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At least 217 records · Page 12Linked to original sources

USGS2021

An eddy-resolving numerical model to study turbulent flow, sediment and bed evolution using detached eddy simulation in a lateral separation zone at the field-scale

Turbulence-resolving simulations elucidate key elements of fluid dynamics and sediment transport in fluvial environments. This research presents a feasible strategy for applying state-of-the-art computational fluid mechanics to the study of sediment transport and morphodynamic processes in lateral separation zones, which are common features in canyon rivers where massive lateral flow separation causes large-scale turbulence that controls sediment erosion and deposition. An eddy-resolving model was developed and tested at the field-scale, coupling a viscous flow and sediment transport solver using Detached Eddy Simulation techniques. A morphodynamic model was applied to the viscous flow/sediment solver to calculate erosion and deposition. A simulation of turbulence was performed at the grid resolution for a straight channel to determine the relative contributions of modeled and resolved diffusivity. The time-dependent, energetically important, correlative, non-stationary signals of the simulated quantities were captured at the lateral separation zone. Strong periodic signals featured by high amplitude were found at the separation zone, while low frequency pulsations were observed at the reattachment zone of the lateral separation zone. Interactions between the eddies and the loose bed boundaries resulted in erosion of sediment at the main channel followed by deposition at the primary eddy and eddy bars. tions elucidate key elements of fluid dynamics and sediment transport in fluvial environments. This research presents a feasible strategy for applying state-of-the-art computational fluid mechanics to the study of sediment transport and morphodynamic processes in lateral separation zones, which are common features in canyon rivers where massive lateral flow separation causes large-scale turbulence that controls sediment erosion and deposition. An eddy-resolving model was developed and tested at the field scale, coupling viscous flow and sediment transport solver using Detached Eddy Simulation (DES) techniques. A morphodynamic model was applied to the viscous flow/sediment solver to calculate erosion and deposition. A simulation of turbulence was performed at the grid resolution for a straight channel to determine the relative contributions of modeled and resolved diffusivity. The time-dependent, energetically important, correlative non-stationary signals of the simulated quantities were captured at the lateral separation zone. Strong periodic signals featured by high amplitude were found at the separation zone, while low frequency pulsations were observed at the reattachment zone of the lateral separation zone. Interactions between the eddies and the loose bed boundaries resulted in massive erosion of sediment at the main channel followed by deposition at the primary eddy and eddy bars.

Arizona↗

USGS2005

Effects of tag loss on direct estimates of population growth rate

The temporal symmetry approach of R. Pradel can be used with capture– recapture data to produce retrospective estimates of a population's growth rate, λ i , and the relative contributions to λ i from different components of the population. Direct estimation of λ i provides an alternative to using population projection matrices to estimate asymptotic λ and is seeing increased use. However, the robustness of direct estimates of λ i to violations of several key assumptions has not yet been investigated. Here, we consider tag loss as a possible source of bias for scenarios in which the rate of tag loss is (1) the same for all marked animals in the population and (2) a function of tag age. We computed analytic approximations of the expected values for each of the parameter estimators involved in direct estimation and used those values to calculate bias and precision for each parameter estimator. Estimates of λ i were robust to homogeneous rates of tag loss. When tag loss rates varied by tag age, bias occurred for some of the sampling situations evaluated, especially those with low capture probability, a high rate of tag loss, or both. For situations with low rates of tag loss and high capture probability, bias was low and often negligible. Estimates of contributions of demographic components to λ i were not robust to tag loss. Tag loss reduced the precision of all estimates because tag loss results in fewer marked animals remaining available for estimation. Clearly tag loss should be prevented if possible, and should be considered in analyses of λ i , but tag loss does not necessarily preclude unbiased estimation of λ i .

Ecology↗

USGS1990

Sediment movement along the U.S. east coast continental shelf-I. Estimates of bottom stress using the Grant-Madsen model and near-bottom wave and current measurements

Bottom stress is calculated for several long-term time-series observations, made on the U.S. east coast continental shelf during winter, using the wave-current interaction and moveable bed models of Grant and Madsen (1979, Journal of Geophysical Research, 84, 1797-1808; 1982, Journal of Geophysical Research, 87, 469-482). The wave and current measurements were obtained by means of a bottom tripod system which measured current using a Savonius rotor and vane and waves by means of a pressure sensor. The variables were burst sampled about 10% of the time. Wave energy was reasonably resolved, although aliased by wave groupiness, and wave period was accurate to 1-2 s during large storms. Errors in current speed and direction depend on the speed of the mean current relative to the wave current. In general, errors in bottom stress caused by uncertainties in measured current speed and wave characteristics were 10-20%. During storms, the bottom stress calculated using the Grant-Madsen models exceeded stress computed from conventional drag laws by a factor of about 1.5 on average and 3 or more during storm peaks. Thus, even in water as deep as 80 m, oscillatory near-bottom currents associated with surface gravity waves of period 12 s or longer will contribute substantially to bottom stress. Given that the Grant-Madsen model is correct, parameterizations of bottom stress that do not incorporate wave effects will substantially underestimate stress and sediment transport in this region of the continental shelf.

