Summer Low-flow Period Research Articles

Sensitivity analyses of a distributed catchment model to verify the model structure

Sensitivity analyses are valuable tools for identifying important model parameters, testing the model conceptualization, and improving the model structure. They help to apply the model efficiently and to enable a focussed planning of future research and field measurement. Two different methods were used for sensitivity analyses of the complex process-oriented model TAC D (tracer aided catchment model, distributed) that was applied to the meso-scale Brugga basin (40 km 2) and the sub-basin St Wilhelmer Talbach (15.2 km 2). Five simulations periods were investigated: two summer events, two snow melt induced events and one summer low flow period. The model was applied using 400 different parameter sets, which were generated by Monte Carlo simulations using latin hypercube sampling. The regional sensitivity analysis (RSA) allowed determining the most significant parameters for the complete simulation periods using a graphical method. The results of the regression-based sensitivity analysis were more detailed and complex. The temporal variability of the simulation sensitivity could be observed continuously and the significance of the parameters could be determined in a quantitative way. A dependency of the simulation sensitivity on initial- and boundary conditions and the temporal and spatial variability of the sensitivity to some model parameters was revealed by the regression-based sensitivity analysis. Thus, the difficulty of transferring the results to different time periods or model applications in other catchments became obvious. The analysis of the temporal course of the simulation sensitivity to parameter values in conjunction with simulated and measured additional data sets (precipitation, temperature, reservoir volumes etc.) gave further insight into the internal model behaviour and demonstrated the plausibility of the model structure and process conceptionalizations.

Validation and Use of a Semidistributed Hydrological Modeling System to Predict Short-Term Effects of Clear-Cutting on a Watershed Hydrological Regime

Abstract The Gestion Integree des Bassins versants a l'aide d'un Systeme Informatise (GIBSI), a semidistributed hydrological modeling system, was evaluated for its ability to simulate the impact of deforestation on the hydrological regime of the Famine River watershed (728 km2), a subwatershed of the Chaudiere River, Quebec, Canada. Annual, spring and summer, and low-water runoff, as well as peak flows, were estimated for both a base-case scenario and a deforestation scenario using 31 annual meteorological series. GIBSI simulated an average increase of annual runoff after clear-cutting of 57% (268 mm) and the proportion of runoff to precipitation increased from 40% to 63%. The average increase in spring runoff was 25%, while in summer it was 138%. For summer low-flow periods, GIBSI simulated an average increase in runoff of 102%. For spring and summer peak-flow rates, hydrographs generated by GIBSI showed that average spring peak flows were increased after deforestation by 26% while summer peak flows were...

Calcite saturation in the River Dee, NE Scotland.

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SIcalcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO2 values indicate strong seasonal patterns, with the highest pH and lowest EpCO2 values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme 'end-member' case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SIcalcite-pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SIcalcite against pH gives a straight line with a gradient in the range 1.6-2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SIcalcite-pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column.

Interaction between shallow groundwater, saline surface water and nutrient discharge in a seasonal estuary: the Swan–Canning system

AbstractThe Swan and Canning Rivers converge to form an estuary that is seasonally forced by wet winter and dry summer conditions. The estuary is also tidally forced due to its contact with the Indian Ocean. The perception that the occurrence of nuisance algal blooms has increased in frequency and severity in recent years has prompted the present investigation into the interaction of the shallow groundwater system with the Swan–Canning Estuary. The extent to which this interaction contributes to nutrient delivery to the river is a focus of the work.Groundwater interaction with the upper reaches of the Swan River is shown to occur at three length scales: (i) the scale of the river‐bed sediments (i.e. <10 m); (ii) the scale over which tidal forcing of the river is transmitted into the adjacent aquifer (10 to 100 m); and (iii) the scale of the regional groundwater field (>1000 m). Two‐dimensional groundwater flow modelling in plan covering the regionally advected groundwater flow domain of the upper Swan River Estuary from the Causeway to Guildford shows that there is a net groundwater discharge to the Swan River of groundwater discharge of about 80 000 m3/day, or about 29 million m3/year. Between 1987 and 1996, the average surface tributary inflow to the Swan River was about 460 million m3/year. Thus groundwater discharge contributed approximately 6% of the total annual river flow. This percentage is clearly small in comparison to the total river flow. However, in the six months from November to April in summer, tributary flow into the Swan River declines sharply to an average total of approximately 12 million m3. Groundwater discharge during this six‐month period is approximately 14 million m3 or about 55% of the surface tributary flow, and thus groundwater is a significant component of the total inflow to the Swan–Canning Estuary during this period. Nutrient concentrations, particularly ammonium, within the sediment pore fluids underlying the river are very high relative to concentrations in the river, such that groundwater discharge rates of this magnitude are capable of introducing significant nutrient loadings to the river. The nitrogen load to the Swan River derived from regionally advected groundwater discharge is conservatively estimated at between 30 and 60 t/year. This ranks groundwater as one of the highest single inputs of nitrogen to the Swan–Canning Estuary when compared to surface water flows. It contributes about 10% of the total nitrogen load entering this reach of the river. Seasonal and tidal variations increase the complexity of the system and may act to increase the presence and availability of groundwater‐derived nutrients in the river and estuary system.Field observations and results from numerical groundwater flow modelling show that groundwater preferentially discharges into the Swan River along the outside of river meanders. Along the inside of meanders, groundwater discharge is either very low or at times the flow direction reverses such that saline river water recharges the aquifer during the late spring, summer and early autumn months. Short‐term, high‐frequency fluctuations such as wave‐induced displacement, seawater intrusion, hyporheic flux and tidal fluctuations may cause small spatial scale mixing and perturbation of the nutrient‐rich groundwater and sediment porewater with either the river water directly or with fresher groundwater as it discharges to the river. These short‐term fluctuations appear to cause nutrient release from the sediment pore fluids, particularly in summer low flow periods such that they are a significant contributing factor in triggering algal blooms. Copyright © 2001 John Wiley & Sons, Ltd.

