Overview of the Mark Twain Lake/ Salt River Basin Conservation Effects Assessment Project

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1 doi: /jswc Overview of the Mrk Twin Lke/ Slt River Bsin Conservtion Effects Assessment Project R.N. Lerch, E.J. Sdler, N.R. Kitchen, K.A. Sudduth, R.J. Kremer, D.B. Myers, C. Bffut, S.H. Anderson, nd C.-H. Lin Abstrct: The Mrk Twin Lke/Slt River Bsin ws selected s one of the USDA Agriculturl Reserch Service benchmrk wtersheds for the Conservtion Effects Assessment Project becuse of documented soil nd wter qulity problems nd brod stkeholder interest. The bsin is locted in northestern Missouri within the Centrl Clypn Region, nd it is the source of wter to Mrk Twin Lke, the mjor public wter supply in the region. At the outlet to Mrk Twin Lke, the bsin drins 6,417 km 2 (2,478 mi 2 ), including 10 mjor wtersheds tht rnge in re from 271 to 1,579 km 2 (105 to 609 mi 2 ). The bsin is chrcterized by flt to gently rolling topogrphy with predominnce of clypn soils tht result in high runoff potentil. The clypn soils re especilly vulnerble to soil erosion, which hs degrded soil nd wter qulity throughout the bsin, nd to surfce trnsport of herbicides. Results from cropping system best mngement prctice studies showed tht no-till cropping systems did not reduce surfce runoff compred to tilled systems, nd no-till led to incresed trnsport of soil-pplied herbicides. A mjor chllenge is the need to develop cropping systems tht incorporte herbicides yet mintin sufficient crop residue cover to control soil erosion. Results of the Soil nd Wter Assessment Tool model simultions showed tht the model ws cpble of simulting observed long-term trends in trzine concentrtions nd lods nd the impct of grss wterwys on trzine concentrtions. Current nd future reserch efforts will continue to focus on best mngement prctice studies, development of needed tools to improve wtershed mngement, nd refinements in the clibrtion nd vlidtion of the Soil nd Wter Assessment Tool model. Key words: Conservtion Effects Assessment Project (CEAP) clypn soils cropping systems simultion modeling wter qulity monitoring wtershed mngement The Slt River Bsin in northestern Missouri ws selected s one of the USDA Agriculturl Reserch Service benchmrk wtersheds for the Conservtion Effects Assessment Project (CEAP). The bsin is the source of wter to Mrk Twin Lke, 7,533-h (18,614-c) US Army Corps of Engineers reservoir tht is the mjor public wter supply in the region (figure 1). The Mrk Twin Lke/Slt River Bsin ws selected s CEAP reserch re becuse of brod stkeholder interest nd documented soil nd wter qulity problems. The bsin encompsses the hert of the Centrl Clypn Region mjor lnd resource re (MLRA 113) (USDA Nturl Resources Conservtion Service [NRCS] 2006). The nturlly formed clypn represents the key hydrologic feture of the bsin, nd it is the direct cuse of the high runoff potentil of these soils. Lnd use is predomintely griculturl within the bsin. The primry rowcrops re soyben (Glycine mx), corn (Ze mys), whet (Triticum estivum), nd sorghum (Sorghum bicolor). Forge production is minly tll fescue (Festuc rundince). Livestock production is minly beef cttle, but swine opertions re incresing. While it is widely recognized tht the conservtion progrms funded by the Frm Security nd Rurl Investment Act of 2002 referred to s the 2002 frm bill cn protect millions of cres, the environmentl benefits hve not previously been quntified. This represents the primry rtionle for the development of CEAP, which is to quntify the environmentl benefits of these conservtion progrms. The specific objectives of the Mrk Twin Lke/Slt River Bsin CEAP encompss three of the five objectives contined in the ntionl CEAP pln (i.e., dtbse development, evlution of best mngement prctices [BMPs], nd computer modeling of contminnt trnsport), with detiled reserch objectives tilored to the Slt River Bsin. Anticipted products of this reserch include estblishment of the publicly vilble Sustining the Erth s Wtersheds Agriculturl Reserch Dt System (STEWARDS), knowledge of the wtersheds contributing the highest contminnt lods to Mrk Twin Lke, nd guidelines for implementing BMPs nd trgeting conservtion prctices. However, the specific objective of this rticle is to provide context for the Mrk Twin Lke/Slt River Bsin CEAP, including the physicl nd historicl setting, primry conservtion concerns, legcy dt nd environmentl infrstructure, reserch nd modeling results, nd needed tools for improving wtershed mngement. Slt River Bsin Physiogrphic Region, Topogrphy, nd Predominnt Soils. The Slt River Bsin drins mjor portion of Northest Missouri into the Mississippi River. The bsin is locted within the Dissected Till Plins Physiogrphic Region (12e) (Fennemn nd Johnson 1946) nd is contined primrily within MLRA 113. Elevtion in the wtershed vries from 305 m (1,000 ft) bove men se level to 158 m (518 ft) bove men Robert N. Lerch, E. John Sdler, nd Newell R. Kitchen re soil scientists, Kenneth A. Sudduth is n griculturl engineer, nd Robert J. Kremer is microbiologist t the Cropping Systems nd Wter Qulity Reserch Unit, USDA Agriculturl Reserch Service (ARS), Columbi, Missouri. D. Brenton Myers is grdute reserch ssistnt in the Deprtment of Soil, Environmentl nd Atmospheric Sciences, University of Missouri, Columbi, Missouri. Clire Bffut is hydrologist t the Cropping Systems nd Wter Qulity Reserch Unit, USDA ARS, Columbi, Missouri. Stephen H. Anderson is professor in the Deprtment of Soil, Environmentl nd Atmospheric Sciences, University of Missouri, Columbi, Missouri. Chung-Ho Lin is reserch ssistnt professor in the Deprtment of Forestry, University of Missouri, Columbi, Missouri. Copyright 2008 Soil nd Wter Conservtion Society. All rights reserved. Journl of Soil nd Wter Conservtion 63(6):

