Influence of NRCS snowcourse measurement date on data accuracy and climatic trends

The Natural Resources Conservation Service measures high elevation snowpack manually at snowcourses across the western US. The date of the measurement is nominally the first and fifteenth of the month although in recent years it averages approximately two days earlier to support timely operational water supply forecasts whose production begins on the first. This study found that the primary factors influencing measurement dates are, 1) the epoch of the measurement, 2) the day of the week of the nominal measurement date, 3) the presence or absence of snow at the site and 4) if the measurement is for the first or the fifteenth of the month. Specifically, the measurement date is less variable if snow is absent from the site. Mid-month data are collected closer to the nominal measurement date and first of month data have a bias towards being several days early. Since 1957, there has been a stronger aversion to collecting data on Fridays and weekends whereas before 1957 snow surveyors mostly avoided measuring on Sunday. Further, measurements are taken today, on average, 1.34 days earlier than before. These factors were modeled and the effect on climate trends was found to be small, on the order of less than 5% although in individual circumstances the effect can be significant.