Chris Soulsby

Fine sediment influence on salmonid spawning habitat in a lowland agricultural stream: a preliminary assessment

Spawning habitat utilized by Atlantic Salmon (Salmo salar) and Sea Trout (Salmo trutta) was characterized in a 1.6-km reach of the Newmills Burn, a small, highly canalized tributary of the River Don in Aberdeenshire. The Newmills Burn is typical of the intensively farmed lower sub-catchments of the major salmon rivers on the east coast of Scotland. Such streams have substantial potential in providing spawning and juvenile habitat for salmonids, with high redd densities resulting in egg deposition rates of > 5 m2. However, in comparison with upland spawning tributaries draining less intensively managed catchments, canalization and intensive cultivation has seriously degraded the physical characteristics of aquatic habitats in many streams. In the Newmills Burn, spawning gravels have a relatively high (> 20% by mass) fine sediment (< 2 mm in size) content. The burn is characterized by hydraulic conditions that are suitable for salmonid spawning, with modal velocities of 0.50-0.65 m s(-1) and depths of 0.20-0.25 m. However, infiltration of fine sediments into gravels is rapid during hydrological events in the winter months. Thus, complete siltation of open gravel matrices (simulated redds) can occur within a week, and probably within a single moderate to large storm event. Appreciable, but small, deposition of organic and silt/clay particles can also affect spawning gravels. Egg mortalities in redds following spawning are variable, but can be as high as 86% in the Newmills Burn. This may be related to fine sediment infiltration, reduced permeability of spawning gravels and reduced oxygen supply to ova. It appears that the main cause of high influx is sediment loads mobilized from intensively managed land. It is suggested that fundamental changes to the management of agricultural land is required if fish habitats are to be improved and degraded streams are allowed to re-naturalize. The need for closely focused investigations of the causal relationships between fine sediment infiltration and egg survival is stressed.

Gamma distribution models for transit time estimation in catchments: physical interpretation of parameters and implications for time-variant transit time assessment.

In hydrological tracer studies, the gamma distribution can serve as an appropriate transit time distribution (TTD) as it allows more flexibility to account for nonlinearities in the behavior of catchment systems than the more commonly used exponential distribution. However, it is unclear which physical interpretation can be ascribed to its two parameters (?, ?). In this study, long?term tracer data from three contrasting catchments in the Scottish Highlands were used for a comparative assessment of interannual variability in TTDs and resulting mean transit times (MTT = ??) inferred by the gamma distribution model. In addition, spatial variation in the long?term average TTDs from these and six additional catchments was also assessed. The temporal analysis showed that the ? parameter was controlled by precipitation intensities above catchment?specific thresholds. In contrast, the ? parameter, which showed little temporal variability and no relationship with precipitation intensity, was found to be closely related to catchment landscape organization, notably the hydrological characteristics of the dominant soils and the drainage density. The relationship between ? and precipitation intensity was used to express ? as a time?varying function within the framework of lumped convolution integrals to examine the nonstationarity of TTDs. The resulting time?variant TTDs provided more detailed and potentially useful information about the temporal dynamics and the timing of solute fluxes. It was shown that in the wet, cool climatic conditions of the Scottish Highlands, the transit times from the time?variant TTD were roughly consistent with the variations of MTTs revealed by interannual analysis.

Storage dynamics in hydropedological units control hillslope connectivity, runoff generation, and the evolution of catchment transit time distributions

