Preparing for a Qualifying Precipitation Event (QPE): A Stormwater Sampling Checklist

An approaching forecast raises an immediate compliance question: does the forecast satisfy the permit’s Qualifying Precipitation Event (QPE) criteria under California’s current Construction General Permit, and what does that require for this project?

The 2022 permit uses QPE terminology, but inspection, stormwater monitoring, and sampling duties are not identical across all permitted sites. The required response depends on whether the forecast satisfies the permit’s QPE criteria, together with the project’s classification, Risk Level, LUP status, and other applicable permit requirements.

Before runoff begins, field teams may need to review the National Weather Service forecast, inspect BMPs and drainage features, prepare equipment and documentation, coordinate personnel, organize sampling supplies where required, and confirm laboratory availability where permit-required sampling applies before monitoring activities begin. During and after the event, the workflow can include observations or representative sampling for projects subject to those requirements, Chain of Custody documentation, corrective follow-up, SMARTS submissions, and project recordkeeping. Waiting until stormwater discharge begins can leave too little time to confirm duties or prepare reliable records.

This stormwater sampling checklist organizes the process into QPE determination, pre-event readiness, active-storm inspection and sampling mechanics, representative collection where required, and post-event documentation. It is designed for construction teams in Los Angeles and nearby California communities such as Anaheim, the City of Orange, Santa Ana, Riverside, San Bernardino, and San Diego operating under the same statewide permit framework. When additional field or administrative support is needed, SoCal Stormwater Solutions can help project teams review site conditions, coordinate monitoring activities, and organize the documentation required under applicable Construction General Permit requirements.

Quantitative Definition of a Qualifying Precipitation Event1

A project team does not need to respond to every rain forecast in the same way. The first operational decision is determining whether the forecast satisfies the California Construction General Permit (CGP) definition of a Qualifying Precipitation Event (QPE). That determination establishes whether the project’s applicable permit provisions require inspections, monitoring, sampling, or other permit-required compliance activities, making accurate forecast evaluation an essential part of construction planning rather than a routine weather check.

A Qualifying Precipitation Event (QPE) is a forecasted rain event identified using National Weather Service forecast data under the California Construction General Permit. Under the current California Construction General Permit and associated State Water Resources Control Board implementation guidance, a QPE is identified using a National Weather Service forecast showing at least a 50% probability of precipitation together with a quantitative precipitation forecast of 0.5 inches or more during the applicable 24-hour forecast period.

Meeting the QPE forecast criteria does not mean every construction project must perform the same inspections, sampling, or monitoring activities. Those obligations depend on the project’s applicable Risk Level, Linear Underground/Overhead Project (LUP) classification, and the specific monitoring provisions of the Construction General Permit.

The purpose of evaluating a National Weather Service Forecast is to determine whether forecast conditions meet the permit-defined threshold before personnel, equipment, and documentation activities begin. A forecast must satisfy both components of the QPE definition. Meeting the required probability of precipitation alone is insufficient, and meeting the required Quantitative Precipitation Forecast (QPF) threshold alone is also insufficient. A QPE is identified only when both forecast criteria are met.

Rather than reacting after rainfall begins, construction teams review forecasts in advance to determine whether the forecasted QPE may require preparation for applicable permit-required inspections, monitoring, sampling, or other compliance activities. This allows personnel, equipment, and documentation to be organized before field activities become time-sensitive while avoiding unnecessary mobilization for forecasts that do not satisfy the QPE definition.

The 2022 Construction General Permit introduced the current Qualifying Precipitation Event (QPE) framework, revising the precipitation triggers and inspection and monitoring requirements used under the previous permit. The updated framework allows regulated projects to evaluate forecast conditions before rainfall begins and determine whether project-specific permit requirements may become applicable if the forecasted QPE occurs. This allows project teams to evaluate applicable monitoring responsibilities before rainfall begins.

Under current Water Board guidance, a QPE may extend across consecutive 24-hour forecast periods when each subsequent 24-hour period has a total Quantitative Precipitation Forecast (QPF) of at least 0.25 inches. The extension of a QPE is based on the forecast precipitation amount rather than the probability of precipitation.

