Topographic effects on flow path and surface water chemistry of the Llyn Brianne catchments in Wales

The effects of digital elevation model (DEM) map scale and data resolution on watershed model predictions of hydrologic characteristics were determined for TOPMODEL, a topography‐based watershed model. The effects of topography on watershed hydrology are represented in TOPMODEL as the distribution of ln (a/tan B), where ln is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the gravitational gradient. The minimum, maximum, mean, variance, and skew values of the ln (a/tan B) distribution were computed from 1:24,000‐scale (24K) DEMs at 30‐ and 90‐m resolutions and from 1:250,000‐scale (250K) DEMs at 90‐m resolution for 71 areas in Pennsylvania, New York, and New Jersey. An analysis of TOPMODEL showed that model predictions of the depth to the water table, the ratio of overland flow to total flow, peak flow, and variance and skew of predicted streamflow were affected by both the DEM map scale and data resolution. Further TOPMODEL analyses showed that the effects of DEM map scale and data resolution on model predictions were due to the sensitivity of the predictions to the mean of the ln (a/tan B) distribution, which was affected by both DEM map scale and data resolution. DEM map scale affected the mean of the ln (a/tan B) distribution through its influence on the mean of the ln (a) distribution, which characterizes land‐surface shape, and the mean of ln (1/tan B) distribution, which characterizes land‐surface slope. DEM resolution, in contrast, affected the mean of the ln (a/tan B) distribution primarily by its influence on the mean of the ln (a) distribution.

The effects of subbasin size on topographic characteristics and simulated flow paths were determined for the 111.5‐km2 Sleepers River Research Watershed in Vermont using the watershed model TOPMODEL. Topography is parameterized in TOPMODEL as the spatial and statistical distribution of the index ln (a/tan B), where In is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the slope gradient. The mean, variance, and skew of the ln (a/tan B) distribution were computed for several sets of nested subbasins (0.05 to 111.5 km2)) along streams in the watershed and used as input to TOPMODEL. In general, the statistics of the ln (a/tan B) distribution and the simulated percentage of overland flow in total streamflow increased rapidly for some nested subbasins and decreased rapidly for others as subbasin size increased from 0.05 to 1 km2, generally increased up to a subbasin size of 5 km2, and remained relatively constant at a subbasin size greater than 5 km2. Differences in simulated flow paths among subbasins of all sizes (0.05 to 111.5 km2) were caused by differences in the statistics of the ln (a/tan B) distribution, not by differences in the explicit spatial arrangement of ln (a/tan B) values within the subbasins. Analysis of streamflow chemistry data from the Neversink River watershed in southeastern New York supports the hypothesis that subbasin size affects flow‐path characteristics.

We undertook the task of determining whether base flow alkalinity of surface waters in the northeastern United States is related to indices of soil contact time and flow path partitioning that are derived from topographic and soils information. The influence of topography and soils on catchment hydrology has been incorporated previously in the variable source area model TOPMODEL as the relative frequency distribution of ln (a/Kb tan B), where ln is the Naperian logarithm, “a” is the area drained per unit contour, K is the saturated hydraulic conductivity, b is the soil depth, and tan B is the slope. Using digital elevation and soil survey data, we calculated the ln (a/Kb tan B) distribution for 145 catchments. Indices of flow path partitioning and soil contact time were derived from the ln (a/Kb tan B) distributions and compared to measurements of alkalinity in lakes to which the catchments drain. We found that alkalinity was, in general, positively correlated with the index of soil contact time, whereas the correlation between alkalinity and the flow path partitioning index was weak at best. A portion of the correlation between the soil contact time index and alkalinity was attributable to covariation with soil base saturation and cation exchange capacity, while another portion was found to be independent of these factors. Although our results indicate that catchments with long soil contact time indices are most likely to produce high alkalinity base flow, a sensitivity analysis of TOPMODEL suggests that surface waters of these same watersheds may be susceptible to alkalinity depressions during storm events, due to the role of flow paths.

