Subsurface Flow Paths Research Articles

Examining diel patterns of soil and xylem moisture using electrical resistivity imaging

Summary The feedbacks among forest transpiration, soil moisture, and subsurface flowpaths are poorly understood. We investigate how soil moisture is affected by daily transpiration using time-lapse electrical resistivity imaging (ERI) on a highly instrumented ponderosa pine and the surrounding soil throughout the growing season. By comparing sap flow measurements to the ERI data, we find that periods of high sap flow within the diel cycle are aligned with decreases in ground electrical conductivity and soil moisture due to drying of the soil during moisture uptake. As sap flow decreases during the night, the ground conductivity increases as the soil moisture is replenished. The mean and variance of the ground conductivity decreases into the summer dry season, indicating drier soil and smaller diel fluctuations in soil moisture as the summer progresses. Sap flow did not significantly decrease through the summer suggesting use of a water source deeper than 60 cm to maintain transpiration during times of shallow soil moisture depletion. ERI captured spatiotemporal variability of soil moisture on daily and seasonal timescales. ERI data on the tree showed a diel cycle of conductivity, interpreted as changes in water content due to transpiration, but changes in sap flow throughout the season could not be interpreted from ERI inversions alone due to daily temperature changes.

Groundwater exchanges near a channelized versus unmodified stream mouth discharging to a subalpine lake

AbstractThe terminus of a stream flowing into a larger river, pond, lake, or reservoir is referred to as the stream‐mouth reach or simply the stream mouth. The terminus is often characterized by rapidly changing thermal and hydraulic conditions that result in abrupt shifts in surface water/groundwater (sw/gw) exchange patterns, creating the potential for unique biogeochemical processes and ecosystems. Worldwide shoreline development is changing stream‐lake interfaces through channelization of stream mouths, i.e., channel straightening and bank stabilization to prevent natural meandering at the shoreline. In the central Sierra Nevada (USA), Lake Tahoe's shoreline has an abundance of both “unmodified” (i.e., not engineered though potentially impacted by broader watershed engineering) and channelized stream mouths. Two representative stream mouths along the lake's north shore, one channelized and one unmodified, were selected to compare and contrast water and heat exchanges. Hydraulic and thermal properties were monitored during separate campaigns in September 2012 and 2013 and sw/gw exchanges were estimated within the stream mouth‐shoreline continuum. Heat‐flow and water‐flow patterns indicated clear differences in the channelized versus the unmodified stream mouth. For the channelized stream mouth, relatively modulated, cool‐temperature, low‐velocity longitudinal streambed flows discharged offshore beneath warmer buoyant lakeshore water. In contrast, a seasonal barrier bar formed across the unmodified stream mouth, creating higher‐velocity subsurface flow paths and higher diurnal temperature variations relative to shoreline water. As a consequence, channelization altered sw/gw exchanges potentially altering biogeochemical processing and ecological systems in and near the stream mouth.

Dual nitrate isotopes clarify the role of biological processing and hydrologic flow paths on nitrogen cycling in subtropical low‐gradient watersheds

AbstractNitrogen (N) is an important nutrient as it often limits productivity but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low‐relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low‐relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flow paths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in two of the three watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flow paths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification and denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, and mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low‐gradient watersheds and variability in N delivery flow paths among adjacent watersheds with similar physical characteristics.

Vegetation influences near-surface hydrological characteristics on a surface coal mine in eastern USA

Mining processes alter natural landscapes worldwide, and methods for restoration of mined areas are widely studied. Establishment of vegetation is essential to mined land restoration. Prior research has addressed vegetative influence on erosion and runoff, but effects of vegetation type on surface hydrologic processes are less studied. We measured infiltration rates and observed subsurface flow paths in mine soils on a reforested area and a grassed area on a former surface coal mine in the eastern United States. The two areas were constructed and reclaimed fourteen years prior to our study using processes similar in all aspects except vegetation. MiniDisk tension infiltrometers and Brilliant Blue FCF dye staining analyses were utilized to evaluate near-surface flow processes in the vegetated mine soils. Mean infiltration rates were 16.5±10.7cmh−1 and 10.4±7.7cmh−1 for the reforested and grassed areas, respectively. Infiltration rates, hydraulic conductivities, dye coverage, and dye stained areas were significantly higher in the reforested area. The grassed area had constant dye coverage with depth but more flow paths than the reforested area. A transition from vegetation-controlled subsurface flow to abiotic-control at greater depths was evident in the stained soil profiles. Results highlight the heterogeneous nature of mine soils and the influence of vegetation type in development of mine soil hydrologic processes.

