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At least 631 records · Page 35Linked to original sources
USGS2002

Stormflow-hydrograph separation based on isotopes: the thrill is gone--what's next?

Beginning in the 1970s, the pr omise of a new method for separating stormflow hydrographs using 18 O, 2 H, and 3 Hp r o v e da ni r r e s i s t i b l e temptation, and was a vast improvement over graphical separation and solute tracer methods that wer e pr evalent at the time. Eventu- ally, hydrologists r ealized that this new m ethod entailed a plethora of assumptions about temporal and spatial homogeneity of isotopic composition (many of which wer e commonly violated). Nevertheless, hydr ologists forged ahead with dozens of isotope-based hydrograph- separation studies that wer e published in the 1970s and 1980s. Hortonian overland flow was pr esumed dead. By the late 1980s, the new isotope-based hydr ograph separation technique had moved into adolescence, accompanied by t ypical adolescent problems such as confusion and a search for identity. As experienced hydrologists continued to use the isotope technique to study stormflow hydr ol- ogy in for ested catchments in humid climates, their younger peers followed obligingly — again and again. W as Hortonian overland flow r e ally dead and forgotten, though? What about catchments in which people live and work? And what about catchments in dry climates and the tropics? How u seful were study r esults when several of the assumptions about the homogeneity of sour c e waters wer e commonly violated? What if two components could not explain the variation of isotopic composition measured i n t he stream during stormflow? And what about uncertainty? As with many new tools, once the initial shine wor e off, the limitations of the method became a concern — one of which was that isotope-based hydrograph separations alone could not r eveal much about the flow paths by which water arrives at a str eam channel during storms.

Hydrological Processes↗
USGS2001

Pesticides in the hydrologic system - What do we know and what's next?

Even though the occurrence and behaviour of pesticides in the environment have been studied for decades, water-quality managers and the public still demand more complete and consistent information, and there are many unanswered questions for environmental scientists. In many respects, the greatest potential for unintended adverse effects of pesticides is through contamination of the hydrologic system, which supports aquatic life and related food chains and is used for recreation, drinking water, and many other purposes. The movement of water is one of the primary mechanisms by which pesticides are transported from targeted application areas to other parts of the environment; thus, there is potential for movement into and through all components of the hydrologic system. Extensive reviews of existing information on pesticides in the hydrologic system, including the atmosphere (Majewski and Capel, 1995), ground water (Barbash and Resek, 1996), surface water (Larson et al., 1997), and fluvial sediments and aquatic biota (Nowell et al., 1999), uncovered volumes of useful information, but also noted critical information gaps. For example: (a) relatively few pesticides have been thoroughly studied, particularly transformation products; (b) most data have been collected for small-scale site and field studies in agricultural areas; (c) urban areas have received little attention for monitoring or research; (d) the geographic and temporal distributions of data collection have been highly uneven; and (e) comparing and synthesizing results from most studies is difficult because of inconsistent approaches to data collection and chemical analysis.

Hydrological Processes↗
USGS2002

Ground water and surface water: the linkage tightens, but challenges remain

Hydr ologists have r ecognized for mor e than a century that ground water and surface water are closely linked, but for most of that time studies of their interaction were carried out largely by single disci- plines. This is s lowly changing, however, as the need for integrated studies involving many disciplines is becoming mor e evident.

Hydrological Processes↗
USGS2004

Evaluating the source of streamwater nitrate using δ 15 N and δ 18 O in nitrate in two watersheds in New Hampshire, USA

The natural abundance of nitrogen and oxygen isotopes in nitrate can be a powerful tool for identifying the source of nitrate in streamwater in forested watersheds, because the two main sources of nitrate, atmospheric deposition and microbial nitrification, have distinct δ 18 O values. Using a simple mixing model, we estimated the relative fractions in streamwater derived from these sources for two forested watersheds with markedly different streamwater nitrate outputs. In this study, we monitored δ 15 N and δ 18 O of nitrate biweekly in atmospheric deposition and in streamwater for 20 months at the Hubbard Brook Experimental Forest, New Hampshire, USA (moderate nitrogen export), and monthly in streamwater at the Bowl Research Natural Area, New Hampshire, USA (high nitrogen export). For rain, δ 18 O values ranged from +47 to +77‰ (mean: +58‰) and δ 15 N from −5 to +1‰ (mean: −3‰); for snow, δ 18 O values ranged from +52 to +75‰ (mean: +67‰) and δ 15 N from −3 to +2‰ (mean: −1‰). Streamwater nitrate, in contrast to deposition, had δ 18 O values between +12 and +33‰ (mean: +18‰) and δ 15 N between −3 and +6‰ (mean: 0‰). Since nitrate produced by nitrification typically has δ 18 O values ranging from −5 to +15‰, our field data suggest that most of the nitrate lost from the watersheds in streamflow was nitrified within the catchment. Our results confirm the importance of microbial nitrogen transformations in regulating nitrogen losses from forested ecosystems and suggest that hydrologic storage may be a factor in controlling catchment nitrate losses.