Single flow direction (sfd) and multiple flow direction (mfd) algorithms were used to compute the spatial and statistical distributions of the topographic index used in the watershed model TOPMODEL. An sfd algorithm assumes that subsurface flow occurs only in the steepest downslope direction from any given point; an mfd algorithm assumes that subsurface flow occurs in all downslope directions from any given point. The topographic index in TOPMODEL is ln (a/tan β), where In is the Napierian logarithm, a is the upslope area per unit contour length, and tan β is the slope gradient. The ln (a/tanβ) distributions were computed from digital elevation model (DEM) data for locations with diverse topography in Arizona, Colorado, Louisiana, Nebraska, North Carolina, Oregon, Pennsylvania, Tennessee, Vermont, and Virginia. The means of the ln (a/tan β) distributions were higher when the mfd algorithm was used for computation compared to when the sfd algorithm was used. The variances and skews of the distributions were lower for the mfd algorithm compared to the sfd algorithm. The differences between the mfd and sfd algorithms in the mean, variance, and skew of the ln (a/tan β) distribution were almost identical for the various DEMs and were not affected by DEM resolution or watershed size. TOPMODEL model efficiency and simulated flow paths were affected only slightly when the ln (a/tan β) distribution was computed with the sfd algorithm instead of the mfd algorithm. Any difference in the model efficiency and simulated flow paths between the sfd and mfd algorithms essentially disappeared when the model was calibrated by adjusting subsurface hydraulic parameters.

This article discusses the generation and migration process of nitrate-N pollution in shallow groundwater caused by agricultural nonpoint source pollution in the catchment area of Shitoukoumen Reservoir in northeast China. By monitoring the shallow groundwater nitrate-N in the low-water period, the normal season, and high-flow period in the study area for a year, it was found that the nitrate-N concentration in the shallow groundwater of this area had a seasonal variation in both spatial and time distribution. In the time distribution, the peak value appeared in July, the high-flow period, and the valley value appeared in April, the low-water period, and showed a significant correlation with the time distribution of fertilization rate and rainfall. In the spatial distribution of nitrate-N pollution, when the distribution in shallow groundwater was analyzed separately in the three different periods (low-water period, the normal season, and high-flow period) and the discipline transference and enrichment of nitrate-N pollution in shallow groundwater was determined, this indicated that the region in the southeast study area where runoff conditions were better was less contaminated, and the region where runoff conditions were poor, as well as the region along the river were seriously polluted. The nitrate-N concentration in shallow groundwater was distributed mainly along the path of groundwater flow and was excreted in the drainage region. This showed that the spatial distribution of nitrate-N concentration in the shallow groundwater of the entire region was mainly controlled by the groundwater flow system. At the same time, in the middle and lower reaches of the study area, the seasonal changes in the recharged-excreted relationship between groundwater and river caused seasonal differences in the spatial distribution of nitrate-N pollution in groundwater. The combined effects of the groundwater mobility and the surface river resulted in a poor correlation between the groundwater nitrate-N concentration and land-use types. Only in the plain area where there was little influence from groundwater runoff and the surface river did the groundwater nitrate-N concentration correlate with land-use types. The spatial and time distribution of nitrate-N concentration in the shallow groundwater of the study area was impacted by agricultural nonpoint source pollution, the groundwater flow system, and the surface river and formed a concentration response system which uses basins as a unit.

Organic carbon in the boreal spring flood from adjacent subcatchments

Location and timing of river-aquifer exchanges in six tributaries to the Columbia River in the Pacific Northwest of the United States

