Regional Effects of Agricultural Conservation Practices on Nutrient Transport in the Upper Mississippi River Basin

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1 This is n open ccess rticle published under n ACS AuthorChoice License, which permits copying nd redistribution of the rticle or ny dpttions for non-commercil purposes. pubs.cs.org/est Regionl Effects of Agriculturl Conservtion Prctices on Nutrient Trnsport in the Upper Mississippi River Bsin An Mrí Grcí,*, Richrd B. Alexnder, Jeffrey G. Arnold, Lee Norfleet, Michel J. White, Dle M. Robertson, nd Gregory Schwrz U.S. Geologicl Survey, 3916 Sunset Ridge Rd., Rleigh, North Crolin 02906, United Sttes U.S. Geologicl Survey, 432 Ntionl Center, Reston, Virgini 20192, United Sttes U.S. Deprtment of Agriculture, Agriculturl Reserch Service, Grsslnd Soil nd Wter Reserch Lbortory, 808 Est Blcklnd Rd. Temple, Texs 76502, United Sttes U.S. Deprtment of Agriculture, Nturl Resources nd Conservtion Service, 101 Est Blcklnd Rd. Temple, Texs 76502, United Sttes U.S. Geologicl Survey, 8505 Reserch Wy, Middleton, Wisconsin 53562, United Sttes S Supporting Informtion * ABSTRACT: Despite progress in the implementtion of conservtion prctices, relted improvements in wter qulity hve been chllenging to mesure in lrger river systems. In this pper we quntify these downstrem effects by pplying the empiricl U.S. Geologicl Survey wter-qulity model SPARROW to investigte whether sptil differences in conservtion intensity were sttisticlly correlted with vritions in nutrient lods. In contrst to other forms of wter qulity dt nlysis, the ppliction of SPARROW controls for confounding fctors such s hydrologic vribility, multiple sources nd environmentl processes. A mesure of conservtion intensity ws derived from the USDA-CEAP regionl ssessment of the Upper Mississippi River nd used s n explntory vrible in model of the Upper Midwest. The sptil pttern of conservtion intensity ws negtively correlted (p = 0.003) with the totl nitrogen lods in strems in the bsin. Totl phosphorus lods were wekly negtively correlted with conservtion (p = 0.25). Regionl nitrogen reductions were estimted to rnge from 5 to 34% nd phosphorus reductions from 1 to 10% in mjor river bsins of the Upper Mississippi region. The sttisticl ssocitions between conservtion nd nutrient lods re consistent with hydrologicl nd biogeochemicl processes such s denitrifiction. The results provide empiricl evidence t the regionl scle tht conservtion prctices hve hd lrger sttisticlly detectble effect on nitrogen thn on phosphorus lodings in strems nd rivers of the Upper Mississippi Bsin. INTRODUCTION Quntifying the environmentl benefit of griculturl conservtion prctices hs been priority to stkeholders nd stewrds of rivers, lkes nd esturies tht hve been deteriorted by the intensifiction of griculturl production in the United Sttes. Agriculturl conservtion progrms, which rnge from voluntry technicl ssistnce only to pymentbsed voluntry nd cross-complince progrms, hve been implemented since the Food Security Act of with n erly focus on the vibility of griculturl production through soil conservtion. The Frm Security nd Rurl Investment Act of substntilly incresed the level of public funding for conservtion nd initited the gol of mximizing environmentl benefit.3,4 Subsequently, the Conservtion Effects Assessment Project (CEAP) ws estblished to provide science-bsed guidnce on the best use of funding for This rticle not subject to U.S. Copyright. Published 2016 by the Americn Chemicl Society conservtion nd to fcilitte the lignment of conservtion progrms with ntionl environmentl protection priorities such s the restortion of the Gulf of Mexico. Much of the experimentl reserch in conservtion documents locl (field nd frm scle) benefits of conservtion prctices5,6 but broder off-frm effects hve been more difficult to observe.7 Reviews of 14 Agriculturl Reserch Service (ARS) benchmrk studies8 nd the 13 Ntionl Institute of Food nd Agriculture wtershed studies9 document the findings of few wtershed ( km2) experimentl studies tht demonstrte improvements in stremwter qulity ttributble to Received: Revised: Accepted: Published: 6991 July 21, 2015 My 11, 2016 My 31, 2016 My 31, 2016 DOI: /cs.est.5b03543

