Base Flow: The flow in a channel due to soil moisture or ground water.
Class Interval: A range into which data may be grouped (a sub-range of the total range).
Contour (for elevation or precipitation): A line connecting points on a land surface having the same elevation or precipitation.
Datum: Geographical reference frame, horizontal plane based on a vertical benchmark (such as mean sea level)
Frequency Factor Table: Table used to determine the frequency factor (K) in a Log Pearson Type 3 statistical distribution. To use the table you need to calculate the skewness coefficient and choose a desired return period. K is a function of these two variables and is listed accordingly in the table.
Gaging Station: An installation at a stream site where discharge and water levels are measured (see also streamgaging).
Gumbel Distribution: A statistical distribution that originated from the theory of extremes. This distribution has a downfall in that the function is unbounded on either side, which could lead to the calculation of negative flows (however, this is unlikely).
Period of Record: The time period for which flow measurements have been recorded. The period of record may be continuous or interrupted by intervals during which no data were collected. For USGS gages, it is usually listed at the top of the page with other key information about the gage.
Population: Complete set of persons, places, or things under statistical analysis.
Rating Curve: Relationship between water stage (elevation) and water discharge in a channel.
Recurrence Interval: Time interval in which an event can be expected to occur once on the average.
Reservoir: A man-made storage area for flood control of water supply.
Return Period and Exceedence Probability: An event has a return period (or recurrence interval) of T years if its magnitude is equaled or exceeded once, on the average, every T years. The reciprocal of the return period is the exceedance probability of the event, that is, the probability that the event is equaled or exceeded in any one year. For example, the 50-year flood has a probability of 0.02 or 2%, of being equaled or exceeded in any single year. It is important to note that the return period implies nothing about the actual time sequence of an event. The 50-year flood does not occur once every 50 years; it is expected, for example, that on the average , about twenty 50-year floods can be expected to be experienced during a 1,000 year period.
Skewness: Statistical term describing the third central moment about the mean, a measure of asymmetry.
Skewness Coefficient: Statistical term describing the third central moment about the mean (skewness) divided by the cube of standard deviation. The skewness coefficient is used with return period in the frequency factor table to determine the frequency factor K, which subsequently determines the shape of the flood frequency diagram (asymmetry).
Standard Deviation: Statistical term describing the measure of spread about the mean for a data set, calculated by taking the square root of the average of the deviations squared (variance).
Streamgaging: Two types of field measurements form the basis for all streamflow work: (1) river stage (water surface elevation) and (2) cross-sectional area. The location along the river where these measurements are taken is referred to as a gaging site. A permanent facility at this site is referred to as a gaging station. Determination of river discharge requires that the velocity and cross-section area be measured at the station in some systematic manner. This process is referred to as streamgaging.
Water Year: Time convention used by the USGS for compiling and reporting their streamflow data. The water year for Oregon (and United States) is from October 1st to September 30th. For example water year 2000 is from October 1, 1999 to September 30, 2000. For Oregon and Western U.S., the water year splits data during a relatively dry period during which streamflow does not change significantly from day to day (end of summer, early autumn), rather than in the midst of the wet season during which flows can change rapidly from day to day.
Watershed Divide: A line or border that defines a watershed topographically.
For a more detailed and complete discussion of hydrologic terms, please refer to the following text:
Bedient, Philip B. and Wayne C. Huber. Hydrology and Floodplain Analysis. Prentice-Hall, Inc., Upper Saddle River, 2002.