H23D-1446
The effect of Soil Freezing on N Cycling: Comparison of two Headwater Subcatchments With Varying Snowpack, Hokkaido, Japan
Over-wintering soil processes affect the leaching of NO3$^{-}$ to surface water especially during snowmelt. In Hokkaido, Japan the snowpack decreases from west (~2-3 m) to east (0.5 to 1 m) providyng a unique opportunity to test the effects of variable snowpack on soil freezing and N cycling. The Upper Nakagawa Catchment (UNC), eastern Hokkaido, had a mean annual stream water NO3$^{-}$ concentration of 2 mg L$^{-}$$^{1}$ with a mean maximum yearly snowpack of about 0.5 m. In contrast, in northwestern Hokkaido, the M3 catchment had a mean annual stream NO3$^{-}$ concentration and mean annual maximum snowpack of about 0.1 mg L$^{-}$$^{1}$ and 2.5 m, respectively. A buried bag experiment was conducted to elucidate whether the difference in stream water N between UNC and the M3 was due to differences in soil freezing between the 2 sites or litter quality. Soil samples from two landscape positions (upper and lower slope) in each catchment were mixed and buried at 0, 5, and 30 cm in five locations at each landscape position. Shibecha soil was buried in Uryu and Uryu soil was buried in Shibecha at each landscape position and was incubated between October 2004 and April 2005. Results suggested litter lignin content was different between Uryu and Shibecha with values of 0.31, 0.25, 0.02, and 0.18 g lignin g$^{-}$$^{1}$ Uryu lower slope, Uryu upper slope, Shibecha lower slope, and Shibecha upper slope, respectively. Under conditions without freezing (5 cm and 30 cm soil buried in Uryu), N cycling in Shibecha soil buried in Uryu was much higher than Uryu soil buried in Uryu. Any NH$_{4}$$^{+}$ that was produced was nitrified as evidence by the high final extractable NO3$^{-}$ in Shibecha soil buried in Uryu at 5 cm and 30 cm depths (~2.5 and 3.5 g N m$^{-}2 at the lower and upper slopes, respectively at both depths). There was a decoupling of nitrification and mineralization when soil freezing occurred. Sites experiencing severe soil freezing had higher net mineralization compared to sites having no or less freezing. This was most pronounced in Uryu soil buried in Shibecha and at 0 cm depth. Uryu lower slope soil buried in Shibecha lower slope at 0 cm had a net mineralization rate of 10.6 mg N m$^{-}2 d$^{-}$$^{1}$ compared to 5.7 mg N m$^{-}2 d$^{-}$$^{1}$ at Uryu soil buried in Uryu at the same depth. Since nitrifiers could not survive in severe conditions there was less subsequent nitrification and less NO3$^{-}$ release. For example, Uryu upper slope soil at 0 cm buried in Uryu had net nitrification of 2.7 mg N m-2 d-1 while Uryu upper slope soil at 0 cm buried in Shibecha had net nitrification of 1.7 mg N m$^{-}2 d$^{-}$$^{1}$. The marked differences in N cycling between Uryu and Shibecha soil were likely a function of differences in litter quality and soil freezing between the sites. This study was completed before the spring thaw. Increases in nitrification would likely occur in the spring in especially Shibecha due to an increase in labile sources of organic N and a rejuvenation of nitrifiers. This would also result in higher leachable NO3$^{-}$ in Shibecha. Freeze frequency should be considered when evaluating differences in N dynamics between catchments having a propensity for soil freezing.
H23D-1447
Spatial variations in the concentration and speciation of nitrogen in an upland blanket peat catchment
Nitrogen-saturation and river water N-chemistry is usually evaluated on the basis of sampling at the catchment outlet. However, recent studies have shown that river water NO3-N concentration can show substantial variation along the length of its channel, even over short distances, and is highly scale-dependent. Therefore in order to successfully determine the N saturation of semi-Natural temperate ecosystems there is a need for knowledge of small-scale variability in stream water N chemistry and an understanding of the factors that control it. In this study, a survey of 33 tributaries of a peatland catchment in the upland North Pennines of the UK, was undertaken to investigate the variation in the concentration of different dissolved form of nitrogen (nitrate (NO3-N), ammonium (NH4-N) and dissolved organic nitrogen (DON)) in relation to catchment attributes. For each catchment, data on soil type, vegetation cover and geology were compiled. The data are presented and discussed with respect to the spatial distribution of soluble nitrogen forms and concentrations in relation to catchment attributes.