2 Environmentl Science & Technology conservtion prctices. Exmples include pired wtershed experimentl pproch tht found tht nutrient mngement reduced nitrte concentrtions in tile drined wtersheds by 30% in smll wtersheds ( 4 8 km2) in Iow. 10 Kuhnle et l. 11 documented 60% reduction in sediment lods over 17-yr time period in n experimentl ctchment in northern Mississippi ( 21 km 2 ) where 20% of croplnd hd been converted to permnent cover by enrollment in the Conservtion Reserve Progrm. Literture reviews hve documented decreses in prticulte phosphorus losses with erosion control nd increses in soluble phosphorus vilbility with some structurl prctices, such s conservtion tillge. 6,12 Overll, few wtershed experimentl or monitoring studies hve demonstrted improvements in wter qulity tht could be ttributed directly to the implementtion of conservtion prctices. 9 At regionl scles (> km 2 ) trend nlyses of wter qulity records t monitored outlets of lrge, predominntly griculturl wtersheds hve shown both increses nd decreses in nutrients during time periods when conservtion prctices were dopted. For exmple, Sprgue et l. 13 found tht flow-normlized strem nitrte concentrtions hd incresed t most sites in the Upper Mississippi during time spn tht included some of the lrgest increses in conservtion funding ( ); however, this time period lso included incresed fertilizer consumption 14,15 nd substntil hydrologic modifiction by rtificil dringe. 16 By contrst, Murphy et l. 17 found recent ( ) decreses in in flow-normlized strem nitrte concentrtions for the Iow nd Illinois Rivers. A key limiting fctor of most of these environmentl nd observtionl studies is the inbility to mesure nd control for simultneous processes such s multiple nutrient sources, trnsport processes, nd hydrologicl vribility. 8 To ddress limittions of observtionl studies nd trend nlysis we present n lterntive empiricl methodology to infer whether conservtion prctices hve hd relized nd sttisticlly detectble effect on nutrient lods in lrge, predominntly griculturl wtersheds of the Upper Mississippi River Bsin. We pply the U.S. Geologicl Survey (USGS) SPARROW (Sptilly Referenced Regression on Wtershed Attributes) model 18,19 to investigte the correltions between wter qulity chnges cross multiple ( 700 for the upper midwest) monitoring sttions nd dopted conservtion prctices. The SPARROW model ws developed to extend regression-bsed dt nlysis by introducing sptilreferencing methodology tht llowed spce-for-time substitution, nd therefore contrst trend studies tht rely on inferences regrding chnges over time. The methodology improves the correltion of explntory vribles with wter qulity mesurements over nonsptilly referenced dt nlysis, thereby improving the bility to discern cusl influences. We employ dt nd frm-scle process modeling tht ws previously developed for the regionl Upper Mississippi River Bsin CEAP ssessment. 20 This includes the Nturl Resources Inventory (NRI)-CEAP Croplnd Survey, 21 n unprecedented documenttion of the loction, extent, nd types of conservtion prctices tht hve been implemented. As prt of the regionl CEAP ssessment, the survey dt were represented in field-scle simultion models using the Agriculturl Productivity nd Extension (APEX) model 22 incorporting the best-vilble process knowledge of the expected effects on nutrient edge-of-field delivery. This 6992 pproch llowed for multiple conservtion prctices to be summrized nd ggregted into nutrient-specific, integrted indices of conservtion ctivities on frms while ensuring tht specific frm-level prctices were not disclosed, in complince with disclosure cluses, specificlly Section 1619 of the Food, Conservtion, nd Energy Act, The indices of conservtion intensity derived from CEAP conservtion modeling were incorported into SPARROW model of the Upper Mississippi River Bsin to sttisticlly test for the downstrem effects of estblished prctices on nitrogen nd phosphorus trnsport in strems. The Agriculturl Reserch Service (ARS) benchmrk studies 8 nd the Ntionl Institute of Food nd Agriculture wtershed studies 9 lso documented the findings of wtershed modeling nd forecsted improvements in stremwter qulity. Most of these studies pplied priori scientific understnding of the physicl system to project response nd provide technologiclly fesible conservtion effects. Yet, the level of complexity fforded by process models cn led to uncertinties in the linkges estblished between conservtion prctices nd instremwter qulity response given. 8,23 The CEAP ssessment 19 for the Upper Mississippi River Bsin coupled the APEX-predicted nutrient lods from cultivted croplnd with the Soil nd Wter Assessment Tool (SWAT) to physiclly represent how prctices function in gronomic systems nd would impct downstrem wter qulity in lrge river bsins. However, it is not currently known whether the downstrem benefits forecsted by process models hve in fct been relized in the wtersheds of the Upper Mississippi River Bsin. Studies tht further integrte observtionl dt nd modeling hve been dvocted to dvnce conservtion science nd policy. 