Hydrological Processes↗
USGS2005

A 24 h investigation of the hydrogeochemistry of baseflow and stormwater in an urban area impacted by mining: Butte, Montana

Changes in water quality during a storm event were continuously monitored over a 24 h period at a single location along an urban stormwater drain in Butte, Montana. The Butte Metro Storm Drain (MSD) collects groundwater baseflow and stormwater draining Butte Hill, a densely populated site that has been severely impacted by 130 years of mining, milling, and smelting of copper-rich, polymetallic mineral deposits. On the afternoon of 26 June 2002, a heavy thunderstorm caused streamflow in the MSD to increase 100-fold, from 0·2 ft 3 s −1 to more than 20 ft 3 s −1 . Hourly discharge and water quality data were collected before, during, and following the storm. The most significant finding was that the calculated loads (grams per hour) of both dissolved and particulate copper passing down the MSD increased more than 100-fold in the first hour following the storm, and remained elevated over baseline conditions for the remainder of the study period. Other metals, such as zinc, cadmium, and manganese, showed a decrease in load from pre-storm to post-storm conditions. In addition to the large flush of copper, loads of soluble phosphorus increased during the storm, whereas dissolved oxygen dropped to low levels (<2 mg l −1 ). These results show that infrequent storm events in Butte have the potential to generate large volumes of runoff that exceed Montana water quality standards for acute exposure of aquatic life to copper, as well as depressed levels of dissolved oxygen. This study has important implications to ongoing reclamation activities in the upper Clark Fork Superfund site, particularly with respect to management of storm flow, and may be applicable to other watersheds impacted by mining activities.

Montana↗
USGS2005

Decadal-scale change of infiltration characteristics of a tephra-mantled hillslope at Mount St Helens, Washington

The cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption.

Washington↗
USGS2006

Multiple injected and natural conservative tracers quantify mixing in a stream confluence affected by acid mine drainage near Silverton, Colorado

The acidic discharge from Cement Creek, containing elevated concentrations of dissolved metals and sulphate, mixed with the circumneutral-pH Animas River over a several hundred metre reach (mixing zone) near Silverton, CO, during this study. Differences in concentrations of Ca, Mg, Si, Sr, and SO42- between the creek and the river were sufficiently large for these analytes to be used as natural tracers in the mixing zone. In addition, a sodium chloride (NaCl) tracer was injected into Cement Creek, which provided a Cl- 'reference' tracer in the mixing zone. Conservative transport of the dissolved metals and sulphate through the mixing zone was verified by mass balances and by linear mixing plots relative to the injected reference tracer. At each of seven sites in the mixing zone, five samples were collected at evenly spaced increments of the observed across-channel gradients, as determined by specific conductance. This created sets of samples that adequately covered the ranges of mixtures (mixing ratios, in terms of the fraction of Animas River water, %AR). Concentrations measured in each mixing zone sample and in the upstream Animas River and Cement Creek were used to compute %AR for the reference and natural tracers. Values of %AR from natural tracers generally showed good agreement with values from the reference tracer, but variability in discharge and end-member concentrations and analytical errors contributed to unexpected outlier values for both injected and natural tracers. The median value (MV) %AR (calculated from all of the tracers) reduced scatter in the mixing plots for the dissolved metals, indicating that the MV estimate reduced the effects of various potential errors that could affect any tracer.

Hydrological Processes↗
USGS2006

The composite method: An improved method for stream-water solute load estimation

The composite method is an alternative method for estimating stream-water solute loads, combining aspects of two commonly used methods: the regression-model method (which is used by the composite method to predict variations in concentrations between collected samples) and a period-weighted approach (which is used by the composite method to apply the residual concentrations from the regression model over time). The extensive dataset collected at the outlet of the Panola Mountain Research Watershed (PMRW) near Atlanta, Georgia, USA, was used in data analyses for illustrative purposes. A bootstrap (subsampling) experiment (using the composite method and the PMRW dataset along with various fixed-interval and large storm sampling schemes) obtained load estimates for the 8-year study period with a magnitude of the bias of less than 1%, even for estimates that included the fewest number of samples. Precisions were always <2% on a study period and annual basis, and <2% precisions were obtained for quarterly and monthly time intervals for estimates that had better sampling. The bias and precision of composite-method load estimates varies depending on the variability in the regression-model residuals, how residuals systematically deviated from the regression model over time, sampling design, and the time interval of the load estimate. The regression-model method did not estimate loads precisely during shorter time intervals, from annually to monthly, because the model could not explain short-term patterns in the observed concentrations. Load estimates using the period-weighted approach typically are biased as a result of sampling distribution and are accurate only with extensive sampling. The formulation of the composite method facilitates exploration of patterns (trends) contained in the unmodelled portion of the load. Published in 2006 by John Wiley & Sons, Ltd.