2 Figure 1 Wtersheds of the Slt River Bsin. Slt River Bsin km Mrk Twin Lke Middle Fk. Slt Long Brnch Cr. Goodwter Cr. Youngs Cr. Elk Fk. Slt km Notes: Red strs indicte monitoring sttions operted by the US Monitoring Geologicl Sttions Survey. Blck strs indicte monitoring sttions operted by USDA Agriculturl Reserch Service or their coopertors. Wtershed colors re included to more esily distinguish between the wtersheds. N. Fk. Slt Blck Cr. Crooked Cr. Otter Cr. N S. Fk. Slt Ely Cr. Lick Creek Mrk Twin Lke se level just below the outlet of Mrk Twin Lke, bout 100 km (62.1 mi) upstrem from the mouth of the Slt River. The topogrphy of the Slt River Bsin is flt nd gently rolling in its upper reches (0% to 2% slope on ridges, 3% to 7% on bckslopes) but becomes steeply rolling nd deeply dissected ner the mjor tributries (2% to 5% slope on ridges, 5% to 20% on bckslopes). Clypn soils re predominnt in the Slt River Bsin. These soils re chrcterized by subsoil horizon with n brupt nd lrge increse in cly content within short verticl distnce in the soil profile (Soil Science Society of Americ 2001). The Midwestern US clypn region encompsses n re of bout h within Missouri, Illinois, nd Knss (Jmison et l. 1968; Anderson et l. 1990). The clypn depth vries from 0.1 to 0.5 m (0.3 to 1.6 ft) with cly content rnging from 350 to 600 g kg 1 (35% to 60%) (Miles nd Hmmer 1989; Blnco-Cnqui et l. 2002; USDA NRCS 2006). Smectite cly minerls with high shrink-swell potentil dominte the rgillic zone. During the winter nd spring periods, the clys re swollen, nd their low sturted hydrulic conductivity impedes infiltrtion nd perches wter bove the clypn, cusing high probbility of runoff (Blnco-Cnqui et l. 2002). There is lso high probbility of nnul shrinkge crcks forming during the lte summer nd erly fll periods. Preferentil flow through these crcks is significnt (Ber nd Anderson 1997). Stte Soil Geogrphic (STATSGO) dtbse soils for the Slt River Bsin re provided in tble 1. The clypn soils in the bsin include the Armstrong, Edin, Mexico, nd Putnm series which cover 68% of the lnd re. Other soil series include the Lindley series, which covers 22% of the bsin, is well drined nd modertely permeble, nd forms in dissected glcil till tht my hve thin loess cp; nd the Ftim series, which is 6.4% of the bsin, is modertely well drined with moderte permebility, nd forms in lluvium. The remining soils ech hve less thn 1% extent. All of the subbsins include predominnce of clypn soils except the Middle Fork subbsin which is dominted by the Lindley series (56%) (tble 1). Most soils in the bsin re clssified in hydrologic groups C nd D (USDA NRCS 2005b). Group C soils re primrily hillslope soils in dissected till. They hve slow infiltrtion rte nd moderte runoff potentil due to rgillic horizons or pleosols tht impede downwrd movement of wter. Group D soils occurring t summits hve very slow infiltrtion rte nd high runoff potentil due to clypns, s described bove. Group D soils formed in lluvium hve sesonlly high wter tble nd/or high flooding potentil. Wtershed Hydrology. The Slt River Bsin drins 6,417 km 2 (2,478 mi 2 ) t the outlet to Mrk Twin Lke, nd the bsin is comprised of 10 wtersheds designted s 11-digit hydrologic units by the US Geologicl Survey (USGS) (Seber et l. 1987) (figure 1). Dringe res of the wtersheds rnge in size from 27,070 to 157,910 h (66,900 to 390,210 c) (tble 1). The mjor wtersheds re the primry forks of the Slt River, including the North, Middle, Elk, nd South Forks, nd these four wtersheds ccount for 67% of the bsin s dringe re (s depicted in figure 1). The reminder of the dringe re is comprised of severl smller wtersheds including Blck Creek, Crooked Creek, Otter Creek, Long Brnch Creek, Lick Creek, nd Ely Creek. The Ely Creek 11-digit wtershed delinetion includes mny smll tributries drining directly to Mrk Twin Lke, nd it lso includes bout 28 km 2 (11 mi 2 ) tht re inundted by the lke. Strem orders within the bsin were determined from 7.5-minute USGS topogrphic mps by the Missouri Deprtment of Conservtion (2004). There re 165 third order nd lrger strems in the entire bsin, nd the min stem of the Slt River is seventh order strem below the dm. All the lrger scle wtershed strems included in the monitoring network (see Experimentl Infrstructure nd Legcy Dt section below) re fourth nd fifth order strems, except the North Fork, which is sixth order strem (figure 1). Chnnel grdients for the mjor strems in the upper Slt River Bsin re rther low (Missouri Deprtment of Conservtion 2004). Of the fifth order nd lrger strems in the bsin, grdients rnge from 0.7 m km 1 (3.7 ft mi 1 ) for the Slt River below the dm to 2.7 m km 1 (14.3 ft mi 1 ) in Ber Creek ( North Fork subwtershed). Although the higher order strems hve low grdients, mny third order strems in the upper portions of these wtersheds hve grdients rnging from 7 to 24 m km 1 (37 to 127 ft mi 1 ). Averge nnul precipittion since 2000 t four wether sttions locted within or ner the bsin (Agriculturl Electronic Bulletin Bord 2007) rnged from 346 Nov/dec 2008 vol. 63, no. 6

3 Tble 1 Lnd use nd soil types for the Slt River Bsin nd its mjor wtersheds. Slt River Blck Crooked Elk Ely Lick Long Middle North Otter South Bsin Creek Creek Fork Creek Creek Brnch Fork Fork Creek Fork Are (h) 591,770 27,630 28,410 72,360 27,950 35,000 46,930 88, ,910 27,070 80,200 Percent of Slt River Bsin 100% 4.7% 4.8% 12.2% 4.7% 5.9% 7.9% 14.9% 26.7% 4.6% 13.6% Lnd use Psture 32.9% 24.3% 26.5% 38.6% 25.6% 17.2% 18.5% 43.4% 41.0% 20.3% 27.6% Wter 2.5% 0.5% 0.4% 1.6% 14.1% 1.7% 1.0% 1.3% 3.3% 1.1% 1.9% Forest 18.0% 14.4% 14.5% 20.0% 27.0% 10.1% 10.9% 25.1% 19.1% 12.6% 14.6% Croplnd 44.2% 59.2% 56.0% 37.0% 31.4% 68.6% 68.1% 27.8% 34.0% 64.6% 52.9% Urbn/residentil 2.4% 1.6% 2.6% 2.8% 1.9% 2.4% 1.5% 2.4% 2.6% 1.4% 3.0% Soils* Lindley 22.5% 6.7% 8.3% 24.6% 3.6% 0.0% 3.6% 56.2% 24.4% 14.2% 21.0% Armstrong 12.3% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 12.7% 39.0% 0.1% 0.0% Weller 1.1% 0.0% 0.0% 0.0% 23.4% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% Putnm 12.0% 0.0% 0.0% 4.5% 18.4% 50.5% 49.0% 0.0% 0.8% 0.0% 25.4% Mexico 43.2% 91.8% 91.7% 61.9% 32.9% 38.9% 43.8% 22.3% 21.5% 85.1% 49.1% Brdley 1.0% 0.0% 0.0% 0.0% 7.6% 9.3% 0.0% 0.0% 0.0% 0.0% 0.0% Ftim 6.4% 1.5% 0.0% 8.8% 0.5% 0.0% 3.6% 8.7% 11.5% 0.0% 4.5% Edin 0.2% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.9% 0.0% 0.0% Other 1.3% 0.0% 0.0% 0.2% 13.6% 1.3% 0.0% 0.1% 1.9% 0.6% 0.0% * Primry soil txonomic groups: Lindley nd Brdley, Typic Hpludlfs; Armstrong, Aquertic Hpludlfs; Weller, Aquertic Chromic Hpludlfs; Putnm, Vertic Albqulfs; Mexico, Vertic Epiqulfs; Ftim, Fluvquentic Hpludolls; Edin, Vertic Argilbolls. 