H51B-1116 0800h
A Nitrogen Budget Model as an Indicator Of Leachable Nitrogen Under Agricultural Catchments
Surplus nitrogen (N) in ground and surface water is of concern to land users and downstream water users in intensive agricultural regions. Surplus N is available for leaching during lengthy periods without crop cover in annual crop systems used in many temperate regions. This paper presents a model to estimate surplus N available for leaching to ground water beneath agricultural systems and demonstrates an application to catchments in western Iowa where the landscape is intensively managed for maize and soybean production. Biochemical processes of N transfer and transformation within agricultural systems are simulated. Stocks of N in soil, crops, and livestock, and external stocks representing fertilizer and the atmosphere are linked using commonly available georeferenced data on soils, crops, livestock, and inorganic fertilizer, making it applicable to catchments and watersheds in many regions. Atmospheric exchanges include volatilization from fertilizer, soil, manure, and senescing crops, denitrification, atmospheric deposition, and symbiotic fixation. Nitrogen flow is centered on exchange between the soil inorganic and organic N stock. Differential N mineralization rates are defined for three soil organic matter pools, crop residue, and manure based on carbon:N ratios. Nitrogen exports from the system are accounted for by quantifying harvested crops and animals in addition to losses to the atmosphere. Application of the model to contiguous watersheds produced watershed-averaged annual surpluses of 7 to 26 kg-N ha-1. Annual nitrate loads in local streams are 17 to 30 kg-N ha-1. Net losses of soil organic N are sufficiently small to suggest that the maize-soybean rotation is in near equilibrium with the soils of the region.
H51B-1117 0800h
A General Paradigm of Modeling Three-Dimensional Subsurface Water Quality
This paper presents the model development of reactive chemical transport in subsurface water systems. Through the decomposition of the system of species transport equations via Gauss-Jordan column reduction of the reaction network, fast reactions and slow reactions are decoupled, which enables robust numerical integrations. Species reactive transport equations are transformed into two sets: algebraic equations (either mass action equations or users' specified) of equilibrium variables and reactive transport equations of kinetic variables. As a result, the model uses kinetic-variables instead of biogeochemical species as primary dependent variables, which reduces the number of transport equations and simplifies reaction terms in these equations. In order to improve the efficiency and robustness of the computation, five options are provided to solve the advection-dispersion transport equations. They are Finite Element Method (FEM) Applied to the Conservative Form of Transport Equations, FEM Applied to the Advective Form of Transport Equations, Modified Lagrangian-Eulerian (LE) approach, LE approach with FEM Applied to the Conservative Form of Transport Equations for Upstream Flux Boundary, and LE approach with FEM Applied to the Advective Form of Transport Equations for Upstream Flux Boundary. Three chemical strategies are employed to deal with the reaction terms. They are Fully-implicit scheme, Mixed Predictor-corrector and Operator-splitting method, and Operator-splitting approach. Three example problems are employed to demonstrate the robustness of the numerical simulations and the design capability of the model.
H51B-1118 0800h
Multivariate Analysis of In-stream Nutrient Loads and Salinity for a Large Regional Basin in Australia
The Glenelg-Hopkins area is a large regional watershed in south-west Victoria, Australia (Area : ~30,000 km2). The region delivers many socio-economic benefits with extensive national park systems as well as playing a major role in Australian agriculture. Within the region extensive clearing of native vegetation has led to a decline in water quality including increased solute loads and salinisation. The relationships between patterns in land use and total in-stream phosphorus (TP), total nitrogen (TN) loads and salt concentration (indicated by EC) is investigated. Multi-temporal satellite imagery was interpreted and water quality data analysed from 5 available gauge stations within the Glenelg-Hopkins region. Geographical Information Systems (GIS) were used to analyse spatial variations of land use for corresponding gauging stations in the catchment. Multiple regression analysis for a wide range of catchment characteristics was applied with spatial analysis to predict total stream nutrients and salt concentration. The multiple regression analysis demonstrated that the variables, Dryland pasture, Areas subject to inundation, Agricultural land on greater than 3 percent slope and the Ratio of Agriculture to Native vegetation were most strongly related to TP and TN loads. The regression model for salt concentration shows Native Vegetation, Bluegum Plantations, Dryland Cropping and Irrigated Horticulture were the significant explanatory variables. This study shows strong relationships between in-stream water quality parameters and a selected set of watershed attributes easily determined from satellite images.
