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At least 19 recordsLinked to original sources
USGS1996

Use of 2D and 3D GIS in well selection and interpretation of nitrate data, central Nebraska, USA

Nonpoint-source contamination of the principal aquifers in a 7800 km2 area of central Nebraska was evaluated utilizing aquifer condition, well depth, soil type, and physiographical and land use settings. A two-dimensional geographical information system linked with a three-dimensional geological visualization and analytical program was used in the random selection of acceptable wells for the monitoring of nitrate concentrations in groundwater. Locations of existing wells were super-imposed on the three-dimensional geological block diagram and more than 200 wells randomly were selected for monitoring. The three-dimensional system also was used to show three-dimensional contours of nitrate concentrations that can be used interactively to determine the volumetric percentage of an aquifer that contains nitrate concentrations exceeding a specified threshold. The two-dimensional geographical information system was used in comparing nitrate concentrations in differing physiographical, soil, and land use settings. Preliminary results suggest that approximately 6% (volumetric) of water in the High Plains aquifer has nitrate concentrations above the US Environmental Protection Agency Maximum Contaminant Level of 10 mg 1-1 as N.

IAHS-AISH Publication↗
USGS1996

Using GIS and logistic regression to estimate agricultural chemical concentrations in rivers of the midwestern USA

Agricultural chemicals (herbicides, insecticides, other pesticides and fertilizers) in surface water may constitute a human health risk. Recent research on unregulated rivers in the midwestern USA documents that elevated concentrations of herbicides occur for 1-4 months following application in spring and early summer. In contrast, nitrate concentrations in unregulated rivers are elevated during the fall, winter and spring. Natural and anthropogenic variables of river drainage basins, such as soil permeability, the amount of agricultural chemicals applied or percentage of land planted in corn, affect agricultural chemical concentrations in rivers. Logistic regression (LGR) models are used to investigate relations between various drainage basin variables and the concentration of selected agricultural chemicals in rivers. The method is successful in contributing to the understanding of agricultural chemical concentration in rivers. Overall accuracies of the best LGR models, defined as the number of correct classifications divided by the number of attempted classifications, averaged about 66%.

IAHS-AISH Publication↗
USGS1996

Simultaneous confidence intervals for a steady-state leaky aquifer groundwater flow model

Using the optimization method of Vecchia & Cooley (1987), nonlinear Scheffe??-type confidence intervals were calculated tor the parameters and the simulated heads of a steady-state groundwater flow model covering 450 km2 of a leaky aquifer. The nonlinear confidence intervals are compared to corresponding linear intervals. As suggested by the significant nonlinearity of the regression model, linear confidence intervals are often not accurate. The commonly made assumption that widths of linear confidence intervals always underestimate the actual (nonlinear widths was not correct for the head intervals. Results show that nonlinear effects can cause the nonlinear intervals to be offset from, and either larger or smaller than, the linear approximations. Prior information on some transmissivities helps reduce and stabilize the confidence intervals, with the most notable effects occurring for the parameters on which there is prior information and for head values in parameter zones for which there is prior information on the parameters.

IAHS-AISH Publication↗
USGS1996

Unrealistic parameter estimates in inverse modelling: A problem or a benefit for model calibration?

Estimation of unrealistic parameter values by inverse modelling is useful for constructed model discrimination. This utility is demonstrated using the three-dimensional, groundwater flow inverse model MODFLOWP to estimate parameters in a simple synthetic model where the true conditions and character of the errors are completely known. When a poorly constructed model is used, unreasonable parameter values are obtained even when using error free observations and true initial parameter values. This apparent problem is actually a benefit because it differentiates accurately and inaccurately constructed models. The problems seem obvious for a synthetic problem in which the truth is known, but are obscure when working with field data. Situations in which unrealistic parameter estimates indicate constructed model problems are illustrated in applications of inverse modelling to three field sites and to complex synthetic test cases in which it is shown that prediction accuracy also suffers when constructed models are inaccurate.

