In Eos, Transactions, American Geophysical Union: AGU 1997 fallmtg. 1997 Dec. 8-12; San Francisco, CA. Washington, DC: AGU. 1997 Fall Meeting F327

H52D-4 1415hTemporal trends in streamwater chemistry at Hydrologic

Benchmark Network watersheds in the NortheasternUnited States, 1984-90

D.W. Clow ; M.A. Mast (U.S. Geological Survey, MS 415, DFC, Lake-wood, CO 80225; e-mail dweloweusgegov)

The U.S. Geological Survey has been monitoring streamwater chem-istry and discharge at a national network of medium-sired headwatercatchments (median area of 142 km 2) since the mid-1960a through theHydrologic Benchmark Network (HBN) program. Because the programis of national scope and the catchments are relatively pristine, the HBNprovides a unique opportunity for researchers to evaluate regional ornational trends in streamwater quality that may be related to changesin atmospheric deposition. In the current study, streamwater data fromfive HBN sites and precipitation data from seven nearby NADP sitesin the Northeastern United States for 1984- 96 were analyzed using theseasonal Kendalls trend test. Despite the low sampling frequency atthe HBN sites (mostly quarterly), a number of statistically significanttrends were detected that were consistent among sites and betweenstreamwater and precipitation data seta. There was a significant de-crease (p<0.1) in sulfate concentration at all of the tested streamwaterand precipitation monitoring sit., indicating that a decrease in atmo-spheric deposition of sulfate is being reflected in streamwater quality inthe Northeast. Most of the declines in sulfate concentrations in precip-itation (-0.9 to -1.9 ueq/L/yr) were matched by decreases in hydrogen,which ranged from -0.7 to -1.8 ueq/L/yr. Base cation concentrationsshowed modest declines at most of the precipitation sites (-0.1 to -0.2ueq/L/yr), and somewhat greater declines at most of the streamwatersites (-1.1 to -6.2 ueq/L/yr). Despite consistent downward trends instreamwater sulfate, statistically significant upward trends in alkalinitywere detected in only two of the five streams. None of the precipitationsites and only our of the steeamwater sites showed a significant trend innitrate. These results indicate that even low-frequency monitoring canprovide valuable information on temporal trends in streamwater chem-istry, especially when compared with trends in precipitation chemistry.

H52D-5 1430hTrend Analysis of Mixed-Frequency Water-Quality Time So-

R.P. Hooper (ph. 1-770-903-9146; e-mail: rphooperfferags.gov );B.T. Aulenbach (both at USGS, 3039 Arnwiler Road, Suite 139, At-lanta, GA 30360-2824, USA; ph. 1-770-903-9148; btaulenb0usmagov)

Increasing emphasis on wet weather streamwater quality to character-ize nonpoint-source contaminant runoff has resulted in strew/meter-quality time aeries that are a mixture of regularly spaced samples andsamples collected more frequently during hydrologic events, such assnowmelt or storms. Standard time-series techniques, in which sampleconcentrations are estimated using a regression model to remove hy-drologic and climatic effects, require that the residuals to the modelare statistically independent and identically distributed. The result-ing mixed-frequency data set has a tendency to violate these statisticalassumptions. In this paper, we examine the residuals to regressionmodels typically used in trend analysis, which relate concentration todischarge (transformed using a hyperbolic function) and day-of-year(using sine and cosine functions). Eight solutes were analyzed from aI6-year data record of atreamwater chemistry which contains weeklydata and intermittent periods of event sampling, during which sampleswere collected every few minutes to every few hours. Samples werecollected at the outlet of Panda Mountain Research Watershed, a 41-ha catchment located approximately 25 km southeast of Atlanta, GA,USA.

Trend analysis of water-quality time-serim data requires that residu-als to regression models conform to the above statistical assumptions.When event and weekly samples were fitted to the regression modeldescribed above, we found that the seasonal terms did not reflect theseasonality of baseflow, rather they reflected the average of the short-term changes during events. This problem may be solved by fitting theseasonal data only to the weekly samples. Time-series plots of residu-als indicated that residuals to event samples had greater variance thanthroe of weekly samples. Thin was not apparent from a plot of residu-als versus transformed discharge. These results suggest that currentlyused discharge transformations do not adequately account for the ef-fects of discharge, and result is two distinct populations of sampleswith different behaviors - baseflow and event. Alternative tread detec-tion techniques are required for mixed-frequency data sets, which donot directly compare these different populations.