809 to 909 mm (32 to 36 in). These recent precipittion dt re very comprble to the historic dt for the bsin, which ws in the rnge of 889 to 940 mm (35 to 37 in) (US Army Corps of Engineers 1975). Wtershed History. Existing soil nd wter conservtion concerns in the Slt River Bsin re best understood within the context of historicl settlement ptterns of the lst two centuries. Prior to Europen settlement, Ntive Americn hbittion of the Slt River Bsin ws focused on hunting, gthering, nd some griculture cultivtion (Henning 1975). The Slt River Bsin underwent drmtic chnge with the 1803 Louisin Purchse. Compred to other prts of the Midwest, movement of settlers into the bsin ws inhibited due to poor trnsporttion routes. Gme trils were grdully worn into wgon rods during the 1820s, nd settlement becme more widespred throughout the bsin. Lnd purchse nd settlement expnded rpidly from 1827 to 1836, with ~80% of the public lnd within the bsin purchsed during this decde (O Brien 1984). Throughout most of the 1800s, homesteds were uplnd to void flooding nd in or djcent to timber, n essentil resource for building nd heting (O Brien 1984). The nerly flt pririe grsslnd ws minly used for free-rom grzing. Commerce ws initilly limited with settlers living simple lifestyle growing corn, whet, rye (Secle cerele), pumpkin nd sqush (Cucurbit spp.), grden vegetbles, nd typiclly only few livestock. From the mid to lte 1800s, lnd clering occurred for lrger fming opertions. Development of rodwys led to commerciliztion nd economic diversifiction tht included grist mills, lumber mills, brick yrds, nd csh crops such s tobcco (Nicotin spp.), hemp (Cnnbis stiv), nd cotton (Gossypium sp.) (O Brien 1984). Cttle were primrily rised nd fttened on grsslnd for the St. Louis mrket. Erly in the 1900s, the fce of the Midwest rurl lndscpe, including the Slt River Bsin, underwent mjor trnsformtion. Mny smller communities begn shrinking, frms incresed in size, nd people begn migrting to the lrger metropolitn cities (Brtton nd Smith 1928). At the sme time, improvements in griculturl mechniztion helped ffluent frmers expnd their enterprises. As frms grew, most were integrted with grin nd niml (cttle, hogs, nd sheep) production. Extensive flt grsslnds were plowed nd put into grin production for the first time, typiclly growing corn, ots (Aven stiv), whet, nd new crop, soyben. During World Wr I, corn grin prices sored, nd so did corn crege. This mjor shift in lnd use nd intensity in the erly 1900s hd n immedite impct on the rivers nd strems. Prior to the wr, much of the Slt River ws noted to hve cler, clen wter during most of the yer (Howrd 1980). Fish nd mussels were plentiful. By the 1930s, the strems were sedimentfilled nd fish were disppering. Not by coincidence, the first soil erosion plot reserch in the United Sttes ws initited in 1917 on similr soils on the cmpus of University of Missouri (~50 km [~30 mi] southest of the Slt River Bsin; Troeh et l. 1980). Grin crop yields for mny fields ctully declined when compred to the previous century (Bennett 1939). The trend of fewer nd lrger frms continued through much of the 20th century s motorized equipment incresed in size nd efficiency. In 1950, griculture employed 33% of the lbor force in the Slt River re (US Army Corps of Engineers 1975). Over the next two decdes, the number of cropped cres incresed lmost 10% while the lbor force employed by griculture decresed to less thn 15%. During the 1970s nd 1980s, mny smll to medium-sized frms bndoned niml opertions becuse of poor profitbility nd focused on grin produc- 347

4 tion. Also during this period, much higher percentge of frmlnd becme lese-frmed insted of owner-frmed. A sequence of eril photos (figure 2) obtined from the USDA Frm Service Agency rchives illustrte for typicl field in the Slt River Bsin some of the importnt chnges during the 20th century: (1) lrger fields by merging of smller fields, (2) loss of frmsteds, (3) loss of integrted grin nd niml production systems, nd (4) return of indigenous trees long wterwys nd field boundries. Current lnd use for the Slt River Bsin nd its subbsins is provided in tble 1. Primry Soil nd Wter Conservtion Concerns Surfce Wter. Strems in the bsin hve well-documented history of herbicide nd sediment contmintion problems (Ntionl Acdemy of Sciences 1986; Donld et l. 1998; Ghidey nd Alberts 1998; Blnchrd nd Lerch 2000; USDA NRCS 2000b; Lerch nd Blnchrd 2003). As previously mentioned, soils within MLRA 113 re chrcterized by the presence of nturlly formed subsurfce clypn tht cretes brrier to percoltion nd promotes surfce runoff. This results in high degree of vulnerbility to surfce trnsport of sediment nd herbicides (Ntionl Acdemy of Sciences 1986; USDA NRCS 2000b; Lerch nd Blnchrd 2003). In the erly 1980s, verge soil erosion rtes for croplnd within MLRA 113 were estimted to be 17.9 Mg h 1 (8.0 tn c 1 ) (Ntionl Acdemy of Sciences 1986), exceeding the tolernce fctor (T) of 7.6 Mg h 1 (3.4 tn c 1 ) by bout 2.4 times. More recently, the USDA NRCS used their Ntionl Resources Inventory (USDA NRCS 2000b) dt nd the Universl Soil Loss Eqution to estimte tht in the stte of Missouri 62% of croplnd within MLRA 113 still exceeded T compred to <50% of croplnd in the other intensively row cropped MLRAs of the stte (figure 3). Prtitioning of the MLRA 113 croplnd exceeding T into discrete multiples of T showed tht 53% ws in the 1T to 2T ctegory, 24% ws in 2T to 3T, 9% ws in 3T to 4T, 5% ws in 4T-5T, nd 9% ws >5T. Despite lower erosion potentil thn severl of the other MLRAs (Ntionl Acdemy of Sciences 1986), croplnd erosion in MLRA 113 remins mjor wter nd soil qulity problem for this re s whole nd for the Slt Figure 2 Aeril photos of Slt River Bsin qurter section (~65 h) showing predominnt chnges over the 20th century, including (1) smller fields combined to mke lrger fields, (2) loss of frmsteds, (3) loss of integrted grin nd niml production systems, nd (4) return of indigenous trees long wterwys nd field boundries River Bsin in prticulr. On clypn soils, number of prctices including reduced tillge, no-till, crop rottions tht include whet or grsses, nd cover crops hve been shown Figure 3 Estimted percentge of croplnd with erosion exceeding T fctor for the intensively row-cropped mjor lnd resource res of Missouri. Croplnd erosion exceeding T fctor 70% 60% 50% 40% 30% 20% 10% 0% to effectively reduce soil erosion compred to conventionl cropping systems (Jmison et l. 1968; Ghidey nd Alberts 1998), nd therefore, these conservtion prctices need Mjor lnd resource re Note: T fctor for mjor lnd resource re 113 is 7.6 Mg h 1 (from USDA NRCS 2000b). 348 Nov/dec 2008 vol. 63, no. 6

