What is the Modeling Module?

The Modeling Module calculates and reports modeling results (water quality and quantity) for inventoried BMPs and WQMP sites based on BMP type, location, drainage area, and modeling attributes entered by users as part of their inventory efforts. It also leverages various reference layers and datasets to develop a full set of model inputs necessary to describe the performance of BMPs. These model inputs are then used to calculate long-term runoff reduction, volume treated, and pollutant load removed. 

Where can I find model results?

Modeling results are reported on the BMP details pages and WQMP detail pages. The modeling results for a BMP account for the condition of the upstream watershed. Therefore, results can change as BMPs are added or verified in the tributary drainage area. Results are typically recalculated every 30 minutes. Model results or WQMP sites reflect the sum of the performance of the BMPs associated with that site. 

Use the Find a BMP tool to navigate to individual BMP detail pages. 

Search Water Quality Management Plans to see results at a site level.

What do model outputs represent?

The modeling results represent the current state of the watershed including verified BMPs, land uses, flow generation rates, and pollutant generation rates. The inventory of BMPs and the reference inputs used by the modeling engine are intended to reflect the conditions as we know them today. However, the model results are not intended to describe a given storm, month, or year. Rather, the results are based on how the current system of BMPs would perform over a long-term average condition accounting for the effect of both wet and dry years. 

Why are results missing for some BMPs and WQMP sites?

Modeling results are only calculated for BMPs that have been verified and fully populated. The inventory is a work in progress. Results will appear automatically once these BMPs are verified and populated. 

How does the Modeling Module work?

The Modeling Module consists of two primary components: A watershed database and a modeling engine. 


Watershed Database

The watershed database defines the modeled BMP assets and the geospatial information associated with these assets. 

Modeling Parameters: For each BMP or WQMP site, the user enters a limited suite of modeling parameters. These are the most important attributes that define the long-term performance of the BMP, such as volume, footprint, and treatment rate. These modeling parameters are part of the BMP detail pages. 

Delineations: For a BMP to be modeled, the user must define a delineation of the area that drains to the BMP.  Delineations can either be distributed (freeform) or can be traced through the Regional Subbasin network to define centralized delineations. The user can also connect BMPs to upstream BMPs to define treatment trains. 

GIS services: A master table of watershed characteristics and routing information is created by intersecting the BMP delineations and the Regional Subbasin polygons with a layer of hydrologic response units (HRUs) maintained by OC Survey. 

HRUs: https://ocgis.com/arcpub/rest/services/Environmental_Resources/Hydrologic_Response_Unit/FeatureServer 

Regional subbasins: https://ocgis.com/arcpub/rest/services/Environmental_Resources/RegionalSubbasins/FeatureServer

Modeling Engine

The modeling engine is a network solver that interprets the Watershed Database and consults several reference datasets to compute long-term hydrologic and pollutant load performance. Key supporting datasets include:

Stormwater Runoff Volume:  Unit area estimates for each HRU are based on the Loading Simulation Program in C++ (LSPC) model that was developed and calibrated as part of the Flow Ecology Special Study. https://www.southocwqip.org/pages/flow-ecology-study

Stormwater Runoff Concentrations: Representative stormwater runoff concentrations for each land use type are based on local wet weather outfall monitoring data and prior regional land use monitoring studies. 

BMP Stormwater Capture and Retention Performance: The modeling engine uses BMP hydrologic performance curves (aka, nomographs) to determine the long-term fraction of runoff retained and treated at each BMP node in the computational network.  These curves are based on a large number of continuous simulation models.

Dry Weather Runoff Flowrate: Dry weather runoff generation rates are based on monitoring studies performed in the South OC WMA. As an initial model input, the Modeling Module uses the average flow generation rate from these studies, expressed as a flowrate per developed acre. This is subject to refinement.

Dry Weather Runoff Concentration. Dry weather runoff concentrations are based on dry weather outfall monitoring data available in the SOC WMA between 2018 and 2020. 

Dry weather BMP hydrologic performance.  Based on the user-entered modeling parameters for each BMP, the modeling engine determines the dry weather loss rate and treatment rate provided by the BMP. A steady-state flow routing operation determines the dry weather runoff volume reduced or treated in each BMP and the remaining flow and load present at each calculation node.

BMP Water Quality Treatment Performance. The San Diego Regional WQE Guidance Document included the development of influent-versus-effluent concentration curves to define the water quality treatment performance of each major BMP type based on monitoring studies in the International Stormwater BMP Database.  The Modeling Module utilizes these curves as inputs to the modeling engine. In addition, it utilizes custom curves for dry weather treatment facilities (ozone and ultraviolet disinfection systems) based on available monitoring data from these systems in the WMA.

Are other model results calculated besides BMP performance?

Yes. The current application primarily focused on reporting at the individual BMP and WQMP site level. However, results are calculated for each Regional Subbasin in the watershed. Additionally, a 2002 baseline condition is simulated. These results are published to a web endpoint and consumed by PowerBI. At present, this is available to jurisdictions only. Check back for additional reporting options in the future.