Surface Flow Paths Research Articles

Groundwater Resources of the Transboundary Quaternary Aquifer of the Lake Chad Basin: Towards a Better Management via Isotope Hydrology

A multi-tracer approach has been implemented in the southwestern part of the Lake Chad Basin to depict the functioning of aquifers in terms of recharge, relationship with surface water bodies, flow paths and contamination. The results are of interest for sustainable water management in the region. The multi-layered structure of the regional aquifer was highlighted with shallower and intermediate to deep flow paths. The shallower aquifer is recharged with rainwater and interconnected with surface water. The groundwater chemistry indicates geogenic influences in addition to a strong anthropogenic fingerprint. The intermediate to deep aquifer shows a longer residence time of groundwater, less connection with the surface and no to only a little anthropogenic influence. Ambient Background Levels (ABLs) and Threshold Values (TVs) show the qualitative status of the groundwater bodies and provide helpful information for water resources protection and the implementation of new directives for efficient and more sustainable groundwater exploitation.

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.

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.

Managed aquifer recharge site assessment with electromagnetic imaging: Identification of recharge flow paths

AbstractSurface spreading recharge, the intentional flooding of the ground surface to replenish a groundwater system, is one approach used to mitigate groundwater overdraft in California's Central Valley (CV). Choosing appropriate sites for surface spreading recharge, in regard to the sites’ ability to convey water from the ground surface to the desired recharge depth, can be challenging because of limited knowledge of the properties of the subsurface. In this study, we present an approach for using a towed time‐domain electromagnetic (tTEM) imaging method to develop three‐dimensional (3D) models of sediment type, map potential flow paths through the subsurface, and evaluate sites for surface spreading. We began with tTEM data from seven sites in the CV along with an existing resistivity‐to‐sediment type transform. We leveraged geostatistical methods to generate multiple 3D models of binary (flow and no‐flow) sediment type from the tTEM data. We then developed two metrics to assess the quality of sites for recharge: (a) the depth to the shallowest no‐flow unit beneath each point at the surface and (b) preferential flow paths lengths measured by finding the shortest distance through connected flow units between surface points and the desired depth. We explored how these metrics can be used to identify optimal areas within a site, then developed a way to compare and assess the relative suitability of each site using the decay in the number of vertical flow paths as a function of depth. Our methods can be used to rapidly identify potential sites for surface spreading recharge.

Novel predictors related to hysteresis and baseflow improve predictions of watershed nutrient loads: An example from Ontario's lower Great Lakes basin

Eutrophication has re-emerged in the lower Great Lakes basin resulting in critical water quality issues. Models that accurately predict nutrient loading from streams are needed to inform appropriate nutrient management decisions. Generalized additive models (GAMs) that use surrogate data from sensors to predict nutrient loads offer an alternative to commonly applied linear regression and may better handle relationship non-linearities and skewed water quality data. Five years (2015–2020) of water quantity and quality data from 11 agricultural watersheds in southern Ontario were used to develop GAMs to predict total phosphorus (TP) and nitrate (NO3−) loads. This study aimed to 1) use GAMs to predict nutrient loads using both common and novel predictors and 2) quantify and examine the variability in seasonal and annual nutrient loads. Along with routine surrogate model predictors (i.e., flow, turbidity, and seasonality), the addition of the baseflow proportion and the hydrograph position of flow observations improved model performance. Conversely, including the antecedent precipitation index minimally affected model performance, regardless of constituent. Seasonal and annual patterns in TP and NO3− load predictions mirrored that of the hydrologic regime. This study showed that parsimonious GAMs featuring novel model predictors can be used to predict nutrient loads while accounting for the partitioning of surface and subsurface flow paths and hysteresis between streamflow and water quality parameters that are frequently observed in a wide range of environments.