IAHS-AISH Publication↗
USGS1996

Integrating a geographic information system, a scientific visualization system and an orographic precipitation model

Investigating natural, potential, and man-induced impacts on hydrological systems commonly requires complex modelling with overlapping data requirements, and massive amounts of one- to four-dimensional data at multiple scales and formats. Given the complexity of most hydrological studies, the requisite software infrastructure must incorporate many components including simulation modelling, spatial analysis and flexible, intuitive displays. There is a general requirement for a set of capabilities to support scientific analysis which, at this time, can only come from an integration of several software components. Integration of geographic information systems (GISs) and scientific visualization systems (SVSs) is a powerful technique for developing and analysing complex models. This paper describes the integration of an orographic precipitation model, a GIS and a SVS. The combination of these individual components provides a robust infrastructure which allows the scientist to work with the full dimensionality of the data and to examine the data in a more intuitive manner.

IAHS-AISH Publication↗
USGS1996

Identification of groundwater parameters at Columbus, Mississippi, using a 3D inverse flow and transport model

An extensive amount of data including hydraulic heads, hydraulic conductivities and concentrations of several solutes from controlled injections have been collected during the MADE 1 and MADE 2 experiments at a heterogeneous site near Columbus, Mississippi. In this paper the use of three-dimensional inverse groundwater models including simultaneous estimation of flow and transport parameters is proposed to help identify the dominant characteristics at the site. Simulations show that using a hydraulic conductivity distribution obtained from 2187 borehole flowmeter tests directly in the model produces poor matches to the measured hydraulic heads and tritium concentrations. Alternatively, time averaged hydraulic head maps are used to define zones of constant hydraulic conductivity to be estimated. Preliminary simulations suggest that in the case of conservative transport many, but not all, of the major plume characteristics can be explained by large-scale heterogeneity in recharge and hydraulic conductivity.

IAHS-AISH Publication↗
USGS1996

Discharges and yields of suspended sediment in the Ob' and Yenisey Rivers of Siberia

The northward-flowing rivers of Siberia deliver immense quantities of water but only relatively small quantities of sediment to the Arctic Ocean. The relatively low delivery of sediment to the ocean by these rivers is explained by the large areas of forest and swamp in their basins. In the Ob' River, sediment yields tend to increase between the headwaters and Kamen' na Obi. Further downstream, sediment yields gradually decrease. Near the mouth of the Ob' River, at Salekhard, sediment yield has remained constant at about 5.3 t km-2 year-1. In the lower Yenisey River, the already small natural sediment yield of 5.4 t km-2 year-1 has been decreased several fold by the construction of massive reservoirs on the main stem and on the tributary Angara River, and presently equals 1.8 t km-2 year-1.

IAHS-AISH Publication↗
USGS1996

Using remote sensing and GIS techniques to estimate discharge and recharge. fluxes for the Death Valley regional groundwater flow system, USA

The recharge and discharge components of the Death Valley regional groundwater flow system were defined by remote sensing and GIS techniques that integrated disparate data types to develop a spatially complex representation of near-surface hydrological processes. Image classification methods were applied to multispectral satellite data to produce a vegetation map. This map provided a basis for subsequent evapotranspiration and infiltration estimations. The vegetation map was combined with ancillary data in a GIS to delineate different types of wetlands, phreatophytes and wet playa areas. Existing evapotranspiration-rate estimates were then used to calculate discharge volumes for these areas. A previously used empirical method of groundwater recharge estimation was modified by GIS methods to incorporate data describing soil-moisture conditions, and a recharge potential map was produced. These discharge and recharge maps were readily converted to data arrays for numerical modelling codes. Inverse parameter estimation techniques also used these data to evaluate the reliability and sensitivity of estimated values.

IAHS-AISH Publication↗
USGS1996

Death Valley regional groundwater flow model calibration using optimal parameter estimation methods and geoscientific information systems

A three-layer Death Valley regional groundwater flow model was constructed to evaluate potential regional groundwater flow paths in the vicinity of Yucca Mountain, Nevada. Geoscientific information systems were used to characterize the complex surface and subsurface hydrogeological conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. The high contrasts and abrupt contacts of the different hydrogeological units in the subsurface make zonation the logical choice for representing the hydraulic conductivity distribution. Hydraulic head and spring flow data were used to test different conceptual models by using nonlinear regression to determine parameter values that currently provide the best match between the measured and simulated heads and flows.

