The primary error source in the measurement of air temperature at the site was due to radiation loading of the naturally ventilated sensor shield. These errors were not evaluated in the field, but testing by Gill (1983) has demonstrated solar heating of greater than 2° C with wind speeds less than 0.5 ms^-1.
A minor error source in a limited number of the daily temperature maxima and minima was due to use of daily extreme hourly mean values (see discussion in data quality control section). On these days, the maximum and minimum temperatures are therefore conservative, and the times of maximum/minimum occurrence are for the extreme hourly means.
Radiation loading of the 207 sensor shield affected the accuracy of both temperature and RH measurements, primarily when wind speeds were less than 0.5 ms-1. Error in these measurements therefore affected the accuracy of calculated vapor pressure.
Wind direction was referenced to true north. An attempt was made each year to orient the sensor using its shadow at solar noon (local apparent time), with varying degrees of success. During both seasons, alignment was checked by recording the shadow orientation at a specific date and time. After the field season, solar azimuth was computed based on latitude, date and the observation time. The difference between the predicted and recorded shadow orientation in all but one case ranged from 2.5° to 8.8°, and adjustments were not made. The exceptional case was the Echo sensor in 1991, whose position had been established by reckoning, due to persistent cloud cover. In this case the difference was 21.3°, and an adjustment, or offset, was made to the wind vector direction.
No measurements made at Echo station.
Solar radiation measurement error arose primarily due to the effect of low sun angle on the model 8-48 detectors. This cosine response causes errors to increase with zenith angle. Eppley gives ( 5 percent for 70-80° angles, and that for 80-90° is no doubt higher.
No measurements made at Echo station.
The greatest error source in the L-down measurement was likely due to the pyrgeometers not being ventilated. Ventilation was not considered feasible, due to logistical considerations such as power requirements. The magnitude of this error was not assessed.
L-up was determined from surface temperature and emissivity. As in any point measurement, the instrument site was not always representative of the entire watershed. The presence or absence of snowcover, for example is particularly influential on both surface temperature and emissivity.
Errors in the determination of net radiation are partially the result of errors in the measurement in the components (or net radiation itself, in 1991). In addition, during precipitation events the radiometer domes were commonly wet or rimed. Occasionally snowfall was heavy enough to partially obscure the dome as well.
Precipitation was measured only a the Delta station, and errors and problems in that data are discussed in the documentation for that station.