TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

Appendix B-10: Analysis of Land Disturbance Index Computation 

Approach for FDEP’s Benchmark WBIDs 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-1 

Appendix B-10. Analysis of Land Disturbance Index Computation Approach 

for FDEP’s Benchmark WBIDs 

Florida Department of Environmental Protection’s (FDEP) approach for computing criteria on 

the basis of benchmark sites relies on screening sites (and waterbody IDs (WBID)) using the 

Stream Corridor Land Disturbance Index (LDI). The Stream Corridor LDI computed by FDEP is 

based on tabulating land use within a 100 meter buffer surrounding all streams upstream of a site 

for 10 kilometers, tracing all stream paths. Such tabulations are used to create an area-weighted 

LDI for the site (and WBID). Sampling locations (sites) were aggregated by FDEP into 

waterbodies, identified by a unique WBID. Sites with sufficiently low LDI scores (< 2.0) were 

identified as potential sites for inclusion into a benchmark WBID. The purpose of this analysis is 

to determine, for the 135 nutrient benchmark WBIDs identified by FDEP, whether alternative 

land inclusion definitions for computing an area-weighted LDI explain inter-WBID variability in 

total nitrogen (TN) and total phosphorus (TP), specifically high nutrient values. This analysis 

does not consider the general applicability of the Stream Corridor LDI. 

For this analysis, two alternate land inclusion rules were developed. These rules are listed below. 

 Drainage basin LDI. LDI computed from all drainage areas within Florida upstream of the 

most downstream site within a WBID.4

 Near-field watershed LDI. LDI computed from all drainage areas (at the 12-digit HUC level) 

in Florida within a 10 kilometer straight-line radius to the most downstream site within a 

WBID. 

Geographic Information System Analysis and Quality Control Checks 

The drainage basin rule includes the entire upland watershed but was necessarily limited to lands 

in Florida because there were not similarly classified land cover data from Georgia and Alabama. 

Thus, there is a potential that significant portions of the upland watershed were excluded from 

the analysis. The near-field watershed rule was focused on the closer portion of the watershed 

and had less influence from differences in land cover data availability. Florida’s stream corridor 

LDI was originally calculated using land cover data from 1995. Since FDEP’s original 

estimation, 2004 land use data became available. To address the issue of whether there was an 

effect of using the 1995 land use for computing the Stream Corridor LDI versus more recent land 

cover data, LDIs were computed using both the 1995 and 2004 land cover data. Although more 

recent land cover data were available for some water management districts, land cover data from 

2004 were chosen since these data were available statewide. The following provides an overview 

of the GIS computation steps: 

1. Land use data as stored by FDEP or individual Water Management Districts, Florida 

Digital Elevation Model (DEM) data, National Hydrography Dataset (NHD), watershed 

(12-digit HUC) data, and Florida’s WBID data were downloaded and integrated into a 

geographic information system (GIS) environment. 

 

4 Because of WBID hydrology, two WBIDs (2120A_ and 3489_) did not have a most downstream site—thus two sites in each of 

these WBIDs were maintained in this analysis. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-2 

2. The most downstream site within a WBID was selected visually on the basis of the list of 

benchmark sites and WBIDs provided by FDEP staff. 

3. The downstream site was snapped to the nearest NHD stream segment and a manually 

delineated watershed was created by visual inspection of the DEM data. 

4. Using available GIS tools, the near-field watershed and drainage basin boundaries were 

created in a manner consistent with the above definitions. 

5. Land use was tabulated for each watershed and made available for post processing to 

compute the LDI. 

The GIS work was distributed among contractor and FDEP staff. FDEP staff processed most of 

the sites in the St. John’s River Water Management District, while contractor staff processed the 

remainder. Inherent analyst-to-analyst differences in manual watershed delineation can 

contribute to differences in results. Also, FDEP staff had more direct knowledge of sites that 

would tend to lead to more informed delineations. Additional differences include FDEP’s 

processing of land cover data in vector format versus contractor processing of land cover data in 

raster format. To verify work between the analysts, some sites were processed by both the 

contractor and FDEP. Quality control steps were used to verify the comparability of the work 

between analysts and utility of the combined work product (see Table B10-1 and Figure B10-1). 

The results of the quality control checks indicate that land cover results from FDEP and the 

contractor can be combined and used for the present purpose. 