H23D-1448
Influence of Hydrological Flow Paths on Rates and Forms of Nitrogen Losses from Mediterranean Watersheds
Patterns of precipitation and runoff in California are changing and likely to influence the structure and functioning of watersheds. Studies have demonstrated that hydrologic flushing during seasonal transitions in Mediterranean ecosystems can exert a strong control on nitrogen (N) export, yet few studies have examined the influence of different hydrological flow paths on rates and forms of nitrogen (N) losses. Here we illuminate the influence of variations in precipitation and hydrological pathways on the rate and form of N export along a toposequence of a well-characterized Mediterranean catchment in northern California. As a part of a larger study examining particulate and dissolved carbon loss, we analyzed seasonal patterns of dissolved organic nitrogen (DON), nitrate and ammonium concentrations in rainfall, throughfall, matrix and preferential flow, and stream samples over the course of one water year. We also analyzed seasonal soil N dynamics along this toposequence. During the transition to the winter rain season, but prior to any soil water displacement to the stream, DON and nitrate moved through near-surface soils as preferential flow. Once hillslope soils became saturated, saturated subsurface flow flushed nitrate from the hollow resulting in high stream nitrate/DON concentrations. Between storms, stream nitrate/DON concentrations were lower and appeared to reflect deep subsurface water flow chemistry. During the transition to the wet season, rates of soil nitrate production were high in the hollow relative to the hillslope soils. In the spring, these rates systematically declined as soil moisture decreased. Results from our study suggest seasonal fluctuations in soil moisture control soil N cycling and seasonal changes in the hydrological connection between hillslope soils and streams control the seasonal production and export of hydrologic N.
H23D-1449
Linking Soils and Streams: Hydrological Controls on Organic and Inorganic Solute Transport in two Mediterranean Catchments
The dissolved chemical load in a particular stream is the result of a complex interplay between upland soil biogeochemistry, hydrology and downstream biogeochemical cycling. Because few studies have coupled biogeochemical and hydrological studies at the sub-catchment scale, we still do not fully understand the processes controlling transport and retention of nutrients as they interact with different hydrologic pathways. To better understand the processes behind observed seasonal trends in both organic (dissolved organic matter (DOM)) and inorganic (NO3, HCO3, Si, Al, Fe, Ca, Mg, ...) solute concentrations entering the stream network, we monitored changes in solution chemistry as rainwater moved through the soils and out into the streams of two small (<2 ha) coastal California catchments of differing rainfalls. In the steeper, more humid and deeply weathered site where vertically infiltrating throughflow dominates everywhere except in the immediate vicinity of the channel head, concentrations of DOM drop rapidly with depth primarily due to adsorption with the fine textured soil resulting in very low concentrations in stream water. At the gentler sloping site, saturated subsurface and saturated overland flow occur during most large storms leading to a rapid transfer of DOM and other biologically active solutes from the surface soils directly to the stream bypassing the deeper fine textured zone where adsorption and other removal mechanisms could occur. Due to this short-circuiting of typical elemental removal mechanisms, we observe a significant jump in both organic and inorganic solute concentrations from base flow levels during these large storm events as the spatial extent of saturated flow greatly expands upslope from the channel head. In the two ecosystems in this study, the hydrologic routing of water from soil to stream plays a critical role in determining the stream water chemistry.
H23D-1450
Hydrochemical responses among nested catchments of the Sleepers River Research Watershed.
