ver any <br />extensive area. Nonetheless, even when spaced more closely than is <br />ordinary, a network of precipitation stations takes only a woefully <br />small sample of the water precipitatorJ. Thus, although the conventional <br />statistical records are a usable'index; to relative amounts (volumes) of <br />water precipitated on an ,area, they afford only a rough measure of that <br />volume. If a hydrologic study involves mean depth or volume of water <br />precipitated on an area, the most probable value is that derived from <br />an isohyetal map constructed with due regard to all, recognized parameters <br />of precipitation. Any value derived by arithmetical procedures alone <br />would be less defendable. (It should be noted that these limitations of <br />precipitation records are inherent; they are independent of the accuracy <br />of measuring devices and procedures.) <br /> <br />In contrast, a stream-gaging station measures the integrated volume <br />of ~ater running off from the drainage' area. Thus, the conventiOnal <br />record of streamflow is limited inherently not by inadequacy of sampling, <br />but by accuracy of techniques for measurement, which are considered later <br />in this manual. <br /> <br />Among the hydrologic variables listed above, and in addition to <br />streamflow, only storage in reservoirs and lakes is measured directly in <br />conventional records. All the other kinds of records produce data that <br />are inherently index values rather than absolute measures of the hydro- <br />lo~c variable being sampled. Probably few samples are integrated in the <br />serise that they reflect, in true proportion, all conditions or aspects <br />of the particular water body for a particular moment or interval of time. <br />Thus, in any hydrologic study that involves correlation among two or <br />more of the variables, and in which the' available data,are inadequ.te fQr <br />computing ,a sound correlation coefficient by statistical procedures, <br />conclusions should be drawn cautiously. . <br />