Stream Nitrogen Inputs Reflect Groundwater Across a Snowmelt-Dominated Montane to Urban Watershed.

Snowmelt dominates the hydrograph of many temperate montane streams, yet little work has characterized how streamwater sources and nitrogen (N) dynamics vary across wildland to urban land use gradients in these watersheds. Across a third-order catchment in Salt Lake City, Utah, we asked where and when groundwater vs shallow surface water inputs controlled stream discharge and N dynamics. Stream water isotopes (δ(2)H and δ(18)O) reflected a consistent snowmelt water source during baseflow. Near-chemostatic relationships between conservative ions and discharge implied that groundwater dominated discharge year-round across the montane and urban sites, challenging the conceptual emphasis on direct stormwater inputs to urban streams. Stream and groundwater NO3(-) concentrations remained consistently low during snowmelt and baseflow in most montane and urban stream reaches, indicating effective subsurface N retention or denitrification and minimal impact of fertilizer or deposition N sources. Rather, NO3(-) concentrations increased 50-fold following urban groundwater inputs, showing that subsurface flow paths potentially impact nutrient loading more than surficial land use. Isotopic composition of H2O and NO3(-) suggested that snowmelt-derived urban groundwater intercepted NO3(-) from leaking sewers. Sewer maintenance could potentially mitigate hotspots of stream N inputs at mountain/valley transitions, which have been largely overlooked in semiarid urban ecosystems.

A review of contaminant losses to water from pastoral hill lands and mitigation options

Pastoral hill lands deliver a range of contaminants to receiving environments that are of concern to the wider sector stakeholder community: principally sediment, phosphorus, nitrogen and faecal microorganisms. Thermal energy may also be considered a contaminant. Pastoral waterways generally have higher concentrations of suspended sediments, nutrients, faecal micro-organisms, and water temperature relative to forested waterways. These effects can be quantitatively linked to animal stocking rates and management. The large variation in the micro-climates, parent materials, soil types and vegetation resources inherent in hill country is the major driver of spatial and temporal dynamics of contaminant losses. This variation is modified by animal behaviour and physiology. Stores of contaminants in surface or sub-surface flow paths create important temporal lags resulting from land use and management change. The concept of critical source areas has become a key focus for the development of mitigation options. A wide range of biophysical options are now available, covering multiple scales and levels of cost-benefit. The use of farm planning tools is critical in balancing the implementation of mitigations with farm system objectives to improve whole-system sustainability. More research is needed on long-term impacts, given spatial and temporal variation in drivers and known spatial and temporal lag effects. There will be ongoing demand for mitigations that have been developed through co-innovation processes. Keywords: environmental mitigations, erosion, hill country, nutrient loss, pastoral, sediment export

Identifying hydrologic flowpaths on arctic hillslopes using electrical resistivity and self potential

Shallow subsurface flow is a dominant process controlling hillslope runoff generation, soil development, and solute reaction and transport. Despite their importance, the location and geometry of these flow paths are difficult to determine. In arctic environments, shallow subsurface flow paths are limited to a thin zone of seasonal thaw above permafrost, which is traditionally assumed to mimic the surface topography. We have used a combined approach of electrical resistivity tomography (ERT) and self-potential (SP) measurements to map shallow subsurface flow paths in and around water tracks, drainage features common to arctic hillslopes. ERT measurements delineate thawed zones in the subsurface that control flow paths, whereas SP is sensitive to groundwater flow. We have found that areas of low electrical resistivity in the water tracks were deeper than manual thaw depth estimates and varied from the surface topography. This finding suggests that traditional techniques might underestimate active-layer thaw and the extent of the flow path network on arctic hillslopes. SP measurements identify complex 3D flow paths in the thawed zone. Our results lay the groundwork for investigations into the seasonal dynamics, hydrologic connectivity, and climate sensitivity of spatially distributed flow path networks on arctic hillslopes.

