Showing papers by "Chris Soulsby published in 2008"

Influence of hydrology and seasonality on DOC exports from three contrasting upland catchments

Abstract: Variation in dissolved organic carbon (DOC) concentrations of surface waters is a consequence of process changes in the surrounding terrestrial environment, both within annual cycles and over the longer term. Long-term records (1987–2006) of DOC concentrations at six catchments (0.44–10.0 km2) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal patterns. The intra-annual mode of DOC export contrasted markedly between catchments and appeared dependent on their hydrological characteristics. Catchments in wetter Central Scotland with high rainfall–runoff ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC concentrations from December to May. This appears unrelated to discharge, and is dependent mainly on higher temperatures driving biological activity, increasing decomposition of available organic matter and solubility of DOC. The drier eastern catchments have lower rainfall–runoff ratios, longer transit times and annual drying–wetting regimes linked to changing connectivity of soils. These are characterised by seasonal DOC concentration–discharge relationships with an autumnal flush of DOC. Temperature influences the availability of organic matter for DOC transport producing a high DOC concentration–discharge relationship in summer/autumn and low DOC concentration–discharge relationship in winter/spring. These two distinct modes of seasonal DOC transport have important implications for understanding changes in DOC concentrations and export brought about by climate change (temperature and precipitation) and modelling of aquatic carbon losses from soil-types under different hydrological regimes.

A comparison of forest and moorland stream microclimate, heat exchanges and thermal dynamics

Abstract: Although the importance of riparian forest in moderating stream temperature variability is recognized, most previous research focuses on conifer harvesting effects and summer maximum temperature with highly variable findings. This article compares stream temperature, microclimate and heat exchange dynamics between semi-natural forest and moorland (no trees) reaches in the Scottish Cairngorms over two calendar years to provide a longer-term perspective. Mean daily water column temperature is warmer for moorland than forest in late winter–early spring, but cooler in summer. Daily water column temperature range is greater for moorland than forest. Streambed temperature dynamics are markedly different between reaches, reflecting contrasting groundwater–surface water (GW–SW) interactions. Mean, minimum and maximum daily air temperature is cooler, humidity is lower, and wind speed is much higher for moorland than forest on average. Net radiation is the dominant heat sink in autumn–winter and major heat source in spring–summer for moorland and summer for forest. Net radiation is greater in summer and lower in winter for moorland than forest. Sensible heat is an energy source in autumn–winter and sink in spring–summer, with loss (gain) greater in summer (winter) for moorland than forest. Latent heat is predominantly a sink for both reaches, with magnitude and variability higher for moorland than forest. Streambed heat flux is much smaller than fluxes at the air–water interface, with moorland and forest illustrating seasonal and between-reach differences attributable to different GW–SW interactions. Seasonal patterns in stream energy budget partitioning are illustrated schematically. To our knowledge, this is the first such study of mixed woodland, which generates notably different results to work on coniferous forest. This research provides a process basis to model stream thermal impact of changes in forest practice, and so inform decision making by land and water resource managers. Copyright © 2008 John Wiley & Sons, Ltd.

Conceptualizing catchment processes: simply too complex?

Abstract: 1 School of Geosciences, University of Aberdeen, Aberdeen, AB24 3UF, UK 2 College of Forest Engineering, Oregon State University, Corvallis, USA 3 UNESCO-IHE Institute for Water Education, Department for Water Engineering, 601 DA Delft, (and Delft University of Technology), The Netherlands 4 Plymouth State University/US Forest Service, Plymouth NH 03264-1595, USA 5 Hydrology and Quantitative Water Management Group, University of Wageningen, The Netherlands 6 Institute for Environmental Engineering, ETH Zurich, Zurich, Switzerland 7 Department of Geography, Queen Mary, University of London, UK, E1 4NS 8 Macaulay Institute, Aberdeen, UK

The influence of riparian woodland on stream temperatures: implications for the performance of juvenile salmonids