Relationship of Alternative LDI Approaches to TN and TP 

The LDIs computed from the above approach were merged with TN and TP summary data 

which had been computed as the geometric mean of benchmark WBID-Years. Thus, the initially 

selected set of 135 WBID LDIs includes multiple WBID-Year values computed for TN and TP. 

The remainder of the discussion in this section is drawn from observations taken from Figures 

B10-2 through B10-12. 

Near-field watershed and drainage basin LDIs using 1995 and 2004 land use data were plotted as 

a function of Stream Corridor LDI (Figure B10-2). Of the 1995 near-field watershed and 

drainage area LDIs, 63% and 61% were less than two, respectively. They decreased to 55% and 

53% for the 2004 land cover data, respectively. More than 92% of all computed LDIs were less 

than three. There is a general increasing trend in each panel, and the correlation coefficients 

ranged from 0.39 for the 2004 drainage area LDI to 0.52 for the 1995 and 2004 near-field LDIs. 

TP and TN were plotted as a function of Stream Corridor LDI (Figure B10-3). The most 

noticeable high concentrations associated with high LDI values were TP concentrations for 

WBID 1653_. This graphic provides evidence that WBID 1653_ should not be included in the 

benchmark site distribution, especially given the high LDI values for this site. The same TP and 

TN data were plotted after removing WBID 1653_ (Figure B10-4). Figures B10-5 through B10-8 

do not include data from WBID 1653_. 

TN and TP were plotted as a function of alternative LDI approaches (Figures B10-5 and B10-6). 

TN and TP were positively correlated (significance level > 95%) with each of the LDIs shown in 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-3 

Figures B10-4, B10-5, and B10-6. Correlation coefficients are listed below. TN and TP were 

most strongly correlated with the near field LDI. 

TN TP 

Stream Corridor LDI 0.121 0.343 

Near-field LDI (1995) 0.264 0.392 

Drainage Area LDI (1995) 0.194 0.241 

Near-field LDI (2004) 0.224 0.353 

Drainage Area LDI (2004) 0.159 0.224 

From that analysis, which was developed to explore potential explanations for elevated nutrients 

among benchmark sites, three WBIDS in particular exhibited higher TP concentrations 

associated with increased near-field LDI values (Figure B10-6). WBID 3401_ in the North 

Central region had multiple observations exceeding 0.45 mg/L. Each of the other two WBIDs 

(1807_ and 1811_) had one observation exceeding 0.45 mg/L. Because those observations were 

also associated with higher near-field LDIs, it cannot be concluded whether these data are from 

WBIDs with naturally high occurring levels of TP or because of higher LDI. 

To additionally explore the potential effect of extra-corridor land cover in explaining nutrient 

excursions, TN and TP were plotted as a function of the incremental difference between the 

alternative LDIs and the Stream Corridor LDI (e.g., 2004 near-field LDI minus Stream Corridor 

LDI, Figures B10-7 and B10-8). In each of the land cover relationships with TN and the nearfield relationship for TP, a statistically significant model can be developed; however, the R2

values were less than 10%. 

The TP in the Bone Valley and North Central regions was further investigated in light of the 

above discussion that highlights three WBIDs in the Bone Valley region (1653_, 1807c_, and 

1811_) and one WBID in the North Central region (3401_). 

Bone Valley TP (including WBID 1653_) is plotted as a function of the alternative LDI 

approaches and the Stream Corridor LDI, including WBID 1653_ (Figure B10-9) and the 

incremental difference between the alternative LDIs and the Stream Corridor LDI (Figure B10-

10). Similarly, North Central TP is plotted in Figures B10-11 and B10-12. These figures 

underscore the observations made earlier in relation to Figures B10-3 and B10-6. 

Conclusions and Recommendations 

This analysis was limited to reviewing LDI computations for the 135 nutrient benchmark WBIDs 

that were identified by FDEP using the Stream Corridor LDI and further screened as documented 

elsewhere. Thus, the below conclusions and recommendations cannot be used to infer the general 

applicability of the Stream Corridor LDI beyond the WBIDs evaluated in this document. 

