Temporal Variations In Flow Research Articles

Dominant physical controls on hourly flow predictions and the role of spatial variability: Mahurangi catchment, New Zealand

We present a systematic approach to the development of models for making hourly flow predictions, where each step provides insight into the relative importance of catchment and climatic properties (rainfall, soil properties, vegetation, topography), their spatial variability, and their influence on temporal flow variability at the outlet. Our modelling approach uses a simple conceptual model design requiring minimal calibration and physically meaningful input parameters estimated a priori from landscape data or from analyses of streamflow recession curves. The model structure allows direct measurement of parameter uncertainty and the ability to investigate its propagation through the model to produce bounds of predictive uncertainty via the Monte Carlo method. This method was applied to a number of model designs to assess the tradeoff between model complexity, accuracy and predictive uncertainty, and to identify the most appropriate model design under specific climatic conditions. With the preferred model, sensitivity analysis was used to identify the dominant controls on streamflow variability at the hourly timescale. In summary the aim is not to present a distributed model of universal applicability, but to generate insights into the climate, soil and vegetation controls on streamflow variability at the hourly timescale, and on predictive accuracy and uncertainty, that can be used in future modelling efforts.

Diurnal variations in vertical strain observed in a temperate valley glacier

During a period of diurnal fluctuations in glacial flow speed, vertical strain was measured with sub‐daily temporal resolution on Unteraargletscher, Switzerland. Mean vertical strain in boreholes up to 300‐m deep in 400‐m‐thick ice was found to fluctuate diurnally. Vertical strain rates were tensile in the daytime and compressive at night, with a magnitude of up to 10−3 day−1. Horizontal surface strain was observed to fluctuate in a manner consistent with the vertical deformation. Diurnal surface flow speed variations correlated well with basal water pressure suggesting a basal control on temporal flow variations. Nevertheless, the strain rate measurements indicated that changes in surface flow speed are affected by internal ice deformation and not a direct measure of local basal motion. Basal conditions in the surrounding neighborhood and their temporal variations take an important role in short‐term glacial flow fluctuations.

Effects of precipitation events on colloids in a karst aquifer

The effects of precipitation events on colloid mobilization were evaluated during several storms from six wells in a karstic aquifer at the Oak Ridge Y-12 Plant in eastern Tennessee (USA). Turbidity increases and rapidly recedes following rain events. Although the magnitude of the turbidity increases are relatively small (≤4.78 NTU), the increased turbidity suggests transient increases in colloid abundance during storm versus non-storm periods. During the larger storms (>19 mm), the increased turbidity is associated with increases in pH, total organic carbon (TOC) and temperature, and with decreases in dissolved oxygen (DO). These larger storms result in flushing of a greater proportion of higher pH, TOC (and lower DO) soil or matrix waters into the fractures and conduits than occurs during smaller storms. Smaller storms also result in increases in turbidity, but show increases in DO and decreases in pH reflecting less influence on the water chemistry from the longer residence time epikarst or and matrix waters, and greater impact from the more dilute, newly recharged waters. Due to the complexity of karst flow and temporal variations in flow and chemistry, controls on turbidity are not consistent through time and space at the wells. During smaller storms, recharge by lower ionic strength waters may promote colloid release and thus contribute to observed increases in turbidity. During larger storms, elevated turbidity may be more related to pH increases resulting from greater influx of matrix and soil waters into fractures and conduits. Chemical factors alone cannot account for the changes in turbidity observed during the various storms. Because of the complicated nature of flow and particle transport in karst aquifers, the presence of colloids during precipitation events is dictated by a complex interplay of chemical reactions and the effects of physical perturbations due to increased flow through the conduits and fractures. Simple trends in water quality parameters could not be identified, and broad generalizations cannot easily be made in karst settings, and some of the expected correlations between chemical parameters during the storms were not observed in this work.

FLOW AND HABITAT EFFECTS ON JUVENILE FISH ABUNDANCE IN NATURAL AND ALTERED FLOW REGIMES

Conserving biological resources native to large river systems increasingly depends on how flow-regulated segments of these rivers are managed. Improving management will require a better understanding of linkages between river biota and temporal variability of flow and instream habitat. However, few studies have quantified responses of native fish populations to multiyear (>2 yr) patterns of hydrologic or habitat variability in flow-regulated systems. To provide these data, we quantified young-of-year (YOY) fish abundance during four years in relation to hydrologic and habitat variability in two segments of the Tallapoosa River in the southeastern United States. One segment had an unregulated flow regime, whereas the other was flow-regulated by a peak-load generating hydropower dam. We sampled fishes annually and explored how continuously recorded flow data and physical habitat simulation models (PHABSIM) for spring (April–June) and summer (July–August) preceding each sample explained fish abundances. Patterns of YOY abundance in relation to habitat availability (median area) and habitat persistence (longest period with habitat area continuously above the long-term median area) differed between unregulated and flow-regulated sites. At the unregulated site, YOY abundances were most frequently correlated with availability of shallow-slow habitat in summer (10 species) and persistence of shallow-slow and shallow-fast habitat in spring (nine species). Additionally, abundances were negatively correlated with 1-h maximum flow in summer (five species). At the flow-regulated site, YOY abundances were more frequently correlated with persistence of shallow-water habitats (four species in spring; six species in summer) than with habitat availability or magnitude of flow extremes. The associations of YOY with habitat persistence at the flow-regulated site corresponded to the effects of flow regulation on habitat patterns. Flow regulation reduced median flows during spring and summer, which resulted in median availability of shallow-water habitats comparable to the unregulated site. However, habitat persistence was severely reduced by flow fluctuations resulting from pulsed water releases for peak-load power generation. Habitat persistence, comparable to levels in the unregulated site, only occurred during summer when low rainfall or other factors occasionally curtailed power generation. As a consequence, summer-spawning species numerically dominated the fish assemblage at the flow-regulated site; five of six spring-spawning species occurring at both study sites were significantly less abundant at the flow-regulated site. Persistence of native fishes in flow-regulated systems depends, in part, on the seasonal occurrence of stable habitat conditions that facilitate reproduction and YOY survival.