We are probing chemical and isotopic tracers of dissolved organic carbon (DOC) and nitrate over both space and time to determine how stream nutrient dynamics change with increasing basin size and differ with flow conditions. At the Sleepers River Research Watershed in northeastern Vermont, USA, 20 to 30 nested sub-basins that ranged in size from 3 to 11,000 ha were sampled repeatedly under baseflow conditions. These synoptic surveys showed a pattern of heterogeneity in headwaters that converged to a consistent response at larger basin sizes and is consistent with findings of other studies. In addition to characterizing spatial patterns under baseflow, we sampled rainfall and snowmelt events over a gradient of basin sizes to investigate scaling responses under different flow conditions. During high flow events, DOC and nitrate flushing responses varied among different basins where high-frequency event samples were collected. While the DOC and nitrate concentration patterns were similar at four headwater basins, the concentration responses of larger basins were markedly different in that the concentration patterns, flushing duration, and maximum concentrations were attenuated from headwaters to the largest basin. We are using these data to explore how flow paths and solute mixing aggregate. Overall, these results highlight the complexities of understanding spatial scaling issues in catchments and underscore the need to consider event responses of hydrology and chemistry among catchments.
H23D-1451
Storm-event water and solute exports across catchment scales
Storm-event exports of water and nitrogen (N) species were studied for four catchments (1.6-696 ha) in a glaciated, forested Western New York watershed. Topography and saturation potential for the catchments was characterized using the downslope index (DWI). End-member mixing analysis (EMMA) showed that catchment runoff was composed of shallow groundwater (SGW) discharged at seeps and throughfall (THF) on the rising limb and THF and riparian water (RW) on the recession limb. Riparian water (RW) contributions increased with increasing catchment size while THF amounts were a function of surface-saturated area percent. New-old water contributions from O18 hydrograph separations are currently being evaluated. Exports patterns of ammonium and dissolved organic N (DON) across the catchment scales will also be presented. Observations on how water and solute responses vary from headwater to large catchment scales are critical for developing scale-relevant paradigms.
H23D-1452
Regional and temporal variation in riverine nitrogen species across the United States: Importance of organic nitrogen
Nitrogen delivery from the landscape to aquatic environments has increased drastically over the last 50 years. Much of the current emphasis on nitrogen export relates to nitrate inputs at annual time scales. However, the riverine nitrogen pool consists of seasonally variable ammonia, nitrate and organic nitrogen. Here we illustrate (1) regional differences in nitrogen speciation across the U.S., (2) temporal changes within the in-stream nitrogen pools, and (3) down-stream changes in nitrogen speciation using 30-year flow and water quality records from over 800 U.S.G.S. monitoring sites. The regional spatial patterns in N-speciation are largely controlled by nitrogen sources (e.g. high inorganic N inputs in the Midwest), but we hypothesize that down-stream N-alterations result in temporal and downstream changes in the riverine N-pool, which in turn alters N-reactivity. Furthermore, organic nitrogen is more than 50% of riverine nitrogen in many regions across the U.S. (southeast; intermountain west). Therefore, we suggest that future research related to riverine N export needs to address interactions between and temporal variation of the organic and inorganic N-species. Such an approach has the potential to improve our understanding of the delivery and impacts of anthropogenic nitrogen in our aquatic environments.
H23D-1453
Are Changes in Biogeochemical or Hydrologic Processes Responsible for Increasing DOC Concentrations in Headwater Streams of Northeastern North America?