Hydrological Processes↗
USGS2006

Effect of bedrock permeability on subsurface stormflow and the water balance of a trenched hillslope at the Panola Mountain Research Watershed, Georgia, USA

The effect of bedrock permeability on subsurface stormflow initiation and the hillslope water balance is poorly understood. Previous hillslope hydrological studies at the Panola Mountain Research Watershed (PMRW), Georgia, USA, have assumed that the bedrock underlying the trenched hillslope is effectively impermeable. This paper presents a series of sprinkling experiments where we test the bedrock impermeability hypothesis at the PMRW. Specifically, we quantify the bedrock permeability effects on hillslope subsurface stormflow generation and the hillslope water balance at the PMRW. Five sprinkling experiments were performed by applying 882-1676 mm of rainfall over a ???5.5 m ?? 12 m area on the lower hillslope during ???8 days. In addition to water input and output captured at the trench, we measured transpiration in 14 trees on the slope to close the water balance. Of the 193 mm day-1 applied during the later part of the sprinkling experiments when soil moisture changes were small, < 14 mm day-1 was collected at the trench and <4 mm day-1 was transpired by the trees, with residual bedrock leakage of > 175 mm day-1 (91%). Bedrock moisture was measured at three locations downslope of the water collection system in the trench. Bedrock moisture responded quickly to precipitation in early spring. Peak tracer breakthrough in response to natural precipitation in the bedrock downslope from the trench was delayed only 2 days relative to peak tracer arrival in subsurface stormflow at the trench. Leakage to bedrock influences subsurface stormflow at the storm time-scale and also the water balance of the hillslope. This has important implications for the age and geochemistry of the water and thus how one models this hillslope and watershed. Copyright ?? 2006 John Wiley & Sons, Ltd.

Hydrological Processes↗
USGS2006

The potential roles of biological soil crusts in dryland hydrologic cycles

Biological soil crusts (BSCs) are the dominant living cover in many drylands of the world. They possess many features that can influence different aspects of local hydrologic cycles, including soil porosity, absorptivity, roughness, aggregate stability, texture, pore formation, and water retention. The influence of biological soil crusts on these factors depends on their internal and external structure, which varies with climate, soil, and disturbance history. This paper presents the different types of biological soil crusts, discusses how crust type likely influences various aspects of the hydrologic cycle, and reviews what is known and not known about the influence of biological crusts on sediment production and water infiltration versus runoff in various drylands around the world. Most studies examining the effect of biological soil crusts on local hydrology are done by comparing undisturbed sites with those recently disturbed by the researchers. Unfortunately, this greatly complicates interpretation of the results. Applied disturbances alter many soil features such as soil texture, roughness, aggregate stability, physical crusting, porosity, and bulk density in ways that would not necessarily be the same if crusts were not naturally present. Combined, these studies show little agreement on how biological crusts affect water infiltration or runoff. However, when studies are separated by biological crust type and utilize naturally occurring differences among these types, results indicate that biological crusts in hyperarid regions reduce infiltration and increase runoff, have mixed effects in and regions, and increase infiltration and reduce runoff in semiarid cool and cold drylands. However, more studies are needed before broad generalizations can be made on how biological crusts affect infiltration and runoff. We especially need studies that control for sub-surface soil features such as bulk density, micro- and macropores, and biological crust structure. Unlike the mixed effects of biological crusts on infiltration and runoff among regions, almost all studies show that biological crusts reduce sediment production, regardless of crust or dryland type.

Hydrological Processes↗
USGS2007

Water table fluctuations under three riparian land covers, Iowa (USA)

Water table depth is known to play an important role in nitrogen cycling in riparian zones, but little detailed monitoring of water table fluctuations has been reported. In this study, results of high-resolution water table monitoring under three common riparian land covers (forest, cool season grass, corn) were analysed to gain a better understanding of the relation of vegetation cover to water table depth. Three riparian wells located at the Neal Smith National Wildlife Refuge in Jasper County, Iowa, were instrumented with data loggers to record hourly water table behaviour from July to December 2004. Water table depth under the forest showed a diurnal pattern of rising and falling water levels, whereas the grass and corn exhibited a stepped pattern of greater drawdown during the day and less drainage at night. Clear daytime and night-time water table signals were related to daily plant water demands and lateral groundwater flow. Using two estimates of specific yield, hourly and daily ET rates were estimated to be higher under the forest cover than the grass and corn, with peak ET rates in July ranging from 5.02 to 6.32 mm day-1 for forest and from 1.81 to 4.13 mm day-1 for corn and grass. Following plant senescence in October, water table declines were associated with lateral flow to Walnut Creek. The results from this study suggest that consideration should be given to monitoring water table behaviour more frequently to capture daily and seasonal patterns related to riparian vegetation type. Copyright ?? 2007 John Wiley & Sons, Ltd.

