Subsurface Flow Paths Research Articles

Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil

AbstractDissolved organic and inorganic carbon (DOC and DIC) influence water quality, ecosystem health, and carbon cycling. Dissolved carbon species are produced by biogeochemical reactions and laterally exported to streams via distinct shallow and deep subsurface flow paths. These processes are arduous to measure and challenge the quantification of global carbon cycles. Here we ask: when, where, and how much is dissolved carbon produced in and laterally exported from the subsurface to streams? We used a catchment‐scale reactive transport model, BioRT‐HBV, with hydrometeorology and stream carbon data to illuminate the “invisible” subsurface processes at Sleepers River, a carbonate‐based catchment in Vermont, United States. Results depict a conceptual model where DOC is produced mostly in shallow soils (3.7 ± 0.6 g/m2/yr) and in summer at peak root and microbial respiration. DOC is flushed from soils to the stream (1.0 ± 0.2 g/m2/yr) especially during snowmelt and storms. A large fraction of DOC (2.5 ± 0.2 g/m2/yr) percolates to the deeper subsurface, fueling deep respiration to generate DIC. DIC is exported predominantly from the deeper subsurface (7.1 ± 0.4 g/m2/yr, compared to 1.3 ± 0.3 g/m2/yr from shallow soils). Deep respiration reduces DOC and increases DIC concentrations at depth, leading to commonly observed DOC flushing (increasing concentrations with discharge) and DIC dilution patterns (decreasing concentrations with discharge). Surprisingly, respiration processes generate more DIC than weathering in this carbonate‐based catchment. These findings underscore the importance of vertical connectivity between the shallow and deep subsurface, highlighting the overlooked role of deep carbon processing and export.

Subsurface hydrological connectivity controls nitrate export flux in a hilly catchment

Subsurface runoff represents the main pathway of nitrate transport in hilly catchments. The magnitude of nitrate export from a source area is closely related to subsurface hydrological connectivity, which refers to the linkage of separate regions of a catchment via subsurface runoff. However, understanding of how subsurface hydrological connectivity regulates catchment nitrate export remains insufficient. This study conducted high-frequency monitoring of shallow groundwater in a hilly catchment over 17 months. Subsurface hydrological connectivity of the catchment over 38 rainfall events was analyzed by combining topography-based upscaling of shallow groundwater and graph theory. Moreover, cross-correlation analysis was used to evaluate the time-series similarity between subsurface hydrological connectivity and nitrate flux during rainfall events. The results showed that the maximum subsurface hydrological connectivity during 32 out of 38 rainfall events was below 0.5. Although subsurface flow paths (i.e., the pathways of lateral subsurface runoff) exhibited clear dynamic extension and contraction during rainfall events, most areas in the catchment did not establish subsurface hydrological connectivity with the stream. The primary pattern of nitrate export was flushing (44.7%), followed by dilution (34.2%), and chemostatic behavior (21.1%). A threshold relationship between subsurface hydrological connectivity and nitrate flux was identified, with nitrate flux rapidly increasing after the subsurface connectivity strength exceeded 0.121. Moreover, the median value of cross-correlation coefficients reached 0.67, which indicated subsurface hydrological connectivity exerts a strong control on nitrate flux. However, this control effect is not constant and it increases with rainfall amount and intensity as a power function. The results of this study provide comprehensive insights into the subsurface hydrological control of catchment nitrate export.

Evaluation of Groundwater Chemistry and Multivariate Statistical Studies in Parts of Tirupur District, India

Groundwater quality at any location depends on the mutual possessions of several progressions beside the subsurface flow path. The natural history and degree of infectivity in a particular region can be ascertained by hydro-chemical statistics. In order to assess the groundwater chemistry and its appropriateness for consumption purposes, sixty-three subsurface water samples were analyzed. Based on the perceptive of the hydro-geological and groundwater environment, the chemical characteristics of the groundwater were evaluated. The favored parameters for the quality assessment were major anions and cations along with pH, turbidity, total dissolved solids, total hardness, fluoride and iron as trace metals. Total dissolved solids distribution, hydro-chemical facies, Piper diagram, factor analysis, correlation studies and cluster analysis were done to monitor the study zone. The total dissolved solids (TDS) were clustered independently. It was concluded from the assessment that the groundwater of the Tirupur region was critically deteriorated by different anthropogenic activities.

