Dominant Flow Pathways Research Articles

Upscaling approach to help determine variability in riparian lowland flow dynamics

Riparian lowlands are important for protecting aquatic ecosystems threatened by contamination. Their ability to attenuate and reduce nutrient rich water provides a useful ecosystem service. Previous field studies have shown that the hydrological conditions within a lowland can affect their reduction capabilities dependent on the dominant flow pathways, i.e., surface runoff, groundwater discharge, and drain flow. For example, the likelihood of nutrient reduction within a riparian lowland dominated by surface runoff is low, conversely if groundwater discharge dominates the likelihood is higher. Hence, knowledge of the flow pathways can be used to provide a qualitative estimate of the reduction capacity in riparian lowlands, information that is vital in assessing catchment scale processes. The objective of this study is to establish a relationship between the dominant riparian flow pathways and lowland features, such as slope and hydrogeology/geology. Previous work had shown the ability of a downscaled high-resolution numerical model to replicate observed annual flow patterns within a riparian lowland. This downscaled numerical model was used to provide quantitative information regarding the riparian flow partitioning. To diversify the dataset, the riparian lowland was segmented to provide flow information at different scales, and topographic and hydraulic properties within the model were perturbed to capture the range of topographic and geological characteristics present at large scale. These data were then used to train a random forest (RF) model, where the target variable was the fraction of overland flow. Applying the RF model to a 12,785.5 km2 large region in Denmark provided a prediction in line with our understanding of the area. This approach can prove useful in enhancing existing nitrate management tools by incorporating variability in lowland nitrate reduction capacity.

Effects of Reforestation of a Degraded Imperata Grassland on Dominant Flow Pathways and Streamflow Responses in Leyte, the Philippines

AbstractReforestation of degraded grasslands can increase the soil hydraulic conductivity and number of preferential flow pathways. However, it is not clear to what extent these changes affect streamflow responses and whether this depends on the event size. We, therefore, studied the hydrological response of two small catchments near Tacloban, Leyte (the Philippines): a degraded Imperata grassland catchment and a catchment that was reforested 23 years prior to our study. Precipitation, stream stage, and electrical conductivity were measured continuously from June to November 2013. Samples were taken from streamflow, precipitation, groundwater, and soil water for geochemical and stable isotope analyses. Streamflow and electrical conductivity changed rapidly during almost every event in the grassland catchment, but in the reforested catchment, these responses were much smaller and only occurred during large events. Streamflow was a mixture of groundwater and precipitation for both catchments, but the maximum event water contributions to streamflow were much larger for the degraded grassland than for the reforested catchment. The differences in the event water contributions and timing of the streamflow responses were observed across all event sizes, including a large tropical storm. Together with the low saturated hydraulic conductivity in the degraded catchment, these results suggest that overland flow occurred more frequently and was much more widespread in the degraded grassland than in the reforested catchment. We, therefore, conclude that reforestation of a degraded grassland can change the dominant flow pathways and restore the hydrological functioning if the forest soil is allowed to develop over a sufficiently long period.

Spatial variation in concentration and sources of organic carbon in the Lena River, Siberia

AbstractGlobal warming in permafrost areas is expected to change fluxes of riverine organic carbon (OC) to the Arctic Ocean. Here OC concentrations, stable carbon isotope signatures (δ13C), and carbon‐nitrogen ratios (C/N) are presented from 22 sampling stations in the Lena River and 40 of its tributaries. Sampling was conducted during two expeditions: the first in July 2012 in the south and southeastern region and the second in June 2013 in the northern region of the Lena basin. The data showed significant spatial differences in concentrations and major sources of OC. Mean subcatchment slopes were correlated with OC concentrations, implying that mountainous areas in general had lower concentrations than lowland areas. δ13C and C/N data from tributaries originating in mountainous areas indicated that both dissolved and particulate OC (DOC and POC) were mainly derived from soil organic matter (SOM). In contrast, tributaries originating in lowland areas had larger contributions from fresh vegetation to DOC, while aquatically produced OC was the major source of POC. We suggest that these differences in dominant sources indicated differences in dominant flow pathways. Tributaries with larger influence of fresh vegetation probably had surficial flow pathways, while tributaries with more SOM influence had deeper water flow pathways. Thus, the future export of OC to the Arctic Ocean will likely be controlled by changes in spatial patterns in hydroclimatology and the depth of the active layers influencing the dominant water flow pathways in Arctic river basins.