Lough Gur is a shallow groundwater fed eutrophic lake situated within a small agricultural catchment containing volcanic and karst rock features in mid‐west Ireland. Seasonally active conduits linking two spring discharge locations from the lake under high flow conditions were revealed using dye tracing and a terrestrial geophysical survey, highlighting the architecture of the conduit flow path from Lough Gur to its discharge spring. A radon survey combined with a lake geophysical survey identified the locations of in‐lake discharge springs and thickness of the lakebed sediments. Falling head hydraulic characterization experiments illustrated the heterogenous nature of lakebed sediments and hydrograph analysis coupled with stable isotopes of water (δ18O andδ2H) revealed significant surface water ‐ groundwater interaction during high flow periods. Significantly,δ18O andδ2H signatures plot above the global meteoric water line and local meteoric water line indicating hydration of silicate minerals and direct isotope exchange ofδ18O between water and rock minerals. Groundwaterδ18O andδ2H signatures during low flow periods indicate that recharge sources are influenced by enriched surface waters and precipitation while a wider range of signatures during high flow periods indicates a greater variation of sources. D‐excess signatures illustrate rapid rainfall infiltration under high flow conditions, thereby demonstrating the vulnerability of the groundwater, while lake water signatures confirm widespread surface water‐groundwater interaction/mixing. Hydrochemical analyses confirm both silicate weathering and carbonate dissolution as primary geochemical processes with Mg/Ca ratios suggesting greater groundwater residence time during low flow periods. Correlations betweenδ13CDICand dissolved organic carbon suggest a seasonal switch in the source of DIC to groundwaters between the oxidation of organic matter in summer and dissolution of carbonate minerals in winter. The SI saturation index for calcite (SIC) illustrates calcium carbonate precipitation along with CO2evasion to be a perennial processes. Finally, the spatial variation for nitrate isotopic signatures (δ18ONO3‐andδ15NNO3‐) suggests a number of nitrate sources to groundwaters including soil organic nitrogen, manure and/or domestic effluent with indications of denitrification processes under low flow conditions.

Soil organic carbon (SOC) in mountainous regions is characterized by strong topography-induced heterogeneity, which may contribute to large uncertainties in regional SOC stock estimation. However, the quantitative effects of topography on SOC stocks in semiarid alpine grasslands are currently not well understood. Therefore, the purpose of this research study is to determine the role of topography in shaping the spatial patterns of SOC stocks. Soils from the summit, shoulder, backslope, footslope, and toeslope positions along nine toposequences within three elevation-dependent grassland types (i.e., montane desert steppe at ~ 2450 m, montane steppe at ~ 2900 m, and subalpine meadow at ~ 3350 m) are sampled at four depths (0–10, 10–20, 20–40, and 40–60 cm). SOC content, bulk density, soil texture, soil water content, and grassland biomass are determined. The general linear model (GLM) is employed to quantify the effects of topography on the SOC stocks. Ordinary least squares regressions are performed to explore the underlying relationships between SOC stocks and the other edaphic factors. In accordance with the present results, the SOC stocks at 0–60 cm show an increasing trend in respect to the elevation zone, with the highest stock being approximately 37.70 g m−2 in the subalpine meadow, about 2.07 and 3.41 times larger than that in the montane steppe and montane desert steppe, respectively. Along the toposequences, it is revealed the SOC stocks are maximal at toeslope, reaching to 14.98, 31.76, and 49.52 kg m−2, which are also significantly larger than those at the shoulder by a factor of 1.38, 2.31, and 1.44, in montane desert steppe, montane steppe, and subalpine meadow, respectively. Topography totally is seen to explain about 84% of the overall variation in SOC stocks, of which 70.61 and 9.74% are attributed to elevation zone and slope position, while the slope aspect and slope gradient are seen to plausibly explain only about 1.84 and 0.01%, respectively. The elevation zone and the slope position are seen to markedly shape the spatial patterns of the SOC stocks, and thus, they may be considered as key indicating factors in constructing the optimal SOC estimation model in such semiarid alpine grasslands.

Variations in salinity and flow rate in the aerial, naturally salty spring of Almyros of Heraklion on Crete were monitored during two hydrological cycles. We describe the functioning of the coastal karstic system of the Almyros and show the influence of the duality of the flow in the karst (conduits and fractured matrix) on the quality of the water resource in the coastal area. A mechanism of saltwater intrusion into this highly heterogeneous system is proposed and validated with a hydraulic mathematical model, which describes the observations remarkably well.