A model of temporal and spatial river network evolution with climatic inputs

Predicting the temporal and spatial evolution of the river network is part of the Earth's critical zone investigations, which has become an important endeavor. However, modeling integration of the river network and critical zone over millions of years is rare. We address the problem of how to predict integrated river length development as a function of time within a framework of addressing the critical zone depth as a function of time. In case of groundwater-river interaction, we find a non-linear spatio-temporal scaling relationship between time, t, and total river length L, given by t≈Lp with power p being near 1.2. The basis of our model is the presumption that groundwater flow paths are relevant to river integration. As river integration may proceed over disconnected basins with irregular relief, the relevant optimal subsurface flow paths are proposed to be defined within a 3D network, with optimal path exponent 1.43. Because the 2D model of the river length has already been shown to relate to a power of the Euclidean distance across a drainage basin with the predicted universal optimal path exponent from percolation theory, Dopt = 1.21, the optimal groundwater paths should relate to the surface river length with an exponent equaling the ratio 1.43/1.21 = 1.18. To define a predictive relationship for the river length, we need to use specific length and time scales. We assume that the fundamental specific length scale is a characteristic particle size (which is commonly used to define the pore scale flow network), and the fundamental time scale is the ratio of the particle size to the regional groundwater flow rate. In this paper, we consider cases of predicting spatio-temporal scaling of drainage organization in the southwestern USA–the Amargosa, Mojave, Gila (and its tributaries) and the Rio Grande, and Pecos Rivers. For the Mojave and Gila Rivers, theoretical results for time scales of river integration since ca. 10 Ma are quite predictive, though the predicted time scales exceed observation for the Rio Grande and Pecos.

The Role of Topography in Controlling Evapotranspiration Age

AbstractEvapotranspiration (ET) age is a key metric of water sustainability but a major unknown partly due to the extreme difficulty in modeling it. Groundwater is found to be important in ET age variations in small‐scale studies, yet our understanding is insufficient because groundwater systems are nested across scales. Here, we conducted GPU‐accelerated particle tracking with integrated hydrologic modeling to quantify the variations in ET age at a regional scale of ∼0.4 M km2. Simulation results reveal topography‐driven flow paths shaping the spatial and temporal patterns of ET age variations. On ridges, where root zone decoupling with deep subsurface storage, ET age is generally young, with seasonal variations dominated by meteorological conditions. In the valley bottom, ET age is generally old, with significant subseasonal variations caused by the convergence of subsurface flow paths. On hillslopes with water table depths ranging from 1 to 10 m, ET age shows strong seasonal variations caused by the connections with lateral groundwater regulated by ET demand. Our modeling approach provides insights into the basic linkages between ET age and topography at large scale. Our work highlights the perspective of multiscale studies of ET age, suggesting new field experiments to test these process connections and to determine if such linkages warrant inclusion in Earth System Models.

Hillslope‐Channel Transitions and the Role of Water Tracks in a Changing Permafrost Landscape

AbstractThe Arctic is experiencing rapid climate change, and the effect on hydrologic processes and resulting geomorphic changes to hillslopes and channels is unclear because we lack quantitative models and theory for rapid changes resulting from thawing permafrost. The presence of permafrost modulates water flow and the stability of soil‐mantled slopes, implying that there should be a signature of permafrost processes, including warming‐driven disturbance, in channel network extent. To inform understanding of hillslope‐channel dynamics under changing climates, we examined soil‐mantled hillslopes within a ∼300 km2 area of the Seward Peninsula, western Alaska, where discontinuous permafrost is particularly susceptible to thaw and rapid landscape change. In this study, we pair high‐resolution topographic and satellite data to multi‐annual observations of InSAR‐derived surface displacement over a 5‐year period to quantify spatial variations in topographic change across an upland landscape. We find that neither the basin slope nor the presence of knickzones controls the magnitude of recent surface displacements within the study basin, as may be expected under conceptual models of temperate hillslope evolution. Rather, the highest displacement magnitudes tended to occur at the broad hillslope‐channel transition zone. In this study area, this zone is occupied by water tracks, which are zero‐order ecogeomorphic features that concentrate surface and subsurface flow paths. Our results suggest that water tracks, which appear to occupy hillslope positions between saturation and incision thresholds, are vulnerable to warming‐induced subsidence and incision. We hypothesize that gullying within water tracks will outpace infilling by hillslope processes, resulting in the growth of the channel network under future warming.