Georges Bank, Mid-Atlantic Bight↗

USGS2001

Nutrient and suspended-sediment concentrations and loads and benthic-invertebrate data for tributaries to the St. Croix River, Wisconsin and Minnesota, 1997-99

Nutrient and suspended-sediment data were collected on major tributaries to the St. Croix River during 1997-99 as part of three studies. The first study, done in 1997 as part of the U.S. Geological Survey?s National Water-Quality Assessment Program Upper Mississippi Study Unit, was a widespread synoptic survey of nutrient and suspended-sediment concentrations, loads, and yields during snowmelt. Runoff from snowmelt in agricultural areas and other areas with low permeability soils had significantly greater nutrient concentrations than forested areas, whereas differences in suspended-sediment loading were not detected. In 1998, synoptic samplings of 11 tributaries were done during snowmelt, base-flow, and storm-runoff periods. These studies showed that the Apple, Willow, and Kinnickinnic Rivers were major contributors of suspended sediments and nutrients to the St. Croix River during base flow and storm-runoff events. Nitrate concentrations were high during base flow in the agricultural tributaries?specifically, the Kinnickinnic (4.83 mg/L), Willow (1.53 mg/L), and Apple (0.79 mg/L) Rivers?possibly from ground-water recharge or point-source contributions. Extensive water-quality sampling was done monthly and during high-flow events in water year 1999 (October 1, 1998 to September 30, 1999) in coordination with continuous streamflow monitoring at 12 sites in the St. Croix River Basin. These data were used to compute annual nutrient and suspended-sediment loads and yields at the monitored sites for water year 1999. Relations among environmental characteristics and calculated annual nutrient and suspended-sediment yields were used to estimate loading from unmonitored parts of the basin. The environmental characteristics found to best estimate annual yields were soil characteristics (clay, permeability of soil, and erodibility), basin slope and area, and the percentages of wetland and urban areas in the basins. Variability in 1999 rainfall intensity resulted in annual yields from several northern, forested basins being higher than those from the southern, agricultural basins. The Sunrise River had the highest annual suspended-sediment and nutrient yields in the basin in 1999. Concentrations and instantaneous loading rates varied as much among various flow conditions at individual sites as among sites during the three years of study. Benthic invertebrates were sampled and indices of water quality were calculated at 16 tributaries in fall 1999. Benthic invertebrate indices indicated excellent to good water quality at all tributaries except Valley Creek, Willow River, and Kettle River. No relations were found between benthic invertebrate indices and the calculated and estimated 1999 annual tributary loads and yields.

Minnesota, Wisconsin↗

USGS1988

Simulation of three lahars in the Mount St Helens area, Washington using a one-dimensional, unsteady-state streamflow model

A one-dimensional, unsteady-state, open-channel model was used to analytically reproduce three lahar events. Factors contributing to the success of the modeling were: (1) the lahars were confined to a channel, (2) channel roughness was defined by field information, and (3) the volume of the flow remained relatively unchanged for the duration of the peak. Manning 's 'n ' values used in computing conveyance in the model were subject to the changing rheology of the debris flow and were calculated from field cross-section information (velocities used in these calculations were derived from super-elevation or run-up formulas). For the events modeled in this exercise, Manning 's 'n ' calculations ranged from 0.020 to 0.099. In all lahar simulations, the rheology of the flow changed in a downstream direction during the course of the event. Chen's 'U ', the mudflow consistency index, changed approximately an order of magnitude for each event. The ' u ' values ranged from 5-2,260 kg/m for three events modeled. The empirical approach adopted in this paper is useful as a tool to help predict debris-flow behavior, but does not lead to understanding the physical processes of debris flows.