8,24,25 With the complementry ppliction of process understnding in the APEX modeling, conservtion dt derived from the CEAP croplnd survey nd the empiricl ssessment frmework in SPARROW we investigte the mesured effect if ny of griculturl conservtion prctices, including those tht were not strtegiclly designed for environmentl benefit. With this, we intend to fill n importnt gp in conservtion reserch nd provide regionl-scle empiricl ssessment of conservtion effects nd n estimte of the mgnitude of downstrem impcts. MATERIALS AND METHODS We perform n inference nlysis by evluting the empiricl ssocitions tht result from the inclusion of conservtion informtion into regression-bsed SPARROW nlyses of nutrient trnsport in the Upper Mississippi River Bsin. The SPARROW model is nonliner lest-squres multiple regression on ctchments of hydrologic frmework to solve mthemticl expression of constituent mss. Men nnul instrem constituent lod t the outlet of ctchment, i, is expressed s function of lndscpe nd in-strem chrcteristics such tht, L = A( δ X ) L + A ( δ X ) α S exp( θ Z ) i k k ik j j Ji k k ik n n ni m nm nmi where S ni is n explntory vrible representtive of direct nutrient source, indexed by n, hving source coefficient α n, Z nmi is source-specific uplnd or lnd-to-wter delivery explntory vrible, indexed by m, medited by source/delivery-vriblespecific coefficient θ nm, nd the functionl term, A( ), ccounts (1) DOI: /cs.est.5b03543

3 Environmentl Science & Technology Figure 1. Mp of mjor river bsins (4-digit HUCs) in the Upper Mississippi River Bsin (UMRB) nd distribution of wter qulity monitoring sites used for SPARROW. Inset: Sptil extent of the Upper Midwest SPARROW models. for in-chnnel nd reservoir processing, depending on set of k-indexed ttenution vribles, X ik, nd ssocited coefficients, δ k. The functionl term, A, represents ttenution pplied to lod entering the rech network t ctchment i, nd is evluted s A if rech i is strem, nd equl to A if rech i is reservoir. The term Σ j Ji L j corresponds to constituent lod leving the set of reches J i, directly upstrem of rech i. In model estimtion, if ny of the contributing upstrem reches re monitored, those reches use the monitored vlue of lod to represent the lod from tht rech; otherwise, for predictions or for unmonitored upstrem reches, the contributing lod is estimted using the modeled lod given by eq 1. If rech i is monitored, residul in logrithm spce, ϵ i, my be determined from monitored lod, L i 0, nd predicted lod, L i, given by eq 1 such tht ϵ i =lnl i 0 ln L i. The model is estimted using nonliner optimiztion methods to determine the vlues of the coefficients α n, θ nm nd δ k tht minimize the sum of squred residuls cross ll monitored sites. The residuls re ssumed to be independent, identiclly distributed, nd hve zero men. Further detils on the theoreticl development of the SPARROW model re provided by Smith et l. 18 nd Schwrz et l. 19 The strting frmework for this study ws bsed on previously published SPARROW models 24 developed for the U.S. portion of the Gret Lkes, Upper Mississippi, Ohio, nd Red River Bsins, km 2 re in the Upper Midwest, represented by pproximtely ctchments of 1 to 6993 DOI: /cs.est.5b03543

4 Environmentl Science & Technology Tble 1. Estimted Source (α n ) Lnd-to-Wter (θ nm ) nd in-chnnel (δ k ) Coefficients for SPARROW Nitrogen Models with nd without Conservtion Vrible SPARROW without conservtion vrible SPARROW with conservtion vrible explntory vrible b coefficient c stndrd error p d coefficient c (95% CI) stndrd error p Source point source (kg/yr) ( ) tmospheric deposition (kg/yr) ( ) croplnd, UM (km 2 ) ( ) croplnd, UMRB (km 2 ) e ( ) mnure from confined sources, UM (kg/yr) ( ) mnure from confined source, UMRB (kg/yr) e ( ) Lnd-to-wter Delivery conservtion intensity (kg/km 2 ) 6.4 ( ) frction of ctchments with tiles ( ) dringe density (km/km 2 ) ( ) temperture (deg. C) ( ) cly content (%) ( ) precipittion (mm) ( ) In-Chnnel Processing loss in smll strems (m 3 /s) ( ) loss in medium strems (m 3 /s) ( ) loss in reservoirs (yr/m) ( ) Model Dignostics number of sites, UM number of sites, UMRB RMSE of residuls R 2 of nutrient yield eigenvlue spred Vribles significnt t the 1% significnce level; UMRB, Upper Mississippi River Bsin; UM, Upper Midwest dringe re exclusive of the Upper Mississippi River; CI, confidence intervl. Descriptions of the dt sets used to derive the explntory vribles re provided s Supporting Informtion (Tble S.1.). c Coefficient units re inverse of explntory vrible units. d Reported p-vlues re for