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Florida Department of Environmental Protection Review of SPARROW: 

How useful is it for the purposes of supporting water quality standards development?

The Florida Department of Environmental Protection (FDEP) conducted a review of the 

SPARROW model with the intent of understanding its technical strengths and limitations for 

water quality management purposes. FDEP utilized State modeling and water quality 

assessment expertise, largely developed through implementation of the Total Maximum Daily 

Load (TMDL) programs, which provides a sound basis for the level of technical veracity 

necessary to support regulatory actions such as setting TMDLs or water quality standards.

General Comments

FDEP believes SPARROW is a useful tool for assessment of general patterns of major nutrient 

sources in watersheds. However, FDEP observed some limitations in the use of SPARROW for 

developing scientifically defensible numeric nutrient values that would be used to drive 

regulatory decisions. We understand that this is not new information as the USGS 

acknowledged that the regional model was not intended to be used for site-specific application 

in regulatory programs without significant development of site-specific information, and also 

acknowledged that the model had geographic limitations, such as in South Florida and areas 

with high groundwater recharge. Beyond that, FDEP recognizes that the SPARROW model 

used only four general source inputs and was calibrated across a large geographic area. This 

coarse spatial resolution resulted in large error bars associated with the model output for any 

specific geographic area. The FDEP also understands that SPARROW was calibrated to only 

one year’s worth of data, and was not validated by testing data from other years. These above 

factors simply raise too many questions regarding output results and offer too much legal 

vulnerability regarding the technical veracity of the decisions supported by the model. 

Because of these technical vulnerabilities, calculating the presumed allowable loading using the 

model cannot be scientifically supported and leads to errors when compared to loads necessary 

to protect the designated use of various estuaries. EPA offered an approach using SPARROW

to estimate natural background loads as a basis for deriving a protective target. There are 

several examples where the proposed values for the allowable loading are considerably lower 

than what is actually necessary to protect the estuaries (e.g, Apalachicola Bay, Apalachee Bay, 

St. Marys River). This error is probably largely due to the simplified source input variables to 

the model, coupled with the limitations on how the model was calibrated but not validated and 

the challenges of estimated background loads, but also raises questions about EPA’s proposal 

to set the protective target at the midpoint between current and background loading..

FDEP also notes that, when comparing the protective long-term averages calculated using 

SPARROW (and the downstream calculator) to the proposed benchmark criteria for streams, it 

is important to take the expression of the criteria into account. To be in compliance with the 

criteria (expressed as an annual geometric mean not to be exceeded more than once in a 3-

year period), the long term mean nutrient concentration must be lower than the 90th percentile 

value of the reference site population (see Table 1). For example, the long-term average TN 

concentration in Panhandle tributaries would need to be 0.56 mg/L in order to meet the 
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proposed standard of an annual geometric mean of 0.82 mg/L not to be exceeded more than 

once in a 3-year period, and this value is well below the protective long-term averages 

calculated by Hagy and Green. Therefore, we conclude that the Department’s proposed 

Panhandle bioregion nitrogen criterion would be considerably lower than what is necessary to 

protect Apalachicola Bay. 

In summary, FDEP believes that the SPARROW model is not appropriate at this time for use as 

the basis for regulatory decisions. A more appropriate approach, short of further calibration and 

validation of models on site specific watersheds, would be to use local scientific judgment and 

analysis for each estuary to estimate protective loads, like was done for the Tampa Bay estuary.

Table 1. Long-term average total nitrogen concentrations necessary to achieve the proposed 

stream benchmark criteria at least 90% of the time. 

Parameter Nutrient Region Magnitude 

of Draft 

Criterion

Required Long-term Concentration to Achieve 

the 1 in 3 Year Test with 90% Confidence

Long-term Geometric Mean

TN (mg/L) Panhandle 0.82 0.56

NE-NC-Pen-BV 1.73 1.11

Specific Comments:

1. The “atmosphere N load to the estuary” estimate from SPARROW needs to be verified 

against site-specific examples to assess the accuracy of these predictions. Also the 

atmospheric (background) contribution needs to take into account the range of loads rather 

than using a static (detrended 2002) value.

