The WaterWatch Summary Table provides a synopsis of streamflow conditions for each day. The table is divided into two parts: a summary of the percentage of streamgages in each WaterWatch percentile class (appearing to the left of the "Total stations" column), and a percentile summary (appearing to the right of the "Total stations" column). The seven percentile classes are defined as follows:
1 = Lowest ever for the date at a streamgage
2 = < 10th percentile
3 = 10th to 24th percentile
4 = 25th to 75th percentile
5 = 76th to 90th percentile
6 = > 90th percentile
7 = Highest ever for the date at a streamgage
The number appearing in each class category for each date is the percentage of the total number of streamgaging stations with a flow in that class. Thus, on January 1, 2006 for example, 1 percent of the 2862 stations reported the lowest streamflow ever observed on any January 1st, 5 percent of the 2862 stations reported streamflow below the 10th percentile for the date, etc.
The second component of the table provides a summary of the observed percentiles for all stations on each date. For example, on January 1, 2006, a percentile value was assigned to each of the 2862 streamgages based on the January 1 flow values in the historical database for each streamgage. Of all 2862 streamgages, the “driest” streamgage had a January 1, 2006 flow value at the 1.10 percentile when compared to all January 1 flow values in the historical record for the streamgage. The “wettest” streamgage was at the 99.04 percentile when compared to all January 1 flow values in its historical record.
Similarly, the median (50th percentile) value for the 2862 streamgages was 63.82, meaning that one-half of the streamgages had streamflow values above the 63.82 percentile, and one-half below. A median streamgage percentile value of 50 would indicate perfectly normal conditions across the country, so the 63.82 value indicates that streamflow across the country was a little above that typically observed on January 1.