Mendocino County Water Agency Temperature Monitoring Equipment and Quality Assurance Measures

The MCWA uses Onset Hobo® Temp water temperature monitors, which can be expected to be accurate within 2.3ºF (1.3ºC). According to the user’s manual, this instrument will respond to temperature changes of between 0.7 to 1.6ºF (0.4 to 0.9ºC). Prior to placement in the streams, the temperature monitors were started and then immersed in an ice bath (0ºC) overnight. Although a National Institute of Standards and Technology (NIST) traceable thermometer is not used to check the temperatures during calibration, the response of this non-NIST thermometer has been checked against a NIST-traceable thermometer. Within 2 weeks, the temperature monitors are placed in the stream at the deepest locations with good shading. A deep, well shaded area is chosen due to an interest in refugia temperature. However, the temperature monitors are not necessarily placed in thermally well-mixed locations. Therefore, the water temperatures they record may vary slightly from average water temperature conditions in their respective thermal reaches. After retrieval, the data set is checked for proper response to the room and ice bath temperatures as well prior to placing the temperature monitors in the field. This extraneous data is then trimmed from the data set prior to analysis. Probes were set to collect temperature measurements every 96 minutes which ensures capture of the daily maximum temperatures (Lewis et al 2000). As the collection interval increases beyond 96 minutes, the peaks may be missed and it becomes increasingly likely that the recorded daily maximum water temperatures are lower than the actual daily maximum water temperatures.