2. Chapter 14 of EPA’s NCD Technical Document (EPA 2009) notes that “Interim loading 

targets could not be developed for 5 estuaries in south Florida because required information 

related to TN loading could not be obtained for these systems in the time available.” The 

technical documentation (Hoos et al., 2008) for the specific application of SPARROW being 

used here has the following caveat: “Nutrient conditions measured in streams affected by 

substantial influx or outflux of water and nutrient mass across surface-water basin divides do 

not reflect nutrient source and transport conditions in the topographic watershed; therefore, 

inclusion of such streams in the SPARROW modeling approach is considered inappropriate. 

River basins identified with this concern include south Florida (where surface-water flow 

paths have been extensively altered) and the Oklawaha, Crystal, Lower Santa Fe, Lower 

Suwannee, St. Marks, and Chipola River basins in central and northern Florida (where flow 

exchange with the underlying regional aquifer may represent substantial nitrogen influx to 

and outflux from the surface-water basins).” FDEP understands the source of these 

limitations and would suggest using a statistical approach using actual data, versus a 

modeling approach, to estimate protective loadings to estuaries. 
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3. The Hoos et al., 2008 document states that “For local-scale interpretations use of loads 

estimated for actual time periods and employing a more detailed regression analysis, such 

as stepwise linear regression and consideration of additional explanatory variables, is 

suggested.” FDEP agrees that more site specific regression and source variables would 

increase the veracity of the estimates. 

4. Subsection 4.1.3 (EPA 2009) states that “across the 13 estuaries in Florida, we find that 

atmospheric N loading levels vary from 20% to 60% of the current TN loading (Table 1). 

Thus, allowable TN loads that could ensure protection of designated use in downstream 

estuarine waters must be constrained between the loading to estuarine waters resulting 

from the atmospheric load and the current total load to estuarine waters from all sources.” 

This makes the presumption that current load may not be protective when there are number 

of estuaries currently in good health across the State. Additionally, the source of 

atmospheric load estimates are calibrated to a limited number of stations that may be 

appropriate on a large geographic scale, but loses a large degree of technical merit when 

then applied to a relatively small geographic area. Estimating natural background using only 

atmospheric loads appears to eliminate loads from natural flora and fauna (e.g., leaf litter 

from the extensive forests throughout Florida). It may be a worthwhile to exercise to verify 

this estimate by comparing such them to actual nutrient loads in relatively undisturbed areas 

to assess its accuracy.

5. Subsection 14.1.5 (EPA 2009) states that “In Florida, recommended reductions from TMDLs 

and PLRGs range from 2% to 63% (mean 36%, median 38%). Larger reductions are 

recommended where anthropogenic loading has increased loads by the largest margin. 

Based on the information herein, we suggest that in the absence of more specific 

information it may be appropriate to reduce the anthropogenic contribution of TN load to 

estuaries by a constant fraction. For example, by reducing the anthropogenic fraction of TN 

loads by 50% results, on average, in a reduction of overall loading by 30%, similar to the 

loading reductions determined via TMDLs and PLRGs in Florida.” We believe that more 

specific information is available for many of the 13 estuaries evaluated in Section 14 and 

that there needs to be a more comprehensive assessment comparing results of that 

information to that set forth in Chapter 14. DEP staff have prepared or in the process of 

preparing such assessments for several Florida estuaries (i.e, Apalachicola Bay, St. Johns 

River, Tampa Bay). Assessments for Apalachicola Bay and the St. Johns River are 

attached.

6. Table 3 (page 192), which contains SPARROW watershed areas and “Watershed” areas for 

the 13 estuaries, states that “The difference between the SPARROW-watershed area and 

the total watershed area reflects areas that are not hydrologically-resolved by the ERF1_2 

file.” It is not clear how the total watershed area is calculated and what is meant by this 

statement. We note that these two watershed area estimates differ by from 1-70 percent 

and that the “Watershed” area is typically larger than the SPARROW watershed area. We 

assume the load shown in the table is calculated from the SPARROW watershed area. 