Forested and agricultural land use impacts on subsurface floodplain storage capacity using coupled vadose zone-saturated zone modeling

A floodplain study was implemented in central Missouri, USA, to better understand land use effects on floodplain storage capacity. Study sites included a heavily instrumented historic agricultural field (Ag) and a remnant bottomland hardwood forest (BHF). The groundwater flow model MODFLOW was used to simulate hydrology for the study. The model was calibrated using observed 2011 and 2012 water year data including precipitation, groundwater level, volumetric water content (VWC), and evapotranspiration (ET) estimated using Penman–Monteith. The model performed similarly well for both sites and is considered robust given optimization constraints, as supported by model fit for head (BHF: Root Mean Square Error or RMSE = 0.40 m; and Ag: RMSE = 0.31 m), VWC (BHF: RMSE = 2.5 %; and Ag: RMSE = 2.3 %), and ET (BHF: RMSE = 0.74 mm; and Ag: RMSE = 0.84 mm). Model predictions for the full vertical profile at the BHF site (median = 520 mm) indicated 28 % more vadose zone storage than the Ag site (median = 409 mm) on a median percent difference basis. On a cumulative basis, the BHF site accepted 117 mm (75 %) more water into storage than the Ag site. Subsurface storage differences between the two sites were likely, at least in part, the result of subsurface flow paths and evapotranspiration rates, which differed by more than 16 % (BHF > Ag). Results highlight several important differences in hydrology between the two land use types and support the re-establishment of riparian forests in floodplains of the Midwest as a strategy to address increasing flood concerns.

Stormflow generation: A meta‐analysis of field evidence from small, forested catchments

AbstractCombinations of runoff characteristics are commonly used to represent distinct conceptual models of stormflow generation. In this study, three runoff characteristics: hydrograph response, time source of runoff water, and flow path are used to classify catchments. Published data from the scientific literature are used to provide evidence from small, forested catchments. Each catchment was assigned to one of the eight conceptual models, depending on the combination of quick/slow response, old/new water, and overland/subsurface flow. A standard procedure was developed to objectively diagnose the predominant conceptual model of stormflow generation for each catchment and assess its temporal and spatial support. The literature survey yielded 42 catchments, of which 30 catchments provide a complete set of qualitative runoff characteristics resulting in one of the eight conceptual models. The majority of these catchments classify as subsurface flow path dominated. No catchments were found for conceptual models representing combinations of quick response‐new water‐subsurface flow (SSF), slow‐new‐SSF, slow‐old‐overland flow (OF) nor new‐slow‐OF. Of the 30 qualitatively classified catchments, 24 provide a complete set of quantitative measures. In summary, the field support is strong for 19 subsurface‐dominated catchments and is weak for 5 surface flow path dominated catchments (six catchments had insufficient quantitative data). Two alternative explanations exist for the imbalance of field support between the two flow path classes: (1) the selection of research catchments in past field studies was mainly to explain quick hydrograph response in subsurface dominated catchments; (2) catchments with prevailing subsurface flow paths are more common in nature. We conclude that the selection of research catchments needs to cover a wider variety of environmental conditions which should lead to a broader, and more widely applicable, spectrum of resulting conceptual models and process mechanisms. This is a prerequisite in studies where catchment organization and similarity approaches are used to develop catchment classification systems in order to regionalize stormflow.

Soil pipe collapses in a loess pasture of Goodwin Creek watershed, Mississippi: role of soil properties and past land use

AbstractLittle is known about the association of soil pipe collapse features with soil properties or land use history. Three loess covered catchments in northern Mississippi, USA were characterized to investigate these relationships. Soil pipe collapses were characterized for their size, type feature and spatial location along with soil properties across the three catchments. Although mapped as the same soil, one of the catchments did not contain pipe collapse features while the other two had 29.4 and 15.4 pipe collapses per hectare. These loess soils contained fragipan layers that are suspected of perching water, thereby initiating the piping processes. Pipe collapses associated with subsurface flow paths were not always consistent with surface topography. The surface layer tended to be non‐erodible while layers below, even the upper fragipan layers, were susceptible to erosion by pipeflow. Soil properties of the lowest fragipan layer were highly variable but tended to prevent further downward erosion of soil pipes and thus formed a lower boundary for gullies. Middle to lower landscape positions in one of the piped catchments contained anthropic soils that were highly erodible. These anthropic soils were previously gullies that were filled‐in in the 1950s when forested areas, assumed to have been established when land was previously converted from crop to forest land, were converted to pasture. Three decades after this land use change from forest to pasture, pipe collapses became evident. In contrast, the adjacent catchment that does not exhibit pipe collapse features experienced severe sheet and rill erosion prior to the 1930s while in cotton production. The surface horizons above the lower fragipan layer were completely removed during this period, thus the top‐soil layer that tends to form a bridge above soil pipes in the more erodible subsoil layers was removed. This study showed that knowledge of soil characteristics or topography alone do not explain the distribution of soil pipe collapses as past land use can play a definitive role. Copyright © 2015 John Wiley & Sons, Ltd.