Coastal wetlands are characterized by strong, dynamic interactions between surface water and groundwater. This paper presents a coupled model that simulates interacting surface water and groundwater flow and solute transport processes in these wetlands. The coupled model is based on two existing (sub) models for surface water and groundwater, respectively: ELCIRC (a three‐dimensional (3‐D) finite‐volume/finite‐difference model for simulating shallow water flow and solute transport in rivers, estuaries and coastal seas) and SUTRA (a 3‐D finite‐element/finite‐difference model for simulating variably saturated, variable‐density fluid flow and solute transport in porous media). Both submodels, using compatible unstructured meshes, are coupled spatially at the common interface between the surface water and groundwater bodies. The surface water level and solute concentrations computed by the ELCIRC model are used to determine the boundary conditions of the SUTRA‐based groundwater model at the interface. In turn, the groundwater model provides water and solute fluxes as inputs for the continuity equations of surface water flow and solute transport to account for the mass exchange across the interface. Additionally, flux from the seepage face was routed instantaneously to the nearest surface water cell according to the local sediment surface slope. With an external coupling approach, these two submodels run in parallel using time steps of different sizes. The time step (Δtg) for the groundwater model is set to be larger than that (Δts) used by the surface water model for computational efficiency: Δtg = M × Δts where M is an integer greater than 1. Data exchange takes place between the two submodels through a common database at synchronized times (e.g. end of each Δtg). The coupled model was validated against two previously reported experiments on surface water and groundwater interactions in coastal lagoons. The results suggest that the model represents well the interacting surface water and groundwater flow and solute transport processes in the lagoons. Copyright © 2011 John Wiley & Sons, Ltd.

Summary Long-period horizontal seismometers, tiltmeters and strainmeters are influenced by inhomogeneous rock properties in the site region. These inhomogeneities interact with applied stress fields to create local strains and tilts that differ from those averaged over greater distances. These effects have been called strain-strain and strain-tilt coupling. For each instrument, the coupling is defined by three coefficients, so that for observations at a particular point a correction may be defined using a third-order tensor for seismometers and tiltmeters, and a fourth-order tensor for strainmeters. These tensors may be evaluated using instrumental data alone, either at tidal periods, or at seismic periods using properties of Rayleigh waves. Long-period seismometers and tiltmeters have usually been sited in specially constructed vaults in hard rock on the grounds that such sites give the lowest levels of background noise. The theory of the detection of seismic waves (see for example Rodgers 1968) implicitly assumes horizontal layering in the region of the site (including the earth-air interface) and no account is taken of lateral inhomogeneities that have dimensions small compared to the seismic wavelength being studied. Since these inhomogeneities do not affect the propagation of seismic waves it is assumed that they can be neglected in the region of a seismometer site. This is rarely satisfactory especially for a hard rock site (King 1971) since hard rocks are usually jointed and hard rock regions have pronounced topography. Topography, inhomogeneities due to fissures and the cavity formed by the seismometer vault all cause the strains associated with incoming seismic waves to produce local tilting; effects of this sort have been described in the tidal literature (Lecolazet & Wittlinger 1975; King & Bilham 1973; Lennon 8z Baker 1973; Itsueli et al. 1975; Harrison 1975) using terms such as ' cavity effect ' and ' topographic effect '. Attempts have been made to study the site effect with finite element models (Itsueli et al. 1975; Harrison 1975) using as input data the visible topography and cavity shape, and rock moduli inferred from geological and seismological information. This approach has two defects. The first is the practical problem of constructing and checking large three-dimensional finite element models. The second arises from the difficulty of obtaining the correct input data for the models. Published results (Itsueli et al. 1975) indicate that fracture patterns exist around excavations and it is