The recent recognition of widespread and significant upward trends in dissolved organic carbon (DOC) concentrations in surface waters of northeastern North America and Europe has stimulated research to understand the cause of these trends. Several factors have been offered to explain these DOC trends including climate warming, chronic atmospheric nitrogen deposition, decreasing atmospheric sulfur deposition, and increasing surface water pH. Changes in these factors have acted to either increase the solubility of DOC or increase the rates of biogeochemical processes that generate labile carbon in the soil. Additionally, it is well known that rain events and snowmelt increase DOC concentrations in many surface waters through flushing along shallow flow paths where most labile carbon is stored. Changes in hydrologic flushing rates have generally not been explored as a possible explanation of these widely reported upward trends in DOC concentrations. Biscuit Brook, a 9.9 km2 catchment in the Catskill Mountains of New York has shown a significant increasing trend in DOC concentrations since 1992, consistent with other streams in this region. Stream chemistry has been monitored at Biscuit Brook on a weekly basis supplemented with event samples since 1983, providing a detailed data set with which to examine the causes of changes in DOC concentrations. Here, we examine the relative roles of climate warming, decreasing sulfate (SO$_{4}$$^{2-}$) and nitrate (NO3$^{-}$) concentrations, and changes in the frequency and size of hydrologic events on the long-term temporal pattern (1992 to 2004) of DOC concentrations in Biscuit Brook. DOC concentrations increased significantly in weekly samples collected primarily during low flow conditions. No similar trend was apparent in the high flow samples. Mean annual SO$_{4}$$^{2-}$ plus NO3$^{-}$ concentrations showed a strong inverse relation (r2 = 0.91, p < 0.01) to DOC concentrations, but these concentrations were not related to stream pH nor to air temperature. These results suggest that the DOC trends largely result from biogeochemical processes associated with the decreasing SO$_{4}$$^{2-}$ and NO3$^{-}$ concentrations such as an increase in the solubility of DOC caused by the decreasing ionic strength of the stream water. The frequency of hydrologic events as determined by hydrograph separation has not changed during 1992 to 2004, so it seems unlikely that changes in hydrologic flushing can explain the upward DOC trends in this stream. Events of similar size in the latter part of the record (2000 to 2004) produced higher DOC concentrations than were evident in the early part of the record (1992 to 1996) suggesting that changes in biogeochemistry and not hydrology are largely responsible for the trends observed at this site, which is representative of upland forested catchments of northeastern North America.
H23D-1454
Hydrological Controls on Nitrogen and DOC Transport at the Plot, Hillslope and Catchment Scale, HJ Andrews Experimental Forest.
While the flushing of nutrients at the catchment scale has been described in many forested environments during the last decade, the flushing mechanisms, flowpaths and geographic sources of different N species (DON, NO3$^{-}$ and NH4$^{+}$) and DOC are still poorly resolved, especially during different storm size and antecedent wetness conditions. We characterized flowpaths of N and DOC at the hillslope scale during and between storm events in WS10, H.J. Andrews, Oregon, USA, for five storms over the period Fall 2004 until Spring 2005. This catchment is dominated by hillslopes with negligible riparian water storage due to 1986 and 1996 debris flows that evacuated the valley bottom. This enabled us to study the hydrological and biogeochemical coupling between the hillslope and catchment in a way unimpeded by riparian zone groundwater dynamics.Through a combination of hydrometric and chemistry data from groundwater wells, tension and zero tension lysimeters at different depths, tensiometers, soil moisture probes and hillslope runoff from a 10 meter wide trench at the hillslope, we were able to resolve the dominant flowpaths. Fluorescence (a proxy for DOC) of hillslope and catchment runoff was monitored continuously with a fluorometer during storms. Preliminary data analysis showed a significant relationship between DOC concentrations and fluorescence values suggesting that fluorescence can be used to characterize DOC dynamics at small time scales. Our high frequency DOC characterization showed a clockwise hystersis pattern of DOC and total N against discharge for both hillslope and catchment runoff. This suggests flushing of nutrients in near and/ or in stream zones during the initial part of the storm. Total N and DOC concentrations in groundwater wells and lysimeters at shallow soil depths were high compared to other potential sources during storms. Our interpretation is that vertical preferential flow of high concentration water drives the groundwater contribution to hillslope- and catchment observed concentrations. Tensiometer data indicates that transient saturation at subtle changes in hydraulic conductivity within the soil profile storm might drive rapid lateral delivery of nutrients to the stream during the storm peak. Overall, our results suggest that increased nutrient concentrations during storms are caused by vertical and lateral preferential flow, and that thresholds at the hillslope scale modulate the connect from plot to the catchment scale.