Routine Weather Monitoring vs. QPE Trigger Conditions
Forecast Condition	Routine Weather Monitoring	QPE Trigger
Precipitation Probability	Forecast does not satisfy the permit-defined probability threshold.	Forecast meets the probability threshold established by the Construction General Permit.
Quantitative Precipitation Forecast	Forecast precipitation amount remains below the permit-defined trigger.	Forecast precipitation amount meets or exceeds the applicable permit threshold for the defined forecast period.
Operational Response	Continue normal weather awareness and routine project planning.	Review applicable permit requirements for the project’s Risk Level or LUP classification and prepare for any monitoring activities required by the permit.
Monitoring Implications	No QPE-triggered monitoring activities are anticipated based on the forecast alone.	Applicable inspection, monitoring, or sampling responsibilities are determined according to the project’s Risk Level, LUP classification, and applicable Construction General Permit requirements.
Documentation Preparation	Routine project documentation continues.	Prepare documentation needed to support any permit-required monitoring activities if the forecasted Qualifying Precipitation Event occurs.

 

Pre-Event Site Inspections and Field Readiness

Once an approaching storm is expected to meet the project’s Qualifying Precipitation Event (QPE) criteria, the focus shifts from forecast evaluation to field preparation.

A well-organized response before rainfall begins allows project teams to verify that erosion and sediment controls remain functional, assign responsibilities, organize equipment, and prepare the documentation needed if the project’s applicable Construction General Permit inspection, monitoring, sampling, or other permit-required activities are triggered.

The Construction General Permit requires all dischargers to have a Qualified SWPPP Practitioner (QSP) conduct a pre-QPE visual inspection within 72 hours before the start of a forecasted Qualifying Precipitation Event. When extended forecast information covering more than three days is available, the pre-QPE inspection may be conducted up to 120 hours before the event. Operational preparation for personnel, equipment, BMP maintenance, and documentation should therefore be coordinated around this permit-defined inspection window.

Not every forecast that calls for rain justifies the same level of field preparation. Once the forecast satisfies the project’s QPE criteria, project teams can begin organizing the activities required by the applicable permit provisions.

Step 1 — Continue Forecast Verification

Forecast review does not end once a QPE has been identified. Project personnel should continue monitoring the National Weather Service Forecast leading up to the event because precipitation timing, location, and forecast intensity can change. Confirming that forecast conditions continue to satisfy the applicable QPE criteria helps determine whether planned inspection and stormwater monitoring activities remain necessary under the project’s permit requirements and prevents unnecessary mobilization if forecast conditions change.

Step 2 — Inspect Existing Best Management Practices (BMPs)

Before rainfall begins, inspect the Best Management Practices (BMPs) that are relevant to current site conditions. 

Depending on the project, the pre-event BMP inspection may include:

• perimeter controls;
• sediment controls;
• stabilized construction entrances;
• fiber rolls;
• silt fences;
• inlet protection devices;
• other temporary or permanent BMPs identified in the project’s SWPPP.

Any deficiencies identified during the inspection should be corrected, where practicable, before forecasted rainfall.

The objective is to confirm that installed controls remain properly positioned, functional, and capable of managing anticipated runoff before precipitation occurs rather than identifying deficiencies after stormwater has already reached disturbed areas.

Step 3 — Review Drainage Paths and Inlet Protection

Drainage features that function under dry conditions may not perform as intended once runoff begins. Reviewing them before rainfall allows crews to identify obstructions while access is still straightforward. Where appropriate, remove accumulated sediment, debris, or construction materials that could obstruct drainage features or interfere with intended drainage patterns. This preparation supports proper runoff conveyance during the storm and helps reduce the likelihood that blocked flow paths will interfere with the performance of installed BMPs.

Step 4 — Prepare Personnel and Field Equipment

Operational readiness depends on assigning responsibilities before the weather changes. Before rainfall begins, confirm:

• inspection and monitoring assignments;
• communication responsibilities;
• inspection forms;
• field monitoring instruments, where required;
• sampling equipment, where required by the project;
• any other project-specific field resources.

Organizing these resources before rainfall reduces confusion once field conditions become more demanding.

Step 5 — Confirm Documentation Readiness

Documentation readiness begins before rainfall rather than after field activities start. Preparing documentation before the event helps field personnel record observations consistently while site activities continue. Depending on the project’s applicable permit requirements, this preparation may include inspection records, weather documentation, project maps, field logs, and monitoring forms. Having these materials organized before rainfall begins makes it easier to document field activities accurately as they occur and helps ensure that required observations are recorded during the event rather than reconstructed afterward.