Washington↗

USGS2011

Changes in low-flow frequency from 1976-2006 at selected streamgages in New York, excluding Long Island

Many Federal, State, and local agencies use low-flow data to establish water-use policy and help determine the total maximum daily loads and effluent limits of point and nonpoint sources of contamination of surface water during periods of decreased streamflow. Low-flow magnitude and frequency are used often by water-supply planners, reservoir managers, and hydroelectric facilities to manage water availability for supply and power generation. Low-flow statistics for eight selected U.S. Geological Survey streamgages in New York State were calculated for the period from 1976 through 2006 and for the entire period of continuous streamflow record. The 7-day, 2-year and 10-year low flows were computed and compared with those low flows published in the1979 U.S. Geological Survey report, Low-flow frequency analysis of streams in New York, Bulletin 74. Observed changes in low-flow frequency at each gage were then examined and compared to changes in precipitation and land use to determine whether a relation between similar patterns could be identified. A statewide U.S. Geological Survey study has not been done to develop equations for estimating low flows on rural unregulated streams in New York. Currently (2010) only one regional study developed for parts of the lower Hudson River Basin in 1986 is available to assist in estimating low flows on rural streams with unregulated streamflow in New York. Low-flow statistics published in the 1979 report need to be updated by using additional data collected since 1976 to determine current low-flow conditions across New York State. At-site low-flow statistics were updated for eight streamgages in New York by using continuous daily streamflow data through 2006 for the future development of a statewide research study. Selection of the eight streamgages used in this study identified a major deficiency in the number of available unregulated long-term U.S. Geological Survey streamgages needed for the development of regional low-flow equations in New York. A limited analysis of the changes in land use for the contributing drainage areas for each streamgage, changes in precipitation, and trends in the annual 7-day minimum flow also are presented. The 7-day, 2-year low flow showed increases of 14 to 35 percent and the 7-day 10-year low flow showed zero to 19 percent increases at rural streamgages with unregulated streamflows when statistics were computed by using data from 1976 through 2006 and compared with published data in Bulletin 74. When the entire period of record was used to compute low flow frequencies, the 7-day, 2-year low flows increased from about 6 to 15 percent whereas the 7-day 10-year low flows showed zero to 5 percent increases. Streamgages affected by urbanization and regulation for water supply showed the most significant changes in the 7-day, 2-year and 10-year low-flow frequencies. These streamgages are included to help identify the effects of urbanization and regulation on streamflow at these locations. The 7-day 10-year low flow increased by 65 percent at the U.S. Geological Survey streamgage Hackensack River at West Nyack, N.Y., and increased 120 percent at the U.S. Geological Survey streamgage Neversink River at Godeffroy, N.Y., when statistics were computed by using data from 1976 through 2006 and compared with the statistics for the regulated period computed in Bulletin 74.

New York↗

USGS2000

Creating a standardized watersheds database for the lower Rio Grande/Rio Bravo, Texas

This report describes the creation of a large-scale watershed database for the lower Rio Grande/Rio Bravo Basin in Texas. The watershed database includes watersheds delineated to all 1:24,000-scale mapped stream confluences and other hydrologically significant points, selected watershed characteristics, and hydrologic derivative datasets. Computer technology allows generation of preliminary watershed boundaries in a fraction of the time needed for manual methods. This automated process reduces development time and results in quality improvements in watershed boundaries and characteristics. These data can then be compiled in a permanent database, eliminating the time-consuming step of data creation at the beginning of a project and providing a stable base dataset that can give users greater confidence when further subdividing watersheds. A standardized dataset of watershed characteristics is a valuable contribution to the understanding and management of natural resources. Vertical integration of the input datasets used to automatically generate watershed boundaries is crucial to the success of such an effort. The optimum situation would be to use the digital orthophoto quadrangles as the source of all the input datasets. While the hydrographic data from the digital line graphs can be revised to match the digital orthophoto quadrangles, hypsography data cannot be revised to match the digital orthophoto quadrangles. Revised hydrography from the digital orthophoto quadrangle should be used to create an updated digital elevation model that incorporates the stream channels as revised from the digital orthophoto quadrangle. Computer-generated, standardized watersheds that are vertically integrated with existing digital line graph hydrographic data will continue to be difficult to create until revisions can be made to existing source datasets. Until such time, manual editing will be necessary to make adjustments for man-made features and changes in the natural landscape that are not reflected in the digital elevation model data.

Texas↗

USGS2004

Contribution of natural history collection data to biodiversity assessment in national parks

There has been mounting interest in the use of museum and herbaria collections to assess biodiversity; information is often difficult to locate and access, however, and few recommendations are available for effectively using natural history collections. As part of an effort to inventory vertebrates and vascular plants in U.S. national parks, we searched manually and by computer for specimens originating within or adjacent to 14 parks throughout the northeastern United States. We compared the number of specimens located to collection size to determine whether there was any effect on detection rate of specimens. We evaluated the importance of park characteristics (e.g., age since establishment, size, theme [natural vs. cultural]) for influencing the number of specimens found in a collection. We located >31,000 specimens and compiled associated records (hereafter referred to as specimens) from 78 collections; >9000 specimens were park-significant, originating either within park boundaries or in the local township where the park was located. We found >2000 specimens by means of manual searches, which cost $0.001–0.15 per specimen searched and $0.81–151.95 per specimen found. Collection effort appeared relatively uniform between 1890 and 1980, with low periods corresponding to significant sociopolitical events. Detection rates for specimens were inversely related to collection size. Although specimens were most often located in collections within the region of interest, specimens can be found anywhere, particularly in large collections international in scope, suggesting that global searches will be necessary to evaluate historical biodiversity. Park characteristics indicated that more collecting effort occurred within or adjacent to larger parks established for natural resources than in smaller historical sites. Because many institutions have not yet established electronic databases for collections, manual searches can be useful for retrieving specimens. Our results show that thorough, systematic searching of natural history collections for park-significant specimens can provide a historical perspective on biodiversity for park managers.