Importance of a nanoscience approach in the understanding of major aqueous contamination scenarios: case study from a recent coal ash spill.

A coal ash spill that occurred from an ash impoundment pond into the Dan River, North Carolina, provided a unique opportunity to study the significance and role of naturally occurring and incidental nanomaterials associated with contaminant distribution from a large-scale, acute aquatic contamination event. Besides traditional measurements of bulk watercolumn and sediment metal concentrations, the nanoparticle (NP) analyses are based on cross-flow ultrafiltration (CFUF) and advanced transmission electron microscopy (TEM) techniques. A drain pipe fed by coal ash impoundment seepage showed a high level of arsenic, with concentrations many times over the EPA limit. The majority of the arsenic was found sorbed to large aggregates dominated by incidental iron oxyhydroxide (ferrihydrite) NPs, while the remainder of the arsenic was truly dissolved. These ferrihydrites were probably formed in situ where Fe(II) was leached through subsurface flowpaths into an aerobic environment, and further act as a significant contributor to the elevated As concentrations in downstream sediments after the spill. In addition, we discovered and describe a photocatalytic nano-TiO2 phase (anatase) present in the coal ash impacted river water that was also carrying/transporting transition metals (Cu, Fe), which may also have environmental consequences.

Maize (Zeamays L.) yield response to nitrogen as influenced by spatio-temporal variations of soil–water-topography dynamics

Applying N fertilizer at rates that satisfy both economic and environmental objectives are critical for sustainable agriculture. The hypothesis of this study was that the spatial variability in maize (Zea mays L.) yield and its response to N rate were influenced by soil–water-topography dynamics. In 2008 and 2009, a study was conducted along an agricultural hillslope in the Northern Appalachian Ridge and Valley Physiographic Province in the USA with Cambisols according to FAO soil classification. Minimum, maximum, and delta yields and optimum N rate at different slope positions were determined using quadratic-plateau maize yield – N rate models. Results confirmed our hypothesis. The spatial variability of maize yield and its response to N rate was influenced by silt content, soil depth, profile curvature, slope, soil wetness and degree of soil water content temporal variation. In both dry year (2008) and wet year (2009), optimum N rates positively correlated (P<0.05) with the temporal variation of soil water content, which is an indicator of subsurface flow paths. In 2008, maize yield was little varied along this hillslope (11.7–12.0Mgha−1), while greater yield response to N rate (represented as delta yield, 5.6Mgha−1) was observed in upper convex and steep slope areas with low minimum yield (6.1Mgha−1). However, in 2009, greater maximum maize yield (13.5Mgha−1) and yield response to N rate (8.7Mgha−1) were observed in lower concave slope areas with deeper soil depth and thus greater water storage. Results from this study suggested that site-specific N applications could be improved by considering within field variability of soil, topography and hydrology.

Potential of MODFLOW to Model Hydrological Interactions in a Semikarst Floodplain of the Ozark Border Forest in the Central United States

