USGS
Maryland, Delaware, D.C.

Water Resources Review - Definitions of Terms

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 set off 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).

Monthly 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 median, 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 the January 1993 increased from previous month (December 1992), the increase is seasonal; if monthly mean flow for the January decreased from December, the decrease is contraseasonal.

Flood frequency analyses define the relation of flood-peak magnitude to probability of occurrence or recurrence interval.

Probability of occurrence is the chance that a given flood magnitude will be exceeded in any one year.

Recurrence interval is the reciprocal of probability of occurrence and is the average number of years between occurrences. For example, a flood having a probability of occurrence of 0.01 (1 percent) has a recurrence interval of 100 years. Recurrence intervals imply no regularity of occurrence; a 100-year flood might be exceeded in consecutive years or it might not be exceeded in a 100-year period.

Statements about ground-water levels refer to conditions near the end of the month. The water level in each observation well is compared with quartiles of water level for the end of the month determined from the entire period of record for that well. Changes in ground-water levels, unless described otherwise, are from the end of the previous month to the end of the current month.

FACTORS FOR CONVERTING INCH-POUND UNITS TO
INTERNATIONAL SYSTEM UNITS (SI)

Multiply inch-pound units	By		To obtain SI units

	                	Length
inches				25.4        	millimeters      
              			0.0254  	meters 
feet				0.3048  	meters 
miles				1.609    	kilometers 

		           	Area
square miles			2.590   	square kilometers 

			        Volume
acre-feet        		1233    	cubic meters

                		Flow
cubic feet per second    	0.02832  	cubic meters per second
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Last modified: 11 Jul 2001 gtf