5 Figure 4 Wtershed vulnerbility to herbicide trnsport expressed s the sum of six herbicide (trzine, cynzine, cetochlor, lchlor, metolchlor, metribuzin) nd four metbolite (deethyltrzine, deisopropyltrzine, hydroxytrzine, cynzine mide) losses on treted re bsis (from Lerch nd Blnchrd 2003). Arel loss (g h 1 ) 115 to to to to 490 > km N to be more widely implemented to decrese erosion rtes. Strems within the Slt River Bsin hve frequent detections of commonly used corn nd soyben herbicides, prticulrly in the spring following herbicide ppliction to fields (Donld et l. 1998; Blnchrd nd Lerch 2000; Lerch nd Blnchrd 2003). In Goodwter Creek (figure 1), pek concentrtions of trzine nd metolchlor were observed from My to July, nd mximum reported concentrtions of trzine nd metolchlor were 62.4 µg L 1 (62.4 prts per billion [ppb]) nd 39.5 µg L 1 (39.5 ppb), respectively (Donld et l. 1998). Similr sesonl trends for herbicide contmintion in strems of the northern Missouri/southern Iow region were lso reported by Blnchrd nd Lerch (2000). The strems of MLRA 113 hd the overll highest trzine nd cynzine concentrtions observed for the northern Missouri/southern Iow region (Blnchrd nd Lerch 2000). Furthermore, MLRA 113 strems nd those strems in wtersheds with similrly high proportions of runoff-prone soils (e.g., extreme northestern Missouri) hd the highest herbicide loss, s percent of pplied, nd were mong the highest in overll vulnerbility to surfce trnsport Tble 2 Suggested indictors nd functionl relevnce for ssessing nd monitoring soil qulity on lndscpes in the Slt River Bsin. Soil qulity indictor Functionl relevnce Wter infiltrtion Runoff, leching, compction nd/or soil erosion potentil Wter-stble ggregtion Wter trnsport, microbil hbitt (nutrient trnsformtions, pesticide degrdtion), plnt root growth Bulk electricl conductivity Topsoil depth, soil erosion, soil restortion Soil orgnic mtter, soil orgnic crbon Key fctor influencing biologicl ctivity, soil structure, soil fertility, plnt nutrition Active soil crbon (prticulte orgnic mtter) Specificlly relted to microbil respirtion nd biomss, soil ggregtion Microbil biomss nd/or respirtion Biologicl ctivity, impct of mngement on soil orgnic mtter or soil orgnic crbon Soil enzyme ctivity Soil microbil ctivity, nutrient trnsformtion, mngement impcts on decomposition, xenobiotic degrdtion potentil Microbil diversity Blnce of principl nutrient cycles, multifunctionl performnce of soil Pesticide degrdtion Environmentl buffer, filtering effects, environmentl contmintion potentil 349

6 Tble 3() Infrstructure nd historicl dt vilble for the Missouri Conservtion Effects Assessment Project. Goodwter Creek Experimentl Wtershed Mrk Twin Lke/Slt River Bsin Originting project North Centrl Mngement Systems Conservtion Effects Assessment Project, Reserch Wtershed, Evlution Are nd Mrk Twin Lke/Slt River Bsin Goodwter Creek Agriculturl Systems for Experimentl Wtershed Environmentl Qulity Period 1971 to present 1990 to present 2005 to present (some records extrcted 1990 to present) Scle 13, 30, nd 72 km h plots (30 units instrumented), 72 to 6,417 km 2 wtersheds (13 units instrumented) wtersheds 20 to 36 h fields (3 units instrumented), 13, 30, nd 72 km 2 wtersheds Mesurements Strem stge nd flow Strem stge nd flow (ll scles) Stge nd flow (USDA Agriculturl Reserch Service, US Geologicl Survey) Rinfll Rinfll Rinfll (Ntionl Wether Service, USDA Frm Service Agency) Temperture mx/min Automted wether sttion: Temperture mx/min (Ntionl Wether Service) ir temperture, rinfll, humidity, solr rdition, wind direction, wind speed, soil temperture Pn evportion Wter qulity: nitrte, mmonium, Automted wether sttions: ir temperture, rinfll, ortho-phosphte, sediment, humidity, solr rdition, wind direction, wind speed, trzine, deethyltrzine, soil temperture (USDA Agriculturl Reserch Service, deisopropyltrzine, cynzine, University of Missouri) lchlor, cetochlor, metolchlor, metribuzin Wter tble elevtion Wter tble elevtion Wter qulity: nitrte, mmonium, ortho-phosphte, sediment, trzine, deethyltrzine, deisopropyltrzine, simzine, lchlor, cetochlor, metolchlor, metribuzin Sediment Sediment Cittion Sdler et l. (2006) Sdler et l. (2006) of six prent herbicides nd four metbolites (Lerch nd Blnchrd 2003) (figure 4). Becuse of this vulnerbility, trzine contmintion in Mrk Twin Lke cused it to be plced on the impired wters list (303d) by the Missouri Deprtment of Nturl Resources from 1998 to A mjor chllenge ssocited with clypn soils is the need to develop cropping systems tht concurrently fcilitte incorportion of herbicides to reduce their trnsport in surfce runoff, but mintin sufficient crop residue cover to control soil erosion. These two gols often conflict since effective erosion control mesures, such s no-till, preclude the incorportion of herbicides. Thus, herbicide ppliction methods must be found tht cn incorporte soil-pplied herbicides nd mintin residue cover to levels t or ner tht of no-till systems. Groundwter. Becuse of slow infiltrtion, wter dringe into groundwter is minor compred to surfce wter dischrge (Blevins et l. 1996; Kitchen et l. 1998). When groundwter rechrge occurs, it is primrily through preferentil pthwys such s decyed root chnnels or soil crcks tht develop during droughts (Blevins et l. 1996). Percoltion through the clypn is especilly low in spring nd erly summer (the period of most herbicide nd fertilizer pplictions) becuse the cly within the rgillic horizon hs swollen with fll nd winter precipittion (Kitchen et l. 1998). Thus, compred to other griculturl res of the US Midwest, groundwter is less vulnerble to contmintion (Power et l. 2001). Still, groundwter surveys found nitrte leching hs contminted the glcil till quifer (Lerch et l. 2005). However, herbicide leching to groundwter ws miniml in this sme setting (Blnchrd nd Donld 1997). Twenty-five percent of the wells in the Goodwter Creek wtershed were found to be contminted with nitrte t concentrtions >10 mg L 1 (Kitchen et l. 1997). The conclusion of this ssessment ws tht elevted groundwter nitrte concentrtions were the result of multiple yers of over-ppliction of N for crops, either from synthetic fertilizers, niml mnure pplictions, or both. Becuse of the buffered nture of the quifer, elevted nitrte concentrtions cn persist for decdes (Kitchen et l. 1997). Soil. The primry soil conservtion concerns of croplnd erosion nd trnsport of herbicides, nutrients, nd sediments in runoff (Lerch et l. 2005) re lso relted to degrdtion of soil qulity nd provide benchmrk for evluting mngement prctices for improving soil qulity. Soil qulity 350 Nov/dec 2008 vol. 63, no. 6

