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At least 163 records · Page 9Linked to original sources

USGS2013

Quantifying wetland–aquifer interactions in a humid subtropical climate region: An integrated approach

Wetlands are widely recognized as sentinels of global climate change. Long-term monitoring data combined with process-based modeling has the potential to shed light on key processes and how they change over time. This paper reports the development and application of a simple water balance model based on long-term climate, soil, vegetation and hydrological dynamics to quantify groundwater–surface water (GW–SW) interactions at the Norman landfill research site in Oklahoma, USA. Our integrated approach involved model evaluation by means of the following independent measurements: (a) groundwater inflow calculation using stable isotopes of oxygen and hydrogen ( 16 O, 18 O, 1 H, 2 H); (b) seepage flux measurements in the wetland hyporheic sediment; and (c) pan evaporation measurements on land and in the wetland. The integrated approach was useful for identifying the dominant hydrological processes at the site, including recharge and subsurface flows. Simulated recharge compared well with estimates obtained using isotope methods from previous studies and allowed us to identify specific annual signatures of this important process during the period of study (1997–2007). Similarly, observations of groundwater inflow and outflow rates to and from the wetland using seepage meters and isotope methods were found to be in good agreement with simulation results. Results indicate that subsurface flow components in the system are seasonal and readily respond to rainfall events. The wetland water balance is dominated by local groundwater inputs and regional groundwater flow contributes little to the overall water balance.

Oklahoma↗

USGS2025

Hydrologic variability and plant composition drive relative abundance of marsh birds at created and reference marshes in southeastern Louisiana, U.S.A.

Coastal marsh loss occurs at an alarming pace globally, with extremely high rates along the northern Gulf of Mexico, particularly in Louisiana. In Louisiana, marsh creation projects combat wetland loss; however, biotic responses of vegetation and wildlife receive little to no consideration during and after construction. Habitat characteristics such as hydrologic processes, plant composition, and habitat structure affect the abundance of marsh birds, and understanding these features is important when creating suitable habitat for marsh birds. Our study compared hydrologic characteristics, plant composition, and habitat structure between created ( n = 10) and reference ( n = 9) sites across southeastern Louisiana and determined the relationship of these habitat characteristics to marsh bird relative abundance. We performed bird surveys ( n = 766), including call-back surveys for secretive marsh birds, at all sites across three breeding seasons (2021–2023). We used drone imagery to determine plant composition and used water level recording devices to assess hydrologic characteristics. Our results indicate that hydrologic variation and plant composition are drivers of marsh bird relative abundance regardless of whether a marsh is created or not. While some habitat features differed between created and reference sites, our results indicated that created marshes can support similar abundances of marsh birds as reference sites, depending on what habitat features are present at the site. Our study demonstrates the importance of creating marshes that promote hydrologic connectivity and water level variability, which in turn supports diverse emergent vegetation communities and provides suitable habitat for a variety of marsh bird species.

Louisiana↗

USGS2005

Hydrologic changes in urban streams and their ecological significance

Urban development modifies the production and delivery of runoff to streams and the resulting rate, volume, and timing of streamflow. Given that streamflow demonstrably influences the structure and composition of lotic communities, we have identified four hydrologic changes resulting from urban development that are potentially significant to stream ecosystems: increased frequency of high flows, redistribution of water from base flow to storm flow, increased daily variation in streamflow, and reduction in low flow. Previous investigations of streamflow patterns and biological assemblages provide a scale of ecological significance for each type of streamflow pattern. The scales establish the magnitude of changes in streamflow patterns that could be expected to produce biological responses in streams. Long-term streamflow records from eight streams in urbanizing areas of the United States and five additional reference streams, where land use has been relatively stable, were analyzed to assess if streamflow patterns were modified by urban development to an extent that a biological response could be expected and whether climate patterns could account for equivalent hydrologic variation in the reference streams. Changes in each type of streamflow pattern were evident in some but not all of the urban streams and were nearly absent in the reference streams. Given these results, hydrologic changes are likely significant to urban stream ecosystems, but the significance depends on the stream's physiographic context and spatial and temporal patterns of urban development. In urban streams with substantially altered hydrology, short-term goals for urban stream rehabilitation may be limited because of the difficulty and expense of restoring hydrologic processes in an urban landscape. The ecological benefits of improving physical habitat and water quality may be tempered by persistent effects of altered streamflow. In the end, the hydrologic effects of urban development must be addressed for restoration of urban streams. ?? 2005 by the American Fisheries Society.