Conservation Biology↗

USGS2009

Identifying sources of dissolved organic carbon in agriculturally dominated rivers using radiocarbon age dating: Sacramento-San Joaquin River Basin, California

We used radiocarbon measurements of dissolved organic carbon (DOC) to resolve sources of riverine carbon within agriculturally dominated landscapes in California. During 2003 and 2004, average Δ 14 C for DOC was −254‰ in agricultural drains in the Sacramento–San Joaquin Delta, −218‰ in the San Joaquin River, −175‰ in the California State Water Project and −152‰ in the Sacramento River. The age of bulk DOC transiting the rivers of California’s Central Valley is the oldest reported for large rivers and suggests wide-spread loss of soil organic matter caused by agriculture and urbanization. Using DAX 8 adsorbent, we isolated and measured 14 C concentrations in hydrophobic acid fractions (HPOA); river samples showed evidence of bomb-pulse carbon with average Δ 14 C of 91 and 76‰ for the San Joaquin and Sacramento Rivers, respectively, with older HPOA, −204‰, observed in agricultural drains. An operationally defined non-HPOA fraction of DOC was observed in the San Joaquin River with seasonally computed Δ 14 C values of between −275 and −687‰; the source of this aged material was hypothesized to be physically protected organic-matter in high clay-content soils and agrochemicals (i.e., radiocarbon-dead material) applied to farmlands. Mixing models suggest that the Sacramento River contributes about 50% of the DOC load in the California State Water Project, and agricultural drains contribute approximately one-third of the load. In contrast to studies showing stabilization of soil carbon pools within one or two decades following land conversion, sustained loss of soil organic matter, occurring many decades after the initial agricultural-land conversion, was observed in California’s Central Valley.

California↗

USGS2019

A review of machine learning applications to coastal sediment transport and morphodynamics

A range of computer science methods under the heading of machine learning (ML) enables the extraction of insight and quantitative relationships from multidimensional datasets. Here, we review some common ML methods and their application to studies of coastal morphodynamics and sediment transport. We examine aspects of ‘what’ and ‘why’ ML methods contribute, such as ‘what’ science problems ML tools have been used to address, ‘what’ was learned when using ML, and ‘why’ authors used ML methods. We find a variety of research questions have been addressed, ranging from small-scale predictions of sediment transport to larger-scale sand bar morphodynamics and coastal overwash on a developed island. We find various reasons justify the use of ML, including maximize predictability, emulation of model components, smooth and continuous nonlinear regression through data, and explicit inclusion of uncertainty. Overall the expanding use of ML has allowed for an expanding set of questions to be addressed. After reviewing the studies we outline a set of ‘best practices’ for coastal researchers using machine learning methods. Finally we suggest possible areas for future research, including the use of novel machine learning techniques and exploring ‘open data’ that is becoming increasingly available.

Earth-Science Reviews↗

USGS2000

MODFLOW-2000, the U.S. Geological Survey modular ground-water model; user guide to the observation, sensitivity, and parameter-estimation processes and three post-processing programs