Preparing documentation in advance helps personnel record required observations as field activities occur, rather than relying on information collected after the event.

Additional guidance on pre-event field readiness and project-specific monitoring activities is available in our inspections and monitoring for construction environments resource for readers seeking more detailed operational information.

 

Active-Storm Monitoring and Technical Sampling Mechanics

During a Qualifying Precipitation Event, all dischargers must have a QSP or properly trained delegate conduct at least one visual inspection during each 24-hour period of the QPE. Sampling obligations are more project-specific. Risk Level 2 and 3 traditional construction projects must obtain one stormwater sample from each active discharge location during each 24-hour period of the QPE, while Type 2 and Type 3 Linear Underground/Overhead Projects follow the corresponding representative-location requirements in Attachment E when active discharge occurs.

Step 1 — Confirm Active Discharge and Applicable Duties

Begin by confirming that active stormwater discharge is occurring at a monitoring location identified in the project’s SWPPP, Construction Site Monitoring Program, maps, or applicable permit attachment. Risk Level 2 and 3 projects require one sample from each discharge location during each 24-hour period of a QPE, while Type 2 and 3 linear projects use the representative locations established for the project.

Before collecting anything, determine whether the required task is a visual observation, field measurement, laboratory sample, or a combination of those activities. Rainfall or flowing water alone does not mean every location follows the same procedure.

Step 2 — Identify the Correct Observation or Sampling Location

Monitoring must occur at the locations established in the project documents. The Construction General Permit requires the SWPPP and site drawings to identify visual inspection locations, discharge locations, and applicable sampling locations. Selecting the mapped point connects the result to the correct drainage area, construction activity, and control measures.

For traditional Risk Level 2 and 3 sites, stormwater grab samples are collected from all discharge locations incorporating runoff from project construction areas. Type 2 and 3 linear projects may use one or more discharge locations that represent the project’s construction activities. Where receiving-water monitoring is required, upstream and downstream monitoring locations must be identified in accordance with the permit.

Terms such as inlet, outfall, runoff point, and discharge location are not automatically interchangeable. Use the terminology and monitoring points shown in the project documents rather than treating inlets, outfalls, runoff points, and discharge locations as interchangeable.

Step 3 — Conduct Required Visual Observations

Before or alongside required sample collection, record the observable conditions specified in the project’s monitoring procedures. 

Record the visual conditions required by the project’s monitoring procedures, which may include:

• discharge color and clarity;
• floating or suspended material, foam, or sheen;
• visible sediment movement;
• BMP damage, overtopping, or bypass;
• visible evidence of pollutants in the discharge.

Visual observations provide immediate information about the field condition, but they do not replace pH, turbidity, non-visible pollutants, or receiving-water analysis when the permit requires those measurements. Non-visible pollutant sampling applies only when the project meets the permit conditions that trigger that monitoring duty.

Step 4 — Collect a Representative Sample Where Required

A representative stormwater sample must reflect the discharge flow and conditions being evaluated. Where site conditions allow, collect directly from a freely flowing portion of the designated discharge to reduce contact with deposited sediment or surrounding debris.

Do not drag the container along the ground, disturb accumulated sediment, collect construction debris, touch the inside of the bottle or cap, or transfer the sample through an unspecified intermediate container. Stagnant ponding may not represent the active discharge identified in the monitoring program. When conditions differ from the documented procedure, record the condition rather than moving to an undocumented location for convenience.

Step 5 — Use the Correct Containers and Field Procedures

For laboratory-analyzed parameters, use the containers, labels, preservation instructions, and shipping method specified or supplied by the laboratory. 

Before sampling, confirm the laboratory or approved method instructions for:

• container type and quantity;
• labeling and sample identification;
• preservation requirements;
• packing and transport;
• blank Chain of Custody forms;
• any additional laboratory instructions applicable to the required analyses.

The Construction General Permit directs dischargers to obtain the correct number and type of containers, blank Chain of Custody forms, and preservation instructions from the laboratory. Containers must be labelled, secured, packed to prevent breakage, and handled in a way that limits contamination.

