Lateral Subsurface Flow Research Articles

Fracture‐Dominated Subsurface Flow and Transport in a Sloping Reclamation Cover

The successful performance of reclamation soil covers over saline‐sodic overburden associated with oil sands mining in northern Alberta, Canada, depends on the dynamics of water and salt migration within these covers. Subsurface flow exerts a significant control on the distribution of soil moisture and salts within reclaimed landscapes. A conceptual model for fracture‐dominated lateral subsurface flow and transport in a sloping clay‐rich reclamation soil cover over saline‐sodic shale overburden was developed based on an interpretation of field observations. This model was then verified through the use of numerical simulations. The conceptual model assumes that lateral subsurface flow is dominated by a preferential flow system and that chemical equilibration between fresh snowmelt water stored in the macropores and higher concentration pore water stored in the soil matrix is nearly instantaneous. Numerical modeling of subsurface flow indicated that the discharge rate and cumulative volume are controlled by the bulk saturated hydraulic conductivity and drainable fracture porosity, respectively. A drainable fracture porosity ranging from 3 to 4% yielded a simulated cumulative discharge similar to measured values. A pseudo‐equivalent porous medium transport model was used to simulate the Na+ concentration of collected subsurface flow with time. General agreement between measured and simulated values demonstrates that discharge concentrations increase as the depth of perched water diminishes with time and water drains through macropores associated with a matrix of higher solute concentrations lower in the cover profile.

Adapting the Water Erosion Prediction Project (WEPP) model for forest applications

There has been an increasing public concern over forest stream pollution by excessive sedimentation due to natural or human disturbances. Adequate erosion simulation tools are needed for sound management of forest resources. The Water Erosion Prediction Project (WEPP) watershed model has proved useful in forest applications where Hortonian flow is the major form of runoff, such as modeling erosion from roads, harvested units, and burned areas by wildfire or prescribed fire. Nevertheless, when used for modeling water flow and sediment discharge from natural forest watersheds where subsurface flow is dominant, WEPP (v2004.7) underestimates these quantities, in particular, the water flow at the watershed outlet. The main goal of this study was to improve the WEPP v2004.7 so that it can be applied to adequately simulate forest watershed hydrology and erosion. The specific objectives were to modify WEPP v2004.7 algorithms and subroutines that improperly represent forest subsurface hydrologic processes; and, to assess the performance of the modified model by applying it to a research forest watershed in the Pacific Northwest, USA. Changes were made in WEPP v2004.7 to better model percolation of soil water and subsurface lateral flow. The modified model, WEPP v2008.9, was applied to the Hermada watershed located in the Boise National Forest, in southern Idaho, USA. The results from v2008.9 and v2004.7 as well as the field observations were compared. For the period of 1995–2000, average annual precipitation for the study area was 954 mm. Simulated annual watershed discharge was negligible using WEPP v2004.7, and was 262 mm using WEPP v2008.9, agreeable with field-observed 275 mm.

A similarity framework to assess controls on shallow subsurface flow dynamics in hillslopes

An understanding of the dominant controls on water fluxes through landscapes at scales smaller than the resolution of a hydrologic model is useful for choosing appropriate parameterizations, and for developing a classification system for landscape hydrology. The links between these controls and the parameters in hillslope‐scale models of subsurface lateral saturated flow have been treated implicitly in both physical (e.g., Characteristic Response Function) and empirical formulations, such as power law storage‐discharge relations. In particular, the controls exerted by the boundary condition at the toe of the hilslope and the temporal variability of the recharge have not been integrated with the understanding of the controls of topography and soil properties. In this work, we develop a dimensionless similarity framework for assessing these controls on the subsurface flow dynamics, based on the Boussinesq equation in an idealized hillslope system. Using this framework we demonstrate the relationship between (1) the exponents of the storage‐discharge relations, (2) the boundary conditions and (3) the characteristic storage thickness used in the Characteristic Response Function parameterization. We use the concept of hydrologic regimes to incorporate information about the population of recharge events that drive the system and set up the initial condition of saturated storage for each storm event. The analysis demonstrates that for planar hillslopes with uniform soil properties, exponents range between 0 (when the flow through the hillslope is dominated by the topographic gradients, and is in the early stages of draining) and 2 (when the flow is driven by water table gradients, and the thickness of the aquifer at the seepage face is small compared to the average thickness). The similarity framework can be used to make first‐order predictions of these exponents, and classify hillslopes using a simple typology of subsurface flows based on two components of the behavior: the relative dominance of the topographic and water‐table gradients, and the responsiveness of the hillslope discharge to temporal variability of the recharge inputs.