IAHS-AISH Publication↗
USGS1995

Source, movement and age of groundwater in the upper part of the Mojave River Basin, California, USA

Water samples from wells were collected and analysed for oxygen-18, deuterium, tritium, carbon-14, and carbon-13 to determine the source, movement and age of groundwater in the upper part of the Mojave River basin. Water in the alluvial aquifer has a median deuterium composition of -66??? and contains tritium, and was recently recharged by water from the Mojave River. Water in the regional aquifer near the Mojave River, near Summit Valley, and underlying several small washes has deuterium compositions heavier than -60???. Although some water in the regional aquifer near the Mojave River contains tritium, most of this water does not contain tritium. Carbon-14 data indicate that this water was recharged less than 2400 years ago. Water in the remainder of the regional aquifer has a median deuterium composition of -84???, which is as much as 20??? lighter than the volume-weighted deuterium composition of present-day precipitation. These data show that this water was recharged under climatic conditions different from average conditions today. Carbon-14 data indicate that some water in the regional aquifer was recharged more than 20 000 years ago.Water samples from wells were collected and analyzed for oxygen-18, deuterium, tritium, carbon-14, and carbon-13 to determine the source, movement and age of groundwater in the upper part of the Mojave River basin. Water in the alluvial aquifer has a median deuterium composition of -66qq and contains tritium, and was recently recharged by water from the Mojave River. Water in the regional aquifer near the Mojave River, near Summit Valley, and underlying several small washes has deuterium compositions heavier than -60qq. Although some water in the regional aquifer near the Mojave River contains tritium, most of this water does not contain tritium. Carbon-14 data indicate that this water was recharged less than 2400 years ago. Water in the remainder of the regional aquifer has a median deuterium composition of -84qq, which is as much as 20qq lighter than the volume-weighted deuterium composition of present-day precipitation. These data show that this water was recharged under climatic conditions different from average conditions today. Carbon-14 data indicate that some water in the regional aquifer was recharged more than 20 000 years ago.

IAHS-AISH Publication↗
USGS1997

Tracing hydrologic pathways at the Panola Mountain Research Watershed, Georgia, USA

An analysis of Cl- concentrations and fluxes at the Panola Mountain Research Watershed indicates that Cl- may be effectively used to differentiate "new" and "old" water flow through the hillslope and their respective contributions to streamwater. Rainfall and throughfall, the "new" water inputs, are marked by low Cl- concentrations (<15 ??eq l-1). Stormwater moves rapidly to depth along preferred pathways in a deciduous forest hillslope, as evidenced by low concentrations (<20 ??eq l-1) in mobile soil water from zero-tension stainless-steel pan lysimeters. "Old" waters, matrix soil waters and groundwater, typically have high concentrations (>20 ??eq l-1). Timing of soil water transport is not sufficiently rapid to suggest that soil water from the hillslope contributes to streamwater for an individual rainstorm. The source of streamflow, therefore, must be a combination of channel interception, runoff from near-channel areas, and runoff from a 3-ha bedrock outcrop in the headwaters. Groundwater contribution to streamflow was estimated using Cl- concentrations of throughfall and groundwater as the two end members for a two-component hydrograph separation. For the study period, groundwater contributed 79% of the runoff and from 1985 to 1995, contributed 75% of the runoff. Rainfall was the source of 45% of the Cl- flux from the watershed in the long term; the remaining Cl- is hypothesized to be derived from dry deposition, consistent with the enrichment noted for throughfall. At peak flow during individual rainstorms, "new" water can contribute 95% of the runoff.