H23D-1455
Hyporheic Exchange and Humic Redox Reactions in an Alpine Stream/Wetland Ecosystem
We studied the influence of hyporheic zone interactions on the redox state of the humic fraction of dissolved organic material and other redox active species in an alpine stream in the Colorado Front Range, USA. A constant-injection tracer experiment was conducted using bromide (Br-) to determine hydrologic characteristics of the stream system. The information gained from this experiment allowed us to determine that there were high rates of exchange between the main stream channel and a storage zone with a large cross-sectional area. Fluorescence spectroscopy results showed that the humic substances in filtered whole water samples from the hyporheic zone were more reduced than humic substances from the stream. Our results suggest that through hyporheic exchange reduced humic substances are transported from the hyporheic zone to the stream where they are being rapidly oxidized. In turn, hyporheic exchange from the stream carries water containing oxidized humic substances back to the hyporheic zone. These results indicate that hyporheic zone interactions influence the oxidation state of dissolved humic substances as well as other redox active species and may play an important role in determining energy flow through the entire stream system.
H23D-1456
Hydrologic Flushing of Forest Soils and the Consequent Leaching of Nitrogen and Calcium During Rainstorms and Snowmelt, Catskill Mountains, New York
Catskill Mountain streams in New York often receive pulses of NO3 during storms and snowmelt from watershed soils and acid deposition. This "flushing effect" of nutrients and acids was documented in forest-soil water through use of sequential lysimetry, in which soil-water was collected in equal-volume increments from zero-tension lysimeters placed within the soil profile. Stormflow in previously unsaturated soils began with percolation of water through the upper soil to the B horizon. NO3 pulses in soilwater during storms and snowmelt were typically delayed in the B-horizon relative to the O-horizon, indicating a percolation process for soil water movement between the lysimeters, but during the fall rains and spring snowmelt periods water-table measurements indicated that the B-horizon lysimeter was overtopped by the water table. Net flux was similar from throughfall and 0-lysimeters for individual events, suggesting that throughfall could either percolate unchanged through the soil column or that microbial release of N was rapid and similar to throughfall inputs. Patterns of change in O-Lysimeter concentrations during individual storms indicate repeated backflushing periods in which percolating acidic deposition or snowmelt from an individual event rose back into the O-horizon with a rising water table. This back-flushing phenomena may be enhancing the leaching of soil calcium beyond what would occur if acidic deposition was only percolating once through the O horizon . The sequential lysimetry method allowed investigators a more detailed look at N dynamics in forest soils than has been possible through the typical monthly lysimeter sampling strategy.
H23D-1457
Changes in Stream Water Quality Related to Land Use and Watershed Scale in the Catskill Mountains of New York State
New York City's West-of-Hudson reservoir system supplies about 85% of the city's water supply. Land use management in this region has strong implications for water quality because it is the water supply for 9 million people. The city needs information on the effects of current and future land use on water quality. To address these needs the U.S. Geological Survey operates 34 stream gaging stations within the West-of-Hudson reservoir system in the Catskill Mountains of New York State. Water quality and stream flow data are collected at 12 of these sites, grab samples are collected bi-weekly and storm samples are collected using automated samplers. The gages are arranged in a nested watershed design with one or more forested ''upper-Nodes'' and a ''lower node'' located at some distance downstream. The watersheds range in size from 2.0 to 177.7 $km^{2}$ and data for this study were collected from 2000 to 2004. The two principal land uses within the region are forest and agriculture. Residential and commercial land use account for less than 1% of the area in the watersheds included in the study. Watersheds with agricultural land use greater than 2% had the highest median concentration of total phosphorus (TP) and total dissolved phosphorus (TDP) in stream water, but there was no relation between the percentage of agricultural land use and TP or TDP concentration. The watershed with the highest percentage of agricultural land use (38%) had TP and TDP concentrations similar to another watershed with 3% agricultural land use. The highest TP and TDP stream water concentrations were measured in a watershed that had 20% agricultural land use, but was the smallest of the lower node watersheds (37 $km^{2}$). Likewise there was no relation between land use and median nitrate concentration, indeed the highest median nitrate concentration was measured in a watershed that was 99% forested. Water quality from forested watersheds had lower nutrient concentrations than that from agricultural watersheds, although not all agricultural watersheds had high nutrient concentrations. Since agriculture in this region typically consists of small, family-operated dairy farms, their influence on water quality is small. Base cations increased from upper nodes to lower nodes as did pH and acid neutralizing capacity, but these increases were not related to land use. Neither changes in flow nor change in basin size was correlated with the change in water chemistry from upper nodes to lower nodes. These results underscore the difficulty in trying to generalize the effect of land use on stream water quality in areas where there are not large variations in land use or when land use does not profoundly affect the landscape. Analysis of individual storms that occurred in all of the watersheds may provide additional insight into watershed responses during high flow.