Effects of hydraulic conductivity variability on hillslope‐scale shallow subsurface flow response and storage‐discharge relations

An important problem in scientific hydrology is accounting for the effect of small‐scale heterogeneity in lumped representations of flow through landscapes. In this work we investigate the effect of lateral variations in hydraulic conductivity on saturated subsurface lateral flow through hillslopes using an idealized model based on the Boussinesq equation. The model simulates flow over an impervious planar base through heterogeneous conductivity fields in response to simple recharge inputs and periodic recharge events. The results indicate that the hillslope‐scale response depends on the relative importance of topographic and water table potential gradients driving flow. The relative magnitude of these drivers is related to a dimensionless variable . Where water table gradients dominate, a network of flow paths channel flow through the hillslope toward high‐conductivity areas, and the response is primarily dependent on the boundary condition at the toe of the hillslope. Where the topographic gradients dominate, the flow is delayed by low‐conductivity areas, producing a power law–like response, suggesting that high exponents in observed storage‐discharge relations may be traced to the presence of heterogeneity in the landscape. These observations suggest that the nature of the lumped hillslope‐scale hydrologic models depends on the emergent behaviors that arise at that scale as a result of the interactions between hillslope properties (including their variability in space), the relationship between the hillslopes and their surrounding landscape elements (through the boundary conditions), and the temporal variability of the recharge that enters the system.

Streamflow generation from snowmelt in semi‐arid, seasonally snow‐covered, forested catchments, Valles Caldera, New Mexico

Streamflow generation in the semiarid, seasonally snow‐covered, and forested mountain catchments of the Valles Caldera, New Mexico, was investigated using chemical tracers. Samples were collected from snow, subsurface flow from hillslopes, and streamflow at Redondo and La Jara Creeks from December 2004 to July 2005. A new modeling procedure was developed by combining diagnostic tools of mixing models and end‐member mixing analysis to evaluate the assumptions of mixing models. This procedure was successfully used to determine conservative chemical tracers, identify the number of end‐members that contribute to streamflow, and evaluate eligibility of end‐members. The results show that streamflow at Redondo Creek was generated from two end‐members: lateral subsurface flow (∼80%) and thermal meteoric water (∼20%). Streamflow at La Jara Creek was primarily from lateral subsurface flow alone. Overland flow of snowmelt was not a significant contributor to streamflow in either catchment. Lateral subsurface flow is an important process of streamflow generation in semiarid environments in the southwest United States and should play a critical role in regulating biogeochemical cycles.

Assessment of Mercury-Polluted Soils Adjacent to an Old Mercury-Fulminate Production Plant

Mercury contamination of soils and vegetation close to an abandoned Hg-fulminate production plant was investigated. Maximum concentrations of Hg (>6.5 gkg−1soil) were found in the soils located in the area where the wastewater produced during the washing procedures carried out at the production plant used to be discharged. A few meters away from the discharge area, Hg concentrations decreased to levels ranging between 1 and 5 gkg−1, whereas about 0.5 ha of the surrounding soil to the NE (following the dominant surface flow direction) contained between 0.1 and 1 gkg−1. Mercury contamination of soils was attributed (in addition to spills from Hg containers) to (i) Hg volatilization with subsequent condensation in cooler areas of the production plant and in the surrounding forest stands, and (ii) movement of water either by lateral subsurface flow through the contaminated soils or by heavy runoff to surface waters.