H52D-6 1515hfew Insights and Perspectives on Trajectories of LandscapeResponse to Treatment from Long-term Small Experimen-tal Basin Pairs, Western Cascade., Oregon

Gordon E Grant (USDA Pacific Northwest Research Lab, 3200 SWJefferson Way, Corvallis, OR, 97331; ph. 541-750-7328; e-mail:grantgOfid.orstedu); Julia A Jon. (Department of Geosciesces,Oregon State University, Corvallis, OR, 97331; ph. 541-737-1224;e-mail: jonesjOccmail.orst.edu); David A Poet (Department of Geo.science., Oregon State University, Corvallis, OR, 97331; ph. 541-758.7767; e-mail: postOfsLorstedu); Sherri L Johnson (Departmentof Geoscie_nces, Oregon State University, Corvallis, OR, 97331; ph.541-758-7771; e-mail: johnmnsOfaloratedu)

Long-term paired basin studies can be viewed no observatories into nat-ure/ phenomena that would otherwise be unapproachable for scientificstudy. Many of them were originally designed to assess the impact ofvegetation removal (forest harvest) on hydrology, sediment transportand nutrient chemistry, in order to Met specific perturbation-responsemechanisms implicit in process models of watershed behaviour. Avail-able records from treated-control small basin pair. permit examinationof long term trajectories of system response to disturbance. Thesetrajectories may take various forma, depending upon the variable be-Mg considered, lag timer in response to the initial perturbation, ratesof process recovery, and the degree to which episodic or progressivechanges in basin condition occur during the measurement period. Weillustrate these key points by comparing the trajectories of responseof several key variables over 20 to 40 years following forest harvest atthe HJ Andrews Experimental Forest in western Oregon. Key variable.include hydrology (peak and low streamflows),.ediment transport (sus-pended sediment and bedload), mass movements (landslides and debrisflows), temperature, and nitrogen dynamics. While some of these vari-ables fie nitrogen, winter peak flows, suspended sediment) describe welldefined peak sad decay trajectories, others (aumnier !mellows, land-slides) exhibit threshold rem:anew and nonlinear behaviour in responseto changing vegetation. Discovering these 'surprises' is one of the chiefvalues of bong-term basin studies.

H52D-7 1530hMaspping and Monitoring a Fresh-Water Lake on the Cana-

dian Shield, 1083-1907

Stephen M. Metzger (DRI QSC, 7010 Dmdini Blvd., Reno, NV,89512, USA; ph. 702-674-7705; e-mail: metzgertlacsaneedn)

Over the past 15 years hydrographic surveys were per- formed on an8 ken long lake in Quebec, Canada, that is underlain by crystallinegranitic gneisars of the Canadian Shield. Although the lake is sur-rounded by a reasonably pristine arboreal ecosystem, the water level iscontrolled to augment the transport of river- rafted log harvests.

Lake measurements consisted of several generations of SONAR surveysto map the submerged landscape and, in conjunction with SCUBAand tethered UW camera platform ground truthing, determine bottomcompositions. Water column conditions were recorded for dissolvedoxygen content, turbidity, temperature, and pH. Sampling locationswere initially deduced using compass tries- gulation to shore markersand later with GPS.

Bathometric data has led to the interpretation of numerous glaciallandforms with subsequent modifies- lion by lacustriee submergence.Water column maly- sis indicates turbid, oxygenated plumes consistentwith active surface wave-land interactions oriented to specific weatherevents. Recent increases in the lake water level may be responsiblefor a more dif- fuse thermocline transition to the hypoliamion. Thesemeasurements have been inexpensive to conduct and have provided realfield data for use in secondary- and university-level Earth mience labexercis..

H52D-8 1545hLong-term Acidification of Streams and Watersheds Revealed

by Catchment Monitoring Data

James W. Kirchner (Dept. of Geology and Geophysics, Universityof California, Berkeley, CA, 94720-4767, USA; ph. (510) 643-8559;e-mail: kirchnerOgeomorph.berkeley.edu )

Acid deposition is now decreasing in many regions, but where acid load-ing is still high enough to leach base cations from catchments fasterthan they are resupplied from mineral weathering, bog-term acidifi-cation of runoff can still occur, even under constant or declining aciddeposition (Kirchner, Geochim. Gosmochim. Acts, 56, 2311-2327,1992). This process has a distinctive geochemical signature, and there-fore streamwater chemistry time series can be used to measure long-term changes in the base cation status of catchment soils. For example,from streamwater chemistry time series, Kirchner (1992) proposed thatCa and Mg were being lost from the soils of Hubbard Brook at a rateof 800±240 eq/ha/ye; this was subsequently confirmed independentlyby Likens et al (Science, 272, 244-246, 1996).

