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The Chesapeake Bay Program Priority Agriculture Watershed GIS Analysis (PAW 2021) created separate maps and tables of Delivered Agriculture Loading Rates (DLRag), of Nitrogen (N) and Phosphorous (P), as well as the fraction of streams carrying an agriculture-related pollutant per Chesapeake Bay (CB) twelve-digit coded Hydrological Unit (USGS and USDA-NRCS-National Cartographic and Geospatial Center's (NCGC): <\/span><\/p>

https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/water/<\/a>) (HUC). DLRag values were calculated using the SPARROW 2018 (https://doi.org/10.3133/sir20185051) release of modeled N and P pollutant movement between hydrologic catchments within the Chesapeake Bay watershed along with USGS National Land Cover data (NLCD 2011). Observed stream pollutant data was available through consolidated State 303(d) reports in the EPA ATTAINS database (<\/span>https://www.epa.gov/waterdata/attains<\/a>), for which each state defines its own set of criteria of sampled stream pollutants that are likely to be related to agricultural sources.<\/span><\/p>


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Each SPARROW catchment is represented by one record in the SPARROW database table as well as spatially by its corresponding catchment GIS polygon. Each catchment polygon is nested within an HUC polygon which is also referenced in each catchment table record. Some SPARROW catchment records show a drainage area value that is larger than the HUC in which it nests. This is true of catchments that represent large rivers or streams. Such catchments were identified as \u201cNon-Local\u201d and were removed from the universe of catchments used to summarize stream lengths within each HUC. <\/span><\/p>


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Of the remaining \u201cLocal Catchments\u201d, moreover, some were further assigned a designation as \u201cAgriculture Catchments\u201d (AgCats) which are those with Nitrogen and Phosphorous loadings having mostly agricultural sources, computed from SPARROW tables as follows:<\/span><\/p>

AgCat = Percent Catchment AG  for a SPARROW NHD catchment = <\/font>((Total N Ag load)/(Total N load) + (Total P Ag load)/(Total P load))/2 > 0.5<\/span><\/div>
where:\t\t<\/span><\/font><\/div>
              Total N Ag load kg = ACCL_s4n + ACCL_s5n + ACCL_s6n <\/font><\/div>
              Total P Ag load kg = ACCL_s3p + ACCL_s4p  <\/font><\/div>
              Total N load = ACCL = Sum(ACCL_s1, ACCL_s2, \u2026 ACCL_s7) = All N Loads<\/font><\/div>
              Total P load = ACCL = Sum(ACCL_s1, ACCL_s2, \u2026 ACCL_s5) = All P Loads<\/font><\/div>
and where:\t<\/span><\/font><\/div>
              ACCL_s4n = Accumulated N load from farm fertilizer (kg) <\/font><\/div>
              ACCL_s5n = Accumulated N  load from manure (kg)<\/font><\/div>
              ACCL_s6n = Accumulated N load from nitrogen fixing crops (kg)<\/font><\/div>
              ACCL_s3p = Accumulated P load from farm fertilizer (kg) <\/font><\/div>
              ACCL_s4p = Accumulated P load from manure (kg)<\/font><\/div><\/div>
To determine the Delivered Agriculture Loading Rate for Nitrogen (DLRag_N) for a HUC the Total N Ag load for N, as defined in the formula below were summed and the result was divided by the number of NLCD 2011 Crop and Pasture hectares tabulated per HUC. The same calculation was performed separately for DLRag_P per HUC<\/div>
Delivered Ag Loading Rate N = (DLRag_N) = Total Nag Load / Total NLCD HUC ag area<\/font><\/div>
Delivered Ag Loading Rate P = (DLRag_P) = Total Pag Load / Total NLCD HUC ag area<\/font><\/div>
Where:<\/font><\/div>
              Total Nag load* = dincl_s4n + dincl_s5n + dincl_s6n<\/font><\/div>
              Total Pag load* = dincl_s3p + dincl_s4p\t<\/span> <\/font><\/div>
and where:\t<\/span><\/font><\/div>
              dincl_s4n* = Delivered N load from farm fertilizer <\/font><\/div>
              dincl_s5n* = Delivered N load from manure<\/font><\/div>
              dincl_s6n* = Delivered N load from nitrogen fixing crops <\/font><\/div>
              dincl_s3p* = Delivered P load from farm fertilizer <\/font><\/div>
              dincl_s4p* = Delivered P load from manure <\/font><\/div>
* Kg summed to HUC12 values from all, local and non-local, SPARROW catchment records<\/font><\/div><\/div>