The study of temporal variations in solute concentrations in streams has attracted the attention of many workers during the last twenty years. They have demonstrated that solute concentration can exhibit marked seasonal and stormbased variation and several important controls on solute levels have been proposed. In order to document variations in solute concentrations over short periods of time, in response to hydrometeorological controls, an intensive sampling programme is required. Results obtained from continuous monitoring and frequent sampling in some Devon streams are presented to illustrate the nature and extent of short-term temporal variations in solute levels. IN a recent paper, Imeson (1973) has pointed to the increasing interest in the solute loads of rivers shown by geomorphologists and ecologists and to the particular need to focus attention upon temporal variations. However, if it is suggested that this particular field has received little attention, then this ignores a considerable volume of literature representing studies by workers in various disciplines and which can be only briefly sampled in this communication. The variation in solute levels associated with fluctuations in discharge has been documented by many workers, particularly in the United States. In 1948, Hem (1948) pointed to interesting temporal variations in specific conductance and chloride concentration in rivers of the southwestern United States and these were attributed primarily to the dilution of baseflow constituents during periods of high flow, although diurnal fluctuations were also described. This dilution effect was subsequently substantiated, elaborated and quantified by many other studies (e.g. Durum, I953; Korven and Wilcox, 1964; Ward, I963). Goto (1961) described detailed stormperiod variations in several chemical components within a Japanese stream and Hendrickson and Krieger (I960) and Toler (1965) reported clockwise and anticlockwise hysteretic effects in the relationship between solute concentrations and discharge during individual storm events in American rivers. Examples have also been cited where certain solute concentrations exhibit either a positive relationship with discharge or no significant relationship at all (e.g. Edwards, I973). The frequent use of the dilution effect to explain temporal variations in solute levels has prompted the application of simple mixing models based on the mass balance equation and involving individual run-off components (e.g. Hart et al., 1964; Hem, I970; Steele, 1968; Walling, I974) or theoretical storage reservoirs (Hall, I970, I97I; Johnson et al., 1969). Other studies have highlighted control by additional factors. Seasonal effects on the relationship between solute concentrations and discharge have been described by Glancy et al. (1972), Gunnerson (1967), Ledbetter and Gloyna (1964) and Steele (1968). Work in Russia (e.g. Skakalskiy, 1966) has focused attention on the flow paths of individual run-off components. Temporarily increased concentrations during the initial periods of storm run-off events have been related to the 'flushing' of accumulated salts by Edwards (i973), Steele (1968) and Walling (1974), and Gburek and Heald (1970) have attempted to distinguish between those ions which show smooth changes during storm events and those which fluctuate more rapidly as a result of control

Canada's post‐mined oil sands will have a higher concentration of salts compared with freshwater peatlands that dominate the landscape. While rare, naturally occurring saline wetlands do exist in Alberta's Boreal Plains and may function as analogues for reclamation, however, little is known about their hydrology. This paper investigates the geochemical and hydrologic characteristics of a natural saline‐spring peatland in Alberta's oil sands region. The fen is located within a saline groundwater discharge area connected to the erosional edge of the Grand Rapids Formation. Na+(195–25,680 mgl−1) and Cl−(1785–56,249 mg l−1) were the dominant salts, and the fen transitioned sharply to freshwater along its margins because in part of subsurface mineral ridges that restricted shallow groundwater exchange. Salinity decreased from hypersaline to brackish along the local groundwater flow path but no active spring outlets were observed over the two‐year study. Vertical groundwater discharge was minimal because of the very low permeability of the underlying sediments. Subsurface storage was exceeded during periods of high flow, resulting in flooding and surface runoff that was enhanced by the ephemerally connected pond network. These findings have implications for reclamation, as mechanisms such as subsurface mineral ridges may function as effective saline groundwater‐control structures in the post‐mined environment. Incorporating saline wetlands into regional monitoring networks will help to better quantify natural discharge, which has implications for belowground wastewater storage related toin situbitumen extraction. Copyright © 2015 John Wiley & Sons, Ltd.

Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi‐channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro‐technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form ‘second‐order’ crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches. Copyright © 2017 John Wiley & Sons, Ltd.