This is a process by which the outputs from a model are made to match some real, observed data, by altering model parameters that can be used to tune the model. For example, using a Horton (ILSAX) model, pavedand grassed area percentages, depression storages, soil types and antecedent moisture condition (AMC) parameters can all be varied to increase or decrease the volumes and peaks of hydrographs. Times of concentration can be decreased to increase flowrates.

If there is information available from a real storm, such as a peak water levels in a channel from which a flowrate can be estimated, or a peak level of ponding in a sag or detention basin, DRAINS can be run with the recorded rainfall hyetograph, and the calculated depths and flowrates can be compared with the observed ones. Parameters such as times of concentration can be altered within reasonable limits to improve the agreement, but sometimes the match may not be good. The main problem may be that the rainfall recording station may be some distance from the catchment being modelled, and its rainfall may not be those that actually occurred over the catchment.

In some cases, you may be lucky and have a recorded hydrograph. DRAINS does not have any direct features for comparison of hydrographs, but this can be done through the spreadsheet inputs and outputs. To enter the observed hydrograph, set up a separate drainage system within the main model for your catchment.  This should consist of a node, a pipe, channel or overflow route, and an outlet node.  At the higher node, enter the recorded hydrograph as an inflow hydrograph, transferring this from a spreadsheet using the Paste option. When DRAINS runs, it will display this user-provided hydrograph in the pipe or channel between the nodes. It can be copied to a spreadsheet along with the hydrograph generated by the DRAINS run, and the plots can be inspected. Spreadsheet functions can be used to compute mean square errors and other statistics.

If there are more than one calibration storms, separate sets of nodes and conduits can be used to enter the various recorded hydrographs.

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