IAHS-AISH Publication↗
USGS1997

Spatial and temporal variability in nutrient concentrations in surface waters of the Chattahoochee River basin near Atlanta, Georgia, USA

Nutrient concentrations from the early 1970s through 1995 were evaluated at several sites along the Chattahoochee River and its tributaries near Atlanta, to determine general patterns and processes controlling nutrient concentrations in the river. A spatial analysis was conducted on data collected in 1994 and 1995 from an intensive nutrient study of the Chattahoochee River and its tributaries by the Georgia Department of Natural Resources, Environmental Protection Division. The 1994-1995 data show step increases in ammonium (NH4-N), nitrite plus nitrate (NO2 + NO3-N), and total-phosphorus (Tot-P) concentrations in the river. The step increases occur downstream of two wastewater treatment facilities (WWTFs) and Peachtree Creek, a small tributary inflow with degraded water quality draining a predominantly urban and industrial area. Median NO2 + NO3-N and Tot-P concentrations in the mainstem increase downstream of these inputs from 0.5 to 1 mg 1-1 and from 0.04 to 0.13 mg 1-1, respectively. NH4-N concentrations were typically low with 95% of the 2575 observations less than 0.2 mg 1-1 throughout the river system, except some high values (>1 mg 1-1) in some tributaries, particularly near the central part of Atlanta. High NH4-N concentrations are attributed to sewage discharge as they also are associated with high biological oxygen demand and faecal coliform bacteria concentrations. Nutrient concentrations vary temporally. An assessment of four sites, two mainstem and two tributaries, from 1970 to 1995 indicates a progressive increase and variability in NO2 + NO3-N concentrations during the period. The progressive increase in NO2 + NO3-N concentrations and their variability is similar to that reported for surface waters throughout the world and for which increased fertilizer usage has been attributed. Tot-P concentrations increase at mainstem sites through the middle to late 1980s and decrease markedly thereafter, due to improvements to WWTFs and a 1990 phosphate detergent ban. NH4-N concentrations, although less pronounced than Tot-P, display a similar decrease from the late 1980s to 1995 at the four sites. Tot-P concentration variability has increased at the tributary sites since 1993, although recent concentrations, on average, are the lowest since 1970 at each of the four sites.

IAHS-AISH Publication↗
USGS1997

Temporal variability in the hydrologic regimes of the United States

Discharge records where flows have not been subject to overt anthropogenic controls have been identified for over 1500 streamflow gauging stations throughout the United States in the US Geological Survey Hydro-Climatic Data Network. These stations fall within all 21 water resources regions of the United States. Analysis of runoff in 20 regions, where long-term daily records are available, shows an increasing trend in 16 regions. Further analysis using a stratified subset of 65 sites shows an increase in baseflow at approximately 90% of the sites during the past 50 years, regardless of the size of the drainage area. Because anthropogenic alterations of watershed characteristics cannot explain these hydrologic changes, then meteorological or climatic forces are implicated.

IAHS-AISH Publication↗
USGS1997

A physically-based method for predicting peak discharge of floods caused by failure of natural and constructed earthen dams

We analyse a simple, physically-based model of breach formation in natural and constructed earthen dams to elucidate the principal factors controlling the flood hydrograph at the breach. Formation of the breach, which is assumed trapezoidal in cross-section, is parameterized by the mean rate of downcutting, k, the value of which is constrained by observations. A dimensionless formulation of the model leads to the prediction that the breach hydrograph depends upon lake shape, the ratio r of breach width to depth, the side slope ?? of the breach, and the parameter ?? = (V/ D3)(k/???gD), where V = lake volume, D = lake depth, and g is the acceleration due to gravity. Calculations show that peak discharge Qp depends weakly on lake shape r and ??, but strongly on ??, which is the product of a dimensionless lake volume and a dimensionless erosion rate. Qp(??) takes asymptotically distinct forms depending on whether ?? < < 1 or ?? > > 1. Theoretical predictions agree well with data from dam failures for which k could be reasonably estimated. The analysis provides a rapid and in many cases graphical way to estimate plausible values of Qp at the breach.

IAHS-AISH Publication↗
USGS1998

The role of near-stream riparian zones in the hydrology of steep upland catchments

Surface and subsurface waters were monitored and sampled at various topographic positions in a 40.5-ha headwater catchment to test several hypotheses of runoff generation and stream chemical and isotopic evolution during snowmelt. Transmissivity feedback was observed on the hillslopes during the melt period. Groundwater levels and stream DOC were highly correlated with stream discharge. Hysteresis in the groundwater-streamflow relation suggests that localized water flux from the riparian areas controlled the rising limb and main peak response of the melt hydrograph, whilst hillslope drainage controlled the timing and volume of the falling limb. Lateral flow from upslope positions was detected in the riparian zone.