American Fisheries Society Symposium↗

USGS1991

Data standardization. The key to effective management

Effective management of the nation's water resources is dependent upon accurate and consistent hydrologic information. Before the emergence of environmental concerns in the 1960's, most hydrologic information was collected by the U.S. Geological Survey and other Federal agencies that used fairly consistent methods and equipment. In the past quarter century, however, increased environmental awareness has resulted in an expansion of hydrologic data collection not only by Federal agencies, but also by state and municipal governments, university investigators, and private consulting firms. The acceptance and use of standard methods of collecting and processing hydrologic data would contribute to cost savings and to greater credibility of flow information vital to responsible assessment and management of the nation's water resources. This paper traces the evolution of the requirements and uses of open-channel flow information in the U.S., and the sequence of efforts to standardize the methods used to obtain this information in the future. The variable nature of naturally flowing rivers results in continually changing hydraulic properties of their channels. Those persons responsible for measurement of water flowing in open channels (streamflow) must use a large amount of judgement in the selection of appropriate equipment and technique to obtain accurate flow information. Standardization of the methods used in the measurement of streamflow is essential to assure consistency of data, but must also allow considerable latitude for individual judgement to meet constantly changing field conditions.

Conference Paper↗

USGS2023

Floodwater drainage assessment of Offutt Air Force Base, Nebraska, 2020–22

Offutt Air Force Base, south of Omaha, Nebraska, experienced major flooding during the March 2019 flood event because of the proximity of the base to the confluence of the Missouri River and nearby tributaries, which exceeded flood stages. Postflood, standing water remained through much of the year, attracting waterfowl and other birds and posing a major safety risk to aircraft. The U.S. Geological Survey, in cooperation with the U.S. Air Force, began a study in 2020 to describe the hydrologic processes that affect the persistence of standing water on Offutt Air Force Base. Existing site data, reviewed in concert with groundwater and surface-water elevation data collected for the study, indicate varying hydrologic responses between two areas of concern (AOCs), which can be linked to differences in subsurface geology and changes in flows of the Missouri River. An inundation map indicated that standing water would be present throughout Papillion Creek Ditch in AOC 1 and would extend upstream to AOC 2 during flow events greater than 771 cubic feet per second. A U.S. Army Corps of Engineers Hydrologic Engineering Center-River Analysis System model and a flow-duration analysis were used to infer that many of the surface-water drainage problems experienced in 2019 were the result of backwater conditions caused by higher streamflows in the Missouri River.

Nebraska↗

USGS2015

Documentation of a restart option for the U.S. Geological Survey coupled Groundwater and Surface-Water Flow (GSFLOW) model

A new option to write and read antecedent conditions (also referred to as initial conditions) has been developed for the U.S. Geological Survey (USGS) Groundwater and Surface-Water Flow (GSFLOW) numerical, hydrologic simulation code. GSFLOW is an integration of the USGS Precipitation-Runoff Modeling System (PRMS) and USGS Modular Groundwater-Flow Model (MODFLOW), and provides three simulation modes: MODFLOW-only, PRMS-only, and GSFLOW (or coupled). The new capability, referred to as the restart option, can be used for all three simulation modes, such that the results from a pair (or set) of spin-up and restart simulations are nearly identical to results produced from a continuous simulation for the same time period. The restart option writes all results to files at the end of a spin-up simulation that are required to initialize a subsequent restart simulation. Previous versions of GSFLOW have had some capability to save model results for use as antecedent condiitions in subsequent simulations; however, the existing capabilities were not comprehensive or easy to use. The new restart option supersedes the previous methods. The restart option was developed in collaboration with the National Oceanic and Atmospheric Administration, National Weather Service as part of the Integrated Water Resources Science and Services Partnership. The primary focus for the development of the restart option was to support medium-range (7- to 14-day) forecasts of low streamflow conditions made by the National Weather Service for critical water-supply basins in which groundwater plays an important role. The spin-up simulation should be run for a sufficient length of time necessary to establish antecedent conditions throughout a model domain. Each GSFLOW application can require different lengths of time to account for the hydrologic stresses to propagate through a coupled groundwater and surface-water system. Typically, groundwater hydrologic processes require many years to come into equilibrium with dynamic climate and other forcing (or stress) data, such as precipitation and well pumping, whereas runoff-dominated surface-water processes respond relatively quickly. Use of a spin-up simulation can substantially reduce execution-time requirements for applications where the time period of interest is small compared to the time for hydrologic memory; thus, use of the restart option can be an efficient strategy for forecast and calibration simulations that require multiple simulations starting from the same day.

