Stormwater Harvesting and BMPFast Software

Stormwater Management Series  |  Florida Water Resources  |  Last updated 2025

1. Stormwater Harvesting Overview

Source slides: 1, 2, 3  |  Core concepts and rationale for stormwater harvesting programs

Stormwater harvesting is the practice of capturing, retaining, and reusing runoff that would otherwise leave a site or watershed as discharge. Rather than treating stormwater purely as a nuisance to be conveyed away, harvesting programs reframe it as a recoverable resource with measurable economic and environmental value. In Florida, where water supply pressures are significant and nutrient loading into surface waters is a regulatory priority, harvesting occupies a dual role: it simultaneously addresses water quantity demands and water quality obligations.

Primary Purpose: Replacing Costlier Water Sources

The most direct economic driver for stormwater harvesting is supply substitution. Retained runoff is applied to non-potable end uses — most commonly landscape and agricultural irrigation, industrial process water, toilet flushing, and dust suppression — displacing water that would otherwise be drawn from potable systems, groundwater wells, or purchased reclaimed water. In regions where those alternative sources carry high treatment, pumping, or procurement costs, harvested stormwater can deliver a meaningful cost-per-gallon advantage, particularly once infrastructure is amortized over a project’s operational life.

Water Quality Benefits

Beyond supply value, harvesting provides water quality benefits by reducing the volume of stormwater that reaches receiving waterbodies. Every gallon retained on-site is a gallon that does not carry its associated pollutant load — nutrients, sediment, metals, and pathogens — into downstream systems. This volume-based pollution reduction is distinct from, and complementary to, the concentration-reduction credit that wet detention ponds provide through settling and biological uptake.

• Reduces stormwater discharge volume and the pollutant loads carried with it
• Decreases nutrient loading to impaired waterbodies, supporting Basin Management Action Plan (BMAP) compliance
• Lowers peak discharge rates, reducing erosion and downstream channel stress

Groundwater and Saltwater Intrusion Benefits

In coastal and low-lying areas of Florida, excessive groundwater withdrawal for irrigation lowers the potentiometric surface of the Floridan Aquifer System, allowing saltwater to migrate inland through the aquifer. Substituting harvested stormwater for groundwater-sourced irrigation reduces the net withdrawal volume and helps maintain aquifer pressure gradients that resist saltwater intrusion. At the watershed scale, this contributes to longer-term freshwater availability and ecosystem health in estuaries and coastal wetlands that depend on freshwater inflows.

Revenue Generation and Regulatory Credits

Stormwater harvesting systems can generate revenue for their operators when harvested water is sold or distributed under a fee structure — for example, a utility that operates a reclaimed water distribution network supplemented by harvested stormwater, or an agricultural operation that charges neighboring users for access to a shared retention reservoir. On the regulatory side, documented harvesting volumes can support applications for Environmental Resource Permits (ERPs) by demonstrating effective stormwater management, and they can be counted toward BMAP pollutant load reduction credits, which may reduce the compliance burden or costs associated with nutrient management obligations.

• Potential revenue stream for utilities and operators who sell or distribute harvested water
• Supports Environmental Resource Permit (ERP) compliance documentation
• Provides quantifiable BMAP load-reduction credits for nitrogen and phosphorus

2. Rainwater vs. Stormwater Harvesting

Source slide: 4  |  Distinguishing system types, estimating use potential, and understanding effectiveness curves

Although the terms are sometimes used interchangeably, rainwater harvesting and stormwater harvesting describe meaningfully different systems with different design assumptions, analytical methods, and regulatory treatment. Understanding the distinction is essential before applying any quantitative estimation tool, including the REV curves discussed in later sections.

Rainwater Harvesting: Single-Catchment Systems

Rainwater harvesting refers to the collection of precipitation from a discrete, controlled catchment surface — most commonly a rooftop — before the water contacts the ground or becomes mixed with other runoff pathways. Because the catchment is bounded and the hydraulic pathway from rain event to storage tank is short, rainwater systems are relatively simple to characterize. The roof area, roof material, and local rainfall statistics are typically sufficient to estimate system yield. Groundwater intrusion, soil infiltration losses, and catchment-scale variability are not significant factors.

Stormwater Harvesting: Watershed-Scale Complexity

Stormwater harvesting involves capturing runoff from land surfaces — parking lots, roads, managed turf, natural areas — that drain into detention or retention ponds, swales, or other conveyance infrastructure. Unlike rooftop systems, stormwater catchments are subject to infiltration variability, antecedent soil moisture conditions, and groundwater interaction. In Florida particularly, shallow water tables mean that wet detention ponds may receive groundwater inflow during dry periods, complicating the relationship between rainfall and available harvest volume.