single-tiled t test for source, chnnel trnsport, nd reservoir-loss coefficients nd two-tiled test for lnd-to-wter coefficients. Source terms tht were ssocited with conservtion scle hydrologic frmework. The models used nonliner lest-squres regression methods to obtin coefficient estimtes tht minimize the squred residuls implied by the model in eq 1. The response vribles in the regressions were men nnul lods t 708 wter qulity sttions for nitrogen nd 810 for phosphorus. Robertson nd Sd 26 compiled dt collected from 1970 to 2006 (with most wter-qulity records spnning the period 1980 to 2004) t sptil density of pproximtely 18 wter qulity monitoring sites per 4-digit Hydrologic Unit Code (HUC) (Figure 1). Nutrient lods were estimted using rting curve estimtion methods nd detrended to represent long-term men nnul conditions centered t 2002, removing the potentilly confounding effects of intr- nd inter- nnul vritions in climte nd hydrology. 25 Further informtion on the previously published Upper Midwest models including the sptil dt sources used s explntory vribles in the models, model robustness discussion nd comments on the use of rting curve estimtion methods re provided in the Supporting Informtion (SI). In order to move the descriptive exercise represented by the published models into the experimentl frmework needed for this study, modifictions were mde to explntory vribles to ccommodte informtion on griculturl conservtion prctices. The STATSGO erodibility fctor (K-fctor) ws dded s n explicit representtion of erosion, n importnt feture given tht mny conservtion prctices hve been developed to support erosion control. The explntory vrible for tile dringe flow ws removed from the published Upper Midwest; the estimted coefficient ssocited lower phosphorus delivery with higher tile-dringe intensity s limited surrogte for erosion processes. This chnge did not necessrily remove the representtion of tile-flow from the models. In contrst to mechnistic models, where explicit representtion is needed to fully ccount for prticulr trnsport mechnism, the empiricl nture of SPARROW implies tht estimted coefficients implicitly ggregte the effect of multiple trnsport processes tht re sptilly correlted with strem nutrient lods. The effects of processes tht re not explicitly described by the explntory vribles of the model nd re uncorrelted with nutrient lods re relegted to the sptilly explicit model error term nd quntified s prt of the prediction uncertinties. As detiled lter in this section, informtion bout conservtion prctices were obtined for crop griculture in the Upper Mississippi River Bsin (UMRB), which is 35% of the Upper Midwest (UM) study re (Figure 1). Conservtion prctices were incorported into the model s lnd-to-wter delivery vribles. To ccommodte this it is necessry to develop regionlly strtified SPARROW model specifiction. This llows for different griculturl source coefficients between the UMRB nd remining portions of the UM bsin. The 6994 DOI: /cs.est.5b03543

5 Environmentl Science & Technology Tble 2. Estimted Source (α n ) Lnd-to-Wter (θ nm ) nd in-chnnel (δ k ) coefficients for SPARROW Phosphorus Models with nd without Conservtion Vrible SPARROW without conservtion vrible SPARROW with conservtion vrible explntory vrible b coefficient c stndrd error p coefficient c (95% CI) stndrd error p Source point source (kg/yr) ( ) urbn nonpoint (km 2 ) ( ) 14.5 fertilizer,um (kg/yr) ( ) fertilizer,umrb (kg/yr) e ( ) mnure from confined sources, UM (kg/yr) ( ) mnure from confined sources, UMRB (kg/yr) e ( ) mnure from unconfined sources (kg/yr) ( ) forest nd wetlnds (km 2 ) ( ) 2.24 Lnd-to-wter Delivery conservtion intensity (kg/km 2 ) 1.49 ( ) soil permebility ( ) K fctor ( ) In-Chnnel Processing loss in smll strems (m 3 /s) ( ) loss in medium strems (m 3 /s) ( ) loss in reservoirs (m/yr) ( ) Model Dignostics number of sites, UM number of sites, UMRB RMSE of log-trnsformed residul R 2 of nutrient yield, UM model eigenvlue spred Vribles significnt t the 1% significnce level; UMRB, Upper Mississippi River Bsin; UM, Upper Midwest dringe re exclusive of the Upper Mississippi River; CI, confidence intervl. Descriptions of the dt sets used to derive the explntory vribles re provided s Supporting Informtion (Tble S.1.). c Coefficient units re inverse of explntory vrible units. d Reported p-vlues re for single-tiled t test for source, chnnel trnsport, nd reservoir-loss coefficients nd two-tiled test for lnd-to-wter coefficients. Source terms tht were ssocited with conservtion. estimtes of the coefficients is sttisticlly informed by subset of the observed lods from monitoring