IAHS-AISH Publication↗
USGS1998

Precipitation interpolation in mountainous regions using multiple linear regression

Multiple linear regression (MLR) was used to spatially interpolate precipitation for simulating runoff in the Animas River basin of southwestern Colorado. MLR equations were defined for each time step using measured precipitation as dependent variables. Explanatory variables used in each MLR were derived for the dependent variable locations from a digital elevation model (DEM) using a geographic information system. The same explanatory variables were defined for a 5 ?? 5 km grid of the DEM. For each time step, the best MLR equation was chosen and used to interpolate precipitation onto the 5 ?? 5 km grid. The gridded values of precipitation provide a physically-based estimate of the spatial distribution of precipitation and result in reliable simulations of daily runoff in the Animas River basin.

IAHS-AISH Publication↗
USGS1998

Numerical modelling of vertically extensive groundwater bodies in Maui, Hawaii: An alternative to perched aquifers

Groundwater in East Maui, Hawaii is traditionally described as a series of discrete aquifers perched on low-permeability units underlain by a basal lens with heads of about 2-3 m. An alternative concept, a fully saturated aquifer to as much as 1400 m elevation, was investigated using a numerical model with various horizontal hydraulic conductivity values and anisotropy ratios. Results indicate that horizontal hydraulic conductivity values between about 0.08 and 1.0 m per day and anisotropy ratios between 1:1 and 100:1 will produce simulated water tables that match observed water tables at 400-1400 m elevation. These values of hydraulic conductivity are consistent with available field data for hydraulic conductivity.Groundwater in East Maui, Hawaii is traditionally described as a series of discrete aquifers perched on low-permeability units underlain by a basal lens with heads of about 2-3 m. An alternative concept, a fully saturated aquifer to as much as 1400 m elevation, was investigated using a numerical model with various horizontal hydraulic conductivity values and anisotropy ratios. Results indicate that horizontal hydraulic conductivity values between about 0.08 and 1.0 m per day and anisotropy ratios between 1:1 and 100:1 will produce simulated water tables that match observed water tables at 400-1400 m elevation. These values of hydraulic conductivity are consistent with available field data for hydraulic conductivity.

IAHS-AISH Publication↗
USGS1998

Tracing solute mobility at the Panola Mountain Research Watershed, Georgia, USA: Variations in Na+, Cl-, and H4SiO4 concentrations

Concentration variations of sodium (N+). chloride (Cl-). and dissolved silica (H4SiO4) in rainfall, throughfall, soil water, groundwater and streamwater were evaluated at the Panola Mountain Research Watershed near Atlanta, Georgia, to determine how variations in concentrations of these solutes provide greater understanding of water quality evolution at the hillslope and catchment scales Stormwater moves rapidly to depth along preferred pathways in a deciduous forest hillslope, but the composition of the mobile unsaturated zone water in the hillslope is not reflected in compositional variations of streamwater during rainstorms. The Na+, Cl-, and H4SiO4 concentrations behave similarly in streamwater, decreasing with increasing discharge and increasing with water residence time. Consequently, the lowest flows are associated with the highest concentrations and the oldest water. Streamwater composition is most similar to groundwater and stormflow variations reflect a dilution of groundwater. Subtle differences in the relations among solute concentrations and discharge reflect different sources, especially for Cl-. For example, the residence time of groundwater, as inferred from landscape position, generally is positively related to Na+ and H4SiO4 concentrations, but not to Cl- concentrations. The Na+ and H4SiO4 are derived from mineral weathering and are continuously supplied along hydrological pathways. In contrast, Cl- is derived from atmospheric deposition and is affected only by evapotranspiration (ET) and transport. ET increases Cl- concentrations in matrix soil waters, which are subsequently transported to the saturated zone where Cl- is effectively isolated from further evaporative concentration.

IAHS-AISH Publication↗