H51B-1119 0800h
Hillslope-Riparian-Streamflow Interactions in a Discontinuous Permafrost Alpine Environment
Hillslope-riparian-streamflow interactions are poorly characterized in mountainous discontinuous permafrost environments. Permafrost underlain soils have a distinct soil profile, characterized by thick near-surface organic horizons atop ice-rich mineral substrates, whereas slopes without permafrost have thinner or absent organic soils overlying well drained mineral horizons. Riparian areas occur at the base of both seasonally frozen and permafrost slopes, yet a stronger hydrologic and soil transition occurs at slope bases with only seasonal frost. In a subarctic alpine catchment within the Wolf Creek Research Basin, Yukon, Canada, experiments were conducted between 2001 and 2003 to evaluate linkages along the slope-riparian-stream continuum during melt and post-melt periods. Water table, hydraulic head, stable isotope (d2H, d18O) and simple geochemical (pH, SpC, DOC) data were collected along transects during melt and summer periods. In soils with only seasonal frost, there was a downward piezometric gradient in slopes and upward gradient in riparian areas during melt. In contrast, permafrost soils did not show a recharge/discharge gradient between the slope and riparian zone. DOC declined and SpC increased with depth at all sites during melt. DOC was lower in riparian zones and areas without organic soils. SpC declined in soils as dilute meltwater entered the soil, yet it was difficult to establish spatial relations due to differences in melt timing. The similarity in stable isotope composition among sites indicated that the slopes were well flushed with snowmelt water to depth. DOC in streamflow was greatest on the ascending freshet hydrograph, and declined rapidly following melt. Streamflow SpC declined dramatically in response to dilute meltwater inputs and a decline in stream pH indicates flowpaths through organic horizons. Following melt, DOC concentrations declined rapidly in both slopes and riparian areas. In summer, water tables lowered in seasonally frozen slopes, yet an upward hydraulic gradient and near-surface water table was maintained in the riparian area. In permafrost slopes, water tables fell into mineral soils, increasing SpC and reducing DOC. Riparian water tables remained high and DOC was greater than the seasonally frozen soils, yet riparian zone hydraulic gradient reversed suggesting a small recharge gradient. In permafrost soil, riparian zone DOC was an order of magnitude higher than seasonally frozen riparian zones, which had DOC concentrations similar to streamflow. The similarity in stable isotope ratios among sites throughout the summer indicated that soil waters were dominated by water supplied during melt period. Rainfall waters had little long-term effect on slope and riparian isotopic ratios. Mixing analysis of geochemical and isotopic parameters indicates that during melt, most water was supplied via near surface organic layers, whereas later in the year, subsurface pathways predominated. Permafrost slope-riparian zones have a different hydraulic and geochemical interaction than seasonally frozen ones, yet their respective contribution to streamflow during different times of the year remains unclear at this time.
H51B-1120 0800h
A multi-basin analysis of runoff generation in a temperate forested watershed
Recent hydrologic and biogeochemical studies emphasise the importance of spatial, in addition to temporal, testing of process models. In doing so, they re-assert the influence of both the spatial and temporal variability in process that lead to difficulties in modelling storm response, contaminant transport and nutrient fluxes. Detailed empirical studies of runoff generation were traditionally confined to temporal aspects of meso-scale, trench and plot scales. More recently, empirical work has moved towards collecting more extensive spatial datasets within and among multiple basins as invaluable tools to seek better understanding of the controls on basin scale runoff processes. In our research our objectives are i) to estimate runoff generation across 8 nested basins in a pristine forested watershed in Eastern Quebec and ii) to examine the influence of antecedent moisture conditions (connectivity) and basin morphology on runoff generation. The 8 nested basins studied range in size from 7 to 147 ha and have considerable relief. Morphological characteristics for each basin are derived from a high-resolution digital elevation model ($<$1m spacing), a product of a 50 KHz LIDAR dataset. Isotopic hydrograph separation and geochemical end-member-mixing-analysis provide estimates of runoff generation for 5 storms of varying in size, intensity and antecedent moisture conditions. In two catchments antecedent moisture conditions and connectivity are evaluated using pre-storm spatial surveys of shallow soil moisture and transects of water table elevations. With an extended spatial dataset, we attempt to separate out how the controls on runoff generation change in space and time, in and across basins.