The role of mobilisation and delivery processes on contrasting dissolved nitrogen and phosphorus exports in groundwater fed catchments

Diffuse transfer of nitrogen (N) and phosphorus (P) in agricultural catchments is controlled by the mobilisation of sources and their delivery to receiving waters. While plot scale experiments have focused on mobilisation processes, many catchment scale studies have hitherto concentrated on the controls of dominant flow pathways on nutrient delivery. To place mobilisation and delivery at a catchment scale, this study investigated their relative influence on contrasting nitrate-N and soluble P concentrations and N:P ratios in two shallow groundwater fed catchments with different land use (grassland and arable) on the Atlantic seaboard of Europe. Detailed datasets of N and P inputs, concentrations in shallow groundwater and concentrations in receiving streams were analysed over a five year period (October 2010–September 2015). Results showed that nitrate-N and soluble P concentrations in shallow groundwater give a good indication of stream concentrations, which suggests a dominant control of mobilisation processes on stream exports. Near-stream attenuation of nitrate-N (−30%), likely through denitrification and dilution, and enrichment in soluble P (+100%), through soil-groundwater interactions, were similar in both catchments. The soil, climate and land use controls on mobilisation were also investigated. Results showed that grassland tended to limit nitrate-N leaching as compared to arable land, but grassland could also contribute to increased P solubilisation. In the context of land use change in these groundwater fed systems, the risk of pollution swapping between N and P must be carefully considered, particularly for interactions of land use with soil chemistry and climate.

Rebuilding soil hydrological functioning after swidden agriculture in eastern Madagascar

Land-use change due to the widespread practice of swidden agriculture affects the supply of ecosystem services. However, there is comparatively little understanding of how the hydrological functioning of soils, which affects rainfall infiltration and therefore flood risk, dry-season flows and surface erosion, is affected by repeated vegetation clearing and burning, the extent to which this can recover following land abandonment and vegetation regrowth, and whether active restoration speeds up recovery. We used interviews with local land users and indicator plant species to reconstruct the land-use history of 19 different sites in upland eastern Madagascar that represent four different land-use categories: semi-mature forests that were never burnt but were influenced by manual logging until 15–20 years ago; fallows that were actively reforested 6–9 years ago; 2–10year old naturally regenerating fallows; and highly degraded fire-climax grassland sites. Surface- and near-surface (down to 30cm depth) saturated soil hydraulic conductivities (Ksat), as well as the dominant flow pathways for infiltration and percolation were determined for each land-cover type. Surface Ksat in the forest sites was very high (median: 724mmh−1) and infiltration was dominated by flow along roots and other preferential flow pathways (macropores), whereas Ksat in the degraded land was low (median: 45mmh−1) with infiltration being dominated by near-surface matrix flow. The total area of blue-dye stains was inversely correlated to the Ksat. Both surface- and near-surface Ksat had increased significantly after 6–9 years of forest regeneration (median values of 203 and 161mmh−1 for reforestation and natural regeneration, respectively). Additional observations are needed to more fully understand the rates at which soil hydrological functioning can be rebuilt and whether active replanting decreases the time required to restore soil hydrological functioning or not.

Assessing the drivers of dissolved organic matter export from two contrasting lowland catchments, U.K

Two lowland catchments in the U.K. were sampled throughout 2010–11 to investigate the dominant controls on dissolved organic matter quantity and composition. The catchments had marked differences in terms of nutrient status, land cover and contrasting lithologies resulting in differences in the dominant flow pathways (groundwater vs. surface water dominated). The Upper Wylye is a chalk stream with a baseflow index of 0.98, draining a catchment dominated by intensive agricultural production. Millersford Brook is a lowland peat catchment with a baseflow index of 0.43, draining a semi-natural catchment with heather moorland and coniferous forest. Samples were collected weekly between October 2010 and September 2011 from eleven sampling locations. Samples were analysed to determine dissolved organic carbon, nitrogen and phosphorus fractions with DOM composition evaluated via the DOC:DON ratio, DOC:DOP ratio, specific UV absorption at 254nm, absorbance ratio (a250:a365) and the spectral slope parameter between 350 and 400nm (S350–400). Significant differences were observed in all determinands between the catchments, over time, and spatially along nutrient enrichment and geoclimatic gradients. Seasonal variation in preferential flow pathways mobilising groundwater-derived DOM were identified as likely controls on the delivery of DOM in the permeable chalk dominated catchment. Steeper S350–400 values and elevated a250:a365 ratios in this catchment suggest material of a lower bulk aromatic C content and molecular weight delivered during the winter months when compared to the summer. DOC:DON ratios were markedly lower in the chalk catchment than the peatland catchment, reflecting the paucity of organic matter within the mineral soils of the chalk landscape, and higher fertiliser application rates. This manuscript highlights that DOM composition varies according to catchment landscape character and hydrological function.