This report documents the Observation, Sensitivity, and Parameter-Estimation Processes of the ground-water modeling computer program MODFLOW-2000. The Observation Process generates model-calculated values for comparison with measured, or observed, quantities. A variety of statistics is calculated to quantify this comparison, including a weighted least-squares objective function. In addition, a number of files are produced that can be used to compare the values graphically. The Sensitivity Process calculates the sensitivity of hydraulic heads throughout the model with respect to specified parameters using the accurate sensitivity-equation method. These are called grid sensitivities. If the Observation Process is active, it uses the grid sensitivities to calculate sensitivities for the simulated values associated with the observations. These are called observation sensitivities. Observation sensitivities are used to calculate a number of statistics that can be used (1) to diagnose inadequate data, (2) to identify parameters that probably cannot be estimated by regression using the available observations, and (3) to evaluate the utility of proposed new data. The Parameter-Estimation Process uses a modified Gauss-Newton method to adjust values of user-selected input parameters in an iterative procedure to minimize the value of the weighted least-squares objective function. Statistics produced by the Parameter-Estimation Process can be used to evaluate estimated parameter values; statistics produced by the Observation Process and post-processing program RESAN-2000 can be used to evaluate how accurately the model represents the actual processes; statistics produced by post-processing program YCINT-2000 can be used to quantify the uncertainty of model simulated values. Parameters are defined in the Ground-Water Flow Process input files and can be used to calculate most model inputs, such as: for explicitly defined model layers, horizontal hydraulic conductivity, horizontal anisotropy, vertical hydraulic conductivity or vertical anisotropy, specific storage, and specific yield; and, for implicitly represented layers, vertical hydraulic conductivity. In addition, parameters can be defined to calculate the hydraulic conductance of the River, General-Head Boundary, and Drain Packages; areal recharge rates of the Recharge Package; maximum evapotranspiration of the Evapotranspiration Package; pumpage or the rate of flow at defined-flux boundaries of the Well Package; and the hydraulic head at constant-head boundaries. The spatial variation of model inputs produced using defined parameters is very flexible, including interpolated distributions that require the summation of contributions from different parameters. Observations can include measured hydraulic heads or temporal changes in hydraulic heads, measured gains and losses along head-dependent boundaries (such as streams), flows through constant-head boundaries, and advective transport through the system, which generally would be inferred from measured concentrations. MODFLOW-2000 is intended for use on any computer operating system. The program consists of algorithms programmed in Fortran 90, which efficiently performs numerical calculations and is fully compatible with the newer Fortran 95. The code is easily modified to be compatible with FORTRAN 77. Coordination for multiple processors is accommodated using Message Passing Interface (MPI) commands. The program is designed in a modular fashion that is intended to support inclusion of new capabilities.

Open-File Report↗

USGS2008

Investigation and hazard assessment of the 2003 and 2007 Staircase Falls rock falls, Yosemite National Park, California, USA

Since 1857 more than 600 rock falls, rock slides, debris slides, and debris flows have been documented in Yosemite National Park, with rock falls in Yosemite Valley representing the majority of the events. On 26 December 2003, a rock fall originating from west of Glacier Point sent approximately 200 m 3 of rock debris down a series of joint-controlled ledges to the floor of Yosemite Valley. The debris impacted talus near the base of Staircase Falls, producing fragments of flying rock that struck occupied cabins in Curry Village. Several years later on 9 June 2007, and again on 26 July 2007, smaller rock falls originated from the same source area. The 26 December 2003 event coincided with a severe winter storm and was likely triggered by precipitation and/or frost wedging, but the 9 June and 26 July 2007 events lack recognizable triggering mechanisms. We investigated the geologic and hydrologic factors contributing to the Staircase Falls rock falls, including bedrock lithology, weathering, joint spacing and orientations, and hydrologic processes affecting slope stability. We improved upon previous geomorphic assessment of rock-fall hazards, based on a shadow angle approach, by using STONE, a three-dimensional rock-fall simulation computer program. STONE produced simulated rock-fall runout patterns similar to the mapped extent of the 2003 and 2007 events, allowing us to simulate potential future rock falls from the Staircase Falls detachment area. Observations of recent rock falls, mapping of rock debris, and simulations of rock fall runouts beneath the Staircase Falls detachment area suggest that rock-fall hazard zones extend farther downslope than the extent previously defined by mapped surface talus deposits.

Natural Hazards and Earth System Sciences↗

USGS2006

Numerical modeling of magnetic moments for UXO applications

The surface magnetic anomaly observed in UXO clearance is mainly dipolar and, consequently, the dipole is the only magnetic moment regularly recovered in UXO applications. The dipole moment contains information about intensity of magnetization but lacks information about shape. In contrast, higher-order moments, such as quadrupole and octupole, encode asymmetry properties of the magnetization distribution within the buried targets. In order to improve our understanding of magnetization distribution within UXO and non-UXO objects and its potential utility in UXO clearance, we present a 3D numerical modeling study for highly susceptible metallic objects. The basis for the modeling is the solution of a nonlinear integral equation describing magnetization within isolated objects. A solution for magnetization distribution then allows us to compute magnetic moments of the object, analyze their relationships, and provide a depiction of the surface anomaly produced by different moments within the object. Our modeling results show significant high-order moments for more asymmetric objects situated at depths typical of UXO burial, and suggest that the increased relative contribution to magnetic gradient data from these higher-order moments may provide a practical tool for improved UXO discrimination.