H51B-1121 0800h
Relationships between Stream Biotic Integrity and Satellite-derived Land Cover Metrics in the Mid-Atlantic region, USA
The condition of an aquatic system is determined in large part by the condition and use of the surrounding drainage basin. Increased knowledge about the relationship between stream health, land use and landscape metrics is thus useful for both water and land resource management. Multimetric stream health indices of aquatic condition reflect water chemistry, biologic condition, and physical habitat. The impact of land uses immediately adjacent to streams is influenced, and can be mitigated by vegetated stream buffers. We report on a multivariate and regression analysis of the relationship between satellite-derived landscape characteristics and stream indices in Maryland. Whole watershed characteristics and riparian characteristics were obtained from a land use/land cover map developed from Landsat satellite data. Landscape variables were derived and analyzed relative to the benthic and fish indices of biotic integrity. Associations among the water and land variables were analyzed using stepwise regression, factor analysis, and regression trees. Percent impervious surface cover and tree cover (whether in the watershed or the buffer zone) explained most of the variation in watershed health. We also considered metrics accounting for the spatial configuration of the landscape, such as the amount of impervious cover in the flow path from surrounding lands to the stream. These were also found to be significant predictors of stream health, but were second-order predictors. Our results suggest that best management practices designed to improve stream water quality should focus on the amount of impervious area and tree cover in both the watershed and within the buffer zone.
http://www.whrc.org
H51B-1122 0800h
Water and solute exports across catchment scales with varying proportions of hillslope and valley-bottom saturated areas
Recently there has been considerable interest in investigating how water and solute fluxes vary across catchment scales and if the relative proportions of hillslope and riparian landscape units can explain this response. We studied the export of water and solutes across four partly nested forested catchments located in Western New York, USA. The size of the catchments were - 696 (s1), 3.4 (s2), 1.6 (s3) and 1.9 (s5) ha. S1 was the main watershed oulet; s2 was nested in s1 with a 0.02 ha valley-bottom riparian area; s3 was nested in s2 and represents a headwater hollow catchment; and s5 was located outside s1 with a 0.07 ha valley-bottom wetland. The topographic index (ln[a/tanB]) was computed using a 2 m DEM and the mean values of ln[a/tanb] for s1, s2, s3, and s5 were 5.16, 5.38, 5.28, and 5.66, respectively. The spatial extent of hillslope and riparian units were mapped in the field and surface-saturation was observed in valley-bottom riparian areas, on hillslope benches, and near groundwater seeps. Streamflow and storm-event stream chemistry were monitored at the outlet of the watersheds. Water samples were analyzed for all base cations and anions, DOC and DON. Streamflow yields for s1, s2, s3 and s5 were 0.37, 0.15, 0.42, and 0.17, respectively. The headwater hollow catchment generated the highest discharge yield, with surface runoff being reduced as it traversed the valley-bottom deeper riparian stores. Average storm-event nitrate (ueq/L) and DOC (umol/L) (within []) concentrations for the four watersheds in order were: 22.7 [361.1], 41.8 [233.9], 43.5 [148.8], and 36.7 [316.9]. Corresponding nitrate and DOC flux (mol/ha) were: 0.42 [8.3], 0.54 [4.0], 1.28 [5.18], 0.50 [4.73]. DOC export increased with increase in saturated area whereas nitrate followed an opposite trend. This study shows that consistently-saturated areas located below groundwater seeps or on hillslope benches are as important source of DOC as the valley-bottom riparian areas. Although high values of ln[a/tanB] matched the valley-bottom saturated areas the topographic index map was not able to correctly identify all the hillslope saturated areas. This study suggests that we need to be cautious while partitioning the catchment into broad landscape units to explain catchment-scale water and solute patterns. Consideration needs to be given to saturated landscape parcels that form at hillslope benches and channel heads and which may be important sources for solute exports.