Hydrological Processes↗
USGS2006

Influence of groundwater pumping on streamflow restoration following upstream dam removal

We compared streamflow in basins under the combined impacts of an upland dam and groundwater pumping withdrawals, by examining streamflow in the presence and absence of each impact. As a qualitative analysis, inter-watershed streamflow comparisons were performed for several rivers flowing into the east side of the Central Valley, CA. Results suggest that, in the absence of upland dams supporting large reservoirs, some reaches of these rivers might develop ephemeral streamflow in late summer. As a quantitative analysis, we conducted a series of streamflow/groundwater simulations (using MODFLOW-2000 plus the streamflow routing package, SFR1) for a representative hypothetical watershed, with an upland dam and groundwater pumping in the downstream basin, under humid, semi-arid, and arid conditions. As a result of including the impact of groundwater pumping, post-dam removal simulated streamflow was significantly less than natural streamflow. The model predicts extensive ephemeral conditions in the basin during September for both the arid and semi-arid cases. The model predicts continued perennial conditions in the humid case, but spatially weighted, average streamflow of only 71% of natural September streamflow, as a result of continued pumping after dam removal.

California↗
USGS2007

Factors controlling nitrogen release from two forested catchments with contrasting hydrochemical responses

Quantifying biogeochemical cycles of nitrogen (N) and the associated fluxes to surface waters remains challenging, given the need to deal with spatial and temporal variability and to characterize complex and heterogeneous landscapes. We focused our study on catchments S14 and S15 located in the Adirondack Mountains of New York, USA, which have similar topographic and hydrologic characteristics but contrasting stream nitrate ($\hbox{NO}_{3}^{-}$) concentrations. We characterized the mechanisms by which $\hbox{NO}_{3}^{-}$ reaches the streams during hydrological events in these catchments, aiming to reconcile our field data with our conceptual model of factors that regulate nutrient exports from forested catchments. Combined hydrometric, chemical and isotopic (δ$\hbox{NO}_{3}^{-}$) data showed that the relative contributions of both soil and ground water sources were similar between the two catchments. Temporal patterns of stream chemistry were markedly different between S14 and S15, however, because the water sources in the two catchments have different solute concentrations. During late summer/fall, the largest source of $\hbox{NO}_{3}^{-}$ in S14 was till groundwater, whereas shallow soil was the largest $\hbox{NO}_{3}^{-}$ source in S15. $\hbox{NO}_{3}^{-}$ concentrations in surface water decreased in S14, whereas they increased in S15 because an increasing proportion of stream flow was derived from shallow soil sources. During snowmelt, the largest sources of $\hbox{NO}_{3}^{-}$were in the near‐surface soil in both catchments. Concentrations of $\hbox{NO}_{3}^{-}$ increased as stream discharge increased and usually peaked before peak discharge, when shallow soil water sources made the largest contribution to stream discharge. The timing of peaks in stream $\hbox{NO}_{3}^{-}$concentrations was affected by antecedent moisture conditions. By elucidating the factors that affect sources and transport of N, including differences in the soil nutrient cycling and hydrological characteristics of S14 and S15, this study contributes to the overall conceptualization of $\hbox{NO}_{3}^{-}$ release from temperate forested catchments.

Hydrological Processes↗
USGS2007

Evapotranspiration rates and crop coefficients for a restored marsh in the Sacramento-San Joaquin Delta, California, USA

The surface renewal method was used to estimate evapotranspiration (ET) for a restored marsh on Twitchell Island in the Sacramento–San Joaquin Delta, California, USA. ET estimates for the marsh, together with reference ET measurements from a nearby climate station, were used to determine crop coefficients over a 3‐year period during the growing season. The mean ET rate for the study period was 6 mm day −1 , which is high compared with other marshes with similar vegetation. High ET rates at the marsh may be due to the windy, semi‐arid Mediterranean climate of the region, and the permanently flooded nature of the marsh, which results in very low surface resistance of the vegetation. Crop coefficient ( K c ) values for the marsh ranged from 0·73 to 1·18. The mean K c value over the entire study period was 0·95. The daily K c values for any given month varied from year to year, and the standard deviation of daily K c values varied between months. Although several climate variables were undoubtedly responsible for this variation, our analysis revealed that wind direction and the temperature of standing water in the wetland were of particular importance in determining ET rates and K c values.

Hydrological Processes↗