Field measurements and laboratory samples follow different procedures. Risk Level 2 and 3 projects must analyze pH on-site using a calibrated pH meter and a U.S. EPA-acceptable method. Turbidity is measured with a calibrated turbidimeter either on-site or at a laboratory accredited by the State Water Board’s Environmental Laboratory Accreditation Program. Other laboratory-analyzed parameters must be handled by an ELAP-accredited laboratory, except for permitted field analysis of pH and turbidity. Instruments must be calibrated and maintained according to manufacturer specifications.

Sampling and preservation must follow the applicable approved analytical method, 40 CFR Part 136, laboratory instructions, and any additional Water Board requirements. Because holding times vary by parameter and method, no single universal holding time applies.

Step 6 — Complete Field Documentation

Record information while the observation or collection is occurring. Contemporaneous documentation should identify the project, mapped monitoring point, date, time, sampler, weather and discharge conditions, visual observations, sample identification, and any required field results.

When instruments are used, record the meter identification or relevant equipment information and document instrument calibration in accordance with project procedures. Note unusual flow, restricted access, safety limitations, location deviations, or other conditions that could affect interpretation of the field activity. These details connect each bottle and measurement to the actual discharge condition without relying on later reconstruction.

Step 7 — Complete Chain of Custody and Laboratory Transfer

Chain of Custody documentation tracks possession of the samples from collection through laboratory receipt. Under Attachment D, the form must include the discharger’s contact information, each sample container and collection point, the sampler, collection date and time, requested analyses, and the signatures of those relinquishing and receiving custody.

Samples requiring laboratory analysis must be preserved and transported using the specified method. Samples requiring laboratory analysis should be shipped or delivered according to the laboratory’s preservation and holding-time requirements so the applicable analytical method can be satisfied. Cooling, preservation, and transport conditions must follow the laboratory and approved method instructions.

Representative Sampling Conditions

The table below shows how common field conditions can affect sample representativeness and the appropriate collection response.

Field Sampling Conditions and Recommended Collection Response
Field Condition	Sampling Implication	Operational Response
Freely Flowing Discharge	More likely to provide a representative sample of the active discharge.	Collect from the designated location using the documented method while avoiding contact with the channel bottom or surrounding debris.
Stagnant or Isolated Ponding	May not represent the active discharge leaving the project or drainage area.	Confirm whether the ponded water represents a designated discharge condition; otherwise document it and use the approved active-discharge location.
Sediment-Disturbed Water	Artificially disturbed material can influence turbidity or other analytical results.	Avoid scraping the container along the ground or disturbing deposited sediment during collection.
Inaccessible or Unsafe Discharge	Personnel safety and access constraints can prevent collection at the planned point.	Do not enter unsafe conditions; document the limitation and follow the project’s non-sampling or alternative-location procedures.
Incorrect or Undocumented Location	The result may not be traceable to the required drainage area or construction activity.	Verify the point against the SWPPP, site map, and monitoring plan before collecting the sample.

Industrial facilities operate under a separate Industrial General Permit and should not use this construction workflow as their controlling sampling procedure. Inspections and monitoring for industrial environments may be added only for readers who also manage separately regulated industrial operations.

A complete monitoring record links the observed discharge, designated monitoring location, representative sample, field measurements, and Chain of Custody documentation for the same monitoring event.

 

Post-Event Evaluation and SMARTS Database Logging

Field work concludes before the compliance process does. Once monitoring activities conclude, project teams shift from field observations to reviewing inspection findings, organizing documentation, tracking laboratory analyses where required, and completing the reporting obligations assigned under the Construction General Permit. Maintaining an orderly administrative workflow throughout this stage helps ensure that field observations, analytical data, and supporting records remain connected to the monitored Qualifying Precipitation Event (QPE).

Step 1 — Complete Required Post-Event Inspections

Post-event inspections confirm how the site performed during the QPE and identify BMP maintenance or corrective work that may be needed before the next storm. Under the 2022 Construction General Permit, all dischargers except Type 1 Linear Underground/Overhead Projects must have a QSP or trained delegate conduct a post-QPE visual inspection within 96 hours after the end of the final 24-hour period of the QPE when the onsite rain gauge records a total of 0.5 inches or more for the event. If the onsite QPE total remains below 0.5 inches, the post-QPE inspection is not required under this provision.

During the inspection, crews should document the condition of installed BMPs, discharge pathways, disturbed areas, precipitation totals, and any maintenance or corrective-action needs identified under the applicable permit requirements.

