The Simple Method 2.0: Enhancing the Results
Saturday, April 30, 2011
By Murat Ulasir, Scott Kaiser
Estimating stormwater pollution loads is an integral part of understanding “big picture” surface water quality within lakes, rivers, and streams. Quantifying the volume of pollution by type can often help the practitioner get a pulse on watershed characteristics. This task can be daunting for the layperson if data are difficult to acquire or the model is confusing and difficult to understand or use.
Nonpointsource (NPS) stormwater pollutant estimation can be achieved via simple and/or complex methods to determine a catchment or watershed load. Often, for a quick estimation, we do not need an overly complex model with multiple variables for a confident understanding of the hydrologic system and pollutants. Understanding the general watershed pollution characteristics or sighting and designing best management practices (BMPs) needs a simple approach that delivers presentable results.
The Simple Method is an effective stormwater pollution load estimation model for planninglevel decision support in smallscale watersheds or catchments. This model was developed by Schueler in 1987 to estimate total suspended solids (TSS), total phosphorous (TP), or total nitrogen (TN). The Simple Method equation allows the user to input parameters such as pollutant load concentration, watershed area, and annual runoff to determine an annual load. Once the parameters are defined, the values are multiplied against a conversion factor to yield the NPS pollutant load (L) for the target catchment. The output typically yields pounds per year of the modeled constituent, where
L = 0.226(R)(P_{c})(A)
where
L = annual load (lb.)
R = annual runoff (in.)
P_{c} = pollutant concentration (mg/l)
A = area (ac.)
Additionally, the pollution concentration (P_{c}) could be determined by field sampling or through documented estimations such as the National Urban Runoff Program. However, there are compiled model averages based on several published mean pollution concentration estimates that are acceptable for the calculation.
When determining estimates for smallerscale geographies, e.g., catchments or subwatersheds, the Simple Method approach is acceptable for determining NPS pollutant loads. However, as quick, easy, and straightforward as this approach is, there may be some overestimation of the output pollution load.

Figure 1. Understanding the Simple Method waterquality correction 

Understanding the Overestimation
The Simple Method equation out of the box uses annual runoff (R) as one of the variables to determine the amount of stormwater pollutants (L). This is a function of the annual rainfall (P), a fraction of the runoff producing events (P_{j},_{ }typically 0.9), and the watershed runoff coefficient (C, percentage of imperviousness):
R = (P)(P_{j})(C)
where
R = annual runoff (in.)
P = annual rainfall (in.)
P_{j} = Fraction of annual rainfall producing runoff (typically 0.9)
C = runoff coefficient
When assessing the pollutioncontributing portion of a storm, or the “dirty water,” we are typically concerned only with the first flush or the portion of the rain event that transports the majority of NPS pollution and accounts for approximately 90% of the total annual rainfall events. The firstflush phenomena can be characterized as the stormwater runoff at the early part of a storm that cleanses the landscape of pollutants. In Michigan, the first 0.5 inch of rain during a weather event is considered the first flush. Note that this amount may vary from region to region. For the purposes of this demonstration, we will consider the first flush to be 0.5 inch. Please note that the calculations presented can easily be modified for larger or smaller firstflush amounts.
With the Simple Method as it is proposed by Schueler, we take into account all portions of the storm when in fact we really need only the first 0.5 inch or less of an event. The difference between the total annual firstflush runoff for all events and the total annual runoff is where the potential overestimation occurs within the pollutant load calculation. The overestimation or exaggeration can create errors when calculating an annual load as well as determining the type, location, and size of mitigation BMPs. Even though the Simple Method is a planninglevel tool, its results are widely demonstrated in the literature and during public forums on waterquality projects. If overestimation is occurring, there may be false expectations for the performance of BMPs. Or, if followup sampling occurs after the completion of a project, the expected goals may be missed. Additionally, BMPs may be overdesigned, therefore resulting in potentially higher engineering and
construction costs.
Removing the Overestimation and Refining Results
Amending the total annual runoff to represent the total annual firstflush runoff in the Simple Method equation would help align the loading rates closer to the actual annual pollutant load. By doing so, the user would be creating a customized waterquality runoff representing the contribution from the total annual firstflush rainfall. The modified annual firstflush runoff (R_{ff}) would be applied to the Simple Method 2.0 in place of the standard annual runoff, and the new equation would appear as follows:
L = 0.226(R_{ff})(P_{c})(A)
where
L = annual load (lb.)