http://www.buffalostate.edu/orgs/glc/gowanda/overview.htm
H51B-1123 0800h
Streamwater hydrochemistry in headwaters: mixing, inter catchment groundwater transfer, and instream processes
To test the understandings about the hydrochemical processes from one small catchment to other catchments, the streamwater chemistries in a 2-order catchment (5.99ha) and four 0- or 1-order subcatchments (0.086-1.75ha) were investigated with the mixing model approach. The catchment is underlain by granite. The total area of the four subcatchments corresponds to 49% of the area of the 2-order catchment. As the result, the streamwater chemistries in these catchments were explained in principle by the one mixing model. Based on the water budget, the evidence of inter catchment transfer of bedrock groundwater, which transferred beyond the topographically delineated watershed and was important for the water and chemical flux, between the four subcatchments was found. Comparing the water budget between the 2-order catchment and the four subcatchments, the total annual discharge from the subcatchments corresponded to 35% of that from the 2-order catchment. The concentrations of conservative tracer, Cl-, and the geochemical substances, SiO2 and Na+, did not changed through the instream processes in the 2-order catchment. The annual fluxes of these solutes from the subcatchments corresponded to 36, 33, and 37% of those from the 2-order catchment, respectively, and equivalent to the percentage of the discharge. On the other hand, the concentrations of biologically active substances, NO3-, Mg2+, and Ca2+, were decreased through the instream processes in the 2-order catchment. However, the concentrations of these solutes increased again at the outlet of the 2-order catchment, and annual fluxes of these solutes from the subcatchments only corresponded to 25, 26, and 14% of those from the 2-order catchment, respectively. There is no perennial spring other than the four subcatchments, and the water budget has been considered to balance at the 2-order catchment. Thus, as well as from the pathways detected in the subcatchments, the water and chemical flux from other pathway(s), which may from the slope side along the mainstream or from deeper bedrock layer, contributed in the 2-order catchment. To consider the hydrological scaling, it is important to clarify the role of these pathways.
H51B-1124 0800h
Connectivity of Timber Harvest Units to Stream Channels in the Sierra Nevada Mountains, California.
Increases in sediment delivery to streams degrade water quality and aquatic habitat. Previous studies have documented how roads, fires, and mass movements can deliver large amounts of sediment to streams, but there have been few studies on the extent to which timber harvest units are connected to streams. There also is a lack of validated models for predicting the downslope delivery of sediment due to surface erosion in forested areas. This information is critical for predicting the cumulative effects of timber harvest activities in forested watersheds. The objectives of this study were to: (1) determine the proportion of timber harvest units with rills or sediment plumes leading to streams; (2) assess the magnitude of rill erosion; and (3) collect site data to develop models for predicting sediment delivery to streams. Surface erosional features and sediment delivery from harvest units was assessed by walking the boundary between the riparian protection zone and about 200 harvest units. The harvest units were on four National Forests in the northern Sierra Nevada Mountains of California and ranged in age from 2 to 18 years. Only 15 rills and 4 sediment plumes emanated from harvest units rather than unpaved roads. The rills varied in length from 11 to 220 m as compared to 10 to 22 m for the sediment plumes. Preliminary analysis indicates that the longer rills were associated with older harvest units, steeper hillslope gradients, and higher annual precipitation. These results indicate that timber harvest alone generally does not initiate downslope surface erosion, and public concerns should focus on the associated skid trails and unpaved roads.
H51B-1125 0800h
Groundwater-Surface Water Exchange as a Hydrologic and Water Quality Buffer
The complex interactions of surface water and groundwater are gaining increasing recognition as an outstanding research need. Specifically, the role of transition zones between alpine headwaters and valley bottom river/lake/wetland systems, common across mountain-valley landscapes, in controlling stream flow quantity, timing, and water quality are poorly understood. In many valleys, streams change in both space and time from gaining water from groundwater to losing water to groundwater as they flow toward the valley-bottom. Alpine-valley transition zones play a key role in regulating the amount, timing, and quality of stream water that arrives in the valley bottom. As such, we hypothesize that valley transitions function as hydrologic and biogeochemical buffers, both groundwater recharge and discharge zones, and reflections of integrated local and alpine hydrologic and climatic processes. To investigate these hypotheses we installed a network of 24 wells, 20 nested piezometers, seven stream gauging stations, and recording soil temperature nests (10 depths in each of 12 nests) across a two kilometer reach of Humphrey Creek in southwestern Montana. This network allowed us to investigate the surface water and groundwater hydrology in the study reach and to further understand the spatial and temporal variability in surface-water/groundwater interactions. We collected regular stream and groundwater samples to determine the relative contributions of groundwater and alpine runoff to downstream hydrographs. Salt tracers were injected during various times of the year to further quantify and elucidate areas of groundwater recharge and discharge. Initial results have shown that groundwater recharge and discharge zones across an alpine-valley transition are dynamic. Strong groundwater recharge gradients occurred during snowmelt, but shifted to discharging water into the stream channel when alpine contributions declined. This shift in source water contributions had a marked effect on the chemistry of water found in the stream channel. Furthermore, groundwater table shape and storage were dynamic and responded to alpine snowmelt, yet rain events had little impact on groundwater recharge but caused short pulsed increases in stream flow. These research efforts will provide a better understanding of the role that transition zones play in buffering the quantity, quality, and timing of water delivered from alpine headwaters to rivers, lakes, and wetlands in valley bottoms. This new understanding will be directly relevant to understanding groundwater recharge controls, irrigation withdrawal impacts, irrigation return flow implications, the surface water and groundwater dynamics controlling moisture status in slope wetlands, and the importance of surface-water/groundwater connections in affecting water quality.
