Benchmarking model performance across large samples of
catchments is useful to guide model selection and future model development.
Given uncertainties in the observational data we use to drive and ...evaluate
hydrological models, and uncertainties in the structure and parameterisation
of models we use to produce hydrological simulations and predictions, it is
essential that model evaluation is undertaken within an uncertainty analysis
framework. Here, we benchmark the capability of several lumped hydrological
models across Great Britain by focusing on daily flow and peak flow
simulation. Four hydrological model structures from the Framework for
Understanding Structural Errors (FUSE) were applied to over 1000 catchments
in England, Wales and Scotland. Model performance was then evaluated using
standard performance metrics for daily flows and novel performance metrics
for peak flows considering parameter uncertainty. Our results show that lumped hydrological models were able to produce
adequate simulations across most of Great Britain, with each model producing
simulations exceeding a 0.5 Nash–Sutcliffe efficiency for at least 80 % of
catchments. All four models showed a similar spatial pattern of performance,
producing better simulations in the wetter catchments to the west and poor
model performance in central Scotland and south-eastern England. Poor model performance
was often linked to the catchment water balance, with models unable to
capture the catchment hydrology where the water balance did not close.
Overall, performance was similar between model structures, but different
models performed better for different catchment characteristics and metrics,
as well as for assessing daily or peak flows, leading to the ensemble of
model structures outperforming any single structure, thus demonstrating the
value of using multi-model structures across a large sample of different
catchment behaviours. This research evaluates what conceptual lumped models can achieve as a
performance benchmark and provides interesting insights into where
and why these simple models may fail. The large number of river catchments
included in this study makes it an appropriate benchmark for any future
developments of a national model of Great Britain.
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.
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•Controls on dissolved organic matter quantity and composition investigated in lowland catchments.•Catchments differed in dominant flow pathways (groundwater vs. surface water dominated).•Significant differences between organic matter composition observed between catchments.•Hydrology and land cover identified as likely controls on organic matter composition.
Monitoring programmes worldwide use biota to assess the “health” of water bodies. Indices based on biota are used to describe the change in status of sites over time, to identify progress against ...management targets and to diagnose the causes of biological degradation. A variety of numerical stressor‐specific biotic indices have been developed based on the response of biota to differences in stressors among sites. Yet, it is not clear how variation in pressures within sites, over what time period, and in what combination has the greatest impact on different biotic groups. An understanding of how temporal variation in pressures influences biological assessment indices would assist in setting achievable targets and help focus catchment‐scale mitigation strategies to ensure that they deliver the desired improvements in biological condition.
Hydrochemical data provided by a network of high‐frequency (15 or 30 min) automated monitoring stations over 3 years were matched to replicated biological data to understand the influence of spatio‐temporal variation in pollution pressures on biological indices. Hydrochemical data were summarised in various ways to reflect central tendency, peaks, troughs and variation over 1–90 days before the collection of each biological sample. An objective model selection procedure was used to determine which hydrochemical determinand, and over what time period, best explained variation in the biological indices.
Stressor‐specific indices derived from macroinvertebrates which purportedly assess stress from low flows, excess fine sediment, nutrient enrichment, pesticides and organic pollution were significantly inter‐correlated and reflected periods of low oxygen concentration, even though only one index (ASPTWHPT, average score per taxon) was designed for this purpose. Changes in community composition resulting from one stressor frequently lead to confounding effects on stressor‐specific indices.
Variation in ASPTWHPT was best described by dissolved oxygen calculated as Q5 over 10 days, suggesting that low oxygen events had most influence over this period. Longer‐term effects were apparent, but were masked by recovery. Macroinvertebrate abundance was best described by Q95 of stream velocity over 60 days, suggesting a slower recovery in numbers than in the community trait reflected by ASPTWHPT.
Although use of ASPTWHPT was supported, we recommend that additional independent evidence should be used to corroborate any conclusions regarding the causes of degradation drawn from the other stressor‐specific indices. The use of such stressor‐specific indices alone risks the mistargeting of management strategies if the putative stressor‐index approach is taken to be more reliable than the results herein suggest.
The dynamic interactions between dissolved organic matter (DOM) and particulate organic matter (POM) are central in nutrient cycling in freshwater ecosystems. However, the molecular-level mechanisms ...of such interactions are still poorly defined. Here, we study spatial differences in the chemical (i.e., individual proteinaceous amino acids) and microbial (i.e., 16S rRNA) composition of suspended sediments in the River Chew, UK. We then applied a compound-specific stable isotope probing (SIP) approach to test the potential assimilation of
13
C,
15
N-glutamate (Glu) and
15
N-NO
3
−
into proteinaceous biomass by particle-associated microbial communities over a 72-h period. Our results demonstrate that the composition of suspended particles is strongly influenced by the effluent of sewage treatment works. Fluxes and percentages of assimilation of both isotopically labelled substrates into individual proteinaceous amino acids showed contrasting dynamics in processing at each site linked to primary biosynthetic metabolic pathways. Preferential assimilation of the organic molecule glutamate and evidence of its direct assimilation into newly synthesised biomass was obtained. Our approach provides quantitative molecular information on the mechanisms by which low molecular weight DOM is mineralised in the water column compared to an inorganic substrate. This is paramount for better understanding the processing and fate of organic matter in aquatic ecosystems.