sttions locted in the UMRB (252 for totl nitrogen nd 324 for totl phosphorus). The remining model coefficients re constrined to hve the sme vlues in both the UMRB nd remining res of the UM bsin. The modified nitrogen model contins four crop griculture-relted vribles: croplnd (UM), croplnd (UMRB), mnure from confined sources (UM) nd mnure from confined sources (UMRB). To limit correlted terms we retined croplnd s n explntory vrible for fertilizer loss nd nitrogen fixtion by legumes in crop rottions. We term the modified Upper Midwest models, SPARROW models without conservtion vrible (columns 1 3, Tble 1 nd Tble 2) lthough we hypothesized tht conservtion effects if ny, were contined in the ggregte of estimted coefficients for croplnd griculture. The models re the comprison bsis for the empiricl test of correltion of conservtion prctices with strem nutrient lods. To perform this test, conservtion intensity vrible ws dded s lndto-wter delivery term in the SPARROW model to be treted s intensive trnsport properties in SPARROW, similr to soil chrcteristic. Lnd-to-wter delivery explntory vribles nd estimted coefficients (Z nmi nd θ nm in eq 1) estblish ssocitions to climtic, nturl or nthropogenic lndscpe processes tht ffect contminnt trnsport to strems in the rech network. A positive coefficient indictes tht ctchments 6995 with lrge vlues of the ssocited lnd-to-wter vrible hve n incresed rte of delivery of nutrient source (S ni ) to the strem, s expressed by the α n exp(σ m θ nm Z nmi ) term in eq 1. Therefore, the mgnitude of ny effect of conservtion vrible on nutrient lod ws not presumed but ws insted inferred from the estimtion process. We used conservtion vrible tht ws sptilly explicit representtion of informtion obtined through the NRCS- CEAP regionl ssessment strtegy. 20,27,28 The NRCS-CEAP regionl ssessment strtegy involved developing predictive scenrios tht represented the frmer prctice informtion obtined through the NRI-CEAP Croplnd Survey. 21 The survey results were modeled with the Agriculturl Productivity nd Extension (APEX) model 22 which simulted bseline of edge-of-field loss of phosphorus, nitrogen, pesticides nd sediment from the documented frming ctivities. A second APEX modeling scenrio (i.e., no prctice) simulted frming ctivities without the voluntry conservtion prctices nd incentives tht re included in the bseline scenrio. For exmple, for the no-prctice scenrio (Figures S1-b nd S1-e in SI), structurl best mngement prctices such s reduced tillge, grss terrces, grss wterwys, riprin buffers were removed nd formerly cultivted lnd tht is currently retired ws brought bck into griculturl production. The results were ggregted to the HUC-8 level nd shred with the USGS for DOI: /cs.est.5b03543

6 Environmentl Science & Technology Figure 2. Sptil distribution of () croplnd, the explntory conservtion intensity vrible for (b) totl nitrogen (TN) nd (c) totl phosphorus (TP) for the Upper Mississippi River Bsin. Intervl bounds re quntiles of the distribution. this nlysis. Further detils on the CEAP modeling frmework nd the HUC-8 APEX model results re provided in the SI. The conservtion vrible ws computed s the reltive difference between the APEX lods for the bseline nd the noprctice scenrio (SI Figures S1-c nd S1-f) such tht, Δ P = ( P* P)/ P k k k k (2) where P k * is the nutrient lod for the k th HUC-8 for the no prctice scenrio in mss units (SI Figure S2-b nd S2-e, Tble S7, columns 5 nd 6); P k is the lod for the bseline scenrio, lso in mss units (SI Figure S2- nd S2-d; Tble S7, columns 2 nd 3). To llocte the dt to the Upper Midwest SPARROW hydrologic frmework, we clculted nutrientspecific conservtion intensity vrible, Z CP,i defined s Z CP, i Acropi = ΔPk Acropk (3) where A cropi /A cropk is the rtio of croplnd the i th SPARROW ctchment to re in croplnd within the k th HUC-8. The conservtion intensity vrible, s defined by eq 3 (Figure 2b nd c) is dependent on both process-model predictions of conservtion effectiveness nd the synthesis of conservtion implementtion from gthered survey dt. Therefore, higher vlues re ssocited with both higher expected effectiveness nd higher levels of overll implementtion. Using the reltive difference between modeled scenrios minimizes the dependence on ssumptions nd uncertinties relted to the APEX model development nd clibrtion. To illustrte this, we cn conceptulize the APEX scenrios s simple loding models where lod P = SLC, with S re nutrient inputs, L represent trnsport processes tht embed process understnding, such s lgorithms tht ccount for tile-dringe, nd C n impct fctor relted to conservtion-specific ctivities. The