Techniques and Methods↗

USGS2018

Integrating real-time subsurface hydrologic monitoring with empirical rainfall thresholds to improve landslide early warning

Early warning for rainfall-induced shallow landsliding can help reduce fatalities and economic losses. Although these commonly occurring landslides are typically triggered by subsurface hydrological processes, most early warning criteria rely exclusively on empirical rainfall thresholds and other indirect proxies for subsurface wetness. We explore the utility of explicitly accounting for antecedent wetness by integrating real-time subsurface hydrologic measurements into landslide early warning criteria. Our efforts build on previous progress with rainfall thresholds, monitoring, and numerical modeling along the landslide-prone railway corridor between Everett and Seattle, Washington, USA. We propose a modification to a previously established recent versus antecedent (RA) cumulative rainfall thresholds by replacing the antecedent 15-day rainfall component with an average saturation observed over the same timeframe. We calculate this antecedent saturation with real-time telemetered measurements from five volumetric water content probes installed in the shallow subsurface within a steep vegetated hillslope. Our hybrid rainfall versus saturation (RS) threshold still relies on the same recent 3-day rainfall component as the existing RA thresholds, to facilitate ready integration with quantitative precipitation forecasts. During the 2015–2017 monitoring period, this RS hybrid approach has an increase of true positives and a decrease of false positives and false negatives relative to the previous RA rainfall-only thresholds. We also demonstrate that alternative hybrid threshold formats could be even more accurate, which suggests that further development and testing during future landslide seasons is needed. The positive results confirm that accounting for antecedent wetness conditions with direct subsurface hydrologic measurements can improve thresholds for alert systems and early warning of rainfall-induced shallow landsliding.

Landslides↗

USGS1989

Summary of the U.S. Geological Survey and U.S. Bureau of Land Management national coal-hydrology program, 1974-84

During the decade 1974-84, the U.S. Geological Survey and the U.S. Bureau of Land Management cooperated on investigations to collect information and to study hydrologic processes related to development and mining of federally owned coal. In addition, the U.S. Geological Survey conducted similar investigations related to nonfederally owned coal. As a result of these nationwide investigations, a large quantity of hydrologic information and data has been collected and compiled in more than 500 reports. This report summarizes the major findings and accomplishments that have resulted from data-collection activities, hydrologic studies, and research concerned with the effects of coal mining on water resources. This summary report includes: (1) A description of the Nation's coal and water-resource issues related to coal development, including history, objectives, and design of the coal-hydrology program; (2) a summary of the hydrologic information collected in the major coal provinces and published in more than 500 reports and journal articles; and (3) a summary and application of results obtained from topical studies undertaken throughout the program, including discussions on watershed modeling, salinity modeling, ground-water flow systems, geochemistry of mine spoils, mine drainage, sedimentation, and aquatic biology. A detailed coal-hydrology reference list concludes the report.