Impacts of groundwater abstraction on the trout fishery of the River Piddle, Dorset; and an approach to their alleviation

The River Piddle, a small chalk stream in Dorset, supports a valuable fishery for brown trout. It is also heavily utilised for water abstraction, with a licensed maximum daily quantity of 197 Ml/d, of which 43·5% can be taken from the groundwater. The largest licensed groundwater uses (by licensed volume) are public water supplies (PWS), of which there are four within the Piddle catchment. Investigations into the impacts of the largest PWS abstraction, at Briantspuddle, on the brown trout population and fisheries of the River Piddle demonstrated a spatial correlation between a zone of reduced river flow and an area of low juvenile trout abundance, and a reduction in the period during which good quality trout fishing was available. Trout habitat was quantified by applying the Physical Habitat Simulation System (PHABSIM) in conjunction with a groundwater model. The resultant habitat duration curves and habitat time-series corresponding with historical and naturalised (zero abstraction) flow, indicated large habitat losses for juvenile trout. Using discharge habitat relationships a preferred flow regime for the river at Briantspuddle, designed to satisfy both ecological and recreational needs, was identified. This flow regime, which will be maintained by stream support, includes a ‘survival flow’ for juvenile trout during the critical summer low flow period. Copyright © 1999 John Wiley & Sons, Ltd.

An evaluation of inter-basin water transfers as a mechanism for augmenting salmonid and grayling habitat in the River Wear, North-East England

Transfers of water from the Kielder system have been used for 12 years to avoid low flow problems in the River Wear. Transfers are scheduled to avoid breaches of the river's statutory Minimum Maintained Flow (MMF). Despite this routine use, the role of transfers in augmenting instream habitat has never been evaluated. A physical habitat simulation (PHABSIM) study was undertaken in 1996 to investigate the influence of transfers and the MMF policy on brown trout Salmo trutta, Atlantic salmon Salmo salar and grayling Thymallus thymallus instream habitat at three sites on the Wear. Transfers support total habitat levels up to 10% greater than unregulated conditions. They impact usable instream habitat (weighted usable area) to a much greater extent. For salmon parr, the species/lifestage whose habitat is most limited by low flows, transfers have maintained relatively stable usable habitat levels during periods when otherwise they would have fallen by as much as 70%. The MMF policy results in minimum flow values which are higher than those which would have been set using the Montana Method and the availability of salmon parr usable habitat does not fall below 10% of its mean annual value. Judged in these terms, the MMF-based transfer regime has played a positive role in avoiding extreme habitat loss in the Wear. Simulations of four alternative Kielder transfer release policies indicate that near-optimum habitat levels could be maintained throughout summer low flow periods. However, this would result in unnatural temporal patterns of flow and habitat availability. © 1998 John Wiley & Sons, Ltd.

Shifts in the Effects of Tuft-weaving Midges on Filamentous Algae

-During summer low-flow periods massive growths of filamentous green algae (dominated by Cladophora glomerata) in the Eel River of Northern California become infested with chironomid larvae that weave the algae into retreats, or Field experiments suggest that these midges (primarily Pseudochironomus richardsoni) have negative early effects, and positive later effects on the biomass of algae they inhabit. During initial colonization and tuft construction by midges, weight loss of Cladophora and associated epiphytes in stream enclosures increased with increasing midge densities. In an in situ incubation experiment, chlorophyll a content and final damp weight of Cladophora and epiphytes were higher with old, feces-filled tufts, or with nitrate fertilizer, and were lower in controls or with new midge tufts. These results suggest that tuft-weaving midges have complex effects on algae they inhabit that change with the ontogeny of midges and the age of the retreats they construct.