Sewage treatment systems are a common feature across the landscape of the United Kingdom, serving an estimated 96% of the population and discharging approximately eleven billion litres of treated ...wastewater daily. While large treatment facilities are ubiquitous across the landscape, they are not the only method employed in domestic wastewater treatment. This study investigates whether differences in nutrient export (carbon, nitrogen and phosphorus) and organic matter composition (determined by optical indices, SUVA254, S350-400 and E2:E3) from treated effluent could be detected between four of the most common facilities employed in the treatment of wastewater across the UK. Set in the context of the River Wylye, a small headwater catchment, treatment facilities studied included; a septic tank system, small packet treatment works, and two large sewage treatment works, one of which employed phosphorus stripping for phosphorus removal. Inorganic N and P concentrations ranged between 7.51 and 42.4 mg N l−1 and 0.22 and 8.9 mg P l−1 respectively, with DOC concentrations ranging between 1.63 and 11.8 mg C l−1. Optical indices were comparable to those observed in catchments where organic matter is dominated by autochthonous production, suggesting the dominance of low molecular weight material when compared to values observed across temperate aquatic systems. Combining data from both the Environment Agency and Ordinance Survey we estimate that only 15% of domestic properties not connected to mains sewerage in the study catchment have an Environment Agency consent/exemption permit. This calculation suggests that the quantity of small point sources are significantly underestimated, undermining efforts under current legislation to improve stream ecosystem health.
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•Variability in effluent discharge from four different wastewater treatment facilities investigated.•Significant differences in C, N and P concentrations and optical indices were observed.•Total numbers of ‘off grid’ point source discharges estimated.•Only 15% were found to have an Environment Agency permit to discharge.
Dissolved organic matter (DOM) represents a key component of carbon (C) cycling in freshwater ecosystems. While the behaviour of bulk dissolved organic carbon (DOC) in aquatic ecosystems is well ...studied, comparatively little is known about the turnover of specific DOC compounds. The aim of this study was to investigate the persistence of
C-labelled low molecular weight (LMW) DOC at a wide range of concentrations (0.1 µM to 10 mM), in sediments and waters from oligotrophic and mesotrophic rivers within the same catchment. Overall, rates of DOC loss varied between compound groups (amino acids > sugars = organic acids > phenolics). Sediment-based microbial communities contributed to higher DOC loss from river waters, which was attributed, in part, to its greater microbial biomass. At higher DOC compound concentrations, DOC loss was greater in mesotrophic rivers in comparison to oligotrophic headwaters. A lag-phase in substrate use within sediments provided evidence of microbial growth and adaptation, ascribed here to the lack of inorganic nutrient limitation on microbial C processing in mesotrophic communities. We conclude that the higher microbial biomass and available inorganic nutrients in sediments enables the rapid processing of LMW DOC, particularly during high C enrichment events and in N and P-rich mesotrophic environments.
Dissolved organic matter (DOM) plays an important role in freshwater biogeochemistry. To investigate the influence of catchment character on the quality and quantity of DOM in freshwaters, 45 ...sampling sites draining subcatchments of contrasting soil type, hydrology, and land cover within one large upland-dominated and one large lowland-dominated catchment were sampled over a 1-yr period. Dominant land cover in each subcatchment included: arable and horticultural, blanket peatland, coniferous woodland, and improved, unimproved, acid, and calcareous grasslands. The composition of the C, N, and P pool was determined as a function of the inorganic nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, and PO₄3−) and dissolved organic nutrient (dissolved organic carbon DOC, dissolved organic nitrogen DON, and dissolved organic phosphorus DOP) concentrations. DOM quality was assessed by calculation of the molar DOC : DON and DOC : DOP ratios and specific ultraviolet absorbance (SUVA254). In catchments with little anthropogenic nutrient inputs, DON and DOP typically composed > 80% of the total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations. By contrast, in heavily impacted agricultural catchments DON and DOP typically comprised 5–15% of TDN and 10–25% of TDP concentrations. Significant differences in DOC : DON and DOC : DOP ratios were observed between land cover class with significant correlations observed between both the DOC : DON and DOC : DOP molar ratios and SUVA254 (r
s = 0.88 and 0.84, respectively). Analysis also demonstrated a significant correlation between soil C : N ratio and instream DOC : DON/DOP (r
s = 0.79 and 0.71, respectively). We infer from this that soil properties, specifically the C : N ratio of the soil organic matter pool, has a significant influence on the composition of DOM in streams draining through these landscapes.
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