Water Resources of the United States
Explanations for the National Water Conditions
The term U.S.Geological Survey "water year" in reports that deal with surface-water supply is defined as the 12-month period October 1, for any given year through September 30, of the following year. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 1999 is called the "1999" water year.
The National Water Conditions are generally depicted on a streamflow map that shows generalized contour patterns of streamflow conditions for any given month on the basis of provisional data from 287 index streamflow-gaging stations-- 12 in Canada, 273 in the United States, and 2 in the Commonwealth of Puerto Rico.
Streamflow-condition classifications shown on the map are based on comparison of streamflow for the current month at each index station with the long-term average streamflow for the same month in the 30-year reference period (1961-90). Shorter reference periods are used for one index station in Utah and for both of the index stations in Puerto Rico.
The comparative data are obtained by ranking the 30 flows for each month of the reference period in order of decreasing magnitude; the highest flow is given a ranking of 1, and the lowest flow is given a ranking of 30. Quartiles (25-percent points) are computed by weighted averaging of the 7th and 8th highest flows (upper quartile), 15th and 16th highest flows (middle quartile or median), and the 23rd and 24th highest flows (lower quartile). The upper and lower quartiles denote the highest and lowest 25 percent of flows, respectively, for the reference period. The median (middle quartile) is the middle value by definition. For the reference period, 50 percent of the flows are greater than the median, 50 percent are less than the median, 50 percent of flows are between the upper and lower quartiles (in the normal range), 25 percent are greater than the upper quartile (above normal), and 25 percent are less than the lower quartile (below normal).
The mean flow for the current month is then classified as: in the above-normal range if it is greater than the upper quartile, in the normal range if it is between the upper and lower quartiles, and in the below-normal range if it is less than the lower quartile. Change in monthly mean flow from the previous month to the current month is classified as seasonal if the change is in the same direction as the change in the median. If the change is in the opposite direction of the change in the monthly median flow, the change is classified as contraseasonal. For example: at a particular index station, the January median is greater than the December median; if monthly mean flow for January 1993 increased from that of the previous month (December 1992), the increase is seasonal; if monthly mean flow for January decreased from that of December, the decrease is contraseasonal.