Cascading effects of climate change and wildfire on a subarctic lake: A 20‐year case study of watershed change

AbstractMean annual air temperature has been increasing in Alaska since the 1970s and is expected to continue to increase through the current century, resulting in significant environmental changes (e.g., permafrost thaw, shifts in vegetation community composition and distribution, increased wildfire frequency and severity). Because there is little long‐term monitoring data available on lake ecosystems in the subarctic it is difficult to predict how lakes will respond. Here we present data from an ~20‐year long‐term lake and climate monitoring program in Interior Alaska. A significant portion of the lake catchment was burned by wildfires in 1986 and again in 2004. As a result, much of the vegetation in the lake catchment was converted from spruce‐dominated forest to deciduous forest, indicating likely permafrost degradation. At a nearby monitoring site absent of fire effects, mean annual ground temperatures at 50 and 90 cm warmed significantly from about −2°C to about −1°C over the 20‐year monitoring period. Warming of similar or greater magnitude is inferred at the study site due to fire effects. Lake water analyses before and after the 2004 fire show a significant postfire increase in dissolved organic carbon, total nitrogen, and total phosphorous that persisted for a short period (~3 years) and was followed by small, but significant, increases in specific conductance and ion concentrations. Approximately 15 years postfire sulfate and cation concentrations in the lake increased exponentially due to the development of a groundwater seep near the lake. The seep likely formed as a result of permafrost thaw creating new subsurface flowpaths in response to long‐term climate warming and fire effects. In 2021, specific conductance and sulfate concentrations reached 657 μS/cm and 71 mg/L respectively, exceeding tolerance thresholds of the water lily Nuphar lutea. In 2021, N. lutea beds that had occupied the lake since 1981 were no longer visible. Depending on local catchment characteristics, source waters and flow paths contributing to small subarctic lakes will continue to evolve uniquely in response to climate change and thawing permafrost. This case study shows the cascading effects of warming soil and wildfires on catchment characteristics as well as terrestrial and aquatic vegetation and lake water chemistry.

Simulating the recession dynamics of Arctic catchments in the context of a thawing permafrost

In cold regions, climate warming is causing permafrost to thaw, which modifies the dynamics of groundwater flow from a local system, constrained by frozen ground, to a regional system of interconnected aquifers. Under the assumption presented in Brutsaert (2005), the recession slope of an arctic catchment hydrograph is linearly related to permafrost thawing depth. The recession analysis of arctic river flow may therefore reflect permafrost thawing dynamics. In areas where permafrost observations are rare, recession analysis appears as a valuable method to be exploited since there are extensive datasets of river-discharge for the Arctic, with an exceptionally large temporal and spatial coverage. Yet, it has been shown that the linear relationship between recession slope and permafrost thaw depth may be complicated by the extent of the permafrost, the landscape topography as well as the hydraulic properties of the aquifer. The integrated surface and subsurface hydrologic model Hydrogeosphere (HGS) is used here to account for hydrological complexifications and mechanisms involved when permafrost extent decreases and landscape hillslope increases. Previous modeling studies have already tested the impacts of these factors on the hydrological signature of arctic catchments. However, few have been able to (1) simulate permafrost extent, catchment hillslope and the resulting groundwater flowpaths in three dimensions, (2) distinguish the impact of permafrost thawing in extent from those of permafrost thawing in thickness and (3) represent the connection between surface and subsurface with a coupled approach. These assets allow to conclude that river-tàlik development and valley incision both clearly increase the vertical connectivity of the subsurface flowpaths as well as the non-linearity of the reservoir. The flexibility and the realism of HGS model allow to compare the modeling outputs with the dynamics of 336 real catchments from the Arctic. The comparison corroborates our results: below continuous, assumptions behind Brutsaert conceptual model are not verified and it is no longer possible to relate recession constant to permafrost thaw depth. Finally, this study gives the keys to start elaborating on the recession analysis method to be able to relate the hydrological signature of Arctic rivers to permafrost thawing rate in any type of permafrost extent and catchment topography.