Flow Partitioning in Fully Saturated Soil Aggregates

Microbes play an important role in facilitating organic matter decomposition in soils, which is a major component of the global carbon cycle. Microbial dynamics are intimately coupled to environmental transport processes, which control access to labile organic matter and other nutrients that are needed for the growth and maintenance of microorganisms. Transport of soluble nutrients in the soil system is arguably most strongly impacted by preferential flow pathways in the soil. Since the physical structure of soils can be characterized as being formed from constituent micro-aggregates which contain internal porosity, one pressing question is the partitioning of the flow among the “inter-aggregate” and “intra-aggregate” pores and how this may impact overall solute transport within heterogeneous soil structures. The answer to this question is particularly important in evaluating assumptions to be used in developing upscaled simulations based on highly resolved mechanistic models. In our synthetic model of soils, firstly we statistically generated a number of micro-aggregates containing internal pores. Then we constructed a group of diverse multi-aggregate structures with different packing ratios by stacking those micro-aggregates and varying the size and shape of inter-aggregate pore spacing between them. We then performed pore-scale flow simulations using computational fluid dynamics methods to determine the flow patterns in these aggregate-of-aggregates structures and computed the partitioning of the flow through intra- and inter-aggregate pores as a function of the spacing between the aggregates. The results of these numerical experiments demonstrate that soluble nutrients are largely transported via flows through inter-aggregate pores. Although this result is consistent with intuition, we have also been able to quantify the relative flow capacity of the two domains under various conditions. For example, in our simulations, the flow capacity through the aggregates (intra-aggregate flow) was less than 2 % of the total flow when the spacing between the aggregates was larger than \(18\,\upmu \hbox {m}\). Inter-aggregate pores continued to be the dominant flow pathways even at much smaller spacing; intra-aggregate flow was less than 10 % of the total flow when the inter- and intra-aggregate pore sizes were comparable. Although the results may not be exactly the same as those obtained from actual soil systems, such studies are making it possible to identify which model upscaling assumptions are realistic and what computational methods are required for detailed numerical investigation of hydrodynamics and microbial carbon cycling dynamics in soil systems.

Preliminary study of flow regimes and stream water residence times in multi-scale forested watersheds of central Cambodia

To elucidate the water cycle in lowland forests of the Mekong River basin, our research group established four experimental watersheds in the Stung Chinit River basin in Kampong Thom Province, Cambodia. The drainage areas of these experimental watersheds ranged from small (4 km2) to mesoscale (3,659 km2). Here, we present the first preliminary results of our rainfall-discharge observations and analyses of temporal variations of stable isotope ratios in rainfall, stream water, and groundwater. This paper focuses on the following three main topics: annual rainfall, discharge, and water balance; stormflow generation and dominant flow pathways; and flow regimes and stream water residence times. All stream water residence times (τ = 1.7–7.5 months) for the four experimental watersheds were shorter than the residence time of the groundwater (τ = 9.4 months) through the soil and regolith layers, implying that the stream waters consisted of not only the groundwater-flow component, but also younger-aged flow components such as saturation-excess overland flow. The smallest (4 km2; O Toek Loork) watershed had longer residence time (τ = 7.5 months) than the three larger watersheds (126–3,659 km2; τ = 1.7–3.9 months). This may suggest differing contributions of the groundwater and younger-aged flow components in the stream water in each watershed. Our approach of multi-scale watershed observation might better contribute to the needs of physically based models and aid in predictions for ungauged basins.

87Sr/ 86Sr as an indicator of flowpaths and weathering rates in the Plynlimon experimental catchments, Wales, U.K.