7 Tble 3(b) Infrstructure nd historicl dt vilble for the Missouri Conservtion Effects Assessment Project (continued). Time series dt Goodwter Creek Experimentl Wtershed Mrk Twin Lke/Slt River Bsin Rinfll Obtined by USDA Agriculturl Reserch Service USDA Frm Service Agency Observer network provided by USDA Ntionl Agriculturl Sttistics Service, Gene Dneks, Director ( Medi/docs/CoCoRHS_2005_Brochure_LR.pdf) Rinfll nd Obtined by USDA Agriculturl Reserch Service Ntionl Wether Service Observer network provided by temperture Ptrick Guinn (Extension ssistnt professor of climtology, mx/min Deprtment of Soil, Environmentl nd Atmospheric Sciences, University of Missouri) Automted Obtined by USDA Agriculturl Reserch Service Agriculturl Electronic Bulletin Bord provided by wether sttions Ptrick Guinn ( history/index.sp) Strem stge Obtined by USDA Agriculturl Reserch Service US Geologicl Survey ( nd flow Common ncillry dt in geogrphic informtion system lyers Digitl Center for Agriculturl, Resource nd Environmentl Systems Missouri Sptil Dt Informtion Service (30-m grid) elevtion (10-m buffered ESRI grid of the 10-digit wtershed ) ( model ( SSURGO, SSURGO ( STATSGO ( STATSGO, products/dtsets/ssurgo) products/dtsets/sttsgo) STATSGO ( products/dtsets/sttsgo) Lnd use Missouri Sptil Dt Informtion Service Missouri Sptil Dt Informtion Service ( ( Boundries Center for Agriculturl, Resource nd Better Assessment Science Integrting Environmentl Systems Point nd Non-Point Sources ( ( is perceived s the bility of soil to perform vrious functions to support optiml biologicl ctivity nd diversity for plnt nd niml productivity, to regulte wter flow nd storge, nd to provide n environmentl buffer to mitigte effects of hzrdous compounds (Krlen et l. 1997; Nortcliff 2002; Locke nd Zblotowicz 2004). Assessment of soil qulity within the Slt River Bsin, specificlly in the Goodwter Creek Experimentl Wtershed (GCEW), hs focused on selected indictors (tble 2) tht encompss ech group of soil properties nd their interctions. Soil qulity reserch hs focused on surveys of selected soils, lndscpes, nd cropping systems to determine mesurble soil ttributes tht best describe the condition of soils within GCEW. The selected soil ttributes (tble 2) were used to monitor soil qulity improvement of estblished mngement prctices. Soil erosion ws suitble guide for selecting the most pproprite indictors to describe previling soil conditions nd monitor soil qulity improvement. Topsoil depth, mesured by pprent electricl conductivity, ws lso useful indictor to demonstrte severe erosion on fields historiclly subjected to intensive tillge nd extensive soil qulity degrdtion (Jung et l. 2005; Lerch et l. 2005). Soil qulity ssessment to monitor gronomic nd conservtion mngement prctices for improving soils in GCEW demonstrted the benefits of mintining vegettion nd crop residues on clypn soils (Kremer nd Li 2003; Lerch et l. 2005). Crop rottion with no-till nd permnent vegettion under the Conservtion Reserve Progrm (CRP) provided higher microbil ctivity, soil orgnic C, nd wter-stble ggregtes thn short rottion (corn-soyben) with minimum tillge nd no cover cropping. Similrly, Stben et l. (1997) reported the most significnt benefits of mngement systems with periods of permnent vegettion re the increses in soil orgnic mtter nd soil tilth, becuse numerous other soil properties re influenced. Increses in soil qulity of cropping systems tht include continuous vegettion improve soil function by providing fvorble ecosystem for biologicl processes tht promotes crop growth nd enhnces environmentl spects, including wter qulity. The bility of soil to function in pesticide dissiption hs lso been suggested s primry soil qulity indictor (Locke nd Zblotowicz 2004). High concentrtions of pesticides in runoff from crop production fields my reflect degrded soil qulity 351

8 becuse the environmentl buffer cpcity of soil to filter nd process contminnts in wter hs been compromised. A study t one field site in GCEW reveled tht trzine degrdtion by microbil consorti in Mexico silt lom ws higher thn in Moniteu silt lom, prtly due to long-term cultivtion of corn on the Mexico silt lom in which corn rhizospheres provided selective environment for microorgnisms highly effective in degrding trzine (Stnley 1999). Subsequent studies verified the impct of plnts on trzine nd crbofurn dissiption t 15 dys fter ppliction when both pesticides were reduced by 50% in the upper 10 cm (4 in) of Putnm silt lom under corn compred with fllow soil (Buynovsky et l. 1995). More importntly, no trzine ws detected below the 20-cm (8-in) depth under corn compred with 15 ng trzine kg 1 soil under fllow. Popultions of presumptive crbofurn- nd trzine-degrding microorgnisms were 3 to 5 times greter in corn rhizospheres, likely responsible for reduced pesticide concentrtions in the 0- to 20-cm depth. These studies provided bseline for estblishing the extent of pesticide dissiption nd prevlence of presumptive pesticide-degrding microorgnisms present in soils within the Slt River Bsin. Our results prllel studies conducted in the Mississippi Delt, which demonstrted tht trzine in soil my be reduced bout 50% by the microbil community when the herbicide ws pplied more thn once every 24 months (Krutz et l. 2007). Development of suitble set of soil qulity indictors (tble 2) representing multiple functions of soil t wtershed scle continues to evolve s environmentl impcts of mngement re documented. These indictors will be used to ssess impcts of gronomic nd conservtion prctices (i.e., grss nd riprin buffers) on soil nd wter qulity on lndscpes within the Slt River bsin. Future development of rpid nd sensitive soil qulity monitoring of soil mngement will contribute to the dtbse for conservtion ssessment. Experimentl Infrstructure nd Legcy Dt In the Mrk Twin Lke/Slt River CEAP project, experimentl infrstructure hs been instlled by USDA Agriculturl Reserch Service s prt of three seprte projects in ddition to severl other ongoing monitoring efforts tht re prt of lrger-scle networks. The GCEW ws creted in 1971, with strem weirs t nd nested within 72 km 2 (28 mi 2 ) dringe re. The instrumenttion emphsized surfce wter hydrology (tble 3). In 1990, the Missouri Mngement Systems Evlution Are (Wrd et l. 1994) project dded n emphsis on surfce nd ground wter qulity nd dded field (20 to 36 h [49 to 89 c]) nd plot (0.35 h [0.86 c]) scles. The follow-up to Missouri Mngement Systems Evlution Are, the Agriculturl Systems for Environmentl Qulity project, continued similr emphsis nd infrstructure. The ggregted infrstructure for these two periods is described in Sdler et l. (2006). Beginning in 2005, the Mrk Twin Lke/ Slt River Bsin CEAP project emphsized much lrger scle ssessment of benefits from conservtion prctices. A wter qulity monitoring network ws estblished t 13 sites within the Slt River Bsin (figure 1). Two bsic pproches re being used (1) multi-scle wter qulity ssessment in Long Brnch Creek wtershed nd (2) bsinscle mss blnce ssessment