The influence of hyporheic exchange with spur dikes: Laboratory experiments

It is important to understand heat transfer changes in the hyporheic zone of spur dikes when assessing ecological restoration and sustainability in rivers. To reveal the rule of hyporheic exchange under the action of a spur dike, the sinusoidal temperature change of surface water was set with a period of 24 h. The porous subsurface flow paths under different riverbed forms were plotted through dye tracing. In addition, the influences of surface water velocity (U), river width narrowing ratio (L/B), the number of spur dikes and riverbed shape on hyporheic exchange were also studied using the temperature tracing method. The results show that the hyporheic exchange path presents a trend of diagonal flow downstream, first conducting turbulent diffusion upstream and then sinking, with different riverbed morphologies having different path lengths. The temperature of surface water has a significant controlling effect on the temperature field of the riverbed. The sediment temperature change from the water–sediment interface to 0.3 m depth is closer to the sinusoidal trend. The change in surface water velocity (U) has the most significant effect on the temperature amplitude ratio. The temperature field below spur dikes is more susceptible to the impact of spur dikes and usually presents a “semielliptical” high-temperature area. With the increase in surface water velocity (U), river width narrowing ratio (L/B), and the number of spur dikes, the range of hyporheic exchange under the dam head also increases, which is positively related to the vertical section hyporheic exchange flux at the dam head. The local backwater area generated by spur dikes also affects pressure field distribution near spur dikes, but the changes caused by the fluctuation of the riverbed itself are greater than the impact of spur dikes on the riverbed. The water–sediment interface velocity at the spur dike increases sharply, and the upstream 1 m of the dam head gradually experiences upflow, drops sharply after reaching the maximum at the dam head, and then approaches zero after 1 m. There is no obvious upwelling and downwelling at the whole interface.

Extended growing seasons and decreases in hydrologic connectivity indicate increasing water stress in humid, temperate forests

Forested headwater catchments are important sources of stable and abundant freshwater resources. Interactions between vegetation and topography influence lateral hydrologic connectivity by altering shallow subsurface flow paths. This in turn influences vegetation density along those paths, and subsequent hydrologic partitioning between localized water use and subsurface flows at catchment scales. Climate change impacts on forests, and the degree to which they reshape feedbacks between evapotranspiration (ET) and hydrologic connectivity, remain unclear. To clarify the extent and drivers of changing lateral hydrologic connectivity, we assessed relative changes in upslope to downslope vegetation density using the Normalized Difference Vegetation Index (NDVI) from 1984 – 2021 in 30,044 forested catchments across the Southern Appalachian Mountains. Increasing upslope NDVI relative to downslope NDVI was used as a proxy for decreasing lateral hydrologic connectivity. We then related changes in connectivity to climate and streamflow dynamics across 28 sub-regional reference watersheds. We found decreases in the ratio of downslope to upslope NDVI in almost half of the catchments (48.5%), primarily due to increasing upslope NDVI. This indicates increasing ET upslope and a decline in lateral hydrologic subsidy to downslope given precipitation. This was also supported by faster streamflow recession and increasing ET estimates relative to precipitation in over half of reference watersheds. The strongest predictor of decreasing connectivity was growing season minimum temperature (Tmin), which increased in 88% of catchments (Mean R2 = 0.27 +/- 0.13). While Tmin is not a dominant atmospheric driver of ET, this pattern has been closely linked to lengthened growing seasons. This suggests that alteration of lateral hydrologic connectivity is mainly driven by ecophysiological responses to changing climate rather than directly by atmospheric drivers. Our results emphasize the importance of vegetation dynamics shifting hydrologic partitioning and driving water limitations even in humid, temperate forests.

Identifying the recharge and salinization mechanisms of the Shekastian saline spring in southern Iran.

To identify the recharge, and the salinization mechanisms of Shekastian saline spring, appearing via thin limestone layers on the Shekastian stream bed, southern Iran, a comprehensive study was conducted using multi-tracing approach. Hydrochemical tracing indicated that the halite dissolution is the main salinity source for Shekastian spring. Similar to surface waters, spring salinity is amplified by evaporation during dry season, indicating that recharge comes from surface waters. The temperature of the spring water varies hourly, which is another sign that surface waters recharge the spring. Applying the discharge tracing method at two low-discharge times in two consecutive years by precise longitudinal discharge monitoring of the Shekastian stream above and below the spring site revealed that the main source of recharge for the Shekastian saline spring is water escape via thin limestone layers, located on a stream bed above the spring site. Results of isotope tracing indicated that Shekastian saline spring is recharged from evaporated surface water, subjecting to CO2 gas along subsurface flow path of recharging water. Geologic/geomorphologic evidences, supported by results of hydrochemical tracing, revealed that halite dissolution of Gachsaran evaporite formation by spring recharging water is the main salinization source for the Shekastian saline spring. To avoid salinization of Shekastian stream by Shekastian saline spring, draining the spring recharging water at downstream vicinity of spring recharging stream by building an underground interceptor drainage system is suggested, in which resulted in flow stopping the spring.