Furthermore, the text on page 193 indicates that “model estimates for some river basins in 
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central and northern Florida ... may be affected by unmodeled nitrogen flux to and from the 

regional aquifer (Hoos et al. 2008). We note that the Suwannee River, which is included in 

the list of such basins, is being included in this assessment. This is a further complication 

that suggests that SPARROW may not be appropriate for supporting regulatory decisions at

a smaller geographic scale at this time.

 

7. The SPARROW estimate of watershed TN load for Apalachicola Bay detrended to 2002 (~ 

16 million kg/year) is within the range of estimates (~ 10 to ~31 million kg/year) for 

Apalachicola Bay given by Frick et al., 1996. However, it is difficult to evaluate model 

performance without comparing the rainfall conditions in the basin in 2002 to the conditions 

in the 1974-1990 time period that Frick et al. reported on. This comparison should be done 

for other estuaries for which there are reported loading values to assess the accuracy of the 

SPARROW model predictions.

8. The “interim allowable TN load protective of downstream use” as it is currently being derived 

(midpoint between current loading and estimated background loading) fails to make a 

connection to ecological health of estuaries and presumes that current load is not protective 

of estuaries across the State, which is false. FDEP would recommend using site-specific

loads derived through the TMDL, National Estuarine Program, or other scientificallydefensible estimate of the assimilative capacity of the estuaries for this analysis.

9. Page 187 of EPA (2009) states that “we applied both Lm models to all 13 of the estuaries in 

Florida. In 10 estuaries, the Lm vs. w model provided a result that was between the 

atmospheric N loading level and current TN loading, whereas the Lm vs. qs model provided 

estimates for 11 estuaries that fell within that range, given the error associated with the Lm

estimates. For those estuaries where the model provided a value within the range between 

current loading and atmospheric loading, we considered it a potentially useful reference 

point for determining interim loading targets. Where the model predicted a loading 

maximum outside that range, we conclude that an alternative approach is needed .” FDEP is 

still not clear what is intended by this. However, it is important to note that each approach 

has its limitations and, as a regulatory agency responsible for decisions with respect to 

nutrient loading, FDEP always prefers to rely on site specific information that is calculated in 

a scientifically defensible manner over more generalized approaches, specifically with 

regards to estuaries. 

A comparison of the maximum loading limits for the 13 estuaries calculated by the two 

versions of this model indicates significantly dissimilar results (Figure 1). Of additional 

concern is that the maximum limit for both models is below the SPARROW-predicted 

background load for Apalachee Bay and Choctawhatchee Bay, and the ~f(w) model 

estimate for the St. Mary’s River is also below SPARROW-predicted background load. 

Conversely, the maximum limit for the ~f(w) model is above the SPARROW-predicted 

current load for the Indian River, St. Andrew Bay and Perdido Bay, while the maximum limit 

for the ~f(qs) model is above the SPARROW-predicted current load for the Tampa Bay, 

Sarasota Bay and Pensacola Bay. These disparities in the respective predictive abilities of 
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the SPARROW and S&LMLL models need to be addressed before use for regulatory 

purposes.

y = 0.7913x

R2 = 0.5712

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Max TN Load Limit, Lm ~f(qs) (gN/m2/y)

Max TN Load Limit, Lm ~f(w) (gN/m2/y)

Comparison of Steward & Lowe Maximum Loading Limits 

10. While FDEP recommends that EPA base the downstream protection target for Tampa Bay 

on the federally approved Total Nitrogen TMDL for the bay, if EPA prefers developing a 

flow-weighted average concentration of TN for the downstream protection target, we 

recommend EPA use the TBEP dataset because the TMDL was also based on the load 

estimation methods of TBEP. Table 2 presents a comparison between the GED SPARROW 

based load and the TBEP load estimates for Tampa Bay, and Table 3 provides the flow 

weight mean concentrations used to develop the TBEP estimates. The SPARROW based 

loads are higher than the TBEP estimates. Since the Tampa Bay TMDL is based on a hold 

the line strategy (because the Bay is meets the established chlorophyll a and seagrass 

targets), both estimates of current loads could be considered protective of downstream

waters. Based on site specific analysis in Tampa Bay, no additional load reductions are 

warranted for Tampa Bay since the loading is already below the SPARROW estimate. 