H23D-1458
Regional Assessment of the Relationship Between Landscape Attributes and Water Quality in Five National Parks of the Rocky Mountains
Atmospheric deposition of pollutants threatens pristine environments around the world. However, scientifically-based decisions regarding management of these environments has been confounded by spatial variability of atmospheric deposition, particularly across regional scales at which resource management is typically considered. A statistically based methodology coupled within GIS is presented that builds on small alpine lake and sub-alpine catchments scale to identify deposition-sensitive lakes across larger watershed and regional scales. The sensitivity of 874 alpine and subalpine lakes to acidification from atmospheric deposition of nitrogen and sulfur was estimated using statistical models relating water quality and landscape attributes in Glacier National Park, Yellowstone National Park, Grand Teton National Park, Rocky Mountain National Park and Great Sand Dunes National Park and Preserve. Water-quality data measured during synoptic lake surveys were used to calibrate statistical models of lake sensitivity. In the case of nitrogen deposition, water quality data were supplemented with dual isotopic measurements of d15N and d18O of nitrate. Landscape attributes for the lake basins were derived from GIS including the following explanatory variables; topography (basin slope, basin aspect, basin elevation), bedrock type, vegetation type, and soil type. Using multivariate logistic regression analysis, probability estimates were developed for acid-Neutralizing capacity, nitrate, sulfate and DOC concentrations, and lakes with a high probability of being sensitive to atmospheric deposition were identified. Water-quality data collected at 60 lakes during fall 2004 were used to validate statistical models. Relationships between landscape attributes and water quality vary by constituent, due to spatial variability in landscape attributes and spatial variation in the atmospheric deposition of pollutants within and among the five National Parks. Predictive ability, model fit and sensitivity were first assessed for each of the five National Parks individually, to evaluate the utility of this methodology for prediction of alpine and sub-alpine lake sensitivity across the catchment scale. A similar assessment was then performed, treating the five parks as a group. Validation results showed that 85 percent of lakes sampled were accurately identified by the model as having a greater than 60 percent probability of acid-Neutralizing capacity concentrations less than 200 microequivalents per liter. Preliminary findings indicate good predictive ability and reasonable model fit and sensitivity, suggesting that logistic regression modeling coupled within a GIS framework is an appropriate approach for remote identification of deposition-sensitive lakes across the Rocky Mountain region. To assist resource management decisions regarding alpine and sub-alpine lakes across this region, screening procedures were developed based on terrain and landscape attribute information available to all participating parks. Since the screening procedure is based on publicly available data, our methodology and similar screening procedures may be applicable to other National Parks with deposition-sensitive surface waters.
