The time-area method of hydrologic catchment routing transforms an effective storm hyetograph into a runoff hydrograph. The method accounts for translation only and does not include storage.

Time-area method is based on the concept of time-area histogram, i.e. a histogram of contributing catchment subareas (segmentations of the catchment). To develop a time-area histogram, the catchment's time of concentration is divided into a number of time intervals based on the hydrology time step. Cumulative time at the end of each time interval is used to divide the catchment into segments delimited by isochrones lines, i.e. the loci of points of equal travel time to the catchment outlet. Based on the end times of each time interval and the area of the corresponding segment the time-area histogram table can be built and used in the hydrologic routing, as shown in the following figure.

As stated above, the method assumes that the hydrology computation has time intervals (time segments) within the time of concentration of the catchment, and based on these time segments the total area is also divided into corresponding sub-areas (area segments). In general a minimum of 2 time/area segments are needed for good results, therefore the hydrology time step should be less than half of the time of concentration. If user specifies a larger time step the model will give a warning and results are less reliable.