OPW: Office of Public Works
Oifig na nOibreacha Poiblí

Climate Change

Guidelines on dealing with Climate Change

Empirical Equations

Examples of empirical equations used to estimate flows include a number of equations developed as part of the flood studies report and subsequent studies by various researchers. The flow calculated is generally Qbar (the statistical annual flood) or Qmed (the median flood).

These equations were developed by a detailed examination of records from flow measurement gauges. The statistically relevant flows were compared with three or more catchment characteristics, using regression, to develop relationships. Examples of catchment characteristics include catchment area, various measures of rainfall amount, catchment slope, density of the drainage network, proportion of the catchment area covered by lakes and the capacity of soil to absorb rainfall.

When the data used to gererate one of these relationships is compared with the estimates from the equation, a range of differences are found between the estimate and the actual values. When these are examined statistically, it is possible, for each equation, to estimate the probability of a prediction being underestimated (or over estimated). Standard error factors have been developed for various levels of confidence for many empirical equations.