We examined the storage dynamics and isotopic composition of soil water over 12 months in three hydropedological units in order to understand runoff generation in a montane catchment. The units form classic catena sequences from freely draining podzols on steep upper hillslopes through peaty gleys in shallower lower slopes to deeper peats in the riparian zone. The peaty gleys and peats remained saturated throughout the year, while the podzols showed distinct wetting and drying cycles. In this region, most precipitation events are <10 mm in magnitude, and storm runoff is mainly generated from the peats and peaty gleys, with runoff coefficients (RCs) typically <10%. In larger events the podzolic soils become strongly connected to the saturated areas, and RCs can exceed 40%. Isotopic variations in precipitation are significantly damped in the organic-rich soil surface horizons due to mixing with larger volumes of stored water. This damping is accentuated in the deeper soil profile and groundwater. Consequently, the isotopic composition of stream water is also damped, but the dynamics strongly reflect those of the near-surface waters in the riparian peats. “pre-event” water typically accounts for >80% of flow, even in large events, reflecting the displacement of water from the riparian soils that has been stored in the catchment for >2 years. These riparian areas are the key zone where different source waters mix. Our study is novel in showing that they act as “isostats,” not only regulating the isotopic composition of stream water, but also integrating the transit time distribution for the catchment. Key Points Hillslope connectivity is controlled by small storage changes in soil units Different catchment source waters mix in large riparian wetland storage Isotopes show riparian wetlands set the catchment transit time distribution

Variation in river water temperatures in an upland stream over a 30-year period.

Stream water temperature data from the Girnock burn, a 30-km2 catchment in Scotland were examined for systematic variation across 30 years of record (1968-1997). The data suggest that there has been no change in mean annual temperature with time, but at a seasonal level there is some indication of an increase in mean daily maximum temperatures during the winter (December to February) and spring (March to May) seasons. For the spring season, there is also evidence that mean temperature has increased. There are no apparent or obvious changes in stream flow to account for this. The strong relationship between air and stream temperatures (r2 = 0.96) implies that changes in the stream are the result of changes in the climate. It is possible that this may occur as a result of the effect of increasing air temperatures which may have also reduced the influence of snow and snowmelt on the catchment during the winter and spring seasons.

Hydrological controls on nutrient concentrations and fluxes in agricultural catchments

Like many streams draining intensively farmed parts of lowland Scotland, water quality in the Newmills burn, Aberdeenshire, is characterized by relatively high nutrient levels; mean concentrations of NO3-N and NH3-N are 6.09 mg l(-1) and 0.28 mg l(-1), respectively, whilst average PO4-P concentrations reach 0.06 mg l(-1). Nutrient concentrations vary spatially and temporally with levels being highest under arable farming during the autumn and winter. Annual fluxes from the 14.5 km2 catchment are estimated at 25.67 and 1.26 kg ha(-1) a(-1) for NO3-N and NH3-N, respectively, and 0.26 kg ha(-1) a(-1) for PO4-P. Hydrological controls exert a strong influence on both nutrient concentrations and fluxes. Over short timescales nutrient concentrations and fluxes are greatest during storm events when P04-P and NH3-N are mobilized by overland flow in riparian areas, particularly where the soils have been compacted by livestock or farm machinery. Delivery of deeper soil water in subsurface storm flow, facilitated by agricultural under-drainage, provide large contributions of NO3-N on the recession limb of hydrological events. In contrast, groundwater inputs generally have lower NO3 concentrations implying that denitrification may be a pathway of N loss in the saturated zone. Approximately 75% of the N loss for the catchment occurs during the autumn and early winter when high flows dominate the hydrological regime. The close coupling of hydrological pathways and biogeochemical processes has major implications for catchment management strategies such as Nitrate Vulnerable Zones (NVZs) as it is likely that significant groundwater stores with long residence times will continue to cause N losses before water quality improvements become apparent.

Riparian zone influence on stream water chemistry at different spatial scales: a GIS-based modelling approach, an example for the Dee, NE Scotland

A geographical information system (GIS-ARC/INFO) was used to collate existing spatial data sets on catchment characteristics to predict stream water quality using simple empirical models. The study, based on the river Dee catchment in NE Scotland, found that geological maps and associated geochemical information provided a suitable framework for predicting chemical parameters associated with acidification sensitivity (including alkalinity and base cation concentrations). In particular, it was found that in relatively undisturbed catchments, the parent material and geochemistry of the riparian zone, when combined with a simple hydrological flow path model, could be used to accurately predict stream water chemistry at a range of flows (Q95 to > Q5) and spatial scales (1-1000 km2). This probably reflects the importance of the riparian zone as an area where hydrological inputs to stream systems occur via flow paths in the soil and groundwater zones. Thus, evolution of drainage water chemistry appears to retain the geochemical characteristics of the riparian area as it enters the channel network. In more intensively managed catchments, riparian land use is a further influential factor, which can be incorporated into models to improve predictions for certain base cations. The utility in providing simple hydrochemical models, based on readily available data sets, to assist environmental managers in planning land use in catchment systems is discussed.