continental United States↗

USGS2000

The importance of ground water in the Great Lakes Region

Ground water is a major natural resource in the Great Lakes Region because it indirectly contributes more than 50 percent of the stream discharge to the Great Lakes. In addition, ground water is the source of drinking water for millions of people in the region, is an important source of supply for agriculture and many industries, and provides a relatively uniform supply of water in some ecologically sensitive areas to sustain plant and animal species. Therefore, to improve our understanding of water-resources issues in the Great Lakes Region, it is important to have a better understanding of the role that ground water plays in the overall hydrologic system of the lakes. The main ground-water resources issues in the Great Lakes Region are related to the amount of ground water, the interaction of ground water and surface water, changes in ground-water quality as development expands, and ecosystem health related to quantity and quality of water. Issues related to the amount of ground water Although the amount of water in the Great Lakes Region is vast, issues related to relatively small quantities of water are being raised more and more often. For example, even though the amount of ground water pumped in the region is small compared to the total amount of water present, ground water is an important source of public-water supply as well as an important source of supply for industrial, agricultural, and domestic needs. Less clearly understood, however, is the relation between the amount of streamflow discharging to the Great Lakes and the large portion of that flow that originates as ground water. The implications of this understanding for water- and land-use practices and, in turn, their effects on water quantity and quality, have not been fully incorporated into a policy framework. To help include information about the implications of the role that ground water plays in addressing regional water issues, a comprehensive analysis of indirect ground-water discharge to the Great Lakes is needed. Direct ground-water discharge to the Great Lakes is not a large factor in water-budget analyses for the Great Lakes. Locally, however, direct ground-water discharge to the Great Lakes may be important, even though the rates and places of discharge are not well known. A long-term evaluation of direct ground-water discharge to the Great Lakes would help place this hydrologic process in proper perspective. Near-shore areas with high rates of direct ground-water discharge may provide valuable habitat for aquatic organisms. Issues related to the interaction of ground water and surface water Withdrawal of ground water removes that water from the watershed when it is consumptively used or when the return flow is discharged to another drainage basin. Under these circumstances, pumping ground water constitutes a diversion of Great Lakes water. Alternatively, ground-water withdrawal could have the opposite effect of diverting ground-water flow into the watershed by altering the ground-water divides. In particular, as withdrawals associated with urban expansion increase, more accurate data on the amount and effects of ground-water use need to be collected. Data on the amounts of ground water pumped both within the watershed and outside, but near the watershed boundaries needs to be collected and evaluated for potential diversion of water to or from the Great Lakes. It is currently thought that both irrigation and ground-water withdrawals near the watershed boundaries constitute relatively small amounts of water; however, both rapidly changing farming practices and rapidly expanding urban communities could alter these amounts in a relatively short timeframe, especially during drought periods. At present, the effects of ground-water withdrawals have been quantified in detail at only a few urban locations. In addition to quantifying the amount of water pumped out of aquifers, it is also important to improve our knowledge of the amount of water that is recharging them. Ground-water recharge rates estimated in earlier studies cover only a small part of the Great Lakes Region. A comprehensive study of ground-water recharge rates for the entire watershed is needed to more completely determine the role of ground water in the hydrologic budget of the Great Lakes. Issues related to changes in ground-water quality as development expands Ground-water quality is as important as quantity for most water uses. As ground-water development proceeds, the possibility of altering the quality of ground water increases. The quality of ground water can be altered when water levels are drawn below the layer that confines the aquifer or by inducing water of lesser quality into an aquifer. Many local studies of these problems have been conducted, but few regional-scale analyses of changes in ground-water quality as a result of ground-water development have been done. Issues related to ecosystem health and quantity and quality of ground water Ground water is essential to maintain wetlands and to provide healthy habitat for other aquatic systems. Wetland hydrology is widely recognized as the primary influence on wetland ecology, development, and persistence, and information about hydrology is essential to understanding and quantifying wetland functions and processes. Studies of the role of ground water in selected wetlands in a range of physiographic settings throughout the Great Lakes watershed are needed to more fully understand the role of wetlands in the Great Lakes Region.

Lake Erie, Lake Huron, Lake Michigan, Lake Ontario↗

USGS2014

Metal stable isotopes in weathering and hydrology

This chapter highlights some of the major developments in the understanding of the causes of metal stable isotope compositional variability in and isotope fractionation between natural materials and provides numerous examples of how that understanding is providing new insights into weathering and hydrology. At this stage, our knowledge of causes of stable isotope compositional variability among natural materials is greatest for the metals lithium, magnesium, calcium, and iron, the isotopes of which have already provided important information on weathering and hydrological processes. Stable isotope compositional variability for other metals such as strontium, copper, zinc, chromium, barium, molybdenum, mercury, cadmium, and nickel has been demonstrated but is only beginning to be applied to questions related to weathering and hydrology, and several research groups are currently exploring the potential. And then there are other metals such as titanium, vanadium, rhenium, and tungsten that have yet to be explored for variability of stable isotope composition in natural materials, but which may hold untold surprises in their utility. This impressive list of metals having either demonstrated or potential stable isotope signals that could be used to address important unsolved questions related to weathering and hydrology, constitutes a powerful toolbox that will be increasingly utilized in the coming decades.

Book chapter↗

USGS2003

Meteorology and hydrology in Yosemite National Park: A sensor network application

Over half of California's water supply comes from high elevations in the snowmelt-dominated Sierra Nevada. Natural climate fluctuations, global warming, and the growing needs of water consumers demand intelligent management of this water resource. This requires a comprehensive monitoring system across and within the Sierra Nevada. Unfortunately, because of severe terrain and limited access, few measurements exist. Thus, meteorological and hydrologic processes are not well understood at high altitudes. However, new sensor and wireless communication technologies are beginning to provide sensor packages designed for low maintenance operation, low power consumption and unobtrusive footprints. A prototype network of meteorological and hydrological sensors has been deployed in Yosemite National Park, traversing elevation zones from 1,200 to 3,700 m. Communication techniques must be tailored to suit each location, resulting in a hybrid network of radio, cell-phone, land-line, and satellite transmissions. Results are showing how, in some years, snowmelt may occur quite uniformly over the Sierra, while in others it varies with elevation. ?? Springer-Verlag Berlin Heidelberg 2003.

