In Eos, Transactions, American ... - Andrews...
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In Eos, Transactions, American Geophysical Union: AGU 1997 fall mtg. 1997 Dec. 8-12; San Francisco, CA. Washington, DC: AGU. 1997 Fall Meeting F327 H52D-4 1415h Temporal trends in streamwater chemistry at Hydrologic Benchmark Network watersheds in the Northeastern United 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 headwater catchments (median area of 142 km 2 ) since the mid-1960a through the Hydrologic Benchmark Network (HBN) program. Because the program is of national scope and the catchments are relatively pristine, the HBN provides a unique opportunity for researchers to evaluate regional or national trends in streamwater quality that may be related to changes in atmospheric deposition. In the current study, streamwater data from five HBN sites and precipitation data from seven nearby NADP sites in the Northeastern United States for 1984- 96 were analyzed using the seasonal Kendalls trend test. Despite the low sampling frequency at the HBN sites (mostly quarterly), a number of statistically significant trends were detected that were consistent among sites and between streamwater and precipitation data seta. There was a significant de- crease (p<0.1) in sulfate concentration at all of the tested streamwater and precipitation monitoring sit., indicating that a decrease in atmo- spheric deposition of sulfate is being reflected in streamwater quality in the 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 concentrations showed modest declines at most of the precipitation sites (-0.1 to -0.2 ueq/L/yr), and somewhat greater declines at most of the streamwater sites (-1.1 to -6.2 ueq/L/yr). Despite consistent downward trends in streamwater sulfate, statistically significant upward trends in alkalinity were detected in only two of the five streams. None of the precipitation sites and only our of the steeamwater sites showed a significant trend in nitrate. These results indicate that even low-frequency monitoring can provide valuable information on temporal trends in streamwater chem- istry, especially when compared with trends in precipitation chemistry. H52D-5 1430h Trend 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; btaulenb0 usmagov) 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 and samples collected more frequently during hydrologic events, such as snowmelt or storms. Standard time-series techniques, in which sample concentrations are estimated using a regression model to remove hy- drologic and climatic effects, require that the residuals to the model are statistically independent and identically distributed. The result- ing mixed-frequency data set has a tendency to violate these statistical assumptions. In this paper, we examine the residuals to regression models typically used in trend analysis, which relate concentration to discharge (transformed using a hyperbolic function) and day-of-year (using sine and cosine functions). Eight solutes were analyzed from a I6-year data record of atreamwater chemistry which contains weekly data and intermittent periods of event sampling, during which samples were collected every few minutes to every few hours. Samples were collected 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 model described above, we found that the seasonal terms did not reflect the seasonality of baseflow, rather they reflected the average of the short- term changes during events. This problem may be solved by fitting the seasonal data only to the weekly samples. Time-series plots of residu- als indicated that residuals to event samples had greater variance than throe of weekly samples. Thin was not apparent from a plot of residu- als versus transformed discharge. These results suggest that currently used discharge transformations do not adequately account for the ef- fects of discharge, and result is two distinct populations of samples with different behaviors - baseflow and event. Alternative tread detec- tion techniques are required for mixed-frequency data sets, which do not directly compare these different populations. H52D-6 1515h few Insights and Perspectives on Trajectories of Landscape Response to Treatment from Long-term Small Experimen- tal Basin Pairs, Western Cascade., Oregon Gordon E Grant (USDA Pacific Northwest Research Lab, 3200 SW Jefferson 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 (Department of 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 scientific study. Many of them were originally designed to assess the impact of vegetation removal (forest harvest) on hydrology, sediment transport and nutrient chemistry, in order to Met specific perturbation-response mechanisms implicit in process models of watershed behaviour. Avail- able records from treated-control small basin pair. permit examination of long term trajectories of system response to disturbance. These trajectories may take various forma, depending upon the variable be- Mg considered, lag timer in response to the initial perturbation, rates of process recovery, and the degree to which episodic or progressive changes in basin condition occur during the measurement period. We illustrate these key points by comparing the trajectories of response of several key variables over 20 to 40 years following forest harvest at the 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 debris flows), temperature, and nitrogen dynamics. While some of these vari- ables fie nitrogen, winter peak flows, suspended sediment) describe well defined peak sad decay trajectories, others (aumnier !mellows, land- slides) exhibit threshold rem:anew and nonlinear behaviour in response to changing vegetation. Discovering these 'surprises' is one of the chief values of bong-term basin studies. H52D-7 1530h Maspping 