of contminnt lods dischrged into nd out of Mrk Twin Lke. The multi-scle ssessment of Long Brnch includes four sites rnging in re from 72 km 2 (27.8 mi 2 ) (Goodwter Creek) to 469 km 2 (181 mi 2 ) (Long Brnch Creek), with two intermedite sized wtersheds. The bsin-scle mss blnce ssessment includes nine of the ten wtersheds (Ely Creek ws excluded) within the bsin nd one site t the outlet of Mrk Twin Lke. This monitoring scheme encompsses 71% of the re bove Mrk Twin Lke, with individul wtersheds hving 63% to 94% of their wtershed res monitored. Smpling protocols t ll sites include collection of bseflow nd runoff smples. Grb smples re collected minimum of twice monthly under bseflow conditions nd fter ech runoff event. Runoff smples re collected with utomted smplers. A single composite smple is collected for ech runoff event with subsmples collected t equl flow intervls for the first 2.54 cm (1 in) of runoff. Autosmplers re deployed from April through November, nd grb smples re tken yer-round. Routine nlyses include totl nd dissolved N nd P, common soil pplied herbicides nd two trzine metbolites (cetochlor, lchlor, trzine, deethyltrzine, deisopropyltrzine, metolchlor, metribuzin, nd simzine) nd totl suspended sediment. This smpling scheme will llow for ssessment of contminnt concentrtions under bseflow nd runoff conditions nd will fcilitte computtion of contminnt lods. In ddition, the monitoring progrm will be used to identify wtersheds contributing the highest proportion of the contminnt lods to Mrk Twin Lke, nd for clibrtion nd vlidtion of the Soil nd Wter Assessment Tool (SWAT) model simultions (see below). Operting over n re the size of the Slt River Bsin necessitted leverging infrstructure nd dt from mny sources, s seen in tble 3. A key resource ws the USGS guging sttion network, which monitors strem stge nd flow for eight of the 13 sites (North Fork t Shelbin, Crooked Creek, Lick Creek, Middle Fork, Elk Fork, Lower Long Brnch, South Fork, Clrence Cnnon Dm outlet), leving five dditionl sites for the current project (figure 1). One of these is GCEW, which ws in plce. Another, Young s Creek, ws former USGS sttion, decommissioned in the 1970s, for which rting curve existed tht hs been confirmed pplicble over the rnge observed recently. Tht left only three sites tht needed stge mesurements instlled nd rting curves developed (Blck Creek, Upper Long Brnch, nd Otter Creek). The CEAP project lso needs wether dt, which exists in this region from three sources. The most detiled (hourly) dt re collected by the University of Missouri utomted wether sttion network t three sites in or ner the Mrk Twin Lke bsin to the est (Auxvsse, Monroe City, Novelty), nd one somewht frther wy to the west (Linneus; Agriculturl Electronic Bulletin Bord 2007). To this list is dded the utomted wether sttion within GCEW. These mesure solr rdition, temperture, rinfll, wind speed, wind direction, nd soil temperture. At the dily time bsis, temperture extremes nd rinfll re reported by the Ntionl Wether Service volunteer observer network, nd rinfll is reported by the county USDA Frm Service Agency offices. In ddition to these time series wether dt, geogrphic informtion system lyers provide sptil informtion needed for interprettion nd computer modeling. These include soil dt from USDA NRCS, digitl elevtion dt from ESRI (Redlnds, Cliforni), the US Environmentl Protection Agency Better Assessment Science Integrting Point 352 Nov/dec 2008 vol. 63, no. 6

9 nd Nonpoint Sources (BASINS) dtbse, lnd use/lnd cover dt from the Missouri Sptil Dt Informtion Service, nd strem networks nd hydrologic re boundries from the Center for Agriculturl, Resource nd Environmentl Systems, University of Missouri (tble 3). Evlution of Best Mngement Prctices Reducing Herbicide Trnsport with Vegettive Buffers. Multiple species vegettive buffer strips (VBS) hve been recommended s n effective pproch to mitigte herbicide trnsport in surfce runoff derived from gronomic opertions (Lin et l. 2003; Krutz et l. 2003; Krutz et l. 2004; USDA NRCS 2000). However, the effect of buffer designs nd species composition on reducing herbicide trnsport hs not been well documented. A rinfll simultion study ws conducted on n eroded Mexico silt lom soil to test the bility of different VBS tretments to mitigte herbicide trnsport in surfce runoff. The study included four VBS tretments: (1) continuous cultivted fllow (control); (2) tll fescue (Festuc rundince Schreb.); (3) tll fescue with 1-m (3.28- ft) wide switchgrss (Pnicum virgtum L.) hedge t the upslope end of the VBS; nd (4) ntive wrm seson grsses-minly estern gmgrss (Tripscum dctyloides) nd switchgrss. The effect of the VBS tretments on trnsport of three commonly used soilpplied herbicides trzine, glyphoste, nd metolchlor ws studied. Full detils of the experimentl design nd procedures cn be found in work by Lin et l. (2007). The results showed tht ll VBS tretments significntly reduced the trnsport of both dissolved nd sediment-bound herbicides in surfce runoff compred to the control. A much higher proportion (40% to 60%) of glyphoste ws trnsported with suspended sediment in surfce runoff s compred to tht of trzine nd metolchlor (<3% ws sediment-bound). Four meters of ntive VBS removed bout 75% to 80% of ll three herbicides in surfce runoff. In ddition, 4 m (13.1 ft) of ntive species VBS resulted in much greter reductions in herbicide trnsport thn 8 m (26.2 ft) of the two tll fescue VBS designs. The primry mechnism by which VBS reduced dissolved-phse herbicide trnsport ws enhnced infiltrtion resulting from plnt evpotrnspirtion nd greter wter holding cpcity in the soil. This study lso showed tht the VBS were cpble of incresing the infiltrtion of surfce runoff for high runoff potentil clypn soil. Crop Mngement System Best Mngement Prctices. Development of cropping system BMPs for clypn soils begn on the cmpus of University of Missouri with the estblishment of Snborn Field (1888) nd lter the Duley-Miller Erosion Plots (1917). These erly nonreplicted plots documented tht clypn soils re sensitive to rpid nd severe soil degrdtion (e.g., topsoil erosion, loss of orgnic mtter, loss of productivity) under some crop mngement prctices (Woodruff 1987; Gntzer et l. 1990; Gntzer et l. 1991). With the Missouri Mngement Systems Evlution Are project, investigtions on crop mngement prctices nd development of BMPs hve predominntly been conducted t the plot scle within GCEW (39 13'N, 92 07'W). The re is divided into h plots, encompssing three lndscpe positions: summit, bckslope, nd footslope. The