The role of hillslope hydrology in controlling nutrient loss

Hydrological controls on DOC and N transport at the catchment scale were studied for five storm events from the fall of 2004 through the spring of 2005 in WS10, H.J. Andrews Experimental Forest in the western Cascade Mountains of Oregon. This catchment is devoid of any riparian zone and characterized by hillslopes that issue directly into the stream. This enabled us to compare a trenched hillslope response to the stream response without the influence of riparian zone mixing. DOC and N concentrations and dissolved organic matter (DOM) quality (specific UV-absorbance (SUVA) and C:N of DOM) were investigated at the plot scale, in lateral subsurface flow from the trenched hillslope and stream water at the catchment outlet at the annual and seasonal scale (transition vs. wet period) during baseflow and stormflow conditions. DON was the dominant form of total dissolved nitrogen (TDN) in all sampled solutions, except in transient groundwater, where DIN was the dominant form. Organic horizon leachate and transient groundwater were characterized by high SUVA, and high DOC and total N concentrations, while SUVA and DOC and DON concentrations in lysimeters decreased with depth in the soil profile. This suggests vertical preferential flow without much soil matrix interaction occurred at the site. Deep groundwater (from a spring at the base of the hillslope) was characterized by low SUVA and low DOC and N concentrations. SUVA was always lower in lateral subsurface flow than in stream water at the seasonal scale, even during the wet period when other solutes were similar between lateral subsurface flow and stream water. This suggested mixing of deep groundwater and shallow transient groundwater was different at the hillslope scale compared to the catchment scale. DOC and DON sources were finite (production of DOC and DON from the hillslope soils appeared to be limited) at the seasonal scale since DOC and DON concentrations were significantly lower during the wet period compared to the transition period during stormflow conditions. This was also reflected in the DOC and DON peak and flow weighted storm event concentrations and antecedent soil moisture relationship where drier conditions (less prior flushing) resulted in the highest DOC and DON peak and flow weighted storm event concentrations. Results from this study showed the importance of the hillslope component in DOC and N transport at the catchment scale and underscore the importance of sampling solutes below the root zone (transient groundwater) and the value of using SUVA to fingerprint DOC sources.

A mechanistic assessment of nutrient flushing at the catchment scale

Quantifying nutrient flushing mechanisms at the catchment scale is essential for model development and prediction of land use change and climate change effects on surface water quality. To date, the description of nutrient flushing at the catchment scale has been largely mechanistically weak. This paper mechanistically assesses the flushing mechanism of DOC, DON and DIN at the hillslope and catchment scale during two storm events, in a small catchment (WS10), H.J. Andrews Experimental Forest in the western Cascade Mountains of Oregon. Using a combination of natural tracer and hydrometric data, and end-member mixing analysis we were able to describe the exact flushing mechanism at the site. This mechanism involved vertical preferential flow to the soil–bedrock interface and then lateral downslope flow, with a finite source of DOC and DON in the organic horizon. Both specific UV absorbance (SUVA, 254 nm) and continuously measured raw fluorescence measured with a fluorometer increased during the storm events, suggesting DOC was more aromatic during stormflow compared to baseflow conditions. SUVA patterns in lateral subsurface flow from the hillslope and stream water in combination with hydrometric data enabled us to infer that the contribution of deep soil water and groundwater was higher during the falling limb compared to the rising limb of the hydrograph and contributed to the dilution of DOC, DON and SUVA values during storm events. Overall this study showed the value of using a combination of hydrometric data, natural tracer data, and in particular DOC quality indices such as SUVA and fluorescence to mechanistically assess nutrient flushing at the catchment scale.

A reference data set of hillslope rainfall‐runoff response, Panola Mountain Research Watershed, United States

Although many hillslope hydrologic investigations have been conducted in different climate, topographic, and geologic settings, subsurface stormflow remains a poorly characterized runoff process. Few, if any, of the existing data sets from these hillslope investigations are available for use by the scientific community for model development and validation or conceptualization of subsurface stormflow. We present a high‐resolution spatial and temporal rainfall‐runoff data set generated from the Panola Mountain Research Watershed trenched experimental hillslope. The data set includes surface and subsurface (bedrock surface) topographic information and time series of lateral subsurface flow at the trench, rainfall, and subsurface moisture content (distributed soil moisture content and groundwater levels) from January to June 2002.