Morphometric Analysis by Using Remote Sensing & QGIS Approach to Evaluate the Aquifer Response of Two Sub Watersheds of Coastal Kerala

Global increase in demand of water resources stepped up due to climate change and increase in population, stimulates an efficient development, management and conservation of water resources by quantifying the available water within the catchment to bridge the gap between its demand and availability. Initiated at a watershed scale, it is important to quantitatively evaluate the geomorphological parameters, since all the hydrologic and geomorphic processes may be accounted and balanced within a watershed. This study presents an excellent morphometric characterization of Payppara and Tikovil sub watersheds of Meenachil basin, in the state of Kerala, India, evaluated using SRTM DEM at 30m×30m spatial resolution as input to QGIS. The important linear morphometric aspects like stream order, length of stream segments, bifurcation ratio, Rho coefficient, the areal morphometric aspects like form factor, drainage density, stream frequency, elongation ratio, length of overland flow and relief aspects like ruggedness number, watershed relief were calculated for a comprehensive understanding of the geomorphic characteristics of the watershed. The general terrain of Tikovil was found to be steep but it was relatively mild for Payppara. The gravity flow paths traced by few 4th order, 3rd order and 2nd order stream segments of Payppara were identified as potential groundwater recharge zones. The derived morphometric characteristics benefits a better understanding of the formation of erosive and permeable surface flow paths in response to precipitation, the principal input to the hydrological budget.

Subsurface Flow Path Modeling From Inertial Measurement Unit Sensor Data Using Infinite Hidden Markov Models

Subsurface flows are challenging to map due to the inaccessibility and extreme nature. This coupled with global positioning system (GPS) denied, sensorially deprived, complex, and locally self-similar environments result in most of the traditional mapping methods failing. This results in extensive subsurface environments that remain unknown and unexplored. In this paper, we show that it is possible to detect features using infinite hidden Markov model (iHMM) from inertial measurement unit data (IMU) collected along surface and subsurface flow paths. By assuming zero velocity at the beginning of each feature, we present a data driven model to reconstruct unknown 2D subsurface flow paths using 9 degrees of freedom (dof) IMU sensor data and two known location points along the path. In addition, we show the advantages of using iHMM compared to simpler feature extraction methods. The model is validated on multiple controlled examples, as well as experimental real world datasets.

A new approach for estimating and delineating within-field crop water stress zones with satellite imagery

ABSTRACT As water scarcity and demand for food both grow, viable solutions to efficiently irrigate crops are needed. The recent advent of variable rate irrigation systems provides technology for growers to water their fields accounting for the spatially heterogeneous demand of their crops for water. Some zones within a field may require less or more water based on varying solar radiation, soil type, or surface or subsurface water flow paths. Since these processes are heterogeneous and complex, they are challenging to model or measure with the spatial resolution necessary to implement successful variable rate irrigation. In this paper, we present a new approach that uses high-resolution (20 to 100 m) satellite imagery obtained from Sentinel-2 and Landsat 8 to estimate zones of crop water stress. Our approach integrates two independent measures of crop water availability – Normalized Difference Water Index, and Evapotranspiration modelled using remotely sensed land surface temperature – with k-means clustering to delineate zones in an automated fashion. Our study area in southern Idaho has been set up experimentally to grow crops that are water-stressed and not water-stressed, thus providing a rigorous design to test and validate our method. The results of our method agree well with our independent suite of crop water stress measures, such as yield, variable irrigation rates, and matric potential. This method can be used to develop data-driven zones for variable rate irrigation, which has the potential to improve the efficiency of water use without sacrificing yield.

Agrogeophysical methods for identifying soil pipes

The direct contribution of internal soil piping to soil losses or their collapse to form ephemeral gullies is a vital component in understanding total soil loss from agricultural lands. The generation and evolution of soil pipes occur below the surface, where survey-based measurements and analysis often fail to provide a comprehensive image of these crucial phenomena or their erosional evolution. The distribution of soil pipe collapses does not always correlate with surface flow paths, as indicated by surface topography. This lack of correlation makes inferences from traditional remote sensing or manual surveying of surface features subject to significant uncertainty and lack of correspondence to the actual subsurface flow path. Non-invasive geophysical methods provide a better alternative for high spatial resolution imaging to delineate, characterize, and map the distribution of soil pipe networks. In addition to being non-invasive, geophysical methods are more expedient and lower in cost than invasive techniques. In this paper, the feasibility and efficiency of two geophysical methods, ground penetrating radar (GPR) and electromagnetic induction (EMI), for delineating soil pipes in cross-sectional and plan view maps is presented. The evaluations are based on field measurements at a research site with established soil pipes and ephemeral gullies. Geophysical signatures from the two methods are evaluated using a composite (combined) cross-section plot that includes results of an invasive technique known as cone penetrologger testing (CPL). These signatures are then used to construct plan view maps of the area. The EMI results allow for the delineation of larger zones of fields having soil pipe networks. The GPR data provides a much better resolution. The GPR data can be used to construct maps (depth slices) representing different depths. The results demonstrate the high potential of EMI and GPR as useful tools for studying internal soil pipes.