Quality of Salmonid Hatchery Effluents during a Summer Low-Flow Season

Abstract I assessed the quality of salmonid hatchery effluents and receiving streams in Washington State during the 1988 summer low-flow period. Relative to hatchery influent waters, effluents showed significant increases in temperature, pH, suspended solids, ammonia, organic nitrogen, total phosphorus, and chemical oxygen demand. Wastewater discharges sometimes violated state water quality standards; effects were exacerbated by low dilution. Hatchery nutrient loads equalled or exceeded receiving water loads; effects of enrichment were most evident in oligotrophic waters. Benthic invertebrates sensitive to organic waste were often replaced by pollution-tolerant forms in the vicinity of hatchery outfalls. Survey findings necessitated revision of existing hatchery wastewater discharge permits in Washington.

Benthic Turfs vs Floating Mats of Algae in River Food Webs

In the Eel River of northern California, as in many other sunlit rivers worldwide, large growths of filamentous green algae accumulate during summer low flow periods. Algae initially grow as turfs, attached to the river bed, but over time detach to form floating mats. Floating mats differ from attached turfs as habitats for invertebrates in a number of respects. In paired trials, larval chironomids were 15-16 times more likely to be consumed by fishes when exposed in algal turfs on the river bed than when exposed in floating algal mats on the river surface. On average, 2.7 and 6.1 times more adult insects emerged from floating mats than from benthic algal turfs in shallow (< 30 cm deep) and deep (30-60 cm deep) habitats, respectively. Differences between algal mats and turfs as cover and habitat for resident insects lead to the hypothesis that the timing, abundance, and duration of floating mat habitats strongly influences the routing of insect production in river food webs, to aquatic consumers or to the land.

Habitat Suitability and Fish Production: A Model Evaluation for Rock Bass in Two Virginia Streams

A habitat suitability index (HSI) model for rock bass Ambloplites rupestris was developed from available suitability curves and field tested by comparing HSI and habitat units with seasonal standing stock, biomass, and production of rock bass in two fourth-order streams in Virginia. Consistent ranking between HSI and mean standing stocks for the study year was observed only for 250-m stream sections, the largest spatial unit of comparison. In contrast, changes in HSI explained no more than 22% of the variability in mean standing stocks in 50-m subsections of Back Creek, and were not significantly correlated with standing stocks in Little Walker Creek. Factors that apparently contributed to low correlations and ranking success included geographically homogeneous habitat and seasonal fish movements, as well as potentially important variables such as depth and cover that are not features of the tested HSI models. Annual discharge and fish distribution data further suggested that physical habitat was more limiting to populations of rock bass in the fall, not during the summer low-flow period as previously assumed. Unmeasured temporal variability in the environment may have been equally important in limiting presumed carrying capacities in study streams. Annual production for rock bass was nearly identical in both streams and averaged 1.04 g˙m−2˙year−1. This similarity in annual production was consistent with the habitat homogeneity implied by the narrow range of observed HSIs. Validation of our HSI model and similar short-term assessments of habitat quality would be more likely if study sections included the home range or annual habitat requirements of all life stages of the target species. Additional quantitative data on species-habitat relationships and systematic evaluation of individual suitability indices should also improve testing of aggregate HSI models.

LOW DISCHARGE PERIODS OF RIVERS IN THE UPPER VISTULA RIVER BASIN, POLAND1

ABSTRACT: A method is presented for determining low discharge periods of rivers based on threshold values defined in terms of MEAN OF MINIMUM SUMMER (WINTER) DISCHARGES. The method is used to determine summer low flow periods in the Vistula River basin in Poland. Analysis is based on daily discharge data for 84 basins of the Upper Vistula River which was collected by the Polish Institute of Meteorology and Water Economy.

Brachycentrus americanusandB. occidentalis(Trichoptera) in a Regulated Stream

ABSTRACT Late instar Brachvcentrus americanus larvae were collected just prior to and during normally high spring runoff; adults were collected immediately following runoff; and early instar larvae appeared during the late summer low flow period of September and October. Brachvcentrus occidentalis adults were collected prior to normal spring runoff and early instar were first collected following runoff during moderate flows of July and August. Both strategies helped these species avoid stresses of high water velocities and sediment scouring often associated with spring runoff. During the study period, yearly high stream flows and maximum sediment movement occurred during summer flushing releases from Electric Lake, rather than the normal spring runoff period. Increased stream discharges with associated sediment movement that occurred when early instar Brachvcentrus larvae were present, appear to have had noticeable negative affects. Whatever the cause, following these occurrences, either species having ea...