Conference Paper↗

USGS2006

Relative importance of magnetic moments in UXO clearance applications

Surface magnetic anomaly observed in UXO clearance is mainly dipolar and, as a result, the dipole is the only moment used regularly in UXO applications. The dipole moment contains intensity of magnetization information but lacks shape information. Unlike dipole, higher-order moments, such as quadrupole and octupole, encode asymmetry properties of magnetization distribution within buried targets. In order to improve our understanding of magnetization distribution within UXO and non-UXO objects and its potential utility in UXO clearance, we present results of a 3D numerical modeling study for highly susceptible metallic objects. The basis for modeling is the solution of a nonlinear integral equation, describing magnetization within isolated objects, allowing us to compute magnetic moments of the object, analyze their relationships, and provide a depiction of the surface anomaly produced by the different moments within the object. Our modeling results show significant high-order moments for more asymmetric objects situated at typical UXO burial depths, and suggest that the increased relative contribution to magnetic gradient data from these higher-order moments may provide a practical tool for improved UXO discrimination. ?? 2005 Society of Exploration Geophysicists.

SEG Technical Program Expanded Abstracts↗

USGS1966

Preliminary flood-frequency relations for small streams in Kansas

Preliminary flood-frequency relations have been defined for small streams in Kansas for floods having recurrence intervals not greater than 10 years. The defined relations will be useful for the design of culverts and other hydraulic structures. The relations are expressed in terms of basin characteristics. Peakflow records at 95 sites in Kansas for an 8-year period provided the basic data. The records were analyzed with respect to 20 basin characteristics by multiple-regression techniques. The resulting formulas relate flood magnitude and frequency to size of contributing drainage area, an index of stream-bed slope, and the average number of days per year when rainfall exceeded 1.0 inch. The other 17 factors had no statistical significance. To illustrate a typical application of the flood-frequency relation, a step-bystep method is presented for computing a frequency curve for Rock Creek near Meriden, Kans. The frequency curve shows that a peak discharge of 3,620 cfs (cubic feet per second) can be expected once every 10 years on the average, and that the 67 percent confidence interval ranges from 1,820 cfs to 7,230 cfs. The large range results from the fact that only 8 years of record have been collected and emphasizes the need for collecting records for a longer period.

Kansas↗

USGS2013

The use of process models to inform and improve statistical models of nitrate occurrence, Great Miami River Basin, southwestern Ohio