Hydrologic modeling of reach scale fluxes from flood irrigated fields

Agricultural water is of considerable interest to water managers and policymakers as irrigation—particularly flood irrigation—accounts for the largest portion of freshwater use. However, characterization of how and when flood applied water contributes to storage and adjacent surface water bodies via return flow remains limited, particularly relative to the volume of studies addressing surface processes. This study focuses on improving our understanding of return flow paths and explicitly quantifying subsurface return flow contributions using hydro-geophysical observations coupled with a vadose zone transport flow model. Our approach, which simulates return flow in agricultural fields in a small, heavily instrumented field site in northern Wyoming, leverages daily measured values of evapotranspiration obtained from a large aperture scintillometer and atmospheric conditions, and uses seismic data to characterize heterogeneous structure of the soil profiles at field scale. We use these high-resolution, site specific datasets to parameterize a process-based vadose zone flow model that quantifies hydrologic response to flood irrigation along the stream reach, allowing us to identify return flow fluxes out of the fields. Using our approach, we found that the majority of applied irrigation water (60%) left the fields through subsurface flow paths, while only 13% was consumed by vegetation. Subsurface runoff over less-permeable soil layers, governed by the subsurface structural characteristics, was identified as the dominant subsurface flow process contributing to fluxes out of the fields. Importantly, our findings about the importance of subsurface return flow paths nuance previous research which has emphasized surface flow paths. By improving our understanding of return flow processes through a high- resolution vadose zone model that leverages climatological, geophysical, and hydrological observations, this study advances our understanding of return flow processes. Understanding how subsurface characteristics influence return flow generation in semi-arid agriculturally productive regions will help managers and policy makers balance desires for improved irrigation efficiency against concerns about irrigation-dependent ecosystem services.

Remote sensing vs. field-based monitoring of agricultural terrace degradation

The degradation of agricultural terraces is considered a major challenge to soil and water conservation in steep-slope viticulture. Although terracing is a widespread conservation practice, its sustainability is threatened by adverse climatic and man-made conditions. Previous studies have shown the impact of terrace designs on the formation of runoff pathways, causing degradation processes on terrace platforms (e.g. sheet erosion) and walls (e.g. piping, landslides, collapse). This study evaluates a remote sensing versus a field-based approach to monitor hydrological processes responsible for terrace degradation, as tested in a north-Italian vineyard. The field-based approach was based on spatially measured Soil Moisture Content (SMC) using a Time Domain Reflectometry (TDR) instrument, which clearly revealed saturation hotspots around two damaged terraces in the study area. Moreover, these zones showed a particular cross-sectional SMC profile, with the highest saturation close to the terrace platform edges. The remote sensing approach was based on aerial imagery acquired by an Unmanned Aerial Vehicle (UAV) and photogrammetric reconstruction of the vineyard geomorphology, allowing terrain-based analysis and physical erosion modelling. In this approach, simulations indicated that terrace damages could be partly explained by the formation of preferential runoff pathways caused by the terrace design. This parallel methodology allowed a comparison of the merits and limitations of either approach, as done in light of published work. The occurrence of two SMC hotspots at terrace edges (and their non-typical cross-sectional profiles) could be better understood from simulated surface flow paths. While the causal relationship between heterogeneous soil saturation and terrace instability has been previously reported in literature, the novelty of the presented study is the use of topsoil SMC as an indicator of potential damages, favouring the scalability compared to fixed, local and often intrusive terrace sub-surface experiments. Remote sensing based approaches, however, tend to offer the most time-efficient solution on larger scales, and aerial acquisition of SMC distribution could thus potentially offer a powerful integrated methodology.