primry soils found t the site re poorly drined Mexico nd Putnm silt loms. Estblished in 1991, these plots llow side-by-side comprison of tillge nd rottion effects on crop production, runoff nd wter qulity, nd soil property chnges. Unique bout the reserch is the bility to look t interctions between mngement nd lndscpe position. Additionl description of the reserch re, crop mngement system opertions, nd smpling methodology hs been documented previously (Kitchen et l. 1998; Ghidey et l. 2005; Jung et l. 2007). Our findings of how crop mngement prctices ffect clypn soil nd wter qulity properties provide bsis for BMP implementtion within the Slt River Bsin. The findings include (1) growing seson (i.e., April-October) runoff verged over yers ws not different between tilled nd no-till grin crop systems (figure 5), finding tht is in contrst to mny other runoff studies of no-till cropping (Ghidey et l. 2005); (2) herbicide concentrtion or mss loss ws gretest for the first runoff event nd decresed rpidly s the seson progressed; (3) herbicide losses were pproximtely 120% higher from no-till systems thn the tilled system (figure 5) primrily due to the lck of herbicide incorportion in the no-till systems (Ghidey et l. 2005); (4) split herbicide ppliction (pre- nd postemergent) incresed trzine loss by creting two vulnerble periods for trzine trnsport; (5) though no significnt differences were found between cropping systems for sesonl losses of nitrte (figure 5) or orthophosphte, losses mesured were mong the highest reported when compred to other cropping system runoff reserch under nturl rinfll conditions; (6) herbicide nd nitrte leching through the root zone ws influenced more by intrinsic site-specific soil hydrology nd climte fctors thn by cropping system input rtes nd ppliction methods (Kitchen et l. 1998); (7) perennil grsses improved ggregte stbility nd soil orgnic crbon over ll lndscpe positions (figure 6) (Jung 2005); (8) field-mesured infiltrtion t the summit position ws higher with perennil grss systems (CRP nd hy) thn with grin crop systems, but it ws especilly improved with the hy cropping system (figure 6) becuse of stimulted growth with N fertiliztion nd biomss hrvesting (Jung et l. 2007); (9) lbortory-ssessed sturted conductivity on the bckslope position for CRP nd hy cropping systems ws 16 nd 10 times higher, respectively, thn for the mulch-tilled grin system (figure 6) (Jing et l. 2007); nd (10) soil biologicl ctivity nd microbil diversity were highest for perennil grss nd integrted cropping systems (Kremer nd Li 2003), indicting their effectiveness in improving soil qulity by enhncing multiple soil functions. In some cses, the BMP for one desired outcome my be in opposition to different desired outcome (e.g., no-till for erosion control cn increse herbicide loss). In these cses, priorities will need to be estblished s described by Kitchen et l. (2005). Precision Agriculture s Best Mngement Prctice. Producers primry motivtion for employing precision griculture is to improve profitbility. However, precision griculture systems (PAS) cn lso provide environmentl protection through reduced grochemicl use, incresed nutrient-use efficiencies, nd diminished off-field movement of soil nd grochemicls. From this premise, Berry et l. (2003) developed the ide of precision conservtion nd proposed tht precision conservtion ties efforts cross scles (zones within fields to between fields to wtershed nd bsin mngement) nd is key tool in chieving conservtion gols. 353

10 In the context of precision conservtion, uses of site-specific informtion (e.g., yield mps, soil smpling) extend beyond the usul vrible-rte mngement of inputs to mnging prts of field in completely different wys (e.g., cropping system, conservtion mesures). Such trgeting of conservtion prctices is prticulrly importnt where erosion processes cting over time hve ccentuted the differences in soil cross the lndscpe, s is the cse with clypn soils (Lerch et l. 2005). As precursor to developing n integrted PAS for profitbility nd conservtion, we intensively monitored crop, soil, nd wter qulity informtion on Missouri clypn soils for over decde (Lerch et l. 2005; Kitchen et l. 2005). These dt showed considerble sptil vrition in crop yield nd soil properties. In generl, soil texture nd topsoil depth (s inferred by pprent soil electricl conductivity [Sudduth et l. 2003]), long with topogrphy, hd the most persistent reltionships with yield becuse of their effect on soil wter holding cpcity nd within field wter redistribution (Drummond et l. 2003). The shpe of the reltionship ws dependent on the climte during the prticulr growing seson, specificlly the mount of rinfll received in July nd August (Jung et l. 2005). We used soil, lndscpe, nd profit mps to develop the PAS crop mngement pln ddressing site-specific problems, nd implemented it on typicl clypn-soil field in 2004 (Kitchen et l. 2005). Bsed on our nlysis of previous dt (Lerch et l. 2005), we gve equl priority to surfce wter qulity nd soil qulity, nd less priority to groundwter qulity when determining wht production nd conservtion mesures were needed nd where they should be plced. Shllow topsoil res of the field were no longer plnted to corn receiving soil-pplied herbicides. Insted, these res were plnted to whet nd cover crop, usully without herbicides, with cover crop lso used fter corn on the reminder of the field. The PAS pln included vrible fertilizer ppliction bsed on grid soil smpling nd fertility mpping for P, K, nd lime. Nitrogen for corn nd whet were pplied vribly, relying on ground-bsed crop reflectnce technologies tht hve been commercilized in recent yers (Shnhn et l. 2003). Additionl PAS detils re given by Kitchen et l. (2005). Figure 5 Growing seson runoff, trzine loss, nd nitrte-nitrogen loss (unpublished) s ffected by three grin cropping systems frmed over clypn soil lndscpe (from Ghidey et l. 2005) Runoff (mm) With the complex, integrtive nture of this reserch effort, definitive results will not be obtined for number of yers. However, preliminry results indicte tht PAS mngement is hving positive environmentl effects. There hs been reduction in profile soil nitrte concentrtions, presumbly through b b Atrzine loss (g h 1 ) Mulch till C-S No-till C-S No-till C-S-W-clover Nitrte-N loss (kg h 1 ) Figure 6 Soil orgnic crbon (Jung 2005), ggregte stbility (Jung 2005), in-field qusi-stedy infiltrtion rte (Jung et l. 2007), nd lbortory sturted hydrulic conductivity (Jing et l. 2007) s ffected by three grin cropping systems, two Conservtion Reserve Progrm systems, nd hy cropping system, verged over clypn soil lndscpe b b b Soil orgnic crbon (g kg 1 ) b Aggregte stbility (%) b b combintion of vrible N ppliction nd removl of excess N in the cover crop. In ddition, the cover crop, long with notill nd the whet stubble nd strw residue on prt of the field, hs begun to reduce soil erosion nd runoff from sensitive sideslope res. Prior to these chnges, lrge rinfll b b c Qusi-stedy infiltrtion rte (mm h 1 ) Mulch till C-S No-till C-S No-till C-S-W-clover CRP (cool) CRP (wrm) Hy b n n Sturted hydrulic conductivity (mm h 1 ) 354 Nov/dec 2008 vol. 63, no. 6

