EFFECTS OF MEASUREMENT FREQUENCY ON WATER-LEVEL SUMMARY STATISTICS

Wetland scientists and managers recognize the need to characterize hydrology for understanding wetland ecosystems. Hydrologic data, however, are not routinely collected in wetlands, in part because of a lack of knowledge about how to effectively measure hydrologic attributes and how frequently to measure water levels. To determine how measurement interval affects interpretation of water-level data, we analyzed data from seven wetlands in Oregon and Pennsylvania. We created subsets of daily data for each wetland, with measurement intervals of 2 to 28 days, then compared those subsets to the daily data for annual water-level summary statistics, monthly mean water levels, and occurrence/duration of threshold conditions (e.g., water in the root zone). Our primary goal was to determine if sampling at low frequencies can provide representative water-level data and accurate perceptions of the occurrence of water levels above thresholds. For annual water-level distributions, small data sets from 28-day measurement intervals provided summary data (e.g., median, quartiles, range) comparable to the 1-day reference data. For measurement intervals of seven days or less, average errors in estimates of stage (minimum, 25th, 50th, and 75th percentiles) were ≤ 0.03 m; for a 28-day interval, average errors were