Lecture Notes in Computer Science (including subse↗

USGS2019

Coastal wetlands: A synthesis

This book and this synthesis address the pressing need for better management of coastal wetlands worldwide because these wetlands are disappearing at an alarming rate; in some countries the loss is 70%–80% in the last 50 years. Managing requires understanding. Although our understanding of the functioning of coastal wetland ecosystems has grown rapidly over the past decade, still much remains to be learned and understood. We have gained insight into the roles of geomorphic processes, hydrologic dynamics, biotic feedback, and disturbance agents in creating and molding a variety of coastal wetland ecosystems across climatic gradients. The variety is expressed not so much in the more obvious differences in vegetation cover, but rather how physical processes both facilitate and constrain a diversity of plant and animal communities. At one level, coastal wetlands are the product of tidal forces and freshwater inputs at the margin of continents. At another level, the plants control the water currents in the tidal creeks draining the wetlands by generating a tidal current asymmetry that controls sediment transport and results in a deep tidal creek surrounded by shallow vegetated wetlands. The vegetation also influences the physics of water and sediment through several other processes including biofilms, bioturbation of sediments, the buffeting of currents and waves, organic enrichment of sediments, and the closing of nutrient cycles. Few ecosystems provide us with so many clear examples of such feedback controls. What we do understand about the structure and functioning of coastal wetlands should provide the theoretical underpinnings for effective management in protecting them for their many contributions to ecosystem goods and services. What we do not understand should compel us to focus our attention and energies toward seeking optimal solutions to some of the most perplexing and urgent problems facing natural resource management.

Book chapter↗

USGS2004

Time-series monitoring in fractured-rock aquifers

Time-lapse monitoring of subsurface processes is an emerging and promising area of hydrogeophysics. The combined use of non-invasive or minimally invasive geophysical methods with hydraulic and geochemical sampling is a cost-effective approach for aquifer characterization, long-term aquifer monitoring, and remediation monitoring. Time-lapse geophysical surveys can indirectly measure time-varying hydrologic parameters such as fluid saturation or solute concentration. Monitoring of time-varying hydrologic processes provides insight into aquifer properties and structure and aquifer responses to natural or induced stresses, such as seasonal fluctuations or fluid injection experiments for active remediation. The U.S. Geological Survey (USGS) Office of Ground Water, Branch of Geophysics, in cooperation with USGS Toxic Substances Hydrology Program, Environmental Protection Agency (USEPA), Department of Defense, the University of Connecticut, and Stanford University researchers, has applied time-lapse geophysics for site characterization and remediation monitoring in a number of studies. Recent and ongoing examples of time-lapse monitoring in fractured-rock aquifers include: 1) application of attenuation-difference, boreholeradar tomography used to monitor a series of sodium chloride tracer injection tests in fractured crystalline rock; 2) application of attenuation- and velocity-difference tomography and radar-reflection data to monitor steam injection in a fractured limestone aquifer; 3) design of an electrical resistivity tomography investigation to monitor the injection of resistive water into brackish water in a fractured limestone aquifer for aquifer storage and recovery (ASR); and 4) combined application of borehole-geophysical logging with long-term discreteinterval monitoring of hydraulic head and water-chemistry in a fractured crystalline-rock aquifer. These investigations demonstrate the application of geophysical methods to provide quantitative information about the subsurface critical for characterizing aquifer structure, flow dynamics, and hydraulic processes.

New Hampshire↗

USGS2022

Influences of channel and floodplain modification on expansion of woody vegetation into Catahoula Lake, Louisiana, USA

Ecosystem structure of wetlands in managed floodplains depends on hydrological processes controlled by geomorphology and water management. Overlapping effects of direct modifications and geomorphic adjustments to management can combine to trigger changes to floodplain ecosystem structure. We examined the case of woody vegetation encroaching into the depressional Catahoula Lake, Louisiana, in the context of regional hydrologic and geomorphic modification in the floodplain of the Mississippi River. Historical aerial photographs indicated woody encroachment into Catahoula Lake for at least 80 years, and the rate of expansion has increased in recent decades. Historical stage analysis revealed that the downstream Red–Atchafalaya–Mississippi River system controls the lower limit of the lake water level when the large rivers are high, but channel enlargement and other hydrological changes there have reduced the frequency of backwater flooding by 42% since 1880. In addition, operation of the water control structure on the lake has altered its hydrological regime to be more regular among years. Historic stage analysis revealed current lake levels are lower in the high-water spring, less variable in the dry period, and lack the extreme high-water events of 100+ years ago, all of which facilitate the expansion of woody vegetation.