Spatially explicit simulation of peatland hydrology and carbon dioxide exchange: Influence of mesoscale topography

Carbon dynamics in peatlands are controlled, in large part, by their wetness as defined by water table depth and volumetric liquid soil moisture content. A common type of peatland is raised bogs that typically have a multiple‐layer canopy of vascular plants over a Sphagnum moss ground cover. Their convex form restricts water supply to precipitation and water is shed toward the margins, usually by lateral subsurface flow. The hydraulic gradient for lateral subsurface flow is governed by the peat surface topography at the mesoscale (∼200 m to 5 km). To investigate the influence of mesoscale topography on wetness, evapotranspiration (ET), and gross primary productivity (GPP) in a bog during the snow‐free period, we compare the outputs of a further developed version of the daily Boreal Ecosystem Productivity Simulator (BEPS) with observations made at the Mer Bleue peatland, located near Ottawa, Canada. Explicitly considering mesoscale topography, simulated total ET and GPP correlate well with measured ET (r = 0.91) and derived gross ecosystem productivity (GEP; r = 0.92). Both measured ET and derived GEP are simulated similarly well when mesoscale topography is neglected, but daily simulated values are systematically underestimated by about 10% and 12% on average, respectively, due to greater wetness resulting from the lack of lateral subsurface flow. Owing to the differences in moss surface conductances of water vapor and carbon dioxide with increasing moss water content, the differences in the spatial patterns of simulated total ET and GPP are controlled by the mesotopographic position of the moss ground cover.

Large-Scale Rainfall Simulation Experiments on Juniper Rangelands

The effects of shrub clearing on surface and subsurface water movement in the Edwards Aquifer region were investigated using two large-scale rainfall simulation plots. Multiple replications of a large (168 mm) rainfall event were applied at an ashe juniper covered plot before and after shrub removal and at a plot with longstanding herbaceous cover. The study sites were equipped for monitoring of throughfall, stemflow, surface runoff, and soil water. Lateral subsurface flow was measured in a trench at the downhill end of each plot. The canopy plot produced high lateral subsurface flow during rainfall but no surface runoff, even for high rainfall intensities. In contrast, hydrologic response at the inter-canopy plot was dominated by rapid surface runoff. Following shrub removal at the canopy plot, water movement beyond the soil layer increased due to reduced canopy interception. Soil water storage capacity at the shrub plot remained small for both conditions, with much water apparently bypassing the litter and soil layers via macropore pathways. This additional water could move off site as macropore flow or remain on site as matrix and conduit storage. Differences in surface runoff and subsurfaceflow are attributable to vegetation and geologic differences.

Erosion of Noncohesive Sediment by Ground Water Seepage: Lysimeter Experiments and Stability Modeling

Seepage may be a significant mechanism of streambank erosion and failure in numerous geographical locations. Previous research investigated erosion by lateral subsurface flow and developed a sediment transport model similar to an excess shear stress equation. As a continuation of this earlier research, slope destabilization driven by lateral, subsurface flow was studied to further verify the recently proposed sediment transport model. Laboratory experiments were performed using a two‐dimensional soil lysimeter. The experiments were conducted on two sandy soils: a field soil (loamy sand) and sieved sand with greater sand content and less cohesion. A series of seven lysimeter experiments were performed for the two different sands by varying the bank slope (90, 60, 45, 36, and 26°). Flow and sediment concentrations were measured at the outflow flume. Pencil‐size tensiometers were used to measure soil pore‐water pressure. A slight modification of the existing seepage sediment transport model adequately simulated lysimeter experiments for both noncohesive soils without modifying the seepage parameters of the excess shear stress equation, especially for bank angles >45°. The research then determined whether integrated finite element and bank stability models were capable of capturing both small‐ and large‐scale sapping failures. The models predicted large‐scale failures for bank angles >45° in which tension cracks formed on the bank surface. The models failed to predict collapses for bank angles <45° in which tension cracks formed on the seepage face. The failure to predict collapse was hypothesized to be due to the assumption of circular arc slip surfaces. More analytically complex stability approaches are needed to capture bank slope undermining.

