Size Distributions and Light Scattering Features of Minerogenic Particles in a Stream during Runoff Events

Inorganic (minerogenic) particles received from streams during runoff events cause undesirable increases in turbidity (Tn; i.e., the light scattering coefficient, b) in many lakes and reservoirs. Quantification of particle inputs in the context of turbidity impacts and representative light scattering calculations (submodel), the necessary components in the development of a mechanistic multiple particle class model to simulate these impacts, are described for Schoharie Creek, N.Y. Light scattering attributes of minerogenic particles, including number concentration (N), size distribution (PSD), composition, and projected area per unit volume (PAVm), are quantified for three runoff events through analyses by scanning electron microscopy interfaced with automated X-ray microanalysis and image analysis (SAX). The combined credibility of the light scattering submodel and SAX to represent the light scattering features of minerogenic particles is depicted by the consistency of calculated scattering efficiencies with theory and the strength of the reported Tn-PAVm relationship. Particles in the size range of 2–10?μm were responsible for b and Tn in the stream, with greater contributions by the larger particles of this range at elevated stream flows. Empirical relationships are developed to predict N and PSD from Tn or stream flow.     

Bayesian Rating Curve Inference as a Streamflow Data Quality Assessment Tool

A streamflow time-series is normally obtained by transforming a time-series of recorded stage to discharge using an estimated rating curve. The accuracy of this streamflow time-series depends on the characteristics of the available stage-discharge measurements used to fit the rating curve. The Norwegian Water Resources and Energy Directorate (NVE) has developed a method based on rating curve uncertainty for performing objective quality assessment of streamflow time-series. The method, which is based on a Bayesian statistical framework, uses the available stage-discharge measurements and the corresponding stage time-series to derive statistics utilised for a quality assurance of the streamflow time-series. Nearly one thousand streamflow time-series periods have been classified using the method. This paper presents the results.     

Tracking changes in nutrient delivery to western Lake Erie: Approaches to compensate for variability and trends in streamflow

Tracking changes in stream nutrient inputs to Lake Erie over multidecadal time scales depends on the use of statistical methods that can remove the influence of year-to-year variability of streamflow but also explicitly consider the influence of long-term trends in streamflow. The methods introduced in this paper include an extended version of Weighted Regressions on Time, Discharge, and Season (WRTDS) modeling that explicitly considers nonstationary streamflow by incorporating information on changes in the frequency distribution of daily measured streamflow (discharge) over time. Soluble reactive phosphorus (SRP) trends in annual flow-normalized fluxes (loads) at five long-term monitoring sites in the western Lake Erie drainage basin show increases of 109 to 322% over the period 1995 to 2015. About one-third of the increase appears attributable to increasing discharge trends, while the remaining two-thirds appears to be driven by changes in concentration versus discharge relationships reflecting higher concentrations for any given discharge during recent years. Trends in total phosphorus and three nitrogen parameters (total nitrogen, nitrate-nitrite, and total Kjeldahl nitrogen) at the 10 sites analyzed were much less pronounced, and commonly show decreases in concentration-discharge relationships accompanied by increases in discharge, resulting in little net change in total flux. Trends in monthly SRP fluxes and discharge, dissolved versus particulate fractions of nutrients, and N:P flux ratios were also evaluated. The methods described here provide tools to more clearly discern the effectiveness of nutrient-control strategies and can serve as ongoing measures of progress, or lack of progress, towards nutrient-reduction goals.     

Case Study: Evaluation of Streamflow Partitioning Methods

Understanding water flow and its relative quantities through different pathways is vital for watershed management. Like many problems in hydrology, numbers of methods have been proposed for streamflow partitioning. Five methods were identified as being the most relevant and least input intensive. This study tested performance of these methods against separately measured surface and subsurface flow data from the coastal plain physiographic region of the southeastern United States. Separately measured surface and subsurface flow were collected for 12 years (1970–1981) in a field scale watershed by the Southeast Watershed Research Laboratory of the USDA-Agricultural Research Service. Results of comparative analysis indicated that Method IV performed the best. Results also indicated that accuracy of this method is highly dependent upon the proper estimation of the “fraction coefficient” that is based on many physical and hydrologic characteristics of the watershed. This study concluded that deterministic/empirical methods such as Boughton’s Method IV, require proper parameter value for increased accuracy.     

Steady Streaming around a Circular Cylinder near a Plane Boundary due to Oscillatory Flow

Steady streaming due to an oscillatory flow around a circular cylinder close to and sitting on a plane boundary is investigated numerically. Two-dimensional (2D) Reynolds-averaged Navier-Stokes equations are solved using a finite element method with a k-ω turbulent model. The flow direction is perpendicular to the axis of the cylinder. The steady streaming around a circular cylinder is investigated for Keulegan-Carpenter (KC) number of 2 ≤ KC ≤ 30 with a constant value of Stokes number (β) of 196. The gap (between the cylinder and the plane boundary) to diameter ratio (e/D) investigated is in the range of 0.0–3.0. The steady streaming structures and velocity distribution around the cylinder are analyzed in detail. It is found that the structures of steady streaming are closely correlated to KC regimes. The gap to diameter ratio (e/D) has a significant effect on the steady streaming structure when e/D<1.0. The magnitude of the steady streaming velocity around the cylinder can be up to about 70% of the velocity amplitude of the oscillatory flow. One three-dimensional (3D) simulation (KC = 10, β = 196, and e/D = ∞) is carried out to examine the effect of three dimensionality of the flow on the steady streaming. Although strong 3D vortices are found around the cylinder, the steady streaming in a cross section of the cylinder span is in good agreement with the 2D results.     