Because of this complexity, stormwater use potential is not reliably estimated on a daily timestep. Instead, a twice-per-week estimation approach — evaluating available volume at two-day intervals — better captures the storage dynamics and intermittent availability that characterize these systems. This difference in estimation frequency has implications for how software tools and analytical frameworks are structured for each system type.

Effectiveness Curves and Concentration Reduction Credit

Both system types are evaluated using effectiveness curves — graphical relationships between design parameters (storage volume and use rate) and annual performance (removal or retention efficiency). However, because the underlying hydrology differs, rainwater and stormwater systems have distinct curve families. Applying a rainwater effectiveness curve to a stormwater system, or vice versa, will produce erroneous efficiency estimates.

An additional layer of credit is available when stormwater harvesting is paired with wet detention pond treatment. Wet detention provides concentration reduction credit through sedimentation and biological nutrient uptake, which operates independently of harvest volume. When these mechanisms are combined in a design, the total pollutant load reduction reflects both the volume retained (harvesting credit) and the concentration of the fraction that is discharged (wet detention credit). The REV curve framework and BMPFast software both account for this combined credit approach.

3. Florida Harvesting Examples

Source slides: 5, 6, 7, 8, 9, 10, 11  |  Documented implementations spanning residential, municipal, agricultural, transportation, and utility contexts

Florida has one of the most extensive documented inventories of operational stormwater and rainwater harvesting systems in the United States. By 2020, more than 700 individual harvesting examples had been catalogued across the state, reflecting the diversity of end uses, scales, and institutional arrangements under which harvesting can be implemented. The examples below illustrate the range of approaches.

Utility-Operated Residential Irrigation Systems

Several Florida water utilities operate stormwater harvesting systems that supply irrigation water to residential customers through a separate, non-potable distribution network. In these arrangements, the utility manages detention or retention ponds within a residential community, harvests accumulated stormwater, treats it to a level appropriate for landscape irrigation, and distributes it via a purple-pipe system. Homeowners benefit from lower outdoor water costs; the utility benefits from reduced demand on its potable supply infrastructure and, in some cases, from the regulatory credits associated with reduced stormwater discharge.

City-Operated Retrofit for BMAP Credits

Some municipalities have retrofitted existing stormwater infrastructure — ponds, swales, and conveyance systems originally built only for flood control — to add harvesting capability as a means of generating BMAP pollutant load reduction credits. In these projects, pumps and distribution piping are added to ponds that were never designed for reuse, enabling the city to demonstrate quantifiable nitrogen and phosphorus load reductions. These retrofits are often cost-effective relative to building new treatment systems because the primary storage infrastructure already exists.

Agricultural Use at Large Scale

Florida’s agricultural sector, particularly large citrus, vegetable, and sod operations in the central and southern parts of the state, represents one of the highest-volume applications of stormwater harvesting. Operations harvesting on the order of 20 million gallons per day (MGD) have been documented. At this scale, on-farm reservoirs — often tens to hundreds of acres in surface area — capture field drainage, tailwater from irrigation applications, and direct rainfall. Harvested water is cycled back through the irrigation system, reducing both the volume of nutrient-laden water discharged to downstream canals and the volume of groundwater or surface water withdrawn for irrigation.

FDOT Highway Runoff for Golf Course Irrigation

The Florida Department of Transportation (FDOT) has participated in harvesting programs where highway runoff collected in roadside ponds and retention systems is piped to adjacent golf courses for irrigation use. This arrangement benefits both parties: FDOT reduces the stormwater it must manage and can document load reductions, while the golf course reduces its dependence on groundwater or reclaimed water. The adjacency of transportation infrastructure to high-demand irrigation users — golf courses, parks, athletic fields — makes this a repeatable model across the state.

Underground Storage Where Land Is Scarce

In densely developed areas where surface land for ponds is not available or prohibitively expensive, underground cistern-based harvesting systems provide an alternative. These systems capture rooftop or parking-lot runoff in subsurface tanks, filter and store it, and pump it for reuse. While unit costs per gallon of storage are higher than for surface ponds, underground systems enable harvesting in urban infill contexts where no other approach is feasible. Several Florida municipalities and commercial developments have implemented underground cisterns specifically to meet stormwater management requirements while harvesting reusable water.