reltive difference between APEX edge-of-field lodings for the bseline scenrio, P o nd the no-prctice scenrio, P* (SI Figures S1-c nd S1-f) becomes Δ P= ( LSC * * LS)/ LS = ( SC * * S)/ S (4) The ctul specifiction of APEX loding is more complicted thn this simple description yet we cn expect tht some multiplictive effect of process understnding, ssuming they re equl in both scenrios, drop out (L drops out in eq 2) nd the quntity depends primrily on reltive source inputs nd conservtion prctice. Becuse of the empiricl nture of SPARROW, correltions between the conservtion vrible nd other explntory vribles with similr sptil geogrphy could limit the interpretbility of the empiricl experiment. The correltion to croplnd griculture is smll (Person correltion = 0.1 p < for totl nitrogen (TN) nd 0.2, p < for totl phosphorus (TP)). We further ssessed these issues by performing multivrite stndrd regression on ll trnsport vribles present in SPARROW models for the centrl U.S.: (1) the published Upper Midwest Model, (2) the Mississippi nd Atchfly River Bsin model. 29 The residuls of the multivrite regression, which we here term conservtion vrible residuls were used s explntory vribles in SPARROW models (Tbles S.5 nd S.6 in the SI). The sttisticl significnce of the correltion between the conservtion vrible residuls nd in-strem lods ws essentilly the sme s the results in Tble 1: p = for both nitrogen SPARROW models nd residul of the phosphorus conservtion intensity vrible ws found to remin sttisticlly insignificnt (p = 0.25 to p = 0.51). We conclude tht 6996 DOI: /cs.est.5b03543

7 Environmentl Science & Technology Figure 3. Comprison for 4-digit HUC of nutrient reductions ttributed to conservtion. multicollinerity does not impede process interprettions of results obtined in the estimtion: model sttistics nd estimted coefficients re essentilly the sme for nitrogen nd phosphorus models when correltions were removed. RESULTS AND DISCUSSION The conservtion intensity explntory vrible ws found to be inversely correlted with totl nitrogen delivery to strems s indicted by sttisticlly significnt (p = 0.003) nd negtive lnd-to-wter delivery coefficient, indicting tht higher levels of conservtion intensity re ssocited with lower levels of nitrogen delivery to nerby strems (columns 4 6, Tble 1). For totl phosphorus the inclusion of the conservtion vrible lso resulted in negtive lnd-to-wter coefficient, but the sttisticl significnce is wek by conventionl sttisticl mesures (p = 0.25) (columns 4 6, Tble 2). In fct, the confidence intervl includes positive numbers ( ) implying tht the men coefficient is sttisticlly indistinguishble from zero. Explntory vribles in Tbles 1 nd 2 differ in units nd the estimted coefficients re not stndrdized nd not directly comprble. The mgnitude of the conservtion effects ssocited with the estimted coefficients ws evluted t regionl scle nd is presented in the next section. The SPARROW with conservtion vrible reinterprets the observed Dtset with n explicit representtion of conservtion effects. Adding the conservtion intensity vribles to SPARROW did not fundmentlly chnge model interprettion: explntory vribles ccount for 86% of the vrince in the logrithm of observed nitrogen yield nd 74% of the phosphorus yield vribility. Explntory vribles representing nongriculturl sources nd trnsport properties, unrelted to conservtion prctices remined unchnged. The eigenvlue spred of ech model incresed slightly but remined well 6997 DOI: /cs.est.5b03543

8 Environmentl Science & Technology under 100 indicting tht multicolinerity would not limit interprettion. Our finding of sttisticlly significnt negtive correltion between strem nitrogen lods nd conservtion intensity is consistent with the hydrologicl nd biogeochemicl processes on lnd surfces nd in the subsurfce tht cn potentilly interct with conservtion prctices to yield nitrogen reductions in strems. The structurl nd erosion control prctices tht hve been implemented in the Upper Mississippi River Bsin hve been shown to reduce runoff nd pek flows, incresing wter infiltrtion nd soil wter holding cpcity. 6 Rective forms of nitrogen (mmoni, nitrte) re redily vilble nd highly mobile in nitrogen-enriched griculturl soils where high rtes of nitrogen minerliztion nd nitrifiction fvor dissolved inorgnic over orgnic, sediment-bound nitrogen. 