Abstract Riparian shallow groundwater and nutrient movement is important for aquatic and forest ecosystem health. Understanding stream water (SW)–shallow groundwater (GW) interactions is necessary for proper management of floodplain biodiversity, but it is particularly confounding in underrepresented semikarst hydrogeological systems. The Modular Three-Dimensional Finite-Difference Ground-Water Flow Model (MODFLOW) was used to simulate shallow groundwater flow and nutrient transport processes in a second-growth Ozark border forest for the 2011 water year. MODFLOW provided approximations of hydrologic head that were statistically comparable to observed data (Nash–Sutcliffe = 0.47, r2 = 0.77, root-mean-square error = 0.61 cm, and mean difference = 0.46 cm). Average annual flow estimates indicated that 82% of the reach length was a losing stream, while the remaining 18% was gaining. The reach lost more water to the GW during summer (2405 m3 day−1) relative to fall (2184 m3 day−1), spring (2102 m3 day−1), and winter (1549 m3 day−1) seasons. Model results showed that the shallow aquifer had the highest nitrate loading during the winter season (707 kg day−1). A Particle-Tracking Model for MODFLOW (MODPATH) revealed significant spatial variations between piezometer sites (p = 0.089) in subsurface flow path and travel time, ranging from 213 m and 3.6 yr to 197 m and 11.6 yr. The current study approach is novel with regard to the use of transient flow conditions (as opposed to steady state conditions) in underrepresented semikarst geological systems of the U.S. Midwest. This study emphasizes the significance of semikarst geology in regulating SW–GW hydrologic and nutrient interactions and provides baseline information and modeling predictions that will facilitate future studies and management plans.

Relationships between stream nitrate concentration and spatially distributed snowmelt in high‐elevation catchments of the western U.S.

AbstractThis study compares stream nitrate ( ) concentrations to spatially distributed snowmelt in two alpine catchments, the Green Lakes Valley, Colorado (GLV4) and Tokopah Basin, California (TOK). A snow water equivalent reconstruction model and Landsat 5 and 7 snow cover data were used to estimate daily snowmelt at 30 m spatial resolution in order to derive indices of new snowmelt areas (NSAs). Estimates of NSA were then used to explain the flushing behavior for each basin over a 12 year period (1996–2007). To identify the optimal method for defining NSAs and elucidate mechanisms underlying catchment flushing, we conducted a series of regression analyses using multiple thresholds of snowmelt based on temporal and volumetric metrics. NSA indices defined by volume of snowmelt (e.g., snowmelt ≤ 30 cm) rather than snowmelt duration (e.g., snowmelt ≤ 9 days) were the best predictors of stream concentrations. The NSA indices were better correlated with stream concentration in TOK (average R2= 0.68) versus GLV4 (average R2= 0.44). Positive relationships between NSA and stream concentration were observed in TOK with peak stream concentration occurring on the rising limb of snowmelt. Positive and negative relationships between NSA and stream concentration were found in GLV4 with peak stream concentration occurring as NSA expands. Consistent with previous works, the contrasting flushing behavior suggests that streamflow in TOK was primarily influenced by overland flow and shallow subsurface flow, whereas GLV4 appeared to be more strongly influenced by deeper subsurface flow paths.

Leachate plume delineation and lithologic profiling using surface resistivity in an open municipal solid waste dumpsite, Sri Lanka

This study presents the use of direct current resistivity techniques (DCRT) for investigation and characterization of leachate-contaminated subsurface environment of an open solid waste dumpsite at Kandy, Sri Lanka. The particular dumpsite has no liner and hence the leachate flows directly to the nearby river via subsurface and surface channels. For the identification of possible subsurface flow paths and the direction of the leachate, DCRT (two-dimensional, three-dimensional and vertical electrical sounding) have been applied. In addition, the physico-chemical parameters such as pH, electrical conductivity (EC), alkalinity, hardness, chloride, chemical oxygen demand (COD) and total organic carbon (TOC) of leachate collected from different points of the solid waste dumping area and leachate drainage channel were analysed. Resistivity data confirmed that the leachate flow is confined to the near surface and no separate plume is observed in the downstream area, which may be due to the contamination distribution in the shallow overburden thickness. The stratigraphy with leachate pockets and leachate plume movements was well demarcated inside the dumpsite via low resistivity zones (1–3 Ωm). The recorded EC, alkalinity, hardness and chloride contents in leachate were averaged as 14.13 mS cm−1, 3236, 2241 and 320 mg L−1, respectively, which confirmed the possible causes for low resistivity values. This study confirms that DCRT can be effectively utilized to assess the subsurface characteristics of the open dumpsites to decide on corridor placement and depth of permeable reactive barriers to reduce the groundwater contamination.