 For the 135 WBIDs evaluated, near-field LDIs were better predictors of TN and TP than the 

Stream Corridor LDI. However, the linear regression models developed using the data 

presented in Figures B10-7 and B10-8, while statistically significant, provided little 

meaningful benefit (R2

 < 10%) in comparison to the Stream Corridor LDI. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-4 

 On the basis of the data presented in Figure B10-3 (and reemphasized in Figures B10-9 and 

B10-10), it is recommended to delete WBID 1653_ from the benchmark data set. 

 On the basis of the data presented in Figure B10-6 (and reemphasized in Figures B10-9 

through B10-12), two WBIDs in the Bone Valley (1807c_ and 1811_) and one WBID in the 

North Central region (3401_) have higher TP observations associated with higher near-field 

LDIs. It cannot be concluded that these data are from WBIDs with naturally high occurring 

levels of TP or from higher LDI. In light of the competing interest to (1) identify benchmark 

WBIDs that are not affected, or at most, minimally affected by anthropogenic effects, and 

(2) sample size, there are three likely approaches that can be considered for these three 

WBIDs. 

o Compute benchmark criteria, excluding the data from all three WBIDs. Such an 

approach might lead to sample size concerns in the Bone Valley region. 

o Compute benchmark criteria including the data from all three WBIDs. That approach 

maintains the sample size in the Bone Valley region but draws into question whether 

WBIDs that are more than minimally impaired WBIDs are included. 

o Remove the North Central region WBID (3401_) from the analysis but maintain the 

two Bone Valley WBIDs. Such an approach eliminates the concern that more than 

minimally impaired WBIDs are included in the North Central Region and would 

allow the North Central Region to be handled in a manner similar to other regions. 

That would not be the case for the Bone Valley region. 

The technical concerns related to following one of the above approaches that maintains a WBID 

that is potentially more than minimally impaired can be addressed by choosing a lower percentile 

for the criteria. 

Table B10-1. GIS Quality Control Checks and Results 

# Check Result 

1 Comparison of watershed size 

using rasterized tabulations and 

those computed from watershed 

polygons. 

Six sites, fully contained in Florida were analyzed. The 

greatest error between the land use tabulation and the 

polygon coverage was 0.05%. 

2 Comparison of land use 

summary between rasterized 

land use tabulations (contractor 

algorithm) and use of vectorized 

tabulations (FDEP algorithm). 

For site 21FLWQSP_ESC012GS, the FDEP approach 

found 38 different land covers, while the contractor 

algorithm found 37. The FDEP approach included land 

cover 1210 of 5.7 m2

. For 29 of the land cover categories, 

the difference in total land cover between the FDEP and 

contractor algorithm was less than 1%. For the eight land 

cover categories with more than 1% difference, the total 

land area associated with these lands (and 1210 discussed 

earlier) was 66.9 acres, 0.39% of the total polygon area. 

3 Comparison of 5 m and 10 m 

raster tabulations. 

Eleven (11) sites were evaluated. A total of 441 site/land 

cover categories were tabulated. For the site/land cover 

categories with an error greater than 1%, the total land 

cover for these areas was 0.18% of the total land area 

summarized. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-5 

# Check Result 

4 Comparison of LDI computed 

from rasterized land cover 

tabulations (contractor algorithm) 

and use of vectorized tabulations 

and LDI values computed by 

FDEP. 

For site 21FLCEN_20010824, the FDEP delineated 

watershed was 751 acres larger than the contractor 

delineated watershed. Because of the small watershed 

size, that represented a 4.8% difference. Nevertheless the 

LDI from the FDEP and contractor analysis was 2.584 and 

2.599, respectively. 

5 Compare computed LDIs in this 

analysis 

The near-field watershed and drainage basin LDIs using 

2004 data were plotted as a function of LDIs computed 

using 1995 data (see Figure B10-1). The four highlighted 

sites were reexamined within the GIS. It was confirmed that 

the noticeable differences in LDI could be accounted for in 

land cover changes present in the source data. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

1995 Land Use

2004 Land Use

Near Field

Drainage Area

Points with "+" and "o" on top 

of each other indicate that nearfield and drainage area 

watersheds are identical.

See LDI 

Exception

s

Figure B10-1. Near-field watershed and drainage basin LDIs using 2004 land cover data as a 

function of LDIs computed using 1995 land cover data. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-6 

Stream Corridor LDI

5

4

3

2

1

1.0 1.5 2.0 2.5

5

4

3

2

1

1.0 1.5 2.0 2.5

Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004)

Figure B10-2. Near-field watershed and drainage basin LDIs using 1995 and 2004 land use data 

as a function of Stream Corridor LDI. 

