『Abstract
　N and P budgets quantify inputs and outputs of nutrients at the catchment scale to allow evaluation of inputs and outputs as well as inferences about transport and processing based on unaccounted-for nutrients. N and P budgets were constructed for two catchments in southeastern Michigan with markedly different numbers of impoundments, over two years, to evaluate the influence of impoundments on nutrient fluxes from each catchment. The Huron, with 88 impoundments＞10 ha, stored 156 kg P km^-2 y-1, while the Raisin (with 14 impoundments) had a net export of 102 kg P km^-2 y-1. The Huron catchment also stored and denitrified more N than the Raisin catchment−2,418 kg N km^-2 y-1 compared to 1,538 kg N km^-2 y-1. Riverine export of N and P also varied markedly between the catchments, with the Huron River exporting 288 kg N and 7 kg P km^-2 y-1 and the Raisin River exporting 1,268 kg N and 34 kg P km^-2 y-1. We then re-calculated budget results from previous studies using the approach of the present study, altering input and outputs fluxes as well as system boundaries to obtain comparable budgets. For these comparable budgets, annual P outputs on average accounted for 77％ of inputs whereas N outputs accounted for only 39％ of N inputs. Across catchments, the percent of inputs exported by the river averaged 16％ for N and 5％ for P, indicating more effective retention of P and N.

Keywords: Catchment; Impoundments; Mass balance; Nitrogen; Nitrogen budget; Phosphorus: Phosphorus budget; Rivers』

Introduction
Methods
　Study area
　Huron and Raisin nutrient budgets
　Atmospheric deposition and NHx volatilization
　Fertilizer application
　Manure application
　Sewage disposal
　Nitrogen fixation
　Crop export and food & feed export
　River export
　Re-calculation of budgets from other studies using the reference method
Results
　Huron and Raisin nitrogen budgets
　Huron and Raisin phosphorus budgets
　Huron and Raisin N and P river exports
　Budget comparisons with other studies
Discussion
　Huron and Raisin nutrient fluxes
　Comparisons with other studies
　Budgeting uncertainty and limitations
Conclusion
Acknowledgements
References

Table 8 Comparison of P budgets for various catchments and geographic areas after adjusting results from other studies so they are omparable to the “reference method” P budget described in this study (kg P km^-2 y-1)

Geographic unit	Rain	Fertilizer	Manure	Sludge	Point source disch.	Total P inputs	Crop export	River export	Total P outputs	％P inputs accounted for as outputs	％P inputs exported by rivers
Huron catchmenta	14	275	90	103	6	489	326	7	333	68	1.3
Raisin catchmenta	14	630	172	62	3	881	949	34	983	112	3.9
Maumee catchmentb	14	1,550	450	43	14	2,071	1,115	135	1,250	60	6.5
Sandusky catchmentb	14	1,470	360	27	10	1,881	1,175	140	1,315	70	7.4
State of Illinoisc	14	1,180	240	136	68	1,638	1,353	100	1,453	89	6.1
Muskingham catchmentd	14	322	372	151	66	925	581	61	642	69	6.6
Minnesota catchmentd	14	872	460	5	5	1,356	1,028	32	1,060	78	2.4
Mississippi basind	14	346	336	41	20	757	466	42	507	67	5.5
Sources: aThis study; bBaker and Richards(2002); cDavid and Gentry (2000); dGoolsby et al. (1999)

Table 9 Comparison of N budgets for various catchments and geographic areas after adjusting results from other studies so they are omparable to the “reference method” N budget described in this study (kg N km^-2 y-1)

Geographic unit	Atmos. dep.	Fert. app.	N fixation	Food & feed import	Total N inputs	Voltal.	Food & feed export	River export	Total N outputs	％N inputs accounted for as outputs	％N inputs exported by rivers
Huron catchmenta	954	1,846	1,586	-	4,386	37	659	288	984	22	6.6
Raisin catchmenta	954	4,171	2,663	-	7,788	55	4,928	1,268	6,250	80	16.3
Mohawk catchmentb	1,130	411	1,309	624	3,474	56	-	795	851	24	22.9
Rappahannock catchmentb	1,028	1,030	1,716	607	4,381	134	-	470	604	14	10.7
All NE U.S. catchmentsb	1,067	474	907	748	3,196	108	-	718	826	26	22.5
State of Illinoisc	520	5,930	3,050	-	9,500	77	6,240	1,670	6,407	68	17.6
Sources: aThis study; bBoyer et al.(2002); cDavid and Gentry (2000)

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