Correlation and analysis of water-temperature data for Oregon streams

Abstract

The increasing demand for water-temperature data for Oregon strerms led to a compilation by the Geological Survey of almost all such data collected by various agencies. These data were released in November 1964 in an open-file report. At some sites continuous records of temperature were obtained by means of a thermograph, but at most sites spot (periodic) measurements were made with a hand thermometer. The present report describes the deve^pment of procedures whereby spot observations of water temperature can be correlated with a representative thermograph record to estimate monthly means and extremes at the site where the spot observations were made. In general, these synthetic figures represent probable water temperatures at the spot-observation site during the period of record for the thermograph site. Tests indicrte that the average errors in the estimates of monthly means and extremes are about 1° and 2° F., respectively. The correlations lead to some interesting findings concerning factors affecting water temperature. During the summer months, east-west orientation of a stream can result in water temperature 4°-8° F. warmer than in a similar stream with north-south orientation. Longer exposure to direct solar radiation is the probable cause of this difference. Also, the correlations are found to be a valuable tool in assessing the effect of reservoirs on downstream water temperatures. The accuracy of the instruments and the methods used by the Geological Survey result in water-temperature records that are considered to be rccurate within ±1° F. A series of tests'in the summer of 1963 showed that tin water temperature at the probe end of the thermograph is closely representative of the mean temperature of the stream cross section if the probe is in moving water. Because water temperature in a given month varies from year to year, two rough guides are developed to assist in appraising short-term records with respect to long-term means and extremes of water temperature. One of these guides is a numerical scale that rates mean water temperature for all Julys and Augusts from 1913 to 1962. The scale ranges from one to six : one represents the cold end and six the warm end. Another guide is a statistical device for determining probable recurrence intervals of certain maximum and minimum water temperatures. Both of these guides indicate that any western Oregon thermograph