A knowledge of the processes involved in streamflow generation are critical to an understanding of solute transport and weathering rates in upland catchments. The determination of specific flow pathways and the formulation of process-based models have proved difficult in such terrains, largely due to the heterogeneous nature of catchments and the necessary limitations of bulked parameter models. Natural geochemical tracers have proved invaluable in developing conceptual models of catchment functioning and for constraining weathering processes and geochemical cycling. Strontium isotopes have been used as a natural tracer to calculate weathering rates for Sr and Ca, and to constrain the dominant flow pathways in two upland forested sub-catchments (Afon Hafren and Afon Hore) of the River Severn at Plynlimon in Central Wales. The dominant source of Sr in the catchments is considered to be from the weathering of silicate minerals. Weathering rates for Sr and Ca in the Afon Hafren, calculated using Sr isotopes, were similar to those derived from mass balance studies. The rates for the Afon Hore were similar for Ca, but significantly different for Sr. The reason for the difference is unclear, but may be due to additional sources (calcite) in the catchment. Strontium isotope ratios for different input sources and compartments within the catchment were characterised and helped to identify potential flow pathways. The data suggest an important role for groundwater inputs in contrast to previous models which indicated a dominant role for soil waters.

Evaluating snowmelt runoff generation in a discontinuous permafrost catchment using stable isotope, hydrochemical and hydrometric data

Research on snowmelt runoff generation in discontinuous permafrost subarctic catchments has highlighted the role of: (i) permafrost in restricting deep percolation and sustaining near-surface water tables and (ii) the surface organic layer in rapidly conveying water to the stream. Conceptual models of runoff generation have largely been derived from hydrometric data, with isotope and hydrochemical data having only limited application in delineating sources and pathways of water. In a small subarctic alpine catchment within the Wolf Creek Research Basin, Yukon, Canada, snowmelt runoff generation processes were studied during 2002 using a mixed methods approach. Snowmelt timing varied between basin slopes, with south-facing exposures melting prior to permafrost-underlain north-facing slopes. The streamflow freshet period begain after 90% of snow had melted on the south-facing slope and coincided with the main melt period on the north-facing slope, indicating that contributing areas were largely defined by permafrost distribution. Stable isotope (δ18O) and hydrochemical parameters (dissolved organic carbon, specific conductivity, pH) suggest that, at the beginning of the melt period, meltwater infiltrates soil pores and resides in temporary storage. As melt progresses and bare ground appears, thawing of soils and continued meltwater delivery to the slopes allows rapid drainage of this meltwater through surface organic layers. As melt continues, soil thawing progresses and pre-event water mixes with melt water to impart streamflow with a gradually decreasing meltwater contribution. By the end of the melt period, the majority of water reaching the stream is displaced water that has resided in the catchment prior to melt. For the entire study period, approximately 21% of freshet was supplied by the snowpack, and the remaining majority was pre-melt water stored in the catchment slopes over-winter and displaced during melt. Hydrochemical data support hydrometric observations indicating the dominant flow pathway linking the slopes and the stream is through the organic horizon on permafrost-underlain slopes.

Plot and small‐watershed scale runoff from two reclaimed surface‐mined watersheds in Alberta

Infiltration excess overland flow has been identified as the dominant flow pathway in recently reclaimed surface mined watersheds as a result of compaction and sorting during the reclamation procedure. Therefore, there could be a fairly direct relationship between runoff generated from the hillslopes to that measured at the watershed outlet. A 3-year study was initiated in 1993 to determine how well surface runoff at a watershed scale could be predicted from 1-m2 runoff frames placed on hillslopes in two reclaimed surface-mined watersheds in central Alberta. Runoff from the hillslope frames suggests outlet discharge should be high from the 3\4-ha Sandy Subsoil Watershed and much less for the 9\8-ha West Watershed, but the opposite occurred. Most of the hillslope runoff from the Sandy Subsoil Watershed infiltrated once it reached the channel and depression storage played an insignificant role in determining runoff. In contrast, most of the runoff from the West Watershed originated from rain falling directly on the saturated channel (depression storage) or near-channel saturated areas, rather than the hillslopes. Neither watershed runoff magnitude nor timing could be predicted from the same parameters for hillslope runoff frames for either reclaimed watershed. Copyright © 2000 John Wiley & Sons, Ltd.