Abstract: Stream temperature was monitored at five mainstem locations and one tributary location on the Girnock Burn, a 31-km 2 tributary catchment of the Aberdeenshire Dee in north-east Scotland. Stream temperature was recorded at 15-min resolution between April 2003 and March 2006 to investigate the influence of semi-natural riparian woodland on the spatial and temporal variability of stream temperature and the influence that this had on the performance of juvenile Atlantic salmon (Salmo salar). Two upstream sites were located in open moorland locations, while three downstream sites were located in areas of mixed deciduous/coniferous woodland areas at progressive distances of 0.75, 1.5 and 2.0 km from the upstream edge of continuous riparian tree cover. The effects of riparian woodland on stream temperature varied through the year, increasing during the spring to a maximum in summer, before decreasing once again in autumn. Maximum, mean and diel temperature amplitude were lower and minimum temperatures higher at woodland sites when compared to those located in open moorland. Although significant differences in fish performance were detected between sites, the influence of temperature variability was unclear due to confounding factors, some of which could not be measured during the study. It is recommended that future studies should combine advances in field-based temperature monitoring with those in hydroclimatology to develop realistic process-based models that can be used for stream temperature prediction. Further advances are needed in understanding the relationship between naturally variable thermal regime and fish performance in order that ecological predictions can be usefully made from field and modelled temperature data.

Catchment data for process conceptualization: Simply not enough?

Abstract: C. Soulsby,1* C. Neal,2 H. Laudon,3 D. A. Burns,4 P. Merot,5 M. Bonell,6 S. M. Dunn7 and D. Tetzlaff1 1 School of Geosciences, University of Aberdeen, Aberdeen, AB24 3UF, UK 2 Centre for Ecology and Hydrology, Wallingford, Oxon, OX10 8BB, UK 3 SLU, Umea, Sweden 4 USGS, Troy, NY 12180, USA 5 INRA, Rennes, France 6 UNESCO HELP Centre, University of Dundee, Dundee, UK 7 Macaulay Institute, Aberdeen, UK

Consequences of invasion by the alien plant Mimulus guttatus on the species composition and soil properties of riparian plant communities in Scotland

Abstract: Invasive plant species are widely recognised to have severe ecological impacts in a wide range of ecosystems throughout the world, yet there are few experimental studies measuring community-level effects of invasive plant species. Thus most evidence is from correlative studies, and as such often cannot easily disentangle cause and effect. Through a combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of a widespread invasive species, Mimulus guttatus, on species richness and soil properties of riparian plant communities. The marked negative association between Mimulus cover and plant species richness identified through correlative multi-site comparisons was consistent with experimental removal studies which indicate Mimulus significantly alters the structure of riparian plant communities. Total C and N and soil moisture were marginally higher in invaded than in uninvaded disturbed sediment plots. Following Mimulus removal, there was an increase in the occurrence and abundance of another non-native species, Claytonia sibirica, as well as germination and establishment of Mimulus seedlings. This highlights that, although removal increased richness, bringing the plant community closer structurally to uninvaded vegetation, the application of removal as a management tool needs to be undertaken with caution, as it may create opportunities for other invaders. The impact of Mimulus appeared restricted to disturbed sediment communities, as addition experiments into herb–grass communities were relatively unsuccessful in establishing Mimulus. These patterns were consistent with the distribution of the species in riparian plant communities. The addition experiments highlight that, as well as competition from the resident vegetation community, mollusc herbivory further hinders the establishment of Mimulus. Many manipulation studies have removed invasive plant species from heavily invaded communities, and it is often thought that invasive species usually affect plant community structure only where their cover is high. This study is unique in demonstrating impacts where cover of the invasive plant is relatively low.