Geoelectric Characterization of Hyporheic Exchange Flow in the Bedrock‐Lined Streambed of East Fork Poplar Creek, Oak Ridge, Tennessee

AbstractA multimethod geoelectric survey was implemented between January and March 2022 along a 220‐m long reach of the bedrock‐lined streambed of East Fork Poplar Creek in Oak Ridge, Tennessee to identify locations of surface‐water and groundwater exchange and characterize the subsurface flow paths that convey water between the stream and flood plain. A waterborne self‐potential (WaSP) survey was completed in January 2022 to measure the electric streaming‐potential field in the stream. Electric resistivity tomography (ERT) was performed in March 2022 on the flood plain adjacent to the WaSP survey reach to map the electric resistivity distribution and characterize the hydrogeology and subsurface flow paths that facilitate surface‐water and groundwater exchange in the bedrock‐lined stream. The combination of WaSP and ERT data support the qualitative interpretation that surface‐water and groundwater exchange likely occurs along fractures in outcropping bedrock and along two fault lines that intersect the limestone creek bed.

Three-dimensional Electric-field Vector Resistivity Imaging for Deep subsurface flow paths in Heterogeneous crystalline rocks

Three-dimensional Electric-field Vector Resistivity Imaging for Deep subsurface flow paths in Heterogeneous crystalline rocks

Water source dynamics influence macroinvertebrate communities across groundwater-fed streams in a glacierized catchment

Groundwater contributions to streamflow significantly influence the structure and function of riverine ecosystems, particularly in glacierized catchments where there are marked differences in water sources and subsurface flow paths. Here, we investigated spatial and temporal variation in relationships between water sources, flow paths, physical and chemical processes, organic matter, microbial biofilms, and macroinvertebrates across groundwater-fed streams in the glacierized Toklat River catchment of Denali National Park, Alaska. Streams fed predominantly by seepage from the valley sides were perennial, whereas streams sustained by glacial meltwater seepage were ephemeral. Differences in environmental conditions between flow regimes appeared to influence spatial and temporal patterns of organic matter, linking to macroinvertebrate community dynamics. Macroinvertebrates in perennial streams were supported by fine particulate organic matter from subsurface flow paths during summer, transitioning to a combination of fine particulate matter and leaf litter in autumn. In comparison, macroinvertebrates inhabiting ephemeral streams, which only flowed during autumn, were supported by leaf litter. Some macroinvertebrate taxa were unaffected by turnover in organic matter, indicating potential plasticity in organic matter resource use. Findings highlight the importance of considering spatial and temporal variation in groundwater-fed streams, considering that projected hydrological changes under a changing climate may have significant implications for these systems.

Seasonal Trends of DOM Character in Soils and Stream Change With Snowmelt Timing

AbstractIn many watersheds, runoff occurring with snowmelt brings the largest pulse of dissolved organic matter (DOM) from soils to streams. Yet, exactly how soil and stream DOM fractions are altered as snowmelt timing changes is not well understood. We studied the optical character of DOM as it moved through a first‐order northern hardwood watershed through the snowmelt period. While stream dissolved organic carbon (DOC; mean = 4.6 mg C L−1) concentrations were not correlated with snowmelt flushing, stream DOM appeared humic‐like throughout the study period (humification index, HIX; mean = 10.9). Within the stream, we measured seasonal increases in an inverse index of DOM molecular weight and processing (spectral slope, S275–295; range = 0.0102–0.0214) and in an index of recently derived DOM (Biological Index, BIX; range = 0.2–1.1). We measured a decrease in an inverse index of DOM oxidation (ratio of Peak C to Peak A, or C/A; range = 0.59–2.01) through the snowmelt period. Stream BIX and C/A tracked those measured in the north aspect soils throughout the study. C/A was unique as it decreased from source to stream, likely reflecting the diagenesis of DOM through soils. These data suggest that changes in the timing of snowmelt and hydrologic connectedness within a watershed affect seasonal changes in stream DOM character. We suggest that watersheds with deep glacial till parent materials have stream connectivity limited by subsurface flow paths, with stream DOM character being determined largely by slow melt and a longer period of infiltration.