Statistical models of nitrate occurrence in the glacial aquifer system of the northern United States, developed by the U.S. Geological Survey, use observed relations between nitrate concentrations and sets of explanatory variables—representing well-construction, environmental, and source characteristics— to predict the probability that nitrate, as nitrogen, will exceed a threshold concentration. However, the models do not explicitly account for the processes that control the transport of nitrogen from surface sources to a pumped well and use area-weighted mean spatial variables computed from within a circular buffer around the well as a simplified source-area conceptualization. The use of models that explicitly represent physical-transport processes can inform and, potentially, improve these statistical models. Specifically, groundwater-flow models simulate advective transport—predominant in many surficial aquifers— and can contribute to the refinement of the statistical models by (1) providing for improved, physically based representations of a source area to a well, and (2) allowing for more detailed estimates of environmental variables. A source area to a well, known as a contributing recharge area, represents the area at the water table that contributes recharge to a pumped well; a well pumped at a volumetric rate equal to the amount of recharge through a circular buffer will result in a contributing recharge area that is the same size as the buffer but has a shape that is a function of the hydrologic setting. These volume-equivalent contributing recharge areas will approximate circular buffers in areas of relatively flat hydraulic gradients, such as near groundwater divides, but in areas with steep hydraulic gradients will be elongated in the upgradient direction and agree less with the corresponding circular buffers. The degree to which process-model-estimated contributing recharge areas, which simulate advective transport and therefore account for local hydrologic settings, would inform and improve the development of statistical models can be implicitly estimated by evaluating the differences between explanatory variables estimated from the contributing recharge areas and the circular buffers used to develop existing statistical models. The larger the difference in estimated variables, the more likely that statistical models would be changed, and presumably improved, if explanatory variables estimated from contributing recharge areas were used in model development. Comparing model predictions from the two sets of estimated variables would further quantify—albeit implicitly—how an improved, physically based estimate of explanatory variables would be reflected in model predictions. Differences between the two sets of estimated explanatory variables and resultant model predictions vary spatially; greater differences are associated with areas of steep hydraulic gradients. A direct comparison, however, would require the development of a separate set of statistical models using explanatory variables from contributing recharge areas. Area-weighted means of three environmental variables—silt content, alfisol content, and depth to water from the U.S. Department of Agriculture State Soil Geographic (STATSGO) data—and one nitrogen-source variable (fertilizer-application rate from county data mapped to Enhanced National Land Cover Data 1992 (NLCDe 92) agricultural land use) can vary substantially between circular buffers and volume-equivalent contributing recharge areas and among contributing recharge areas for different sets of well variables. The differences in estimated explanatory variables are a function of the same factors affecting the contributing recharge areas as well as the spatial resolution and local distribution of the underlying spatial data. As a result, differences in estimated variables between circular buffers and contributing recharge areas are complex and site specific as evidenced by differences in estimated variables for circular buffers and contributing recharge areas of existing public-supply and network wells in the Great Miami River Basin. Large differences in areaweighted mean environmental variables are observed at the basin scale, determined by using the network of uniformly spaced hypothetical wells; the differences have a spatial pattern that generally is similar to spatial patterns in the underlying STATSGO data. Generally, the largest differences were observed for area-weighted nitrogen-application rate from county and national land-use data; the basin-scale differences ranged from -1,600 (indicating a larger value from within the volume-equivalent contributing recharge area) to 1,900 kilograms per year (kg/yr); the range in the underlying spatial data was from 0 to 2,200 kg/yr. Silt content, alfisol content, and nitrogen-application rate are defined by the underlying spatial data and are external to the groundwater system; however, depth to water is an environmental variable that can be estimated in more detail and, presumably, in a more physically based manner using a groundwater-flow model than using the spatial data. Model-calculated depths to water within circular buffers in the Great Miami River Basin differed substantially from values derived from the spatial data and had a much larger range. Differences in estimates of area-weighted spatial variables result in corresponding differences in predictions of nitrate occurrence in the aquifer. In addition to the factors affecting contributing recharge areas and estimated explanatory variables, differences in predictions also are a function of the specific set of explanatory variables used and the fitted slope coefficients in a given model. For models that predicted the probability of exceeding 1 and 4 milligrams per liter as nitrogen (mg/L as N), predicted probabilities using variables estimated from circular buffers and contributing recharge areas generally were correlated but differed significantly at the local and basin scale. The scale and distribution of prediction differences can be explained by the underlying differences in the estimated variables and the relative weight of the variables in the statistical models. Differences in predictions of exceeding 1 mg/L as N, which only includes environmental variables, generally correlated with the underlying differences in STATSGO data, whereas differences in exceeding 4 mg/L as N were more spatially extensive because that model included environmental and nitrogen-source variables. Using depths to water from within circular buffers derived from the spatial data and depths to water within the circular buffers calculated from the groundwater-flow model, restricted to the same range, resulted in large differences in predicted probabilities. The differences in estimated explanatory variables between contributing recharge areas and circular buffers indicate incorporation of physically based contributing recharge area likely would result in a different set of explanatory variables and an improved set of statistical models. The use of a groundwater-flow model to improve representations of source areas or to provide more-detailed estimates of specific explanatory variables includes a number of limitations and technical considerations. An assumption in these analyses is that (1) there is a state of mass balance between recharge and pumping, and (2) transport to a pumped well is under a steady state flow field. Comparison of volumeequivalent contributing recharge areas under steady-state and transient transport conditions at a location in the southeastern part of the basin shows the steady-state contributing recharge area is a reasonable approximation of the transient contributing recharge area after between 10 and 20 years of pumping. The first assumption is a more important consideration for this analysis. A gradient effect refers to a condition where simulated pumping from a well is less than recharge through the corresponding contributing recharge area. This generally takes place in areas with steep hydraulic gradients, such as near discharge locations, and can be mitigated using a finer model discretization. A boundary effect refers to a condition where recharge through the contributing recharge area is less than pumping. This indicates other sources of water to the simulated well and could reflect a real hydrologic process. In the Great Miami River Basin, large gradient and boundary effects—defined as the balance between pumping and recharge being less than half—occurred in 5 and 14 percent of the basin, respectively. The agreement between circular buffers and volume-equivalent contributing recharge areas, differences in estimated variables, and the effect on statisticalmodel predictions between the population of wells with a balance between pumping and recharge within 10 percent and the population of all wells were similar. This indicated process-model limitations did not affect the overall findings in the Great Miami River Basin; however, this would be model specific, and prudent use of a process model needs to entail a limitations analysis and, if necessary, alterations to the model.