Combined Reclaimed and Harvested Water for IQ Use

A number of Florida utilities have developed blended systems in which reclaimed water (treated wastewater effluent) and harvested stormwater are combined in a shared storage reservoir or distribution system for irrigation-quality (IQ) reuse applications. This approach provides supply resilience: when stormwater is abundant (wet season), it supplements or replaces reclaimed water in the IQ system; when stormwater availability is low (dry season), reclaimed water fills the gap. The blended approach also allows utilities to optimize the use of both resource streams and meet IQ water quality standards more consistently than either source alone might achieve during extreme conditions.

4. REV Curves for Harvesting

Source slides: 12, 21, 23  |  The REV framework, regional calibration, and integration with wet detention credit

The REV curve framework is the quantitative foundation for evaluating stormwater harvesting performance in Florida’s regulatory and design context. REV curves express the relationship among three variables — use rate, removal efficiency, and storage volume — and allow designers and reviewers to determine the annual performance of a harvesting system from basic site parameters.

The Three REV Variables

Development from 25 Florida Locations

The REV curves used in Florida practice were developed from continuous hydrologic simulation at 25 locations distributed across the state, representing diverse rainfall patterns, soil types, and seasonal moisture regimes. The simulation approach — running long-term daily or sub-daily water balance models for each location — generates statistically robust relationships between R, E, and V that reflect actual Florida conditions rather than generic design assumptions. This site-specific empirical basis is what makes the REV curves appropriate for use in Florida regulatory submissions.

Five Meteorological Regions

Because rainfall intensity, seasonality, and dry-period length vary significantly across Florida — from the panhandle’s more uniform annual distribution to South Florida’s intensely seasonal wet/dry pattern — the state is divided into five meteorological regions for REV curve purposes. Each region has its own family of curves reflecting the local rainfall regime. Selecting the correct regional curve set for a project location is a prerequisite for obtaining accurate efficiency estimates; using a curve from the wrong region can substantially overestimate or underestimate annual performance.

Combining REV Efficiency with Wet Detention Concentration Reduction

When a harvesting system is operated in conjunction with a wet detention pond, the total pollutant load reduction is calculated in two steps. First, the REV curve provides E — the fraction of annual runoff that is harvested and receives 100% load-reduction credit. Second, the remaining fraction (1 − E) is treated by the wet detention pond, which receives concentration-reduction credit for the nutrients and solids it removes from the discharged volume. The combined credit is always greater than either component alone, and BMPFast automates this calculation when both BMPs are entered in series.

5. BMPFast Software Walkthrough

Source slides: 13, 14, 15, 16, 17, 18, 19, 20  |  Step-by-step input sequence, BMP combination logic, and output interpretation

BMPFast is Florida’s primary software tool for quantifying stormwater best management practice (BMP) performance for regulatory purposes. It implements the REV curve framework along with performance curves for other BMPs — wet detention ponds, dry retention systems, swales, and others — and produces standardized removal efficiency and load reduction outputs suitable for inclusion in ERP applications and BMAP compliance reports. The walkthrough below follows the typical input sequence for a project that includes both a wet detention pond and a harvesting component.

Step 1: Site Characteristics and Catchment Data

The first inputs in BMPFast define the drainage area that contributes runoff to the BMP system being evaluated. Required site data includes:

• Total catchment area (acres) — the full drainage area contributing to the BMP
• Directly connected impervious area (DCIA) — the fraction of the catchment where impervious surface drains directly to the conveyance system without opportunity for infiltration
• Soil-based Curve Number (CN) — the SCS/NRCS curve number reflecting soil type and land cover for the pervious portions of the catchment
• Project location — used to assign the applicable meteorological region and retrieve the corresponding REV and BMP curve sets

Step 2: Calculating the Annual Runoff Coefficient from CN and DCIA

BMPFast uses a combined annual runoff coefficient derived from the DCIA fraction and the CN-based runoff coefficient for the pervious portion of the site. The DCIA areas are assumed to generate runoff at a coefficient of 1.0 (all rainfall becomes runoff), while pervious areas generate runoff according to the curve number relationship. The composite coefficient is calculated as:

Accurate DCIA estimation is critical because it directly drives the runoff volume calculation. Over-estimating DCIA inflates the predicted runoff volume and can lead to oversized BMP designs or overstated load-reduction credits.