Examining spatial variation in soil solutes and flowpaths in a semi-arid, montane catchment

Biogeochemical properties of soils play a crucial role in soil and stream chemistry throughout a watershed. How water interacts with soils during subsurface flow can have impacts on water quality, thus, it is fundamental to understand where and how certain soil water chemical processes occur within a catchment. In this study, ~200 soil samples were evaluated throughout a small catchment in the Front Range of Colorado, USA to examine spatial and vertical patterns in major soil solutes among different landscape units: riparian areas, alluvial/colluvial fans, and steep hillslopes. Solutes were extracted from the soil samples in the laboratory and analyzed for major cation (Li, K, Mg, Br, and Ca) and anion (F, Cl, NO2, NO3, PO4, and SO4) concentrations using ion chromatography. Concentrations of most solutes were greater in near surface soils (10 cm) than in deeper soils (100 cm) across all landscape units, except for F which increased with depth, suggestive of surface accumulation processes such as dust deposition or enrichment due to biotic cycling. Potassium had the highest variation between depths, ranging from 1.04 mg/l (100 cm) to 3.13 mg/l (10 cm) sampled from riparian landscape units. Nearly every solute was found to be enriched in riparian areas where vegetation was visibly denser, with higher mean concentrations than the hillslopes and fans, except for NO3 which had higher concentrations in the fans. Br, NO2, and PO4 concentrations were often below the detectable limit, and Li and Na were not variable between depths or landscape units. Ratioed stream water concentrations (K:Na, Ca:Mg, and NO3:Cl) vs. discharge relationships compared to the soil solute ratios indicated a hydraulic disconnection between the shallow soils (<100 cm) and the stream. Based on the comparisons among depths and landscape units, our findings suggest that K, Ca, F, and NO3 solutes may serve as valuable tracers to identify subsurface flowpaths as they are distinct among landscape units and depth within this catchment. However, interflow and/or shallow groundwater flow likely have little direct connection to streamflow generation.

Subsurface flow paths in a chronosequence of calcareous soils: impact of soil age and rainfall intensities on preferential flow occurrence

Abstract. Soil hydrologic processes play an important role in the hydro-pedo-geomorphological feedback cycle of landscape evolution. Soil properties and subsurface flow paths both change over time, but due to a lack of observations, subsurface water flow paths are often not properly represented in soil and landscape evolution models. We investigated the evolution of subsurface flow paths across a soil chronosequence in the calcareous glacier forefield at the Griessfirn glacier in the Swiss Alps. Young soils developed from calcareous parent material usually have a high pH value, which likely affects vegetation development and pedogenesis and thus the evolution of subsurface flow paths. We chose four glacial moraines of different ages (110, 160, 4 900, and 13 500 years) and conducted sprinkling experiments with the dye tracer Brilliant Blue on three plots at each moraine. Each plot was divided into three equal subplots, and dyed water was applied with three different irrigation intensities (20, 40, and 60 mm h−1) and an irrigation amount of 40 mm. Subsequent excavation of soil profiles enabled the tracing of subsurface flow paths. A change in flow types with increasing moraine age was observed from a rather homogeneous matrix flow at 110 and 160 years to heterogeneous matrix and finger-shaped flow at 4 900 and 13 500 years. However, the proportion of preferential flow paths is not necessarily directly related to the moraine age but rather to soil properties such as texture, soil layering, organic matter content, and vegetation characteristics such as root length density and biomass. Irrigation intensity had an effect on the number of finger-shaped flow paths at the two old moraines. We also found that flow paths in this calcareous material evolved differently compared to a previous study in siliceous material, which emphasizes the importance of parent material for flow path evolution. Our study provides a rare systematic dataset and observations on the evolution of vertical subsurface flow paths in calcareous soils, which is useful to improve their representation in the context of landscape evolution modeling.