Interpretation of homogeneity in δ18O signatures of stream water in a nested sub-catchment system in north-east Scotland

Abstract: The isotope hydrology of a set of nested sub-catchments in the north-east of Scotland has been studied to examine the mixing processes and residence times of water in the catchments. The measured δ18O in stream waters was found to be exceptionally uniform both temporally and spatially. Hydrochemical mixing analyses showed that groundwater contributes between 62 and 90% of the stream flow in all sub-catchments. Model analysis indicated that the δ18O in stream water is indicative of a highly mixed system in which near surface runoff appears to be mixed with groundwater, within the soil profile, before being released from the catchment. Small fluctuations in the stream water δ18O response are generated by a small proportion (<10%) of less-well mixed water in infiltration excess runoff during storm events. A comparative application of the model to a nearby catchment, which has a lower proportion of groundwater runoff, demonstrated contrasting behaviour, with significantly less mixing of waters occurring and a more distinct difference in the age of runoff generated by different flow paths. This highlighted that standard methods for characterization of mixing mechanisms are often insufficient and may not discriminate between systems that have retained quite distinct flow paths throughout catchment transit, and those which have been mixed at some stage. Model sensitivity analysis also indicated that the simulated mean residence time of water varies most strongly in response to different parameters compared with the δ18O response. This has implications for estimating water residence times from isotope data. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of hydrological regimes on the pre-spawning entry of atlantic salmon (salmo salar l.) into an upland river

Abstract: The Girnock Burn is an Atlantic salmon (Salmo salar) spawning tributary of the river Dee, Aberdeenshire, Scotland. The dates, sexes and numbers of adult salmon entering the stream in the three months prior to spawning were determined between 1969 and 2005 using a permanent fish-trap. During this time, the number of fish returning to spawnvaried markedly. Here we analyze the whole of this unique long term data set in order to characterize and classify the hydrologic regimes of the pre-spawning migration period and assess the extent towhich these regimes help explain the timing and patterns of arrival of adult salmon into the stream. A range of flow parameters was calculated for the arrival period (September‐November) for each of the 37 years. These parameters were used to ordinate and classify years according to their flow regimes. The analysis yielded six distinct groups or ‘flow regime types’ which separated principally in relation to flow magnitude and frequency parameters. The number of fish entering the burn on a given day was not significantly correlated to discharge on that day. However, patterns of arrival of fish differed markedly between years with different regime types, with the number of fish arriving on a given day related to patterns of discharge over the preceding part of the arrival period. Overall inter-annual variability in entry corresponded to the defined regime types. In wetter years, fish entry to the stream usually starts relatively early and continues throughout the pre-spawning period. In contrast, dry years may result in fish entry being delayed; this may result in lower numbers of spawning females entering the stream and sub-optimal utilization of juvenile habitat in the following spring. Copyright # 2008 John Wiley & Sons, Ltd.

Does the incorporation of process conceptualization and tracer data improve the structure and performance of a simple rainfall‐runoff model in a Scottish mesoscale catchment?

Abstract: A geomorphological instantaneous unit hydrograph (GIUH) rainfall-runoff model was applied in a 31 km2 montane catchment in Scotland. Modelling was based on flow path length distributions derived from a digital terrain model (DTM). The model was applied in two ways; a single landscape unit response based on the DTM alone, and a two-landscape unit response, which incorporated the distribution of saturated areas derived from field-validated geographic information system (GIS) analysis based on a DTM and soil maps. This was to test the hypothesis that incorporation of process-information would enhance the model performance. The model was applied with limited multiple event calibration to produce parameter sets which could be applied to a spectrum of events with contrasting characteristics and antecedent conditions. Gran alkalinity was used as a tracer to provide an additional objective measure for assessing model performance. The models captured the hydrological response dynamics of the catchment reasonably well. In general, the single landscape unit approach produced the best individual model performance statistics, though the two-landscape unit approach provided a range of models, which bracketed the storm hydrograph response more realistically. There was a tendency to over-predict the rising limb of the hydrograph, underestimate large storm event peaks and anticipate the hydrograph recession too rapidly. Most of these limitations could be explained by the simplistic assumptions embedded within the GIUH approach. The modelling also gave feasible predictions of stream water chemistry, though these could not be used as a basis for model rejection. Nevertheless, the study suggested that the approach has potential for prediction of hydrological response in ungauged montane headwater basins. Copyright © 2007 John Wiley & Sons, Ltd.