H51B-1126 0800h
Influence of Wildfire-Created Hydrophobicity on Watershed Flow Paths and Surface Water Quality in City Creek, San Bernardino Mountains, CA
Wildfires alter surface water quality by changing the physical hydrology and biogeochemical interactions in a watershed. Lignin and other waxy organic compounds vaporize during intense fires. Recondensation of these waxy compounds form coatings on soil particles, which can create a hydrophobic layer. The presence of a hydrophobic layer changes flow paths to the stream channel by increasing overland flow and decreasing flow to subsurface reservoirs. Stream water chemistry is altered by the change in flow paths, as overland flow has shorter travel and mineral/vegetation interaction times than subsurface pathways. Nitrogen, sulfur, potassium and carbon are affected by burning the vegetation. The stream water temporarily receives increased fluxes of these solutes after storm events. Post-fire changes in vegetation also contribute to changes in water quality. The 21 square mile City Creek watershed, located in the San Bernardino Mountains, CA, was burned by the Old Fire in fall 2003. A recently developed air permeameter technique was used to estimate the hydrophobicity of the burned soils. The air permeability was determined by measuring the flow rate and change in pressure and by using the equation: Kair = [(flow rate)(air viscosity)(length of sample)]/[(change in pressure)(area of sample)]. Air permeability was correlated to soil hydraulic conductivity by a relationship developed in previous studies: Log (Ksat)= 1.27 x log(Kair) + 14.11. Vegetation in City Creek watershed consists of chaparral, woodland and conifer forests, with the greatest increases of hydrophobicity in the chaparral. Increased hydrophobicity in the City Creek watershed increased the contribution of overland flow to City Creek, as indicated by the decrease in delta-18O during storm events. This approximately 2 per mil decrease in delta-18O is indicative of the direct influence of precipitation and overland flow on the stream, rather than the continuous groundwater contribution during baseflow periods. Increased overland flow also causes increases in nitrogen, sulfur, potassium and carbon in the creek immediately after storm events during the first few months after the fire.
H51B-1127 0800h
Hyporheic Processes Regulating Trace Element Cycling Under Differing Hydrologic and Biogeochemical Regimes
The overall goal of our research is to improve our understanding of processes responsible for regulating trace metal and DOC cycling in the ground water - stream interfacial region. The hyporheic zone is a dynamic interface region likely to be important in determining metal speciation and fluxes, yet few studies have characterized this zone with respect to trace elements. The area under investigation, a stream catchment in the Northern Temperate Lakes Region of north-central Wisconsin, encompasses contrasting sampling sites with regards to redox and ground water recharge. A hyporheic sampler, specifically designed for low-level trace metal sampling, was used to sample depths of 2, 5, 7, 10, and 15 cm below the sediment-water interface at two locations in the watershed, one characterized by downwelling conditions with oxic streamwater, and the other upwelling with anoxic groundwater. The strength of the upwelling regime varies seasonally, becoming dominant during the late spring and summer but weakening during the fall and winter. Results show that hyporheic sulfide levels during summer 2004 were twice as high at the upwelling, anoxic site than at the downwelling, oxic site. Higher sulfide levels were seen in early (mid-June) versus mid summer (late July) at both sites. Sulfide levels decreased with depth in early summer but increased with depth in mid summer at the upwelling site while the opposite trend was observed at the downwelling site. The downwelling site exhibits higher DOC levels (driven by DOC-rich stream water) than the upwelling (ground water dominated) site and values increased with depth for the former while they decreased with depth for the latter site. Both sites saw an initial increase in DOC from spring to summer, but levels at the downwelling site decreased in mid summer (in concert with stream DOC levels) while levels at the upwelling site continued to increase. Levels of Zn were higher than Cu at both sites. There was little difference in Cu levels between sites while Zn levels were higher at the downwelling site. Iron was also higher at the downwelling site ranging from 77-190 micromolar and increasing with depth while at the upwelling site levels ranged from 30-77 micromolar and decreased with depth. Voltametric data (ASV) addressing the quantity and strength of metal-binding ligands is being synthesized to aid our interpretation of processes impacting metal levels and speciation.