Delineation of water flow paths in a tropical Andean headwater catchment with deep soils and permeable bedrock

AbstractTraditional hydrometric data combined with environmental tracers such as water stable isotopes contributes to improve the understanding of catchment hydrology. Nevertheless, the application of isotopic tracers in headwater catchments of the tropical Andes with deep soils and permeable parent material influenced by recent volcanism remains limited. In this study, the stable isotopic composition of precipitation, soil water, wetlands, and streamflow was studied to provide insights into the hydrology of a small tropical Andean catchment with deep and permeable volcanic soils, ash layers, and fractured bedrock resulting from Holocene volcanic activity. Although local precipitation forms under isotopic equilibrium conditions, the stable isotopic composition of precipitation is influenced by atmospheric moisture recycling processes. The spatial and temporal variability of isotopic signals and the analysis of inverse transit time proxies (ITTPs) of surface (streamflow) and subsurface (soil and wetlands) waters indicate that vertical flow paths through the deep volcanic ash soils are dominant across the catchment. The strongly damped isotopic composition of these waters points to high soil and wetlands water storage, increasing the transit time or age of stream water in the hydrological system. These findings indicate that water mobilizing through subsurface flow paths–that is, volcanic soils, ash layers, and cracks in the fractured bedrock resulting from Holocene volcanism–is the main contributor to streamflow generation. Comparison with previously published work from Andean catchments and other volcanic areas shows the diversity of hydrological conditions that can be found as a result of pedological and lithological differences shaped by volcanic activity. Therefore, site‐specific strategies may be needed to improve water resources management.

Topology and spatial-pressure-distribution reconstruction of an englacial channel

Abstract. Information about glacier hydrology is important for understanding glacier and ice sheet dynamics. However, our knowledge about water pathways and pressure remains limited, as in situ observations are sparse and methods for direct area-wide observations are limited due to the extreme and hard-to-access nature of the environment. In this paper, we present a method that allows for in situ data collection in englacial channels using sensing drifters. Furthermore, we demonstrate a model that takes the collected data and reconstructs the planar subsurface water flow paths providing spatial reference to the continuous water pressure measurements. We showcase this method by reconstructing the 2D topology and the water pressure distribution of a free-flowing englacial channel in Austre Brøggerbreen (Svalbard). The approach uses inertial measurements from submersible sensing drifters and reconstructs the water flow path between given start and end coordinates. Validation of the method was done on a separate supraglacial channel, showing an average error of 3.9 m and the total channel length error of 29 m (6.5 %). At the englacial channel, the average error is 12.1 m; the length error is 107 m (11.6 %); and the water pressure standard deviation is 3.4 hPa (0.3 %). Our method allows for mapping of subsurface water flow paths and spatially referencing the pressure distribution within. Further, our method would be extendable to the reconstruction of other, previously underexplored subsurface fluid flow paths such as pipelines or karst caves.

Impacts of seasonally frozen ground on streamflow recession in the Red River of the North Basin

AbstractThe projected rising temperatures due to climate change can alter the frozen ground's duration and thickness in the cold regions of the Northern Hemisphere, leading to changes in surface and subsurface contributions to streamflow. The initial step in understanding the impacts of the changing climate on the hydrologic response of the cold climate watersheds is to investigate the effects of the cold climate processes (e.g., freeze–thaw processes) on watershed response. However, complexities associated with the spatiotemporal data collection of cold climate processes have limited the capabilities of hydrologic models in simulating cold climate watershed responses. This study aims to evaluate the impacts of seasonally frozen soil on the streamflow recession in the Red River of the North Basin (RRB). To the best of our knowledge, this study is one of the first attempts to quantify the impacts of frozen soil on the storage‐discharge relationship of the RRB. We define a set of hypotheses to evaluate how cold climate processes influence the surface and subsurface flow paths and affect the linearity/non‐linearity of the storage‐discharge relationship in spring and summer. The results confirmed that the cold climate processes control the recession behavior of the RRB. It was found that the streamflow discharge is linearly related to the storage during spring, whereas during summer, the relationship was less linear. Statistical analyses revealed that the antecedent winter soil temperature and snowpack control the seasonal dynamics of hydrograph recession. Particularly, results indicated a more linear storage‐discharge relationship for antecedent winters characterized by colder soil temperatures and larger snowpack. These findings highlight the importance of the cold climate hydrologic processes in cold climate basins such as the RRB.