Application of watershed deposition tool to estimate from CMAQ simulations the atmospheric deposition of nitrogen to Tampa Bay and its watershed

Figures & data

Figure 1. Tampa Bay and its watershed. Black outlines indicate watershed basin and bay segments boundaries, and darker shading shows municipal boundaries for major cities.

Figure 2. Watershed Deposition Tool (WDT) display for 2002 atmospheric reactive nitrogen (N) deposition by basins and bay segments within Tampa Bay's watershed.

Figure 3. Reactive nitrogen (N) atmospheric deposition rates for Tampa Bay and its watershed from 2002 CMAQ model simulations: (a) gridded N deposition rates (kg N ha⁻¹); (b) average N deposition rates by watershed basin and bay segment (kg N ha⁻¹).

Table 1. Estimated 2002 direct and indirect atmospheric N loading rates within Tampa Bay's watershed. Water area includes embayment, canals, and major tributaries apportioned by bay segment; land area includes watershed drainage surface by bay segment

Bay segment	Water area, ha	Land area, ha	Direct atmospheric loading, metric tons N	Indirect atmospheric loading,* metric tons N	Atmospheric loading, metric tons N
Old Tampa	22,451	67,326	261	181	441
Hillsborough	10,852	330,364	211	843	1,050
Middle Tampa	29,450	79,924	286	194	480
Lower Tampa	25,265	8,437	218	19.2	237
Boca Ciega	6,188	21,353	43.6	52.5	96.1
Terra Ceia	1,650	2,851	20.6	6.48	27.1
Manatee River	3,460	92,416	42.1	196	238
Totals	99,317	602,669	1,080	1,490	2,570
Notes: Assumes an 18% transfer rate from watershed to bay. Note: Arithmetic operations were performed prior to rounding numbers for presentation in the table and as such the sums of columns and rows may not match exactly the sums shown.

Table 2. Modeled atmospheric deposition (AD) and total N loading to Tampa Bay, 2002–2006

	Previous estimates*			Current estimates
Year	Direct AD N loading, metric tons	Total N loading, metric tons	^§Adjusted Total N loading, metric tons	Direct AD N loading, metric tons	Indirect AD N loading, metric tons	AD N loading, metric tons	Total N loading that is AD loading, %
2002	840	3,400	3,640	1,080	1,490	2,570	70.6
2003	1,150	6,690	6,440	903	1,340	2,240	34.8
2004	801	5,930	6,010	885	1,240	2,120	35.4
2005	603	3,230	3,570	950	1,310	2,260	63.3
2006	552	2,980	3,230	803	1,150	1,950	60.5
Notes: *Poe et al., 2005; JEI, 2008; TBNMC, 2010. ^§Previous estimate of total N loading – previous estimate of direct AD N loading + current estimate of direct AD N loading.

Table 3. Statistics to assess agreement between modeled versus observed monthly rainfall and wet N deposition rates for observations made at NADP's Gandy Bridge site (FL18) for 2002 to 2006. Units are cm on mean modeled, mean observed, and RMSE precipitation; units are kg N ha⁻¹ mo⁻¹ on mean modeled, mean observed, and RMSE deposition rates

Species	Mean modeled	Mean observed	r, %	NMB, %	NME, %	RMSE
Precipitation	12.5	10.0	58.4	26.8	71.1	10.6
NH4 ^+ -N	0.127	0.158	48.9	–18.5	55.8	0.144
NO3 ^− -N	0.232	0.171	80.1	38.0	59.8	0.157
Total N	0.365	0.329	69.1	10.8	53.9	0.265

Table 4. Statistics to assess agreement between modeled versus observed monthly rainfall and wet N deposition rates for observations made at NADP's Verna Wellfield site (FL41) for 2002 to 2006. Units are cm on mean modeled, mean observed, and RMSE precipitation; units are kg N ha⁻¹ mo⁻¹ on mean modeled, mean observed, and RMSE deposition rates

Species	Mean modeled	Mean observed	r, %	NMB, %	NME, %	RMSE
Precipitation	16.3	12.3	73.7	32.6	66.8	12.6
NH4 ^+ -N	0.113	0.108	50.1	2.91	62.1	0.103
NO3 ^− -N	0.173	0.166	78.6	0.416	43.4	0.107
Total N	0.287	0.274	73.7	1.39	43.3	0.186

Figure 4. Atmospheric average (a) dry and (b) wet N deposition rates (kg N ha⁻¹) for Tampa Bay and its watershed, by watershed basin and bay segment, from 2002 CMAQ model simulations.