An Aridity Index‐Based Formulation of Streamflow Components

AbstractDirect runoff and baseflow are the two primary components of total streamflow, and their accurate estimation is indispensable for a variety of hydrologic applications. While direct runoff is the quick response stemming from surface and shallow subsurface flow paths and is often associated with floods, baseflow represents the groundwater contribution from stored sources (e.g., groundwater) to streams and is crucial for environmental flow regulations, and water supply, among others. L'vovich (1979) proposed a two‐step water balance partitioning, where precipitation is divided into direct runoff and catchment wetting, followed by the disaggregation of the latter into baseflow and evapotranspiration. Here, we investigate the role of the aridity index (ratio between mean‐annual potential evapotranspiration and precipitation) in controlling the long‐term (mean‐annual) fluxes of direct runoff and baseflow. We present an analytical solution beginning with similar assumptions as proposed by Budyko (1974), leading to two complementary expressions for the two fluxes. The aridity index explained 77% and 89% of variability in direct runoff and baseflow from 378 catchments within the continental United States, while our formulations were able to reproduce the patterns of water balance partitioning proposed by L'vovich (1979) at the mean‐annual timescale. Our approach can be used to further understand how climate and landscape controls the terrestrial water balance at mean‐annual timescales, while also representing a step toward the prediction of baseflow and direct runoff at ungauged basins.

An agent-based model that simulates the spatio-temporal dynamics of sources and transfer mechanisms contributing faecal indicator organisms to streams. Part 2: Application to a small agricultural catchment

The new Model for the Agent-based simulation of Faecal Indicator Organisms (MAFIO) is applied to a small (0.42 km2) Scottish agricultural catchment to simulate the dynamics of E. coli arising from sheep and cattle farming, in order to provide a proof-of-concept. The hydrological environment for MAFIO was simulated by the “best” ensemble run of the tracer-aided ecohydrological model EcH2O-iso, obtained through multi-criteria calibration to stream discharge (MAE: 1.37 L s−1) and spatially-distributed stable isotope data (MAE: 1.14–3.02‰) for the period April–December 2017. MAFIO was then applied for the period June–August for which twice-weekly E. coli loads were quantified at up to three sites along the stream. Performance in simulating these data suggested the model has skill in capturing the transfer of faecal indicator organisms (FIOs) from livestock to streams via the processes of direct deposition, transport in overland flow and seepage from areas of degraded soil. Furthermore, its agent-based structure allowed source areas, transfer mechanisms and host animals contributing FIOs to the stream to be quantified. Such information is likely to have substantial value in the context of designing and spatially-targeting mitigation measures against impaired microbial water quality. This study also revealed, however, that avenues exist for improving process conceptualisation in MAFIO (e.g. to include FIO contributions from wildlife) and highlighted the need to quantitatively assess how uncertainty in the spatial extent of surface flow paths in the simulated hydrological environment may affect FIO simulations. Despite the consequent status of MAFIO as a research-level model, its encouraging performance in this proof-of-concept study suggests the model has significant potential for eventual incorporation into decision support frameworks.

Evaluating erosional impacts on open-air archaeological sites along the Doring River, South Africa: methods and implications for research prioritization

Open-air sites in arid and semi-arid landscapes are often subject to prolonged periods of exposure and episodes of erosion that can lead to the redistribution of artefacts and the loss of behaviorally significant spatial information. This is true along the Doring River, South Africa, where archaeologically rich sediment stacks with records exceeding 200,000 years are undergoing rapid erosion in response to modern climatic conditions and land use practices. This paper evaluates the impacts of past and future erosion on the disaggregation of artefacts from these open-air sites and the resulting loss of stratigraphic context and behaviorally significant spatial information. We use low elevation aerial images of the Klein Hoek 1 locality captured by an unmanned aerial vehicle (UAV) to develop a high-resolution local digital terrain model (DTM), which we use to model surface flow paths and quantify the potential for future sediment loss using the Revised Universal Soil Loss Equation (RUSLE). We compare the results of these analyses to the distribution of artefacts of different ages to assess artefact dispersion and to guide future research priorities at the locality. We find that some artefact clusters retain significant spatial integrity, whereas others are dispersed and likely out of primary context. The results also indicate that the geomorphic stability of a large part of Klein Hoek 1 has been compromised by erosion, with limited prospects for long-term survival given the present climate and land use practices.

