Description of Study Area

Figure 2.1 illustrates the study area, including the detention pond, which was modeled as storage unit SU1 in Tutorial 03. The pond design includes three orifices and one weir, configured to regulate discharge for design storms of specific duration and return period (e.g., 2-hour, 10-year storms). For this tutorial, the pond configuration (SU1 and its outlet structures) remains unchanged; however, the system is evaluated using a continuous 10-year rainfall record.

Unlike single-event simulations, continuous simulation involves modeling multiple rainfall events over an extended period—spanning months to years. In single-event modeling, rainfall data is first analyzed to create an Intensity-Duration-Frequency (IDF) curve, which is then used to define synthetic design storms. In contrast, continuous simulation in GeoSWMM uses actual historical rainfall data as input to simulate system behavior over time, producing a continuous record of runoff. This record is then statistically analyzed to assess long-term performance and inform design decisions. It is advantageous to use continuous simulation because it accounts for antecedent soil conditions and other initial values of variables, such as the initial water level in storage units, which affect the response of the drainage system to individual storm events. It also allows the representation of actual storm events of varying magnitudes, durations, and occurrence intervals. The main disadvantages of continuous simulation are the additional computation time required and the lack of high quality, long-term rainfall records for many locations. Table 2.2 lists the physical element quantity summary used in this tutorial.

Table 2.2 : Network Object Summary

Continuous Rainfall

GeoSWMM can utilize long-term rainfall data stored in external files. The program recognizes several different file formats for these data: (1) the National Climatic Data Center (NCDC) DSI-3240 format for hourly rainfall and the DSI-3260 format for 15 min rainfall, (2) HLY03 and HLY21 formats for hourly rainfall and the FIF21 format for 15 min rainfall at Canadian stations, available from Environment Canada (EC), and (3) a standard user-prepared format as described in the GeoSWMM User’s Manual. In this tutorial, daily rainfall from the station USW00024217 is considered. The quality and quantity of the record will vary from station to station, but it is unusual to find long precipitation records with no missing or incorrect data. It is very important to check the quality of records before using them.

Evaporation

Evaporation is typically neglected in single-event simulations because its influence is minimal during short-duration rainfall events. However, in continuous simulations, evaporation becomes a critical component of the overall water budget, particularly for recovering depression storage and reducing water levels in extended detention basins and wet ponds over time. Several options are available for representing evaporation data in GeoSWMM, including: (1) a single constant value, (2) historical daily average values stored in an external file, (3) a time series when high temporal resolution is available, and (4) monthly averages, finally (5) calculated from temperature data. In this tutorial, Option 5—temperature-based evaporation estimation—is used to account for variable atmospheric conditions over the simulation period.

Although evaporation conceptually influences the recovery of infiltration capacity in pervious areas, SWMM’s infiltration models do not explicitly incorporate this effect. Instead, empirical functions are employed. Specifically, the Horton infiltration model uses an exponential recovery function during dry periods, where the rate coefficient is inversely proportional to the soil drying time—the number of days required for saturated soil to return to its initial infiltration capacity. For this site, a drying time of 7 days is applied, which is a typical value for silt loam soils as noted in Tutorial 01.

Statistical Tools

Continuous simulations produce a large volume of output data, making it necessary to use statistical analysis tools to extract meaningful insights. To support this, GeoSWMM provides an interactive statistical query tool that enables users to analyze any output variable associated with a specific model element—such as a subcatchment, node, link, or the entire system. The tool performs the following steps when analyzing the statistics for a specific output variable:

1. It first segregates the simulation period into a sequence of non-overlapping events either by day, month, year or cluster of consecutive reporting periods. The definition of these events is based on the following minimum threshold values:
2. • Analysis Variable Threshold - the minimum value of the variable under analysis that must be exceeded for a time period to be included in an event.
   • Event Volume Threshold - the minimum flow volume (or rainfall volume) that must be exceeded for a result to be counted as part of an event.
   • Separation Time - the minimum amount of time that must occur between the end of one event and the start of the next event. Events with fewer hours are combined together. This value applies only to events formed from consecutive reporting periods, not to daily, monthly or annual event periods.
2. It then computes a user-specified event statistic for the selected variable across all reporting time periods that fall within each event period. This could be the event mean value, the peak value, the total volume, etc. Thus, each event is characterized by a single value for whatever variable is being analyzed.
3. Summary statistics are the computed across the entire set of the event values. These statistics include the maximum, minimum, mean, standard deviation and skewness.
4. Finally, a frequency analysis is performed on the event values. This includes both a histogram and a cumulative frequency plot, offering insight into the distribution and recurrence characteristics of the modeled variable.

As an example, a typical query might ask GeoSWMM to segment the outfall flow record into events where the discharge exceeds 5 CFS and at least 12 hours pass between events. It would then compute the peak flow for each event and generate the corresponding summary statistics and frequency distribution.