The groundwater–land-surface–atmosphere connection: Soil moisture effects on the atmospheric boundary layer in fully-coupled simulations

This study combines a variably-saturated groundwater flow model and a mesoscale atmospheric model to examine the effects of soil moisture heterogeneity on atmospheric boundary layer processes. This parallel, integrated model can simulate spatial variations in land-surface forcing driven by three-dimensional (3D) atmospheric and subsurface components. The development of atmospheric flow is studied in a series of idealized test cases with different initial soil moisture distributions generated by an offline spin-up procedure or interpolated from a coarse-resolution dataset. These test cases are performed with both the fully-coupled model (which includes 3D groundwater flow and surface water routing) and the uncoupled atmospheric model. The effects of the different soil moisture initializations and lateral subsurface and surface water flow are seen in the differences in atmospheric evolution over a 36-h period. The fully-coupled model maintains a realistic topographically-driven soil moisture distribution, while the uncoupled atmospheric model does not. Furthermore, the coupled model shows spatial and temporal correlations between surface and lower atmospheric variables and water table depth. These correlations are particularly strong during times when the land-surface temperatures trigger shifts in wind behavior, such as during early morning surface heating.

Linking fragipans, perched water tables, and catchment-scale hydrological processes

Abstract Soils with very slowly permeable fragipans and fragipan-like argillic horizons are extensive throughout the Palouse Region of northern Idaho and eastern Washington, USA. These soils develop seasonal perched water tables (PWTs) under the xeric moisture regime of the region. The objective of this study was to utilize a hydropedology approach to examine the linkages between fragipans, PWTs, and catchment-scale hydrological processes such as soil water storage, runoff, and lateral throughflow. A 1.7-ha catchment dominated by Fragixeralfs (Fragic Luvisols) was instrumented with 135 automated shallow wells to monitor PWTs. Soil water content was measured with water content reflectometry probes, and catchment outflow was measured with a flume. A 35 m × 18 m plot was isolated hydrologically from the surrounding hillslope using tile drains and plastic sheeting to measure perched water outflow. Results show that during the wet winter and spring months, the transition from unsaturated to saturated conditions is accompanied by changes in volumetric water storage of only 4–5%. PWT levels are at the surface of ∼ 26–45% of the catchment soils during periods of high rainfall and snowmelt, thereby generating saturation-excess surface runoff from hillslopes. Observed solute movement via subsurface flow is very rapid and ranges between 2.9 and 18.7 m d − 1 when PWTs are maintained in more-permeable Ap and Bw horizons. Subsurface lateral flow accounts for as much as 90% of the incident precipitation and snowmelt during early spring. Data indicate that the relatively shallow depth to the fragipans and high K sat in surface soil layers combine to create a very flashy hydrological system characterized by considerable temporal and spatial variation in patterns of saturation-excess runoff.

Micro‐Trench Experiments on Interflow and Lateral Pesticide Transport in a Sloped Soil in Northern Thailand

During recent decades, a change in land use in the mountainous regions of Northern Thailand has been accompanied by an increased input of agrochemicals. We identified lateral water flow and pesticide transport pathways and mechanisms in a Hapludult on a sloped litchi orchard in Northern Thailand. During two rainy seasons, two micro-trench experiments were performed at the plot scale (2 by 3 m). The first experiment was performed at the footslope of the orchard; the second was performed at a midslope position. Two salt tracers (bromide and chloride) and two pesticides {methomyl [S-methyl-N-(methylcarbamoyloxy)thioacetimidate] and chlorothalonil (2,4,5,6-Tetrachlor-1,3-benzdicarbonitril)} were applied in stripes parallel to the slope 150 and 300 cm away from the trench. At the trench, soil water was collected by wick samplers. Tensiometers and time-domain reflectometry probes were installed. At the end of the experiment, soil samples were taken and analyzed for residual concentrations of tracers and pesticides. Lateral subsurface flow of water occurred exclusively along preferential flow paths and was mainly observed at 0- to 30- and 60- to 90-cm depth. Lateral transport of pesticides was negligible, but both pesticides were found beneath the application area at 90 cm depth. Therefore, they may pose a groundwater contamination risk. The amount of wick flow and the location of interflow were mainly a function of rain amount and antecedent soil water suction. During dry periods, water flow was restricted to the topsoil. After heavy rain events and wet periods, interflow was mainly observed in the subsoil. The cumulative rain amount between samplings necessary to induce interflow was 20 mm. At the footslope, the interflow was seven times higher, and the network of water-bearing pores increased compared with the midslope position.