Small Arctic rivers mapped from Sentinel-2 satellite imagery and ArcticDEM

Small rivers (<30 m) are significant components of terrestrial river networks and remote sensing is a necessary tool to study them at the global scale. However, current global hydrography data products generally include only rivers that are wider than 30 m, neglecting smaller waterways and thus limiting our understanding of surface water processes, especially in remote areas. We present a new automated methodology to map two Arctic actively-flowing river networks (Kotuy and Coleen Rivers, with study areas of ~12,000 km2 and 10,000 km2, respectively), including small rivers as narrow as 10 m from Sentinel-2 multispectral satellite imagery and high-resolution ArcticDEM digital elevation data. First, preliminary river network masks were generated by using Gabor filtering and path opening morphological operations on Normalized Difference Water Index (NDWI) images. Second, ArcticDEM data were employed to simulate surface flow paths, and the resultant drainage networks were used to create river areas of interest (AOI) and subsequently eliminate non-water features outside the river AOI. Third, gaps along remotely sensed small river channels were filled using the ArcticDEM-modeled drainage networks, yielding continuous river networks with high spatial resolution. We compared the 10 m Sentinel-ArcticDEM merged River Network (SARN) with the National Hydrography Dataset (NHD) and four 30 m Landsat-derived hydrography data products (GSW, GRWL, FROM-GLC 2017, and G3WBM). We conclude: (1) SARNs have similar quality to the >1st order NHD (>70% of NHD streams appear in SARNs). (2) At a reach scale, SARNs include more small rivers with the drainage densities at least ~4.5 times larger than Landsat-derived hydrography data products. Similarly, coupling ArcticDEM data with 30 m Landsat satellite images increases resultant drainage densities approximately three-fold. (3) At a catchment scale, merging remotely sensed river networks with ArcticDEM-modeled drainage networks significantly improves river connectivity (defined as the longest connected river length) with at least a ten-fold increase over the other hydrography data products examined here. In sum, merging high-resolution Sentinel-2 imagery with ArcticDEM data enable effective, continuous mapping of complex fluvial drainage patterns of Arctic rivers.

Riparian catchments: A landscape approach to link uplands with riparian zones for agricultural and ecosystem conservation

V egetated riparian buffers can trap and treat pollutants in runoff from agricultural uplands, thereby protecting stream water quality and the integrity of aquatic systems (Liu et al. 2008; Dosskey et al. 2010). The effectiveness of a buffer in achieving these goals depends on buffer design, buffer extent (i.e., stream length occupied), distributions of hydrologic flows through the buffer in time and space, and the conservation management of upslope agricultural lands. Soil survey and topographic data can be used to prioritize buffer implementation, and to design buffers that effectively intercept surface and/or subsurface flow paths according to landscape characteristics (Tomer et al. 2015; Schultz et al. 2009). However, buffer effectiveness can be enhanced through conservation management of agricultural uplands, using (as examples) zero/zonal tillage, field borders, and/or contour buffers, practices that can attenuate peak runoff flows that may bypass buffer vegetation (Dabney et al. 2006). In other words, riparian buffer performance depends on the use of upland practices that can effectively reduce erosion and slow or detain runoff from uplands. Benefits of integrated strategies for co-managing upland and riparian systems at watershed scale have been discussed, but no framework has been suggested to enable full landscape planning to be undertaken in an extensive yet detailed way. A methodology for landscape analysis that provides a way to …

Wetland Connectivity Thresholds and Flow Dynamics From Stage Measurements

AbstractDepressional wetlands are dominant features in many low‐gradient landscapes, where they provide water storage and exchange. Typical basin morphology enables water storage during drier periods, when surface flow paths are disconnected and exchange is limited to slower groundwater flow paths. Under wetter conditions, wetland stage can exceed surface connection thresholds, activating surface flow paths to downstream waters. Empirical methods are needed to quantify these dynamics and thus to assess their role in landscape hydrology and associated functions. We developed a new water budget‐based approach to enumerate connectivity thresholds and flows from stage measurements. We propose that this approach, termed Connectivity and Flow from Stage (CFS), has broad applicability across wetlandscapes. We applied the CFS method in the Big Cypress National Preserve, where we hypothesized that surface connectivity episodes control water and solute flux, with consequences for exported carbonate weathering products and thus for karst landform evolution. Across five study wetlands, this analysis detected surface connectivity thresholds and assessed temporal flow dynamics. Imputed connectivity thresholds were clear from stage‐dependent net flow dynamics and aligned well with LiDAR‐derived thresholds. Water export occurred overwhelmingly when stage exceeded these thresholds, indicating that water and solute export from these wetlands is dominated by periods of enhanced landscape connectivity. Notably, the presented CFS method can quantify wetland connectivity thresholds from stage data, even without supporting geomorphic information. This approach is useful for understanding hydrologic controls on biogeomorphic evolution in this particular karst landscape, and more broadly for inferring wetland connectivity patterns and magnitudes in other wetlandscape settings.