Contrasts in storm event hydrochemistry in an acidic afforested catchment in upland Wales

The hydrochemistry of stream water in an acidic afforested catchment in the Welsh uplands was monitored routinely between 1985 and 1990. Nineteen storm episodes were sampled intensively during this period. Although the general storm response of the stream can be characterised by increased concentrations of H+, Al and dissolved organic carbon (DOC), and a dilution of Ca and SiO2, the detailed hydrochemistry of individual acid episodes exhibited marked contrasts. The minimum pH reached during specific episodes ranged from 4.1 to 5.0, and peak dissolved Al concentrations varied from 9 to 44 μmol l−1. The reasons for such differences in the hydrochemical response can be identified for each individual episode by examining the complex interactions between (1) the quantity and quality of event precipitation, (2) antecedent patterns of weather and atmospheric deposition and (3) the hydrological processes which dominate the storm runoff response. The dynamic nature of catchment hydrology was found to exert a particularly strong influence on the hydrochemistry of specific acid episodes.

Hydraulic and sedimentary controls on the availability and use of Atlantic salmon (Salmo salar) spawning habitat in the River Dee system, north-east Scotland

Abstract The hydraulic and sedimentary characteristics of the spawning habitat of Atlantic salmon ( Salmo salar ) in tributary and mainstem locations in a river system in north-east Scotland are described. Salmon used spawning sites with a relatively wide range in sediment characteristics, although measures of central tendency were all in the gravel (2–64 mm) size-class. The dominant factor differentiating the sediment characteristics of study sites was the level of fine sediment, which accounted for significant differences between tributary and mainstem samples. The ranges of depth and velocity in areas used for spawning by salmonids were found to be similar in all tributary study sites. However, due to the interdependence of depth and velocity, major differences were observed between tributary and mainstem study sites in that spawning in larger streams tended to be associated with deeper, faster flowing water. Spawning locations were shown to have similar Froude number, despite different sized streams and species of salmonid. Due to its dimensionless nature and significance in characterising flow hydraulics, the Froude number is proposed as a potentially useful variable for describing the habitat of aquatic organisms.

Stream water age distributions controlled by storage dynamics and nonlinear hydrologic connectivity: Modeling with high‐resolution isotope data

Abstract To assess the influence of storage dynamics and nonlinearities in hydrological connectivity on time‐variant stream water ages, we used a new long‐term record of daily isotope measurements in precipitation and streamflow to calibrate and test a parsimonious tracer‐aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands, and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both streamflow and isotope variations are generally well captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a nonlinear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting nonstationary ages. This approach is well suited for constraining process‐based modeling in a range of northern temperate and boreal environments.

Do time‐variable tracers aid the evaluation of hydrological model structure? A multimodel approach

[1] In this paper we explore the use of time-variable tracer data as a complementary tool for model structure evaluation. We augment the modular rainfall-runoff modeling framework FUSE (Framework for Understanding Structural Errors) with the ability to track the age distribution of water in all model stores and fluxes. We therefore gain the novel ability to compare tracer/water age signatures measured in a catchment with those predicted using hydrological models built from components based on four existing popular models. Key modeling decisions available in FUSE are evaluated against streamflow tracer dynamics using weekly observations of tracer concentration which reflect the tracer transit time distribution (TTD). Model structure choice is shown to have a significant effect on simulated water age characteristics, even when simulated flow series are very similar. We show that for a Scottish case study catchment, careful selection of model structure enables good predictions of both streamflow and tracer dynamics. We then use FUSE as a hypothesis testing tool to understand how different model characterization of TTDs and mean transit times affect multicriteria model performance. We demonstrate the importance of time variation in TTDs in simulating water movement along fast flow pathways, and investigate sensitivity of the models to assumptions about our ability to sample fast, near-surface flow.