Reynolds Stress in Open Channel Flow with Upward Seepage

Nonuniform-unsteady flow in open channels with streamwise sloping beds having uniform upward seepage is theoretically analyzed. Expressions for the Reynolds stress and bed shear stress are developed, assuming a modified logarithmic law of velocity profile due to upward seepage, and using the Reynolds and continuity equations of two-dimensional open channel flow. The computed Reynolds stress profiles are in agreement with the experimental data.     

Calibrating hourly rainfall-runoff models with daily forcings for streamflow forecasting applications in meso-scale catchments

The absence of long sub-daily rainfall records can hamper development of continuous streamflow forecasting systems run at sub-daily time steps. We test the hypothesis that simple disaggregation of daily rainfall data to hourly data, combined with hourly streamflow data, can be used to establish efficient hourly rainfall-runoff models. The approach is tested on four rainfall-runoff models and a range of meso-scale catchments (150–3500 km²). We also compare our disaggregation approach to a method of parameter scaling that attains an hourly parameter-set from daily data.Simple disaggregation of daily rainfall produces hourly streamflow models that perform almost as well as those developed from hourly rainfall data. Rainfall disaggregation performs at least as well as parameter scaling, and often better. For the catchments and models we test, simple disaggregation is a very straightforward and effective way to establish hydrological models for continuous sub-daily streamflow forecasting systems when sub-daily rainfall data are unavailable.     

Comparing single- and two-segment statistical models with a conceptual rainfall-runoff model for river streamflow prediction during typhoons

This study examined various regression-based techniques and an artificial neural network used for streamflow forecasting during typhoons. A flow hydrograph was decomposed into two segments, rising and falling limbs, and the individual segments were modeled using statistical techniques. In addition, a conceptual rainfall–runoff model, namely the Public Works Research Institute (PWRI)-distributed hydrological model, and statistical models were compared. The study area was the Tsengwen Reservoir watershed in Southern Taiwan. The data used in this study comprised the observed watershed rainfalls, reservoir inflows, typhoon characteristics, and ground weather data. The forecast horizons ranged from 1 to 12 h. A series of assessments, including statistical analyses and simulations, was conducted. According to the improvements in errors, among single-segment statistical models, the multilayer perceptron achieved superior prediction accurary compared with the regression-based methods. However, the pace regression was the most favorable according to an evaluation of model complexity and accuracy. To examine the robustness of the results for forecast horizons varying from 1 to 12 h, statistical significance tests were performed for the single- and two-segment models. The prediction ability of the two-segment models was superior to that of the single-segment models. In addition, Typhoon Sinlaku in 2008 was considered in a comparison between the conceptual PWRI model output and that of the developed statistical models. The results showed that the PWRI model yielded the least favorable results.     

Assessment of three dimensionless measures of model performance

Pertinent characteristics of three dimensionless and comparable model-performance or model-efficiency measures are examined and compared. Our model-assessment recommendations apply to many types of environmental models. Representing measures based on sums-of-squared errors or “quadratic measures” (by far, the most widely used class) is Nash and Sutcliffe's (1970) well-known efficiency measure (E). Our assessments of E, by and large, also apply to similar-in-form benchmark measures (Seibert, 2001). Legates and McCabe's (1999) version of E (E₁) and Willmott et al.'s (2012) refined index of agreement (d_r) represent the less-often-employed but more interpretable class of measures based on sums of error magnitudes. Conceptual and algebraic arguments are used in conjunction with assessments of many parameter sets of the Soil and Water Assessment Tool (SWAT) hydrologic model, over the period 1950–2005. Our findings suggest that, of the three measures, d_r has the broadest utility, followed in order by E₁ and E.     

Flow Depletion Induced by Pumping Well from Finite Length of Stream

A procedure for calculating the flow depletion from a finite length of a stream induced by a pumping well in an adjacent aquifer is developed. Four management cases of finite length of the stream including a basic case are considered. A “basic flow depletion factor” is defined, in terms of which the flow depletion factors for all cases are expressed. The basic flow depletion factor is twice the Hantush M function. A computationally simple and accurate practical approximation of the basic flow depletion factor is presented that encompasses the full practical range of the solutions. Using this approximation, an optimization method is proposed for the estimation of the aquifer hydraulic diffusivity and effective distance from the pumping well to the line of recharge from the measured temporal variation of stream flow depletion between two sections. During optimization, repeated computation of stream flow depletion is required; use of the proposed approximation simplifies the computation.