Step 2 — Review Site Conditions and Corrective Needs

Inspection findings should be reviewed while site conditions remain readily observable. The objective is to identify operational issues that could affect future stormwater performance before the next qualifying rain event.

Typical review activities include evaluating:

• sediment-control performance;
• erosion or scour;
• accumulated sediment requiring maintenance;
• damaged or displaced BMPs;
• drainage pathways and runoff routing;
• areas requiring repair or additional controls;
• documented maintenance or corrective actions required under the permit.

Observations should be recorded promptly so they accurately reflect field conditions observed after the event.

Step 3 — Organize Field Documentation

Before reporting begins, assemble the project records generated during stormwater monitoring into a complete documentation package. Keeping records organized throughout the reporting year simplifies later reporting activities and supports consistent project documentation.

Typical records include:

• inspection reports;
• field observation notes;
• rainfall measurements and weather records;
• monitoring logs;
• sampling location documentation;
• site photographs;
• instrument calibration records where applicable;
• laboratory analytical reports and associated Chain of Custody records;
• maintenance or corrective-action documentation, when required.

Step 4 — Track Laboratory Results

Where laboratory analysis is required by the applicable monitoring program, project personnel remain responsible for following the status of submitted samples after laboratory delivery. Analytical reports should be reviewed promptly after receipt to confirm that requested analyses have been completed and that the results correspond to the correct project, monitoring location, and sampling event.

For field sampling analytical results required under the Construction General Permit, dischargers generally must submit the results through SMARTS within 30 days of the precipitation event. If the analytical results demonstrate an exceedance of a Numeric Action Level (NAL) or Numeric Effluent Limitation (NEL), the results must be submitted within 10 days. Sampling results are reported through SMARTS using the applicable Ad Hoc reporting workflow. Other specialized monitoring programs, including receiving-water or TMDL-related requirements, may carry additional or different reporting obligations under the applicable permit attachment.

Project teams, not the laboratory, are responsible for ensuring that required information is submitted accurately and within the applicable timeframe.

Step 5 — Complete the SMARTS Reporting Workflow

SMARTS serves as the California State Water Resources Control Board’s electronic reporting platform for Construction General Permit administration. After monitoring records and laboratory reports have been reviewed, project personnel should complete the reporting workflow by:

• confirming that required monitoring information is complete;
• reviewing laboratory analytical reports for accuracy;
• entering required monitoring information into SMARTS;
• uploading or entering supporting monitoring information as required by SMARTS and the applicable permit provisions;
• confirming successful electronic submission;
• retaining supporting project records in accordance with the Construction General Permit recordkeeping requirements.

SMARTS reporting does not replace project recordkeeping. Inspection records, laboratory reports, field documentation, and other supporting materials remain part of the project record and should be retained in accordance with the Construction General Permit.

Post-Event Documentation Checklist

Use this checklist to help organize the key records and supporting documentation needed after stormwater monitoring activities are complete.

• Post-event inspection records (where required)
• Rainfall measurements and supporting weather information
• Field observation notes
• Site photographs
• Stormwater monitoring logs
• Laboratory analytical reports (where applicable)
• Calibration records for field instruments (where applicable)
• Maintenance or corrective-action documentation (where applicable)
• SMARTS submission confirmation and associated reporting records (where applicable)

Projects requiring broader context on California construction stormwater obligations can refer to The Ultimate California Stormwater Compliance Handbook: SWPPP, Permits, and Risk Management for additional permit background. Consistent documentation and timely reporting help create a complete project record that supports ongoing Construction General Permit implementation.

Sources:

1. California State Water Resources Control Board – Qualifying Precipitation Event Guidance
2. California State Water Resources Control Board – Construction Stormwater General Permit Order WQ 2022-0057-DWQ
3. California State Water Resources Control Board – Attachment E: Linear Underground and Overhead Project Requirements
4. California State Water Resources Control Board – Attachment D: Risk Level Requirements
5. California State Water Resources Control Board – Fact Sheet: ORDER WQ 2022-0057-DWQ
6. California State Water Resources Control Board – NPDES 2022 Construction Stormwater General Permit
7. California State Water Resources Control Board – Construction Stormwater General Permit: Attachment B – Glossary
8. California State Water Resources Control Board – Attachment 1 to Tentative Order No. R9-2025-0002