R_{ff} = annual firstflush runoff (in.)
P_{c} = pollutant concentration (mg/l)
A = area (ac.)
The analyst should start by gathering local daily precipitation data, for example from the National Oceanic and Atmospheric Administration (NOAA), to calculate the corrected value. The data can quickly be gathered and downloaded from the Internet via the NOAA Satellite and Information Service
(www.ncdc.noaa.gov). NOAA has organized the text base data by gage station and year on its site. With the daily precipitation data in a spreadsheet, all events less than 0.5 inch and the first 0.5 inch of all events larger than the first flush should be summed. This total precipitation would be the most polluting annual stormwater for that given dataset. The new firstflush precipitation value (R_{ff}) would replace the annual rainfall (R) in the Simple Method annual runoff equation (Figure 1). The improved annual firstflush runoff (R_{ff}) equation would appear as follows:
R_{ff} = (P_{ff})(P_{j})(C)
where:
R_{ff} = annual firstflush runoff (in.)
P_{ff} = annual firstflush rainfall (in.)
P_{j} = fraction of annual rainfall producing runoff (typically 0.9)
C = runoff coefficient
To understand the potential for model overestimation, the new summed value divided by the total annual precipitation would be the waterquality correction factor (WQCF). This represents the percentage of the annual runoff carrying NPS pollutants when considering the firstflush phenomena. Note: A “ruleofthumb” WQCF option was evaluated. Regional variability associated with rainfall amount and firstflush amount made this value difficult to determine and potentially inaccurate. It is best to gather the precipitation data to determine the annual
firstflush rainfall for the target region or locality. Additionally, averaging several years of firstflush precipitation data together may create a more refined estimation for the pollution model.
Use This Approach
Integrating the WQCF should result in a reduced annual runoff value. The percent of runoff reduction will vary from year to year and region by region. Therefore, it is important to capture a large enough multiyear sample to give an adequate representation of the waterquality or firstflush runoff. This amendment to the Simple Method, now the Simple Method 2.0, was yielding approximately 20% lower annual runoff for the sampled southeast Michigan precipitation data. The results, with the lower firstflush annual runoff value, reduced the theoretical calculated annual pollution load for the subwatershed. Additionally, the waterquality correction factor is customizable to any area with gage data, therefore making local NPS pollution forecasting region specific and dynamic.
Southeast Michigan Example: Annual TSS
The annual precipitation data for this example were acquired from NOAA for Ann Arbor, MI, for 2000 through 2009 (Table 1). The assumed watershed characteristics will be for a 300acre urbanized residential area with a runoff coefficient of 0.60. Based on the land cover, the TSS pollution concentration would be 100 mg/l.
Simple Method With Annual Precipitation (P)
P = 28.21 in.
P_{j}= 0.9 (typically 0.9)
C = 0.60 (percent impervious)
P_{c} = 100 mg/l (TSS concentration from literature)
A = 300 ac. (watershed area)
1. Calculate annual runoff:
R = (P)(P_{j})(C)
R = (28.21)(0.9)(0.60)
R = 15.23 in.
2. Calculate pollution load:
L = 0.226(R)(P_{c})(A)
L = (0.226)(15.23)(300)(100)
L = 103,259 lb/yr
Simple Method 2.0 With Annual FirstFlush Precipitation (P_{ff})
P_{ff} = 22.32 in.
P_{j} = 0.9 (typically 0.9)
C = 0.60 (percent impervious)
P_{c} = 100 mg/l (TSS concentration from literature)
A = 300 ac. (watershed area)
1. Calculate annual firstflush runoff:
R_{ff} = (P_{ff})(P_{j})(C)
R_{ff} = (22.32)(0.9)(0.60)
R_{ff} = 12.05 in.
2. Calculate pollution load:
L = 0.226(R_{ff})(P_{c})(A)
L = (0.226)(12.05)(300)(100)
L = 81,699 lb/yr
This value for TSS is about 21% smaller than the value calculated using the entire annual precipitation.
Author's Bio: Scott Kaiser, GISP, CFM, is a GIS analyst with OHM Inc. in Livonia, MI. 
Author's Bio: Murat Ulasir, P.E., Ph.D., is a technical specialist with OHM Inc. in Livonia, MI. 
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