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 an 8 ken long lake in Quebec, Canada, that is underlain by crystalline granitic gneisars of the Canadian Shield. Although the lake is sur- rounded by a reasonably pristine arboreal ecosystem, the water level is controlled to augment the transport of river- rafted log harvests. Lake measurements consisted of several generations of SONAR surveys to map the submerged landscape and, in conjunction with SCUBA and tethered UW camera platform ground truthing, determine bottom compositions. Water column conditions were recorded for dissolved oxygen content, turbidity, temperature, and pH. Sampling locations were initially deduced using compass tries- gulation to shore markers and later with GPS. Bathometric data has led to the interpretation of numerous glacial landforms with subsequent modifies- lion by lacustriee submergence. Water column maly- sis indicates turbid, oxygenated plumes consistent with active surface wave-land interactions oriented to specific weather events. Recent increases in the lake water level may be responsible for a more dif- fuse thermocline transition to the hypoliamion. These measurements have been inexpensive to conduct and have provided real field data for use in secondary- and university-level Earth mience lab exercis.. H52D-8 1545h Long-term Acidification of Streams and Watersheds Revealed by Catchment Monitoring Data James W. Kirchner (Dept. of Geology and Geophysics, University of 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 faster than they are resupplied from mineral weathering, bog-term acidifi- cation of runoff can still occur, even under constant or declining acid deposition (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 that Ca and Mg were being lost from the soils of Hubbard Brook at a rate of 800±240 eq/ha/ye; this was subsequently confirmed independently by Likens et al (Science, 272, 244-246, 1996). Data from the Soiling catchment in Germany provide an opportunity to directly test the usefulness of runoff chemistry time series for inferring internal geochemical changes in catchments. At Selling successive in- ventories of exchangeable cations from 1968 through 1990 show that Ca has 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±1.9 ueq/L/yr. To observe this geochemical signal in seepage waters at Soiling, one must first statistically subtract the 'salt effect' produced by large changes in acid anion concentrations over the period of record (1973-present). Doing so reveals an ongoing Ines of Ca from seepage waters at a rate of 7.7±0.9 ueq/L/yr, which is quantitatively consistent with the trend expected from the measured depletion of exchangeable Ca. This demonstrates that depletion of base rations from catchment soils produce. measurable (and quantitatively predictable) changer runoff chemistry, and thus that runoff chemistry time series can be used to detect and quantify trends in exchangeable bases stored in catch- ment soils. H52D-9 )600h A 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 as well as for downstream receiving waters such as Chesapeake Bay. Pre- vious studies of N leakage have suggested that annual dissolved N fluxes from small watersheds can vary by several orders of magnitude and may be increasing as second-growth forests gradually become N-saturated from the long-term accrual of atmospheric N loadings. Alternatively, patterns of N leakage during the late 1980's and early 1990's have been shown to display considerable temporal and spatial synchrony with outbreaks of defoliation by the gypsy moth larva (a non-endemic forest pest), suggesting that forest disturbance may be a major contributor to N leakage in the region. A logical first step in linking regional-scale forest disturbance with N export is an evaluation of the applicability of a simple, linear unit nitrogen export response function (UNERF) model to long-term N export data from intensively-studied forested watersheds in the region. The use of the UNERF model is completely analogous to the use of the unit hydrograph for modeling storm runoff and ki based on the solution of the differential equations representing operation 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 moth defoliation (from state defoliation maps) within the period of data col- lection. Decouvolution can be accomplished by matrix algebra or linear regression, but the preferred method is linear programming (LP) given the usual case of a sequence of multiple defoliations of the same water- shed. le this situation, the LP solution is formulated as an objective function which minimizes the absolute error between measured and predicted values of annual N export. H52D-10 1615h Observation 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 in the Blue Ridge Mountains of Virginia were consistent with geochemical response 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 Oak Run; 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 responses were 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 of response and collapse of the gypsy moth population, the composition of the effected streams appears to be returning to predefoliation con- centration levels. It is remarkable that the water quality response to forest defoliation, especially the decrease in sulfate concentrations and the increase in nitrate concentrations, occurred during a period of de- creasing sulfate deposition and rising concern about ecosystem effects of nitrate deposition. Correct attribution of water quality changer as- sociated with defoliation may have been difficult without the long-term data record provided by regional catchment monitoring. This page may be freely copied.
Transcript of In Eos, Transactions, American ... - Andrews...
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.
This page may be freely copied.