11. The data used by USGS for TN concentrations has not yet been received and reviewed; 

however there appear to be numerous flow stations that have little concentration data over 

the period of record used. The estimation methods used to fill these data gaps require 

further analysis. The data period used by SPARROW SAGT (1974-2005) may not be 

reflective of current water quality conditions in the Tampa Bay watershed. If the intent of the 

approach is to evaluate whether the proposed freshwater criteria are protective of 

downstream waters, there needs to be a clear demarcation of the water quality data used to 

characterize the fresh- and estuarine areas. Tidal tributaries themselves are often a ‘pinchpoint’ for nutrient load-ecological response in a system, and would seem to be more 

responsive to load and duration rather than an annual flow-weighted average concentration.

Figure 1. Comparison of Steward and Lowe Maximum Loading Limits.
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12. DEP has prepared an assessment of the protectiveness to the Apalachicola Bay estuary of 

the state’s proposed benchmark stream criteria for the Panhandle. This assessment 

included an evaluation of the SPARROW model estimated loadings to the Bay. The key 

conclusions of this analysis included:

a) Available nutrient data do not show an increasing trend, and recent chlorophyll a data 

indicate that Apalachicola Bay is not impaired for nutrients.

b) While the SPARROW model estimated average TN concentration for the base year 

(0.67 mg/L) compared favorably with the period of record average concentration of 0.74 

mg/L TN, the averages do not take into account the natural variability in nitrogen loads 

and concentrations reaching Apalachicola Bay, which varied from under 5 million kg/year 

to almost 22.5 million kg/year. Given this variability, the target load/concentration would 

be considered very conservative, particularly given that nutrient impairment is not 

evident in Apalachicola Bay throughout this range of nitrogen loads.

 

c) It should be noted that to achieve the Department’s proposed reference site-based 

criteria (no more than one yearly exceedance in a three year period), the actual long 

term mean nutrient concentration must be substantially lower than the 90th percentile 

value of the reference site population. For example, using a probabilistic approach to 

calculate the long-term average nutrient concentrations necessary to achieve the 

proposed stream benchmark criteria at least 90% of the time in tributaries to 

Apalachicola Bay, the Department determined that the long-term average TN 

concentration in these tributaries would need to be 0.56 mg/L. This value is well below 

the protective long-term averages calculated by Hagy and Green. Therefore, we 

conclude that the Department’s proposed Panhandle bioregion nitrogen criterion would 

be protective of Apalachicola Bay.

13. DEP also prepared an evaluation of the protectiveness of the proposed stream criteria to the 

Lower St. Johns River (attached) and the proposed benchmark-based stream TP and TN 

criteria and associated long term average concentrations are similar to the TMDL based 

nutrient concentrations in the Lower St. Johns River. Based on this comparison, the 

proposed benchmark-based criteria for streams are inherently protective.