Plot and small-watershed scale runoff from two reclaimed surface-mined watersheds in Alberta

Infiltration excess overland flow has been identified as the dominant flow pathway in recently reclaimed surface mined watersheds as a result of compaction and sorting during the reclamation procedure. Therefore, there could be a fairly direct relationship between runoff generated from the hillslopes to that measured at the watershed outlet. A 3-year study was initiated in 1993 to determine how well surface runoff at a watershed scale could be predicted from 1-m2 runoff frames placed on hillslopes in two reclaimed surface-mined watersheds in central Alberta. Runoff from the hillslope frames suggests outlet discharge should be high from the 3\4-ha Sandy Subsoil Watershed and much less for the 9\8-ha West Watershed, but the opposite occurred. Most of the hillslope runoff from the Sandy Subsoil Watershed infiltrated once it reached the channel and depression storage played an insignificant role in determining runoff. In contrast, most of the runoff from the West Watershed originated from rain falling directly on the saturated channel (depression storage) or near-channel saturated areas, rather than the hillslopes. Neither watershed runoff magnitude nor timing could be predicted from the same parameters for hillslope runoff frames for either reclaimed watershed. Copyright © 2000 John Wiley & Sons, Ltd.

The contribution of old and new water to a storm hydrograph determined by tracer addition to a whole catchment

Two tracer experiments have been carried out at an enclosed catchment in southern Norway. The catchment was brought to steady state with respect to rainfall and runoff prior to the tracer addition. A known concentration of lithium bromide was then added to the rainfall for the duration of each event. The tight control on tracer concentration and rainfall amount enabled assessment of the contribution of old and new water to runoff, the dominant flow pathways and soil water residence times during a storm event. A significant volume of ‘old’ water contributes to runoff despite the hydrologically responsive nature of the catchment and several hours of tracer injected rainfall are required before ‘new’ water becomes the dominant runoff source. After 34 h of tracer injection, ‘new’ water apparently contributes c. 83% to instantaneous flow and c. 55% of the total tracer input to the catchment has been lost in runoff. Recovery of the tracer from soil water indicates that the organic soil surface layer is the dominant flow pathway for rainwater through the catchment and that a significant pathway also exists at the soil–bedrock interface. New water is retained in deep pockets of soil for several days. Assessment of the conservative behaviour of the tracer suggests that 10–14% of the input Br− is retained in the soil and the tracer is not conservative. Laboratory experiments indicate that sorption of Br− to organic soil is the likely mechanism of retention. This process is probably concentration dependent and will have occurred predominantly during the initial period of tracer application. Copyright © 2000 John Wiley & Sons, Ltd.

Modelling water and solute transport in macroporous soil. II. Chloride breakthrough under non-steady flow : J Soil Sci V42, N1, March 1991, P71–81

SUMMARY A model of water and solute transport in macroporous soils (Jarvis et al., 1991) has been evaluated in column breakthrough experiments under field conditions. Hydraulic properties were first measured in replicate soil monolith lysimeters sampled from grass ley and continuous barley treatments in a clay soil. A pulse input of 0.05 M KCl was then supplied by drip irrigation and measurements made of the water discharge and chloride leaching resulting from the natural rainfall over a 1-month period. The results showed that the macropores constituted the dominant flow pathway (accounting for 80% of the total water outflow) and that diffusive exchange of chloride between the two flow domains was the main factor causing variability in leaching. Larger hydraulic conductivities and macroporosities in the lower topsoil and at plough depth in the grass ley monoliths were taken as evidence of structural amelioration. Less of the applied chloride was leached in the grass monoliths than in the barley (means of 20% and 31% respectively). This was mainly due to a smaller effective aggregate size and thus a more efficient diffusion-controlled retention.

Modelling water and solute transport in macroporous soil. II. Chloride breakthrough under non‐steady flow

SUMMARYA model of water and solute transport in macroporous soils (Jarvis et al., 1991) has been evaluated in column breakthrough experiments under field conditions. Hydraulic properties were first measured in replicate soil monolith lysimeters sampled from grass ley and continuous barley treatments in a clay soil. A pulse input of 0.05 M KCl was then supplied by drip irrigation and measurements made of the water discharge and chloride leaching resulting from the natural rainfall over a 1‐month period.The results showed that the macropores constituted the dominant flow pathway (accounting for 80% of the total water outflow) and that diffusive exchange of chloride between the two flow domains was the main factor causing variability in leaching. Larger hydraulic conductivities and macroporosities in the lower topsoil and at plough depth in the grass ley monoliths were taken as evidence of structural amelioration. Less of the applied chloride was leached in the grass monoliths than in the barley (means of 20% and 31% respectively). This was mainly due to a smaller effective aggregate size and thus a more efficient diffusion‐controlled retention.