Step 3: Entering Wet Detention Pond Parameters

If the project includes a wet detention pond, its parameters are entered before the harvesting system to reflect the typical physical arrangement in which runoff first passes through the pond before being harvested from pond storage. Key wet detention inputs include:

• Permanent pool volume (acre-feet or inches over EIA) — determines the hydraulic residence time and treatment effectiveness
• Surface area of the permanent pool — used to assess biological and physical treatment capacity
• Littoral zone percentage — the fraction of permanent pool area occupied by emergent or aquatic vegetation, which affects nutrient uptake credit
• Outfall structure type — controls the drawdown rate and determines peak discharge attenuation

Step 4: Entering Harvesting Parameters

After the wet detention pond parameters are entered, the harvesting component is defined. The harvesting inputs map directly to the REV framework:

• Harvesting storage volume (V) — the dedicated reuse storage volume in inches over EIA; this must be clearly separated from the wet detention permanent pool volume so that credits are not double-counted
• Average daily use rate (R) — the anticipated daily withdrawal rate for the intended end use, in inches per day over EIA; users should base R on documented demand data (irrigation schedules, process water needs) rather than arbitrary assumptions
• Meteorological region — confirms the regional REV curve set to be applied

Step 5: Combining BMPs in Series

BMPFast allows multiple BMPs to be entered and analyzed in series, reflecting the fact that many real projects route runoff through two or more treatment or retention systems sequentially. When BMPs are combined, the software calculates the cumulative removal efficiency by applying each BMP’s performance to the fraction of pollutant load remaining after the upstream BMP. This approach correctly avoids adding individual BMP efficiencies arithmetically (which would overstate combined performance) and instead compounds them multiplicatively.

Typical series combinations in Florida practice include:

• Wet detention pond followed by a harvesting system drawing from the permanent pool
• Dry retention swale followed by a wet detention pond
• Harvesting system followed by a supplemental wet detention pond for overflow treatment

Step 6: Interpreting the Summary Report

The BMPFast summary report presents results in a standardized format that can be submitted directly as supporting documentation for ERP applications and BMAP compliance reports. Key outputs include:

• Annual removal efficiency (E) — percentage of annual runoff volume captured and credited as fully removed
• Annual retained volume — total volume of runoff harvested per year, in acre-feet or million gallons
• Nitrogen load reduction — annual mass of total nitrogen (TN) removed or retained, in pounds per year, calculated from event mean concentration (EMC) data
• Phosphorus load reduction — annual mass of total phosphorus (TP) removed or retained, in pounds per year
• Combined BMP series performance — when multiple BMPs are analyzed in series, the report shows individual and cumulative efficiencies for each BMP stage

The nitrogen and phosphorus loading outputs use land-use-specific event mean concentrations (EMCs) drawn from Florida monitoring data, which are embedded in the software’s reference tables. Users can review and, in some cases, override default EMC values if site-specific monitoring data are available and have been approved for use by the applicable Water Management District.

6. Water Use Permit Impacts

Section 6 of 7 — Regulatory Integration and Consumptive Use Accounting

When a rainwater harvesting system captures precipitation before it enters a natural watercourse, the harvested volume may be classified as a consumptive use under state water law. Understanding how the Runoff Coefficient (ROC) value is selected — and how that selection propagates through the mass-balance calculation — is therefore not merely a modeling concern: it has direct consequences for permit applications, compliance thresholds, and long-term water rights.

Harvested Water and Consumptive Use Permits

Many jurisdictions require that any intentional capture of precipitation from a defined catchment area be disclosed as part of a consumptive use permit. The annual volume reported to the permitting authority is derived directly from the modeled harvest estimate. If the model overstates the available volume, the permit application will request more water than the system can realistically deliver — and, conversely, the applicant may be held to a higher consumptive use ceiling than is warranted by actual hydrologic conditions.

ROC Value Selection: Land Use vs. Generic Default

Two ROC values appear repeatedly in practice and are worth comparing directly:

The difference between these two values is not trivial. Across a multi-acre contributing area receiving typical annual rainfall, the higher coefficient can overstate harvested volume by a factor of two or more. Because permit applications must represent intended consumptive use accurately, the land-use-derived ROC of 0.325 is the more appropriate and defensible choice.

Why a Higher ROC Overestimates Available Water

The rational method formula used in harvest estimation multiplies rainfall depth by contributing area and the ROC. A coefficient of 0.80 implicitly assumes that 80 percent of all rainfall becomes runoff — a condition associated with heavily impervious urban surfaces such as rooftops and paved plazas. When the actual watershed includes pervious lawns, vegetated buffers, or mixed land cover, far less precipitation reaches the collection point. Applying a high ROC to a partially pervious catchment therefore introduces systematic overestimation into every downstream calculation: projected tank fill frequency, annualized yield, and reported consumptive use all carry the same inflated bias.

REV Curves as Scenario Discovery Tools

Rainfall–Efficiency–Volume (REV) curves extend the single-scenario output of a mass-balance model into a continuous decision surface. By plotting harvest efficiency and cumulative volume across a range of storage sizes and demand assumptions, REV curves allow practitioners and regulators to identify the storage capacity at which marginal gains in yield diminish — the “knee” of the curve that represents the optimal harvest scenario for a given site.