<\/div>
Percent Rank (PR) of these values, DLRag_N_PR and DLRag_P_PR, was calculated relative to all HUCs across the watershed. A set of Percent Rank values for each States\u2019 DLRag values were also calculated.<\/div>

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A second set of maps was created by applying the above defined AgCats to the stream data. ATTAINS assessed streams were identified that had at least one agriculture-related pollutant, as defined by each State. Of these Ag-Impaired streams, only those that were found within an AgCat were selected. This was because Ag-Related stream pollutants could be found which originated in city parks and other non-agricultural stream reaches. The combined length of such Ag-Impaired streams per HUC was divided by the sum of all local ATTAINS assessed catchment stream lengths found within the same HUC. This resulted in a map of the fraction of All-Assessed HUC local stream lengths that were ag-impaired and fell within an AgCat.<\/div>

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The resultant data layers were created using ESRI ArcGIS Pro Version 2.7.2 software. It consists of two feature class File Geodatabase entries, one for the HUC12 Percent Rankings Watershed-Wide (WW) and one for the State-Specific (SS) State HUC12 rankings, named as PAW_WW_2021 and PAW_SS_2021, respectively. The Attribute Table Columns for these features are as follows:<\/div><\/font><\/div>

\n \n \n \n \n \n \n \n \n \n \n \n
\n

Field Name<\/span><\/u><\/b><\/p>\n <\/td>\n

\n

Description<\/span><\/u><\/b><\/p>\n <\/td>\n <\/tr>\n

\n

HUC12 or HUC12_ST<\/span><\/p>\n <\/td>\n

\n

Twelve-Digit\nHydrologic Unit Name or HUC Name with State Suffix<\/font><\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

WW_AgImp_Fract<\/font><\/span>
<\/p>\n <\/td>\n

\n

Watershed-Wide\nAg Impaired Stream Length Fraction<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

WW_DLRag_N<\/span>
<\/p>\n <\/td>\n

\n

Watershed-Wide\nDelivered Loading Rate for Nitrogen<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

WW_DLRag_PR_N<\/span>
<\/p>\n <\/td>\n

\n

Watershed-Wide\nDelivered Loading Rate Percent Rank for Nitrogen<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

WW_DLRag_P<\/span>
<\/p>\n <\/td>\n

\n

Watershed-Wide\nDelivered Loading Rate for Phosphorous<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

WW_DLRag_PR_P<\/span>
<\/p>\n <\/td>\n

\n

Watershed-Wide\nDelivered Loading Rate Percent Rank for Phosphorous<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

SS_DLRag_N<\/span>
<\/p>\n <\/td>\n

\n

State\nSpecific Delivered Loading Rate for Nitrogen<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

SS_DLRag_PR_N<\/span>
<\/p>\n <\/td>\n

\n

State\nSpecific Delivered Loading Rate for Nitrogen Percent Rank<\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

SS_DLRag_P<\/span>
<\/p>\n <\/td>\n

\n

State\nSpecific Delivered Loading Rate for Phosphorous <\/span>
<\/p>\n <\/td>\n <\/tr>\n

\n

SS_DLRag_PR_P<\/span>
<\/p><\/td>

State\nSpecific Delivered Loading Rate for Phosphorous Percent Rank<\/span><\/td><\/tr>\n<\/tbody><\/table>
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