RZWQM simulation of long-term crop production, water and nitrogen balances in Northeast Iowa

Agricultural system models are tools to represent and understand major processes and their interactions in agricultural systems. We used the Root Zone Water Quality Model (RZWQM) with 26 years of data from a study near Nashua, IA to evaluate year to year crop yield, water, and N balances. The model was calibrated using data from one 0.4 ha plot and evaluated by comparing simulated values with data from 29 of the 36 plots at the same research site (six were excluded). The dataset contains measured tile flow that varied considerably from plot to plot so we calibrated total tile flow amount by adjusting a lateral hydraulic gradient term for subsurface lateral flow below tiles for each plot. Keeping all other soil and plant parameters constant, RZWQM correctly simulated year to year variations in tile flow ( r 2 = 0.74) and N loading in tile flow ( r 2 = 0.71). Yearly crop yield variation was simulated with less satisfaction ( r 2 = 0.52 for corn and r 2 = 0.37 for soybean) although the average yields were reasonably simulated. Root mean square errors (RMSE) for simulated soil water storage, water table, and annual tile flow were 3.0, 22.1, and 5.6 cm, respectively. These values were close to the average RMSE for the measured data between replicates (3.0, 22.4, and 5.7 cm, respectively). RMSE values for simulated annual N loading and residual soil N were 16.8 and 47.0 kg N ha −1, respectively, which were much higher than the average RMSE for measurements among replicates (7.8 and 38.8 kg N ha −1, respectively). The high RMSE for N simulation might be caused by high simulation errors in plant N uptake. Simulated corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] yields had high RMSE (1386 and 674 kg ha −1) with coefficient of variations (CV) of 0.19 and 0.25, respectively. Further improvements were needed for better simulating plant N uptake and yield, but overall, results for annual tile flow and annual N loading in tile flow were acceptable.

Sensitivity of tile drainage flow and crop yield on measured and calibrated soil hydraulic properties

Process-based agricultural system models require detailed description of soil hydraulic properties that are usually not available. The objectives of this study were to evaluate the sensitivity of model simulation results to variability in measured soil hydraulic properties and to compare simulation results using measured and default soil parameters. To do so, we measured soil water retention curves and saturated soil hydraulic conductivity ( K sat) from intact soil cores taken from a long-term experimental field near Nashua, Iowa for the Kenyon–Clyde–Floyd–Readlyn soil association. The soil water retention curves could be well described using the pore size distribution index ( λ). Measured λ values from undisturbed soil cores ranged from 0.04 to 0.12 and the measured K sat values ranged from 1.8 to 14.5 cm/h. These hydraulic properties were then used to calibrate the Root Zone Water Quality Model (RZWQM) for simulating soil water content, water table, tile drain flow, and crop yield (corn and soybean) by optimizing the lateral K sat (L K sat) and hydraulic gradient (HG) for subsurface lateral flow. The measured soil parameters provided better simulations of soil water storage, water table, and N loss in tile flow than using the default soil parameters based on soil texture classes in RZWQM. Sensitivity analyses were conducted for λ, K sat, saturated soil water content ( θ s) or drainable porosity, L K sat, and HG using the Latin Hypercubic Sampling (LHS) and for L K sat and HG also using a single variable analysis. Results of sensitivity analyses showed that RZWQM-simulated yield and biomass were not sensitive to soil hydraulic properties. Simulated tile flow and N losses in tile flow were not sensitive to λ and K sat either, but they were sensitive to L K sat and HG. Further sensitivity analyses using a single variable showed that L K sat in the tile layer was a more sensitive parameter compared to L K sat in other soil layers, and HG was the most sensitive parameter for tile flow under the experimental soil and weather conditions.