30 The routing of lrger quntities of wter to the subsurfce by conservtion prctices contributes to incresed hydrulic storge tht cn led to higher denitrifiction rtes 31,32 nd reductions in nitrogen delivery to strems when compred to equivlent res without conservtion prctices. In res where extensive tile dringe short circuits nturl flow pths nd contributes to lrger nitrte lods in strems, 33,34 structurl conservtion prctices, such s strem riprin buffers, cn lso increse hydrulic storge nd reduce nitrogen delivery to strems Soil conditions conducive to denitrifiction hve been observed in the shllow subsurfce throughout the Upper Mississippi region 38,39 but substntil leching to groundwter hs lso been noted, which cn dely the delivery of nitrogen to strems by severl decdes (10 40 yers). 40,41 Therefore, the effectiveness of conservtion prctices in reducing nitrogen delivery to strems is highly dependent on subsurfce hydrologicl nd biogeochemicl conditions tht fvor the permnent removl of nitrogen vi denitrifiction nd/or pprecibly dely the trnsport nd delivery of nitrogen to strems by leching to deep ground wters. The wek negtive correltion between conservtion intensity nd strem phosphorus lods my be explined by the effects of severl key fctors. First, the downstrem response to conservtion-relted reductions in prticulte phosphorus 6 is subject to long time lgs: sediment-bound phosphorus cn be stored on frm fields nd cn tke decdes ( 25 yers) to move downstrem s prticles re repetedly deposited nd resuspended, which cn dely mesurble instrem responses to chnges in uplnd prticulte phosphorus delivery. 42 In our nlysis, observing the effects of conservtion prctices on prticulte phosphorus my be limited by the use of stremwter qulity dt detrended to bse yer (2002) tht is pproximtely coincident with the time period represented by the conservtion intensity mesure. Additionl yers of observtions tht cn support the use of lter bse yer for detrending (post 2005) in SPARROW could provide more complete evlution of lgged conservtion response in strem prticulte phosphorus lods. Second, some erosion control prctices, specificlly, no-till nd reduced tillge hve been shown to increse soluble phosphorus levels in frm runoff, 12,43 which cn potentilly offset expected downstrem benefits derived from conservtionrelted reductions in prticulte phosphorus. In soils with notill nd other forms of reduced tillge where there is less verticl mixing of the soil nd integrtion of phosphorus fertilizers, phosphorus cn ccumulte ner the surfce, where desorption processes cn led to elevted levels of soluble phosphorus in overlnd runoff. 12 Reduced tillge cn lso fcilitte the 6998 development of soil mcropores with connections to tile dringe systems tht speed the delivery of soluble phosphorus to strems. 43 The vilbility of soluble phosphorus is lso enhnced in frm soils where the phosphorus sorption cpcity hs been sturted by legcy pplictions of phosphorusenriched mnure nd fertilizers. 44 With substntil frction of totl phosphorus delivered in soluble form, nd incresingly vi subsurfce pthwys, mny conservtion prctices designed to remedite erosion loss hve limited effectiveness t controlling dissolved phosphorus losses. This collection of confounding processes my explin the lck of sttisticl evidence in the SPARROW nlysis of the effects of conservtion prctices on totl phosphorus in strems of the Upper Mississippi. Predictions of Regionl-Scle Conservtion Effects. To quntify the mgnitude of possibly relized conservtion effects in mjor rivers in the UMRB, nutrient lods were predicted by using the estimted coefficients from the SPARROW with conservtion columns (Tbles 1 nd 2) nd Eqution 1. Conservtion effects were subtrcted from the SPARROW strem lods bsed on n ssumed no prctice condition where the conservtion intensity vrible ws set to reflect the bsence of conservtion prctices (Z cp, i = 0). A prmetric bootstrp tht presumes normlity for ll model coefficients ws performed to provide 90% confidence intervls for the SPARROW predictions. The results were then summrized to the 4-digit HUC level by including the effects of nutrient processing during in-strem trnsport nd computing lods delivered to the outlet of the bsin (Figure 3). The SPARROW predictions re presented longside HUC- 4 conservtion effects predictions documented in the Upper Mississippi CEAP 20 ssessment. The CEAP results were developed using the Soil nd Wter Assessment Tool 45 to route APEX-derived croplnd lods downstrem nd simulte nonpoint source lodings from lnd uses other thn croplnd. While the comprison presented in Figure 3 is of two differing ssessment frmeworks, it llows us to mke severl conclusions. Our process inferences regrding the conservtion impct of nitrogen re explicitly represented in the mechnistic models. Most of the reduction presented by the CEAP ssessment is the result of representing hydrologic modifictions, such