H23D-1459
Groundwater-Stream Interactions in a Mountain - Valley Transition: Impacts on Catchment Hydrologic Response and Stream Water Chemistry
As mountain headwater catchments increase in size to the meso-scale, they incorporate new landscape elements including mountain-valley transition zones. Mountain-valley transition zones form part of the mountain front, and influence groundwater (GW)-stream interactions and impact hydrologic response and stream water composition. Mountain front recharge (MFR) in mountain-valley transition zones and subsequent GW discharge to streams in the valley bottom are important hydrological processes. These GW-stream interactions are dynamic in both space and time, playing a key role in regulating the amount, timing, and chemistry of stream water exiting the mountains and reaching the valley floor. We hypothesize that mountain-valley transitions function as hydrologic and biogeochemical buffers via GW recharge and subsequent GW discharge. More specifically, we propose that streams often recharge GW near the mountain front and receive stored GW further downstream. To investigate these processes we applied physical hydrology techniques (four stream gauging stations, 19 wells, and 18 piezometers), tracer injections, and geochemical hydrograph separations in the Humphrey Creek watershed in southwestern Montana. Our intensive instrumentation network allowed us to assess the spatial and temporal variability of mountain front GW recharge and GW-stream interactions across a mountain-valley transition. Geochemical signatures were used to partition stream flow into alpine and GW sources. Tracer injections were used to quantify GW recharge/discharge over multiple reaches. We found multiple lines of evidence necessary to investigate complex GW-stream interactions - single lines of evidence would have been misleading. Our results indicate that much of the alpine stream water recharged GW at the mountain front and that stored GW of a different chemical composition sustained down-valley stream discharge. Down-valley stream discharge was dominated by GW inputs and responded to GW stage more closely than upstream reaches. A critical GW stage height was necessary for down-valley channel flow, as this was the only major input to channel flow during early and late season base flow. Conversely, GW contributed little to stream flow in the upper reaches of the study area. GW-stream water exchange served as a flow and geochemical buffer, which caused significant changes in stream water chemistry from the alpine, to the MFR zone, to the valley bottom and muted fluctuations in channel flow, both at high and low flow. Implications are that mountain front GW recharge magnitudes can control valley aquifer storage state which combined with alpine runoff magnitude and valley bottom GW discharge controls stream water quantity and geochemical composition downstream.
H23D-1460
Conceptual Modeling Approach to Explain the Spatial Variability of Streamwater Chemistry in a Meso-Scale Headwater Catchment
Even within homogeneous geological and climatological settings, the spatial variability of specific discharge and some solute concentrations in headwater catchments often increases with decreasing stream order and sub-catchment area, and there is a certain catchment size where the variability is minimized. A simulation model based on the subsurface-bedrock groundwater mixing concept was proposed to determine mechanisms of this scale-variability relationship. Generally, it is difficult for the headwater small catchment to be determined its catchment area of the bedrock groundwater discharge while that of the subsurface groundwater can be measured based on topographical information, and the actual catchment area for the bedrock groundwater is often different from that for the subsurface runoff. The model presented in this paper focuses on this _gdiscrepancy_h in the actual catchment area between the subsurface and bedrock groundwater runoff as an origin of the variability among the lower-order sub-catchments. The model simulates the relationship between the sub-catchment size and the variability of specific discharge and stream solute concentrations of the catchment as expressed as an aggregate of the zero-order catchments. The model assumes that 1) the variability of specific discharge is generated by the discrepancy of the actual catchment area between subsurface and bedrock groundwater runoff, 2) the variation of the stream solute concentrations is determined by the mixing proportion of subsurface and bedrock groundwater discharge, 3) {\it n} th order catchment consists of {\it m} pieces of {\it n}-1 th order catchments, and {\it m} can be a function of {\it n}. The catchment simulation is made by _gaccumulating_h zero-order catchments that have different subsurface and bedrock groundwater contributions that are assigned for each zero-order catchment using a random number generator. The model was applied to a dataset from the Fudoji Experimental Catchment located in weathered granitic mountains of central Japan. The area-variability relationship of the geochemical solutes such as SiO2 in streamwater was reproduced by this model using values for subsurface and bedrock groundwater concentrations that were determined from the observations. The simulated results suggested that the major controls are 1) the size of the zero-order catchment, and 2) the range of the difference in catchment area contributed from subsurface or bedrock groundwater. The simulations also imply that the catchment size where the variability is minimized depends on the scale of the zero-order catchments.