Ohio↗

USGS2009

An introduction to standardized sampling

It was probably one of the oddest riots in the history of the United States. In Erie, Pennsylvania during 1853. federal marshals were called to restore order during bloody uprisings. A mob of women, equipped with sledgehammers, was tearing up railroad rack to protest standardization of track width (Nesmith 1985). All across the United States, standardization of rail gauges was talking place to improve transportation across the country,but many people did not want consistency. Jobs moving freight from, a train running on one gauge of track to a train running on another gauge were plentiful at this time, and standardization would mean these jobs would disappear. Fortunately, for us today, the riots were quelled and standardization of railroad tack gauges went ahead. The magnificent transportation system of North America was aided by the standardization of rails, contributing to robust economies. Standardization of industrial processes, languages, measurements, and data collection methods has been essential for world progress (Figure 1.1). Today , we are often unaware of the degree of standardization of the most basic elements of our society--from bolts and nuts where thread sizes are standard to computer components that can be used interchangeably to the standard sizes of photos we carry in our wallets or purses. Data collection and presentation are standardized in many disciplines, including medicine, meteorology, geology, and water chemistry. For example, our cholesterol, body temperature, and blood pressure are measured by standard medical tests and compared to averages calculated from the results of the same standard tests for many other people to determine if individuals are higher, lower, or average compared to the population in general. If these diagnostic tests were not standardized, it is unlikely that we would be able to evaluate eve the most basic data about our health. In fact, if standardization was not used in countless other facets of our society our lives would be much more difficult. For data collection purposes, standardization means to collect data in one way so comparisons can be easily made. Although routine data collection has been standardized in many other disciplines, data from freshwater fish sampling across North America have not. Previously, most data collection has been standardized only at local, state, and provincial levels (Bonar and Hubert 2002). Several years ago, when one of the authors (Bonar) was a biologist for a state agency, he was asked to compile as much data as he could about the state's warmwater fish communities to provide information to managers developing fishing regulations. These data had been collected by many biologists over time using different methods, including rotenone, electrofishing, gill netting, and hook-and-line sampling. Data were written carefully on detailed data sheets or in scribbled notes in a biologists's notebook. As you can imagine, these data were a nightmare to compile. However, they were even worse to interpret. How could length-frequency distributions be compared among lakes if the methods used to catch the fish were dissimilar with differing efficiencies in sampling fish of various species and lengths? How could catch per unit effort (CPUE), a common index of population density, be compared when samples were collected one year using fyke nets and the next year by electrofishing? Ultimately, how could one compare if fish population were high, low, or average in growth, body condition, or abundance if there was no compilation of distributions of standard data to facilitate comparison? Months were spent trying to interpret these data, and finally a body of comparable data gathered by similar methods was assimilated. However, much of the nonstandard data had to be discarded--data that had taken thousands of hours to collect but were essentially useless. If all data jhad been collected and recorded in a standard manner, whoch would have required very little extra work, all of these hours of survey effort would not have gone to naught and isights regardimng the fisheries would have been imporived by a larger number of samples.

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USGS2008

Estimating sediment discharge: Appendix D

Sediment-discharge measurements usually are available on a discrete or periodic basis. However, estimates of sediment transport often are needed for unmeasured periods, such as when daily or annual sediment-discharge values are sought, or when estimates of transport rates for unmeasured or hypothetical flows are required. Selected methods for estimating suspended-sediment, bed-load, bed- material-load, and total-load discharges have been presented in some detail elsewhere in this volume. The purposes of this contribution are to present some limitations and potential pitfalls associated with obtaining and using the requisite data and equations to estimate sediment discharges and to provide guidance for selecting appropriate estimating equations. Records of sediment discharge are derived from data collected with sufficient frequency to obtain reliable estimates for the computational interval and period. Most sediment- discharge records are computed at daily or annual intervals based on periodically collected data, although some partial records represent discrete or seasonal intervals such as those for flood periods. The method used to calculate sediment- discharge records is dependent on the types and frequency of available data. Records for suspended-sediment discharge computed by methods described by Porterfield (1972) are most prevalent, in part because measurement protocols and computational techniques are well established and because suspended sediment composes the bulk of sediment dis- charges for many rivers. Discharge records for bed load, total load, or in some cases bed-material load plus wash load are less common. Reliable estimation of sediment discharges presupposes that the data on which the estimates are based are comparable and reliable. Unfortunately, data describing a selected characteristic of sediment were not necessarily derived—collected, processed, analyzed, or interpreted—in a consistent manner. For example, bed-load data collected with different types of bed-load samplers may not be comparable (Gray et al. 1991; Childers 1999; Edwards and Glysson 1999). The total suspended solids (TSS) analytical method tends to produce concentration data from open-channel flows that are biased low with respect to their paired suspended-sediment concentration values, particularly when sand-size material composes more than about a quarter of the material in suspension. Instantaneous sediment-discharge values based on TSS data may differ from the more reliable product of suspended- sediment concentration values and the same water-discharge data by an order of magnitude (Gray et al. 2000; Bent et al. 2001; Glysson et al. 2000; 2001). An assessment of data comparability and reliability is an important first step in the estimation of sediment discharges. There are two approaches to obtaining values describing sediment loads in streams. One is based on direct measurement of the quantities of interest, and the other on relations developed between hydraulic parameters and sediment- transport potential. In the next sections, the most common techniques for both approaches are briefly addressed.

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