11 Tble 4 Finl vlues of goodness of fit indictors for the Goodwter Creek experimentl wtershed (Ghidey et l. 2007). events would often result in significnt rilling nd movement of soil from sideslopes to n lluvil re immeditely bove the field dringe outlet. After PAS estblishment, similr rinfll events hve cused much less rilling nd no observed new deposition in the lluvil re. A chllenge in optimizing precision mngement is tht no two fields re the sme nd therefore priori specifiction of the best mngement system is difficult. Becuse of this, the PAS concept is dptive, relying on the underlying sptil dtsets to guide the ppliction of conservtion mngement to the most pproprite subfield res. Rther thn explicitly defining precision griculture BMP for ppliction to the lndscpe, the prctice in this cse is the decisionmking sequence tht, in concert with the sptil chrcteristics of the field, llows the mnger or dvisor to uniquely define nd plce prctices in mnner tht is optimum for tht prticulr field. Modeling Efforts nd Needed Improvements to Models Soil nd Wter Assessment Tool Results to Dte. Modeling efforts in the Mrk Twin Lke/Slt River Bsin strted with the clibrtion nd vlidtion of the SWAT model for GCEW becuse of the extensive nd detiled precipittion, flow, nd wter qulity dt vilble in the experimentl wtershed. Initil clibrtion nd vlidtion of SWAT results for GCEW were presented by Ghidey et l. (2005b). These results were preliminry in tht they were bsed on flow dt from 1993 to 2003 prior to retrospective dt qulity nlysis. A subsequent repet of the model clibrtion nd vlidtion highlighted modeling nd monitoring issues (Sdler et l. 2006b). Herbicides nd sediment concentrtions nd lodings were found to be sensitive to fctors relted to the model nd to errors ssocited with the stge-dischrge rting Clibrtion 1993 to 1997 Vlidtion 1998 to 2003 Time step r 2 E NS r 2 E NS Annul Flow Sediment Monthly Flow Sediment Dily Flow Sediment Note: NS = Nsh-Sutcliffe coefficients. curve developed for the 72 km 2 (28 mi 2 ) dringe re of GCEW. The modeling fctors included (1) yer-specific plnting dtes, nd consequently herbicide ppliction nd tillge dtes; (2) the spred of herbicide ppliction dtes over period throughout the wtershed, whose distribution is unknown; nd (3) yer-to-yer crop distribution, which is function of climte nd economic fctors. With respect to the dischrge dt, the confidence of the rting curve decreses s flow increses for vrious resons, including fewer opportunities to mke n ctul mesurement, incresed difficulty mking the mesurement, nd higher likelihood to hve modifictions of the higher bnk profile over severl decdes. This uncertinty propgtes to uncertinties in the model clibrtion nd in the estimtion of event, monthly, nd nnul pollutnt lodings. The best model results were obtined using 12-yer rottion in order to specify yerly plnting nd trzine ppliction dtes nd to more ccurtely simulte the temporl spred in frming opertions throughout the wtershed. However, lnd use nd crop distributions re fixed nd cnnot be chnged in the model. Finl vlues of the r 2 nd the Nsh-Sutcliffe coefficients for flow nd sediment concentrtion in GCEW re presented in tble 4. A recent comprison of modeling results with SSURGO nd STATSGO dt showed tht lthough the finl set of input prmeter vlues were different, both sources of soil dt led to similr goodness of fit during the clibrtion nd vlidtion periods (Ghidey et l. 2007). This result is encourging in tht clibrtion results in smll wtershed of the region with STATSGO soil dt cn be scled up to lrger wtershed or to the river bsin. Erlier studies show vriety of results concerning this issue. Di Luzio et l. (2004) showed n dvntge in using SSURGO rther thn STATSGO dt. Wng nd Melesse (2006) obtined slightly better overll results with SSURGO, with the dvntge being lessened when looking t specific rnges of flow conditions. Peschel et l. (2006) found differences in the results but did not evlute the goodness of fit with either dt set. Modeled Effects of Best Mngement Prctices. Sensitivity of SWAT to relevnt mngement prctices in the GCEW ws lso investigted (Sdler et l. 2005). Thirtyyer simultions were performed to contrst pre-herbicide tillge prctices ginst current prctices tht utilize herbicide rther thn tillge to control weeds. The nlysis suggested, on verge, sufficient sensitivity for continued study. Specific yers tht showed no differences my suggest venues for exploring chrcteristics within the model or the mesurements tht drmticlly ffected sensitivity. In ddition, unexplined differences relted to phosphorus losses were detected during the 1970s in comprison to the lter yers (1980 to 1990s). Sttisticl nlyses of mesured trzine concentrtions in smples collected t the outlet of GCEW showed significnt concentrtion decreses for June nd the combined months of April, My, nd June from 1993 to Numericl, but not sttisticlly significnt, increses nd decreses in trzine concentrtions were lso observed in April nd My, respectively (Bockhold et l. 2006). Severl fctors explin these trends. First, incresed mximum nd minimum dily tempertures in the spring led to erlier corn plnting nd trzine pplictions. This llows more time for the chemicl to degrde, leving less to be detected in June. Second, severl grss wterwys hve been implemented protecting s much s 9% of the wtershed re by We estimted tht 652 h (1,611 c) in corn-soybens hd grss wterwys dded to them from 1993 to Other fctors such s chnges in corn nd sorghum crege or in trzine ppliction rtes were considered but not investigted becuse we did not hve enough informtion to conclude whether there hd been chnge. The SWAT model developed nd clibrted for GCEW ws utilized to estimte the impct of these climte nd mngement fctors. First, the simulted trzine concentrtion dt set ws evluted to determine if SWAT hd reproduced the trends found in the mesured trzine dt. This would men tht the results were ffected by only 355