Study of Soil Erosion for Evaluation of Long-term Behavior of Radionuclides Deposited on Land

The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) resulted in the deposition of large quantities of radionuclides over parts of eastern Japan. Radioactive contaminants have been observed over a large area including forests, cities, rivers and lakes. Due to the strong adsorption of radioactive cesium by soil particles, radioactive cesium migrates with the eroded soil, follows the surface flow paths, and is delivered downstream of population-rich regions and eventually to coastal areas. In this study, we developed a model to simulate the transport of contaminated sediment in a watershed hydrological system and this model was compared with observation data from eroded soil observation instruments located at the Korea Atomic Energy Research Institute. Two methods were applied to analyze the soil particle size distribution of the collected soil samples, including standardized sieve analysis and image analysis methods. Numerical models were developed to simulate the movement of soil along with actual rainfall considering initial saturation, rainfall infiltration, multilayer and rain splash. In the 2019 study, a numerical model will be used to add rainfall shield effect by trees, evaporation effect and shield effects of surface water. An eroded soil observation instrument has been installed near the Wolsong nuclear power plant since 2018 and observation data are being continuously collected. Based on these observations data, we will develop the numerical model to analyze long-term behavior of radionuclides on land as they move from land to rivers, lakes and coastal areas.

A watershed-scale model for depressional wetland-rich landscapes

Wetlands are often dominant features in low relief, depressional landscapes and provide an array of hydrologically driven ecosystem services. However, contemporary models do not adequately represent the role of spatially distributed wetlands in watershed-scale water storage and flows. Such tools are critical to better understand wetland hydrological, biogeochemical, and biological functions and predict management and policy outcomes at varying spatial scales. To develop a new approach for simulating depressional landscapes, we modified the Soil and Water Assessment Tool (SWAT) model to incorporate improved representations of depressional wetland structure and hydrological processes. Specifically, we refined the model to incorporate: (1) water storage capacity and surface flowpaths of individual wetlands and (2) local wetland surface and subsurface exchange. We utilized this model, termed SWAT-DSF (DSF for Depressional Storage and Flows), to simulate the ∼289 km2 Greensboro watershed within the Delmarva Peninsula of the US Coastal Plain. Model calibration and verification used both daily streamflow observations and remotely sensed surface water extent data (ca. 2-week temporal resolution), allowing us to assess model performance with respect to both streamflow and watershed inundation patterns. Our findings demonstrate that SWAT-DSF can successfully replicate distributed wetland processes and resultant watershed-scale hydrology. SWAT-DSF provides improved temporal and spatial characterization of watershed-scale water storage and flows in depressional landscapes, providing a new tool to quantify wetland functions at broad spatial scales.

Hysteresis analysis of nitrate dynamics in the Neuse River, NC

Anthropogenic activities have caused N saturation in many terrestrial ecosystems. The transfer of nutrients and sediments to freshwater environments has resulted in water quality impairments including eutrophication, increased turbidity, ecosystem acidification, and loss of biodiversity. Storm events account for the transport of a large proportion of nutrients and sediments found in watersheds on an annual basis. To implement effective water-quality management strategies, the importance of surface and subsurface flow paths during storm events and low flow conditions need to be quantified. The increased availability of optical in-situ sensors makes high-frequency monitoring of catchment fluxes practical. In this study, we present a high-resolution nitrate monitoring record over a 10-year period in the Neuse River Basin near Clayton, North Carolina. The relationship between discharge and nitrate concentration for 365 storm events are categorized into hysteresis classes that indicate different transport mechanisms into the river. Storm events over the entire period of this study are divided between clockwise, counter-clockwise, and complex hysteresis patterns, indicating multiple nitrate flow paths during different seasons and years. Logistic regression of a suite of environmental variables demonstrates that antecedent soil moisture is a significant factor in determining the storm hysteresis class, with the odds of counter-clockwise hysteresis increasing by 10.3% for every 1 percentage point increase in the soil moisture. There is also an overlying seasonal effect, which indicates that dry soil conditions and frequent small storms during summer leads to greater nitrate transport on the rising limb, in contrast to slower, groundwater-driven inputs during the rest of the year.