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An intensity-duration-frequency curve (IDF curve) is a mathematical function that relates the intensity of an event (e.g. rainfall) with its duration and frequency of occurrence. [1] Frequency is the inverse of the probability of occurrence. These curves are commonly used in hydrology for flood forecasting and civil engineering for urban ...
According to the U.S. National Weather Service (NWS), PoP is the probability of exceedance that more than 0.01 inches (0.25 mm) of precipitation will fall in a single spot, averaged over the forecast area.
The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. [1] The curve number method was developed by the USDA Natural Resources Conservation Service , which was formerly called the Soil Conservation Service or SCS — the ...
In hydrology, the log-normal distribution is used to analyze extreme values of such variables as monthly and annual maximum values of daily rainfall and river discharge volumes. [ 78 ] The image on the right, made with CumFreq , illustrates an example of fitting the log-normal distribution to ranked annually maximum one-day rainfalls showing ...
The theoretical return period between occurrences is the inverse of the average frequency of occurrence. For example, a 10-year flood has a 1/10 = 0.1 or 10% chance of being exceeded in any one year and a 50-year flood has a 0.02 or 2% chance of being exceeded in any one year.
Rainfall-runoff models need to be calibrated before they can be used. A well known runoff model is the linear reservoir , but in practice it has limited applicability. The runoff model with a non-linear reservoir is more universally applicable, but still it holds only for catchments whose surface area is limited by the condition that the ...
A number of methods can be used to calculate time of concentration, including the Kirpich (1940) [2] and NRCS (1997) [3] methods. Time of concentration is useful in predicting flow rates that would result from hypothetical storms, which are based on statistically derived return periods through IDF curves .
The upper figure shows mean distributions of stratiform and convective rainfall. The linear part of the distributions can be adjusted with particular of the Marshall-Palmer distribution. The bottom one is a series of drop diameter distributions at several convective events in Florida with different precipitation rates.