What are Continuous Simulation Models?

Hydrologic simulation models generally are designed to transform input rainfall into a hydrograph for a particular watershed. Many models are designed to do this for single storm events. Continuous simulation models are designed to transform  rainfall data for a long period of time (a year of several years) into  a streamflow hydrograph for an equivalent time period.

Continuous simulation models differ from other simulation models in that they are of necessity “water balance models”. In order to operate on rainfall data over long time period, the model model must contain a feedback mechanism which continuously update soil moisture storages and modifies various process of the model which are dependent on the quantity of water in storage such as infiltration and evapotranspiration. In order to achieve this, a continuous simulation model requires an input of potential ET, so that a continuous accounting of water inputs and losses from the watershed may be maintained.

Event hydrologic modeling for a basin characterizes finer-scale hydrologic processes and reveals how the basin responds to an individual rainfall event e.g., quantity of surface runoff, peak, timing of the peak, and detention. Thus, event hydrologic modeling is useful for better understanding the underlying hydrologic processes and identifying the relevant parameters. Also, intensive fine-scale hydrologic monitoring data for certain rainfall events, which are essential to the calibration of the event hydrologic model, are easily obtained. In contrast, continuous hydrologic modeling synthesizes hydrologic processes and phenomena i.e., synthetic responses of the basin to a number of rain events and their cumulative effects over a longer time period that includes both wet and dry conditions.

Continuous simulation is important because it allows actual historic data to be used to analyze the performance of drainage systems and their components. Drainage systems are typically designed using synthetic design storms. These single event design storms do not take into account varying patterns of rainfall duration and intensity, variation of time between storms, changing antecedent soil and storage conditions within the watershed, and the effect of evaporation. Continuous simulation considers all of these factors and allows for a more accurate and robust comparison of the long-term water balance and hydrologic performance of alternative stormwater management scenarios.

1. R.Linsley, N.Crawford (1974) Continuous simulation models in urban hydrology
2. Xuefeng Chu, A.M.ASCE,Alan Steinman (2009) Event and continuous modelling with HEC_HMS

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