s chnges in soil moisture content nd relted effects on nitrogen cycling processes including enhnced denitrifiction processes. The close greement in predictions of nitrogen lod reductions with the two pproches is likely relted to the high mobility of rective nitrogen tht is fvored in griculturl systems, especilly those with tile dringe, which leds to rpid downstrem wter qulity response. Thus, the results cn be considered to provide limited vlidtion of existing process understnding nd we isolte possible benefit of conservtion: n overll increse in hydrulic storge, which when coupled with denitrifiction could led to permnent nitrogen losses tht reduce nitrogen delivery to strems. Although the predictions for totl phosphorus reductions differed substntilly (Figure 3b) they illustrte the difference in ssessment pproches. The wek empiricl ssocition obtined with SPARROW led to the minor reductions: on verge 4% reduction t mjor river bsin outlets with some lower bounds for the 90% confidence intervl below 0. The verge reduction for the CEAP pproch ws 30%, with significnt portion ttributed to nutrient mngement. The Upper Mississippi croplnd CEAP survey documented substntilly more phosphorus mngement thn for nitrogen. 20 The process models simulte nutrient reductions s DOI: /cs.est.5b03543

9 Environmentl Science & Technology immedite nd sustined which leds technologiclly fesible forecst of wter qulity improvements. With the SPARROW nlysis n ssessment of lredy relized benefits during the 1990s nd erly 2000s, it is not possible to forecst response tht is not represented in the observtionl dt set. Nutrient mngement ws fully incentivized in the Frm Security nd Rurl Investment Act of 2002 through the Environmentl Qulity Incentives Progrm. In comprison to well estblished soil mngement prctices tht hve been lndscpe fetures for yers, if not decdes, nutrient mngement prctices hve dynmic implementtion ptterns tht would led to lgs in mesured effects. Therefore, it is possible tht dditionl wter qulity records, beyond the erly 2000s, my be needed to ctully observe the downstrem benefits of the mnged phosphorus reductions. Our combined process-bsed nd empiricl ssessments compliment temporl nlyses of ggregted wter qulity monitoring records by providing n cross the lndscpe exercise ssessment tht leverges spce over time nd provides some degree of experimentl control. The process knowledge contined in the field-scle APEX lods fcilitted the development of nutrient-specific, integrted mesures of multiple conservtion ctivities on frms tht llowed n empiricl regionl-scle ssessment of both our process-bsed understnding conservtion effects nd documented levels of doption nd implementtion. Applictions of this methodology in other environmentl settings re needed to further evlute the impcts of conservtion prctices. With this study we identify mitigting effect of conservtion prctices for the delivery of totl nitrogen strems. However, these reltively short-term benefits re potentilly offset in future yers by the influence of deep leching tht introduces delys in the delivery of nitrogen to strems nd postpones the impcts of griculture on wter qulity. For phosphorus, the chemistry of griculturl soils nd more recent reserch on soluble phosphorus mobility nd trnsport re leding to shifts in understnding tht re either not fully incorported in process modeling or modeled with uncertinties. Obtining n improved understnding of the reltive importnce of these different processes in U.S. wtersheds is needed to optimize the mngement of nutrients for environmentl benefits. ASSOCIATED CONTENT *S Supporting Informtion The Supporting Informtion is vilble free of chrge on the ACS Publictions website t DOI: /cs.est.5b Additionl detils on modifictions to previously published SPARROW models, comments on the use of men nnul nutrient lods, description of CEAP conservtion effects dt sets nd nutrient lods from APEX models for scenrios (PDF) AUTHOR INFORMATION Corresponding Author *Phone: ; fx: ; e-mil: grci@ usgs.gov. Author Contributions The mnuscript ws written through contributions of ll uthors. All uthors hve given pprovl to the finl version of the mnuscript. Notes The uthors declre no competing finncil interest. ACKNOWLEDGMENTS This reserch ws funded by the U.S. Geologicl Survey Ntionl Wter Qulity Progrm nd the U.S. Deprtment of Agriculture Conservtion Effects Assessment Progrm. REFERENCES (1) U. S. Congress. Food security ct of 1985; 1985; p 198. (2) U. S. Congress. Frm security nd rurl investment ct of 2002; (3) Becker, G. S. The 2002 frm lw t glnce. 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