The record at a station has to be checked for consistency if the gage has been relocated, a new gage has been installed, etc. Double mass analysis It is used to detect if data at a site have been subjected to significant change in magnitude due to external factors. This is a more common adjustment. All these four parameters are needed to adequately describe an event and specify its magnitude. Intensity is the slope of this diagram.
Duration is the width of the hyetograph.
In design, these long duration rainfalls although produce large total volumes of runoff, may not be the most conservative. Usually, when designing for the runoff peak, we consider rainfalls of shorter duration at or close to the time of concentration of the particular catchment which produce higher intensities and higher runoff peaks. We use longer duration rainfalls only when we design storage structures because they produce more volume. The engineer should judge and decide for the particular case. Duration of the storm also varies.