Runoff sources and flow paths in a partially burned, upland boreal catchment underlain by permafrost

AbstractBoreal soils in permafrost regions contain vast quantities of frozen organic material that is released to terrestrial and aquatic environments via subsurface flow paths as permafrost thaws. Longer flow paths may allow chemical reduction of solutes, nutrients, and contaminants, with implications for greenhouse gas emissions and aqueous export. Predicting boreal catchment runoff is complicated by soil heterogeneities related to variability in active layer thickness, soil type, fire history, and preferential flow potential. By coupling measurements of permeability, infiltration potential, and water chemistry with a stream chemistry end‐member mixing model, we tested the hypothesis that organic soils and burned slopes are the primary sources of runoff, and that runoff from burned soils is greater due to increased hydraulic connectivity. Organic soils were more permeable than mineral soils, and 25% of infiltration moved laterally upon reaching the organic‐mineral soil boundary on unburned hillslopes. A large portion of the remaining water infiltrated into deeper, less permeable soils. In contrast, burned hillslopes displayed poorly defined soil horizons, allowing rapid, mineral‐rich runoff through preferential pathways at various depths. On the catchment scale, mineral/organic runoff ratios averaged 1.6 and were as high as 5.2 for an individual storm. Our results suggest that burned soils are the dominant source of water and solutes reaching the stream in summer, whereas unburned soils may provide longer term storage and residence times necessary for production of anaerobic compounds. These results are relevant to predicting how boreal catchment drainage networks and stream export will evolve given continued warming and altered fire regimes.

Dual‐domain mass‐transfer parameters from electrical hysteresis: Theory and analytical approach applied to laboratory, synthetic streambed, and groundwater experiments

AbstractModels of dual‐domain mass transfer (DDMT) are used to explain anomalous aquifer transport behavior such as the slow release of contamination and solute tracer tailing. Traditional tracer experiments to characterize DDMT are performed at the flow path scale (meters), which inherently incorporates heterogeneous exchange processes; hence, estimated “effective” parameters are sensitive to experimental design (i.e., duration and injection velocity). Recently, electrical geophysical methods have been used to aid in the inference of DDMT parameters because, unlike traditional fluid sampling, electrical methods can directly sense less‐mobile solute dynamics and can target specific points along subsurface flow paths. Here we propose an analytical framework for graphical parameter inference based on a simple petrophysical model explaining the hysteretic relation between measurements of bulk and fluid conductivity arising in the presence of DDMT at the local scale. Analysis is graphical and involves visual inspection of hysteresis patterns to (1) determine the size of paired mobile and less‐mobile porosities and (2) identify the exchange rate coefficient through simple curve fitting. We demonstrate the approach using laboratory column experimental data, synthetic streambed experimental data, and field tracer‐test data. Results from the analytical approach compare favorably with results from calibration of numerical models and also independent measurements of mobile and less‐mobile porosity. We show that localized electrical hysteresis patterns resulting from diffusive exchange are independent of injection velocity, indicating that repeatable parameters can be extracted under varied experimental designs, and these parameters represent the true intrinsic properties of specific volumes of porous media of aquifers and hyporheic zones.

A simple, regionally parameterized model for predicting nonpoint source areas in the northeastern US

Abstract Study Region Ten watersheds in New York, New Jersey, and Pennsylvania, USA. Study Focus A three parameter model based on variable source areas (VSAs) that does not require calibration within the region, was developed and tested. The model maintains a lumped daily water budget, but distributes runoff spatially using a soil topographic index (STI). We used ten gauged watersheds across the region to establish the model parameters, and compared model predicted VSAs against shallow water table depth and surface soil moisture field measurements in three sites. New Hydrological Insights for the Region The model was able to correctly predict VSAs at all three monitoring sites, indicating that saturation-excess runoff is important in these watersheds. The pattern of error in the model suggests that lateral subsurface flow paths could be exerting an influence on overland runoff generation in a way that is not captured by a static STI. The model has potential to be used as part of a strategy to limit nonpoint source pollution from saturation-excess runoff dominated areas in the region, and has been incorporated into an online decision support tool in central NY (www.hsadss.bee.cornell.edu).