Simulating Tritium Fluxes in the Vadose Zone under Transient Saturated Conditions

A phytoremediation project was established at the U.S. Department of Energy's Savannah River Site (South Carolina) to reduce fluxes of tritium‐contaminated groundwater to surface waters. Contaminated groundwater was collected in a pond and applied by spray irrigation to a catchment of mixed forest on Atlantic Coastal Plain soils. The objectives of this research project were to simulate tritium uptake by the vegetation and to determine if subsurface lateral flow at the sand‐clay interface impacts tritium uptake by forest vegetation. To simulate water and tritium fluxes within the catchment, we developed a spatially explicit water and solute transport model. Vertical water flow was simulated within a grid using a layered capacitance model with lateral flow between cells dependent on head development and the local slope of the impeding clay layer. Tritium movement was simulated on a daily basis assuming complete mixing within a cell. The model was evaluated by comparing biweekly measurements of soil tritium activity and soil water content in 18 measurement clusters distributed across the irrigated portion of the catchment. Although lateral flow was predicted to occur locally, after 3 yr of tritium irrigation, the model predicted that lateral transfer of tritium was <70 m downslope of the irrigation area. Transient saturated conditions were observed and simulated to occur within parts of the catchment. Including the simulation of subsurface lateral flow of water and tritium under transient saturated conditions, however, did not significantly alter average tritium uptake predictions for the catchment.

Hydrologic effects on methane dynamics in riparian wetlands in a temperate forest catchment

To understand how hydrological processes affect biogeochemical and methane (CH4) cycles in temperate riparian wetlands, we measured CH4 fluxes and dissolved chemical constituents and CH4 concentrations in groundwater, and monitored several environmental factors in wetlands located within a forested headwater catchment in a warm, humid climate in Japan. Variation in redox components dissolved in groundwater, including nitrate (NO3−), Mn, Fe, and sulfate (SO42−), depended on temperature and soil‐water conditions. Strongly reducing conditions usually occurred in the high‐temperature months of July, August, and September. Dissolved CH4 in groundwater changed with redox conditions and was highest in summer and lowest in winter. CH4 emissions from riparian wetlands were observed almost throughout the year and displayed clear seasonality. Occasionally in summer, emission rates were more than 4 orders of magnitude greater than hillslope uptake rates. Although CH4 emissions increased markedly during most of the summer, they were constrained by (1) fluctuation of the water table, which when lowered can shift the subsurface zone to a more oxidized condition, and (2) the oxygen‐rich water such as precipitation and lateral subsurface flow from the hillslope. These results suggest that hydrological processes in forest headwater catchments play an important role in supplying oxygen to soils and consequently affect biogeochemical cycles, including CH4 formation, and that small wetlands in forest watersheds function as large sources of CH4, especially in regions with warm humid summers.

Groundwater Nitrate Spatial and Temporal Patterns and Correlations: Influence of Natural Controls and Nitrogen Management

To use shallow groundwater NO3–N concentration as an indicator of groundwater quality requires understanding its patterns, correlations, and controls across space and time. Within a study comparing variable‐rate and uniform N management, our objectives were to determine groundwater NO3–N patterns and correlations at various spatial and temporal scales and their association with natural controls and N management. Experiments in a random, complete block design were conducted in a 2‐yr crop rotation in North Carolina that included one variable‐rate and two uniform N management treatments to wheat (Triticum aestivumL.) and corn (Zea maysL.). We measured groundwater NO3–N and depth every 2 wk at 60 well nests, sampling the 0.9‐ to 3.7‐m depth. Field‐mean NO3–N varied with time from 5.5 to 15.3 mg NO3–N L−1These variations were correlated primarily with concurrent changes in water table elevation and depth. Mean NO3–N exhibited two preferred states: high when the water table was shallow and low when the water table was deep. Temporal NO3–N fluctuations greatly exceeded treatment effects. Treatments appeared to affect NO3–N temporal covariance structure. Groundwater NO3–N spatial patterns and correlations were associated mostly with saturated hydraulic conductivity and water table fluctuations and appeared influenced by subsurface lateral flow. When treatment effects became consistently significant later in the study, they overrode natural controls, and NO3–N was spatially uncorrelated or exhibited shorter spatial correlation ranges and patterns associated predominantly with treatments.