Fine‐scale field measurement of benthic flow environments inhabited by stream invertebrates

We used hot‐film anemometry to quantify fine‐scale spatial and temporal flow variations near the surfaces of stones inhabited by suspension‐feeding larval blackflies (Simulium vittatum). We focused especially on within‐stone patterns of covariation between patchy microdistributions of larvae and local spatial variations in current speed. Current speeds were sampled at 256 Hz for heights between 1 and 10 mm above the bed. Profiles of current speed exhibited complex shapes, and boundary‐layer thicknesses ranged from <1 to >5 mm. Average current speeds measured 2 mm above the bed (the approximate height of larval feeding appendages) ranged between 7 and 59 cm s−1. Current speeds measured 10 mm above the bed were very poor predictors of speeds measured at the 2‐mm height. Larval abundance exhibited a significant positive relationship to current speed at 2‐mm height, and within‐stone variations in speed explained ~59% of the variation in abundance. Time series of current speed exhibited marked fine‐scale temporal heterogeneity, fluctuating by as much as 80 cm s−1 in <0.1 s. Maximum accelerations sometimes exceeded 1 × 104 cm s−2, which suggests that the forces tending to dislodge benthic organisms from the bed may be greater than previous estimates based on assumptions of steady flow. Observed levels of turbulence were greater than predicted from traditional boundary‐layer theory. We suggest that much of the turbulence evident on individual stones is not produced by local shear but is inherited from upstream roughness elements that cause flow separation.

The Morphology, Magnitude and Regime of a Carboniferous Fluvial-Distributary Channel

ABSTRACT A meandering fluvial-distributary was initiated into the interdistributary bay of a Carboniferous (Yoredale) delta lobe by avulsion. Initiation immediately preceded abandonment of the lobe, and lateral migration of the channel produced a meander belt sandstone in the short period of channel activity. The channel fill comprises an 8 m fining upwards sequence with lateral accretion surfaces in the coarse member which permit calculation of palaeochannel parameters. The coarse member also exhibits inclined, erosive discontinuities which reflect temporal variations in flow. Paleochannel regime was dominated by frequently recurring normal floods which produced the lateral accretion surfaces, and less frequent exceptional floods which were primarily effective in scouring the bar surface, pro ucing the discontinuities.

A Theoretical Approach to Interspecific Competition in Phytoplankton Communities

A three-compartment mathematical model was developed to represent a phytoplankton population with the capability of storing nitrogen in a nitrate-limited environment. Parameters were estimated by fitting the model to equilibrium data from two chemostat experiments reported by Caperon (1968, 1969). The model was then run to simulate the transient chemostat conditions, and the model response was compared with the observed data. The model provides a reasonable representation of the sudden population surges associated with intracellular nutrient storage. One of the most interesting phenomena associated with phytoplankton communities is the ability for many different genera and species to coexist in the same apparently homogeneous environment. Because of the sequential blooms observed in nature, it is intuitively obvious that temporal variations in environmental conditions are a predominant mechanism influencing an organism's ability to compete and coexist with its competitors. However, in the past, true coexistence based on transient conditions has proved difficult to describe mathematically. In this paper competition was studied by using a mathematical model which incorporates intracellular nutrient storage. Thus, an organism's ability to compete at any given time is based not only on current environmental conditions, but also on the past history of nutrient uptake. The model demonstrates the effect of flow variations on the competitive ability of two hypothetical species. Temporal variations in flow were characterized by three parameters: period, amplitude, and percentage of time high flow occurs. Regions of coexistence were described in terms of these parameters. Temporal variations in nutrient supply also affect competitive ability. It is suggested that a region of coexistence can be thought of as multidimensional, with each dimension described by a parameter associated with temporal variations in some environmental characteristic. The reason that "niches" are so difficult to find and describe may be that each is defined by a hyperspace of temporal variation. Many environmental forces are known to influence growth dynamics in natural systems. The intensities of these forces fluctuate widely, regularly or irregularly, with different periods and amplitudes, thus providing an unlimited number of intensity combinations as the system passes through time. At one point in time the combination of environmental conditions may be most suitable for one particular species and at another time most suitable for an entirely different species. Provided that a suitable combination occurs with sufficient frequency and duration, a species may be able to survive indefinitely. One of the most subtle dangers to a dynamically balanced system of this type is the removal of the complex temporal variations. Considering man's encroachment on the environment, concern must be shown not only for what is added to or withdrawn from a natural system, but also for the possibility of significantly dampening the fluctuation of environmental forces. We are presently applying the concepts and the above model to data obtained from natural populations in a stratified marine environment.