Diurnal fluctuations in remote precipitation gage readings

Precipitation gage readings from hundreds of remote sites could dramatically improve the accuracy of western streamflow forecasts. Unfortunately, diurnal fluctuations of more that 3 cm have been observed when precipitation did not occur. Reliable data must be available for forecasts to be accurate, since errors of 3 cm could indicate catastrophic flooding. This study attempted to determine the cause of the diurnal fluctuations. A simulated remote precipitation gage site was installed so that temperature and pressure readings could be monitored at 15-minute intervals, and visual observations and changes to the system could be made frequently. Results indicated that: (1) thermal expansion and contraction produced daily pressure changes of less than 0.3 cm of H20; (2) concentration of methyl alcohol-ethylene glycol affects the magnitude of the fluctuations; and (3) diurnal fluctuations (etc.).