Figure 5. Atmospheric average (a) oxidized and (b) reduced N deposition rates (kg N ha⁻¹) for Tampa Bay and its watershed, by watershed basin and bay segment, from 2002 CMAQ model simulations.

Figure 6. CMAQ-modeled 2002–2006 annual average N deposition rates for Tampa Bay and its watershed.

Figure 7. CMAQ-modeled 2002–2006 monthly average wet and dry N deposition rates for Tampa Bay and its watershed.

Figure 8. Comparison of CMAQ-modeled and observed precipitation rates for NADP's (a) AIRMoN site at Gandy Bridge (FL18) and (b) NTN site at Verna Wellfield (FL41).

Figure 9. Scatter plots of CMAQ-modeled and observed (a) precipitation rates, (b) oxidized N wet deposition rates, and (c) reduced N wet deposition rates for NADP's NTN Verna Wellfield (FL41) site.

Figure 10. Comparison of CMAQ-modeled and observed atmospheric N wet deposition rates for NADP's (a) AIRMoN site at Gandy Bridge (FL18) and (b) NTN site at Verna Wellfield (FL41).

Table 5. Comparison of annual average CMAQ-modeled and measurement-based direct dry deposition rates to Tampa Bay for 2002–2003

Species	CMAQ, kg N ha^−1 yr^−1	BRACE, kg N ha^−1 yr^−1
HNO3	1.42	0.81
NH3	0.38	2.29
NO3 ^−	0.48	0.07
NH4 ^+	0.10	0.01
Other Ox N	0.54	—
Total	2.91	3.18

Table 6. Spreadsheet calculation of combined uncertainty for atmospheric N loading to Tampa Bay using eq 6, where

			Standard deviations, metric tons N yr^−1
Term ID	N loading term	Estimate, metric tons N yr^−1	s1	s2	s3	s4	s5
1	Fwet_land	3,490	6,980	3,490	3,490	3,490	3,490
2	Fdry_land	4,800	4,800	9,600	4,800	4,800	4,800
3	Fwet_bay	474	474	474	948	474	474
4	Fdry_bay	608	608	608	608	1,216	608
5	EC	0.18	0.18	0.18	0.18	0.18	0.36
	LRAD+	—	3,202	3,438	3,048	3,182	4,066
	− LRAD	2,574	2,574	2,574	2,574	2,574	2,574
		—	628	864	474	608	1,492

Figure 11. CMAQ-modeled changes in watershed N deposition for 2002 base case and 2020 future scenario emissions for Tampa Bay and its watershed, processed with WDT from CMAQ 2002 12-km gridded scale output: (a) change in gridded N deposition rates (kg N ha⁻¹); (b) change in average N deposition rates by watershed basin and bay segment (kg N ha⁻¹).

Poe , A. , Hackett , K. , Janicki , S. , Pribble , R. and Janicki , A. 2005 . Estimates of Total Nitrogen, Total Phosphorus, Total Suspended Solids, and Biochemical Oxygen Demand Loadings to Tampa Bay, Florida: 1999–2003, Final Report, March 2005 , St. Petersburg , FL : Tampa Bay Estuary Program Technical Publication 02-05 .

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Tampa Bay Nitrogen Management Consortium . 2010 . 2009 Reasonable Assurance Addendum: Allocation and Assessment Report. Tampa Bay Estuary Program Technical Publication 03-10 , St. Petersburg , FL : Tampa Bay Estuary Program .

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