Data from the Soiling catchment in Germany provide an opportunity todirectly test the usefulness of runoff chemistry time series for inferringinternal geochemical changes in catchments. At Selling successive in-

ventories of exchangeable cations from 1968 through 1990 show that Cahas been lost at a rate of 450+/-90 eq/ha/yr. If heterogeneous equilib-rium prevaila between catchment soils and porewaters, this loss of ex-changeable Ca should reduce porewater Ca concentration. by 8.4±1.9ueq/L/yr. To observe this geochemical signal in seepage waters atSoiling, one must first statistically subtract the 'salt effect' producedby large changes in acid anion concentrations over the period of record(1973-present). Doing so reveals an ongoing Ines of Ca from seepagewaters at a rate of 7.7±0.9 ueq/L/yr, which is quantitatively consistentwith the trend expected from the measured depletion of exchangeableCa. This demonstrates that depletion of base rations from catchmentsoils produce. measurable (and quantitatively predictable) changerrunoff chemistry, and thus that runoff chemistry time series can be usedto detect and quantify trends in exchangeable bases stored in catch-ment soils.

H52D-9 )600hA Linear Model of the Effects of Disturbance on Nitrogen

Leakage from Forested Watershed.

K N Eshleman (Appalachian Laboratory, University of Maryland Cen-ter for Environmental Science, Frostburg, MD 21532; ph. 301-689-3115; e-mail: eshlenumOalumces.edu )

The leakage of dissolved nitrogen (N)-primarily in the form of nitrate-from forested watersheds in the mid-Appalachian region may hare im-portant water quality ramifications for small acid-sensitive streams aswell as for downstream receiving waters such as Chesapeake Bay. Pre-vious studies of N leakage have suggested that annual dissolved N fluxesfrom small watersheds can vary by several orders of magnitude and maybe increasing as second-growth forests gradually become N-saturatedfrom the long-term accrual of atmospheric N loadings. Alternatively,patterns of N leakage during the late 1980's and early 1990's have beenshown to display considerable temporal and spatial synchrony withoutbreaks of defoliation by the gypsy moth larva (a non-endemic forestpest), suggesting that forest disturbance may be a major contributorto N leakage in the region. A logical first step in linking regional-scaleforest disturbance with N export is an evaluation of the applicabilityof a simple, linear unit nitrogen export response function (UNERF)model to long-term N export data from intensively-studied forestedwatersheds in the region. The use of the UNERF model is completelyanalogous to the use of the unit hydrograph for modeling storm runoffand ki based on the solution of the differential equations representingoperation of a linear system. It is shown that UNERF's can be identi-fied by deconvolution of time series of long-term annual N export data,given information on both the timing and magnitude of gypsy mothdefoliation (from state defoliation maps) within the period of data col-lection. Decouvolution can be accomplished by matrix algebra or linearregression, but the preferred method is linear programming (LP) giventhe usual case of a sequence of multiple defoliations of the same water-shed. le this situation, the LP solution is formulated as an objectivefunction which minimizes the absolute error between measured andpredicted values of annual N export.

H52D-10 1615hObservation of Change in Regional Stream-Water Composi-

tion Following Forest Defoliation

JR Webb • BJ Cosby; FA Deviney; IN Galloway (Dept. of Environ-mental Sciences, University of Virginia, Charlottesville, Virginia; ph.1-804-924-7817; e-mail: rwebbOvirginiaedu)

During the 1980s changer in the solute composition of two streams inthe Blue Ridge Mountains of Virginia were consistent with geochemicalresponse to atmospheric deposition of sulfur. Regression analysis per-formed with annual discharge-weighted mean concentrations indicated:(I) increasing concentrations of sulfate and magnesium for White OakRun; and (2) increasing concentrations of sulfate, magnesium, calcium,and hydrogen ion, as well as decreasing arid-neutralization capacity(ANC), for Deep Run. However, by the early 19904 these responseswere obscured when the expanding range of the gypsy moth (Lyman-tria diaper L.) resulted in severe forest defoliation in western Virginia.Changes in stream-water composition for White Oak Run, Deep Run,and other monitored catchments in the region, included increasing con-centrations of nitrate, have cations, and hydrogen ion, as well as de-creasing concentrations of ANC and sulfate. After several years ofresponse and collapse of the gypsy moth population, the compositionof the effected streams appears to be returning to predefoliation con-centration levels. It is remarkable that the water quality response toforest defoliation, especially the decrease in sulfate concentrations andthe increase in nitrate concentrations, occurred during a period of de-creasing sulfate deposition and rising concern about ecosystem effectsof nitrate deposition. Correct attribution of water quality changer as-sociated with defoliation may have been difficult without the long-termdata record provided by regional catchment monitoring.

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