14. Department modeling staff have the following questions/comments/recommendations, 

based on their initial review of the SAGT model:

a) The Florida database of NPDES facilities includes over 100 facilities that are not 

included in the SAGT model. Additional review is warranted to determine whether some 

of these facilities represent significant nutrient loads. Similarly, approximately 65 

facilities in the SAGT model had zero nutrient loads, and some of these facilities may 

have flow and concentration data available and loads could be calculated.
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b) Some NPDES facilities have multiple discharges to different waterbodies and it was not 

clear how this was addressed in the model. For example the Iron Bridge facility has 

discharges to both the Little Econhatchee and the St. Johns River, and it seemed to be 

input as a single discharge.

c) There are a number of facilities that have surface discharges as well as reuse, and it is 

not clear how the reuse portion of the discharge was addressed in the model, which 

could have significant implications on the accuracy of the point source loads.

d) Estimated atmospheric deposition may be significantly underestimated because dry 

deposition of nitrogen was not included. Based on some previous USGS reports, dry 

deposition of nitrogen may equal or exceed wet deposition was not considered. It was 

also not clear whether atmospheric deposition was directly applied to the surfaces of 

larger streams and/or lakes.

e) Nitrogen fixation by blue green algae can be a significant source of nitrogen in a number 

of waterbodies and was not considered as a source in the model.

f) Only three level 1 land classes were considered in the model input, and it is unclear why 

all eight classes were not considered, particularly if fertilizer and other loads were 

distributed among some of the other land types. Wetlands, scrub/shrub, or grasslands 

could represent a substantial fraction of the catchment area in many watersheds. 

g) It does not appear that the model took into account the effects of control structures, 

which would have significant impacts on flow and water quality. There are also a 

number of gaging sites that are influenced by discharger flows and it is unknown 

whether any special adjustments were made, especially if discharger flows and loads 

varied during the period.

h) Given the variability in rainfall and flows over the period of record, some further 

evaluation of the use of 2002 as a base year with respect to rainfall and other long-term 

attributes may be warranted.

i) Many catchments in Florida (approximately 168) do not seem to have loads or reach 

nodes.

j) Some additional verification of mean annual stream velocities should be considered 

since it determines reach time of travel and influences first order decay calculations.

k) We recommend that EPA run the model with upstream concentrations set at the 

proposed criteria to evaluate whether the resultant load is consistent with the estimated 

allowable loading. We would expect that the fraction delivered would be different (lower) 

when upstream concentrations are lower.
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Table 2. Comparison of SPARROW flows and loads to those used for TBEP loading estimates

Gaged Site (1) SPARROW TBEP

System Number (2) Location 

Data 

Yr (3)

TN Tot 

Ld_kg (4)

meanq 

(5) Data Yr

TN Total 

Load (kg/yr)

Annual Ave 

Flow (cfs)

Hillsborough River

02303330 @ Morris Br Rd 2002 300,304 282 -- -- --

21FLHILL.137

@ Columbus Dr 

Bridge 2002 243,116 606 -- -- --

02304500 over Reservoir Dam -- -- -- 2002 247,830 240

Alafia River 02301500 @ Lithia 2002 508,003 382 2002 357,854 266

Manatee River 02299550 Near Myakka Head 2002 127,908 157 -- -- --

From Reservoir -- -- -- 2002 144,452 122

Little Manatee River 02300500 Near Wimauma 2002 382,964 230 2002 226,442 142

Horse Creek 02297310 Near Arcadia 2002 417,108 260 2002 398,634 298

1) Sites are USGS unless noted.

2) 21FLHILL.137 = EPCHC site #137. Discharge estimate methods unknown.

3) The model predictions of the mean annual load for the calibrated model are standardized to a single year referred to as the 

“base year” to give an estimate of the mean nutrient load that would have occurred in streams during that year if long-term 

mean annual flow conditions had prevailed. The base year used for the Southeast US analysis was 2002.
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4) TotLd_kg = Instream load transported past the downstream end of the reach, in kilograms (for monitored reaches, predicted 

load is adjusted to match observed load)

5) meanq = long-term mean annual flow for site (period of record ?) (assumed cfs)

Table 3. Flow-weighted average TN concentrations based on data used to develop TBEP loads. Total nitrogen were data collected 

by EPC at stations 105 (Hillsborough River) and 114 (Alafia River) began in 1981.

1981-2004 2003-2007

Hillsborough River at Dam 1.10 mg/L 0.91 mg/L

Alafia River at Lithia 2.38 mg/L 1.54 mg/L