Stream Corridor LDI

1.0 1.5 2.0 2.5

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.0 1.5 2.0 2.5

2.5

2.0

1.5

1.0

0.5

0.0

GeoMeanTP GeoMeanTN

Figure B10-3. TP (GeoMeanTP, mg/L) and TN (GeoMean TN, mg/L) as a function of Stream 

Corridor LDI. Includes WBID 1653_ with TP values greater than 0.75 mg/L. 

WBID 1653_ 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-7 

Stream Corridor LDI

1.00 1.25 1.50 1.75 2.00

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1.00 1.25 1.50 1.75 2.00

2.5

2.0

1.5

1.0

0.5

0.0

GeoMeanTP GeoMeanTN

Figure B10-4. TP (GeoMeanTP, mg/L) and TN (GeoMean TN, mg/L) as a function of Stream 

Corridor LDI. Excludes WBID 1653_ with TP values greater than 0.75 mg/L. GeoMeanTN

2.5

2.0

1.5

1.0

0.5

1 2 3 4

2.5

2.0

1.5

1.0

0.5

1 2 3 4

Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004)

Figure B10-5. TN as a function of the alternative LDI approaches. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-8 

Figure B10-6. TP as a function of the alternative LDI approaches with selected WBIDs 

highlighted. GeoMeanTN

2.5

2.0

1.5

1.0

0.5

0 1 2 3

2.5

2.0

1.5

1.0

0.5

0 1 2 3

1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

Figure B10-7. TN as function of the incremental difference between the alternative LDIs and the 

Stream Corridor LDI. GeoMeanTP

0.60

0.45

0.30

0.15

0.00

1 2 3 4

0.60

0.45

0.30

0.15

0.00

1 2 3 4

Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004)

1807

3401

1811
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-9 

GeoMeanTP

0.60

0.45

0.30

0.15

0.00

0 1 2 3

0.60

0.45

0.30

0.15

0.00

0 1 2 3

1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

Figure B10-8. TP as function of the incremental difference between the alternative LDIs and the 

Stream Corridor LDI. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-10 

GeoMeanTP

1.2

0.9

0.6

0.3

0.0

54321

54321

1.2

0.9

0.6

0.3

0.0

54321

Near Field LDI(1995) Drainage Area LDI(1995) Stream Corridor LDI

Near Field LDI(2004) Drainage Area LDI(2004)

1653_

1807C_

1811_

1978_

WBID

Figure B10-9. TP as a function of the alternative LDI approaches and the Stream Corridor LDI 

for the Bone Valley region. GeoMeanTP

1.2

0.9

0.6

0.3

0.0

3.02.52.01.51.00.50.0

1.2

0.9

0.6

0.3

0.0

3.02.52.01.51.00.50.0

1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

1653_

1807C_

1811_

1978_

WBID

Figure B10-10. TP as function of the incremental difference between the alternative LDIs and 

the Stream Corridor LDI for the Bone Valley region. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-11 

GeoMeanTP

0.60

0.45

0.30

0.15

0.00

4321

4321

0.60

0.45

0.30

0.15

0.00

4321

Near Field LDI(1995) Drainage Area LDI(1995) Stream Corridor LDI

Near Field LDI(2004) Drainage Area LDI(2004)

3504_

3504A_

3506_

3506B_

3530_

3578_

3598_

3605_

3605D_

3641_

3324_

3341A_

3341B_

3351_

3368_

3388_

3401_

3448_

3477_

WBID

Figure B10-11. TP as a function of the alternative LDI approaches for the North Central region. GeoMeanTP

0.60

0.45

0.30

0.15

0.00

0.0 0.5 1.0 1.5 2.0

0.60

0.45

0.30

0.15

0.00

0.0 0.5 1.0 1.5 2.0

1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

3504_

3504A_

3506_

3506B_

3530_

3578_

3598_

3605_

3605D_

3641_

3324_

3341A_

3341B_

3351_

3368_

3388_

3401_

3448_

3477_

WBID

Figure B10-12. TP as function of the incremental difference between the alternative LDIs and 

the Stream Corridor LDI for the North Central region.