Hyporheic Exchange and Nutrient Uptake in A Forested and Urban Stream in the Southern Appalachians

The hyporheic exchange (HE) is defined as the interaction between stream water and groundwater, where the stream water passes back and forth between the active channel and subsurface flow paths (Runkel, 1998). The zone of this exchange is characterized by saturated pore spaces, under sand or gravel stream beds (Hancock, 2002) and near the adjacent banks of the stream (Ryan, 2010), that contain some amount of water from the main stream channel (Boulton, Welty, & Larson, 2010). An indication of channel complexity within streams (Grimm, 2005), this transitional zone exposes surface water solutes to alternating oxic and anoxic conditions as they are mixed within the groundwater flow system (Lautz & Siegel, 2007). This plays a crucial role in stream ecosystem functioning with regard to physical characteristics (e.g. stream temperature), biogeochemical processes, (Ryan, 2010), and nutrient cyclings, which ultimately control the water quality of the stream (Lautz & Siegel, 2007). HE is driven by many physical attributes. Parent lithology of watersheds affects HE by controlling sediment porosity and hydraulic conductivity (Morrice, Valett, Dahm, & Campana, 1997). In addition, bed topography (i.e.dunes and ripples in sediments) drives HE through changes in local hydraulics (Harvey & Bencala 1993). Stream slope, morphology, and bed form (e.g. pool-step sequence, channel sinuosity, etc) also influence HE (Wondzell, 2005). As a result, extent of HE depends on a wide variety of factors including, but not limited to, sedimentporosity and hydraulic conductivity, channel morphology, as well as strength of groundwater upwelling, discharge, and size of the channel (Hancock, 2002). Size of HE in the vertical and lateral directions can range anywhere from a few centimeters to tens of meters depending on all of the above stated factors (Kasahara & Hill,2000). From a hydrological perspective, the hyporheic zone also has the potential to reduce peak discharge during storm events (Kasahara & Hill, 2002), a quality of particular importance in urban areas with extensive impervioussurfaces prone to storm runoff. The ecological significance of HE is even greater than just hydrologic flux. HE brings oxygen and other substrates in and flushes wastes out (Hancock, 2002). This process lends itself to a buffer having the potential to impede pollutant transport within both surface water flow paths as well as groundwater (Hester & Gooseff, 2010). HE increases the contact time of stream water with chemically reactive sediments and microbial communities, which creates hot spots for biogeochemical processes (Findlay, 1995).Given the permeable nature of hyporheic sediments, as nutrients flow through the hyporheic zone, it is reactively filtered (Trimmer et al., 2012). Hyporheic zones also provide a means for nitrogen (N) and other nutrients to be temporarily stored before they are lost downstream (Reidy & Clinton, 2004). This longer residence time for materials and reactive filtration allows for nutrient processing and plays a crucial role in regulating theconcentrations and forms of N exported downstream (Ensign & Doyle, 2006).

Tracing variability of run‐off generation in mountainous permafrost of semi‐arid north‐eastern Mongolia

AbstractThe headwaters of mountainous, discontinuous permafrost regions in north‐eastern Mongolia are important water resources for the semi‐arid country, but little is known about hydrological processes there. Run‐off generation on south‐facing slopes, which are devoid of permafrost, has so far been neglected and is totally unknown for areas that have been affected by recent forest fires. To fill this knowledge gap, the present study applied artificial tracers on a steppe‐vegetated south‐facing and on two north‐facing slopes, burned and unburned. Combined sprinkling and dye tracer experiments were used to visualize processes of infiltration and water fluxes in the unsaturated zone. On the unburned north‐facing slope, rapid and widespread infiltration through a wet organic layer was observed down to the permafrost. On the burned profile, rapid infiltration occurred through a combusted organic and underlying mineral layer. Stained water seeped out at the bottom of both profiles suggesting a general tendency to subsurface stormflow (SSF). Ongoing SSF could directly be studied 24 h after a high‐intensity rainfall event on a 55‐m hillslope section in the burned forest. Measurements of water temperature proved the role of the permafrost layer as a base horizon for SSF. Repeated tracer injections allowed direct insights into SSF dynamics: A first injection suggested rather slow dispersive subsurface flow paths; whereas 18 h later, a second injection traced a more preferential flow system with 20 times quicker flow velocities. We speculate that these pronounced SSF dynamics are limited to burned slopes where a thermally insulating organic layer is absent. On three south‐facing soil profiles, the applied tracer remained in the uppermost 5 cm of a silt‐rich mineral soil horizon. No signs of preferential infiltration could be found, which suggested reduced biological activity under a harsh, dry and cold climate. Instead, direct observations, distributed tracers and charcoal samples provided evidence for the occurrence of overland flow. Copyright © 2014 John Wiley &amp; Sons, Ltd.