This capability is directly relevant to permit applications in two ways:

• Right-sizing the permit request: Rather than applying for the maximum conceivable volume, the applicant can demonstrate — using the REV curve — that a smaller storage size captures nearly the same annual yield, thereby limiting consumptive use to a well-justified amount.
• Supporting adaptive management conditions: If a permit includes trigger-based restrictions (e.g., reduced harvest during drought), the REV curve can illustrate how harvest efficiency shifts under reduced rainfall inputs, giving regulators confidence that the system will respond predictably to conditions of record.

BMPFast’s Role in Permit Documentation

BMPFast automates the generation of annual mass-balance outputs that can be attached directly to water use permit applications. The tool produces tabular and graphical summaries of:

• Annual inflow to the storage system by month and water year
• Annual demand met from harvested supply vs. supplemental sources
• Overflow and bypass volumes (water that exits the system without being harvested)
• Net consumptive use, disaggregated by application type if multiple end uses are served

Because BMPFast uses a continuous simulation approach driven by long-term rainfall records rather than single-event design storms, its outputs reflect the statistical distribution of wet and dry years at the project location. This is the standard most permitting agencies now expect for consumptive use quantification.

7. Learning Summary

Section 7 of 7 — Consolidation of Core Concepts and Tools

This module introduced the foundational concepts, analytical methods, and software tools needed to assess, design, and document rainwater harvesting systems. The topics progressed from definitional distinctions through hydrologic modeling to regulatory integration, building toward a complete workflow that practitioners can apply to real projects. The following summary consolidates the key learning outcomes from each section.

1 — Rainwater vs. Stormwater Harvesting: Why the Distinction Matters

The module opened by establishing that rainwater harvesting and stormwater harvesting are not interchangeable terms. Rainwater harvesting captures precipitation close to where it falls — typically from rooftop or impervious collection surfaces — before it mingles with runoff from other land areas. Stormwater harvesting, by contrast, intercepts runoff that has already traveled across the landscape and may carry sediment, nutrients, and other contaminants.

This distinction affects:

• Water quality assumptions and the treatment train required before end use
• The ROC values appropriate for modeling inflow
• Permit classification under state and local water law
• Siting constraints and contributing area delineation methodology

2 — BMPFast for Annual Mass Removal Analysis

BMPFast provides a continuous-simulation platform for evaluating rainwater harvesting system performance across the full range of historical rainfall conditions at a project site. Its annual mass-balance outputs quantify inflow, storage behavior, demand satisfaction, overflow, and net consumptive use in a format suitable for both engineering design and regulatory submission.

3 — REV Curves for Discovery and Scenario Comparison

Rainfall–Efficiency–Volume curves transform a point estimate into a continuous performance envelope. By plotting harvest efficiency against storage volume across a range of design configurations, REV curves enable practitioners to:

• Identify the storage size at which marginal efficiency gains diminish (the optimal “knee”)
• Compare alternative scenarios — different contributing areas, demand profiles, or ROC assumptions — on a single graph
• Communicate system performance to non-technical stakeholders and permitting authorities in an accessible visual format
• Demonstrate how performance changes under drought or reduced-rainfall conditions

4 — Output Supporting Water Use Permit Documentation

The combined outputs of BMPFast and REV curve analysis constitute the evidentiary core of a water use permit application for a rainwater harvesting system. A complete documentation package includes:

• Contributing area delineation map with land cover characterization and derived ROC values
• BMPFast input parameters and continuous simulation results (annual and monthly tabulations)
• REV curves illustrating performance across the selected storage range
• Narrative connecting modeled consumptive use to the proposed permitted volume
• Description of intended end uses, demand schedule, and any supplemental supply relationships

5 — Example Project: Real-World Application

The example project presented in the module demonstrated how each component of the analytical workflow connects in practice. Starting from a defined catchment area with mixed land cover, the example walked through ROC derivation, BMPFast setup, annual mass-balance output interpretation, and REV curve generation. The resulting documentation illustrated how a practitioner moves from raw site data to a permit-ready submission — and how sensitivity to ROC selection propagates through every reported metric.

The project also highlighted the importance of iterative scenario testing: no single combination of contributing area size, storage volume, and ROC is inherently correct. The goal is to identify the configuration that best balances system performance, cost, and regulatory compliance for the specific site conditions and end-use requirements at hand.

Appendix: Quick-Reference Cards

Condensed reference material for field use, permit preparation, and peer review