H23D-1461
Scale-Dependence of Natural Variability of Flow Regimes in a Forested Landscape
The natural flow paradigm states that rivers should be managed to preserve their natural flow regimes. Maintaining natural variability in the flow regime is critical for conserving the structure and function of riverine ecosystems. This research seeks to determine relations between natural variability in the flow regime and basin scale. A distributed hydrologic model was used to characterize the natural flow regime of basins from first to fifth order within tributaries of the Batchawana River in the Algoma Highlands of central Ontario using the Range of Variability Approach (RVA). A thirty-year simulated flow record was used to calculate natural variability in the flow regime, defined as the S80 ((90th percentile - 10th percentile) / median). A scale-dependence in the S80 of these flows, and particularly low-flow parameters, was observed. Basins less than a threshold between ca. 400 and 600 ha had a large range in S80, while basins greater than 600 ha had a smaller range that converged towards a constant with increasing area. This represents the potential for a representative elementary area (REA) to exist with regard to interannual variability of some flow parameters. Below the REA, the mean of the ln ( /To tan B) distribution was significantly related to the S80 mean summer flow and 90-day minimum flow (p<0.001). This research demonstrates the scale-dependence of natural variability of flows, important for establishing reference conditions against which impacts of disturbance on flows throughout a drainage basin may be measured.
H23D-1462
Hydrochemical and hydrological dynamics of bedrock groundwaters in a weathered granite headwater
The importance of the bedrock groundwater to the hydro- biogeochemistry in forested headwater catchments has been stated. However, there are few studies which directly measured the bedrock groundwater dynamics and their chemistry because of the difficulties to access to the bedrock groundwater. We directly collected their samples within 1m depths from the bedrock surface at the headwater subcatchment (0.086ha) within the Kiryu Experimental Watershed (KEW; 5.99ha) and measured their chemistry and the stable isotope ratio of water to evaluate the role of bedrock groundwater to the streamwater chemistry in a weathered granite catchment. Half of the total rainfall infiltrated into the bedrock at the hillslope plot (0.024ha). This bedrock groundwater was the dominant in the baseflow. The SiO2 concentrations in the bedrock groundwater were highest within the catchment, and those in the streamwater were similar to or lower than them. The NO3- concentrations in the streamwater were clearly lower than those in the groundwater above the bedrock, and therefore, the bedrock groundwater with low NO3- may be contributing to the streamwater. The NO3- concentrations in the bedrock groundwater decreased with the time and along the infiltration processes, though these were higher than those in the streamwater. These mean that there are some processes of which the NO3- concentrations decrease within the bedrock. The isotope data showed that the deeper bedrock groundwater had the longer mean residence time. Thus, the bedrock groundwater seems to be affected much by the biogeochemical reactions before it contributed to the stream. Considering the water budget, the contributions of the bedrock groundwater increased at the outlet of KEW, though the streamwater had similar SiO2 concentrations there. Therefore, the bedrock groundwater, contributing to the streamwater in the weathered granite headwaters, has uniform geochemistry within the catchment; in other word, the hydrologic pathways within the bedrock may be relatively shallow.
H23D-1463
Origin for Kuparuk River Basin Springs, North Slope of Alaska
In the foothills of the Brooks Range of northern Alaska, an abundance of spring discharge occurs year-round. This current research focuses on hydrologic conditions, source of the springs, its residence time and geometry. Possible sources for the spring water are sub-permafrost groundwater and baseflow and groundwater flow immediately adjacent to the Kuparuk River. Water samples were collected in 2005 from the Kuparuk River Watershed, including Imnavait Creek, prior to snowmelt (April), during/after snowmelt (June) and late summer (August). Electric conductivity (EC) values ranged between 50-65 mS/cm for spring water, while EC values at the Kuparuk River ranged from 34.6-235 mS/cm. Alkalinity readings of water from the Kuparuk River ranged from 13.9-64 mg/L; this is compared with spring water values of 24.3-39.9 mg/L. Variations in chemical properties (total and dissolved organic and inorganic carbon, pH, alkalinity, electrical conductivity, and dissolved oxygen) suggest that local spring water is related to the baseflow of Kuparuk River during summer periods. Preliminary data collected in the 2005 field season demonstrates interaction between the flow of the Kuparuk River, nearby spring discharge, aufeis development and permafrost dynamics.