Louisiana↗

USGS2022

Assessing placement bias of the global river gauge network

Knowing where and when rivers flow is paramount to managing freshwater ecosystems. Yet stream gauging stations are distributed sparsely across rivers globally and may not capture the diversity of fluvial network properties and anthropogenic influences. Here we evaluate the placement bias of a global stream gauge dataset on its representation of socioecological, hydrologic, climatic and physiographic diversity of rivers. We find that gauges are located disproportionally in large, perennial rivers draining more human-occupied watersheds. Gauges are sparsely distributed in protected areas and rivers characterized by non-perennial flow regimes, both of which are critical to freshwater conservation and water security concerns. Disparities between the geography of the global gauging network and the broad diversity of streams and rivers weakens our ability to understand critical hydrologic processes and make informed water-management and policy decisions. Our findings underscore the need to address current gauge placement biases by investing in and prioritizing the installation of new gauging stations, embracing alternative water-monitoring strategies, advancing innovation in hydrologic modelling, and increasing accessibility of local and regional gauging data to support human responses to water challenges, both today and in the future.

Nature Sustainability↗

USGS1998

Effects of landcover, water redistribution, and temperature on ecosystem processes in the South Plate Basin

Over one-third of the land area in the South Platte Basin of Colorado, Nebraska, and Wyoming, has been converted to croplands. Irrigated cropland now comprises 8% of the basin, while dry croplands make up 31%. We used the RHESSys model to compare the changes in plant productivity and vegetation-related hydrological processes that occurred as a result of either land cover alteration or directional temperature changes (−2°C, +4°C). Land cover change exerted more control over annual plant productivity and water fluxes for converted grasslands, while the effect of temperature changes on productivity and water fluxes was stronger in the mountain vegetation. Throughout the basin, land cover change increased the annual loss of water to the atmosphere by 114 mm via evaporation and transpiration, an increase of 37%. Both irrigated and nonirrigated grains became active earlier in the year than shortgrass steppe, leading to a seasonal shift in water losses to the atmosphere. Basin-wide photosynthesis increased by 80% due to grain production. In contrast, a 4°C warming scenario caused annual transpiration to increase by only 3% and annual evaporation to increase by 28%, for a total increase of 71 mm. Warming decreased basin-wide photosynthesis by 16%. There is a large elevational range from east to west in the South Platte Basin, which encompasses the western edge of the Great Plains and the eastern front of the Rocky Mountains. This elevational gain is accompanied by great changes in topographic complexity, vegetation type, and climate. Shortgrass steppe and crops found at elevations between 850 and 1800 m give way to coniferous forests and tundra between 1800 and 4000 m. Climate is increasingly dominated by winter snow precipitation with increasing elevation, and the timing of snowmelt influences tundra and forest ecosystem productivity, soil moisture, and downstream discharge. Mean annual precipitation of <500 mm on the plains below 1800 m is far less than potential evapotranspiration of 1000–1500 mm and is insufficient for optimum plant productivity. The changes in water flux and photosynthesis from conversion of steppe to cropland are the result of redistribution of snowmelt water from the mountains and groundwater pumping through irrigation projects.

Ecological Applications↗

USGS1994

Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma, Colorado

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants. In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.

Colorado↗

USGS1995

Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited

Simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow (3-5 m) subsurface datum were made in selected coastal salt marshes of Louisiana, Florida, and North Carolina to quantitatively test Kaye and Barghoorn's contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate. Rates of subsidence of the upper 3-5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation-erosion table. Surface elevation change was significantly lower than vertical accretion at each site after 2 years, indicating a significant amount of shallow subsidence had occurred, ranging from 0.45 to 4.90 cm. The highest rate of shallow subsidence occurred in the Mississippi delta. Results confirm Kaye and Barghoorn's contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate, including not only compaction but also apparently shrink-swell from water storage and/or plant production--decomposition at some sites. Indeed, surface elevation change was completely decoupled from vertical accretion at the Florida site. The assumption of a 1:1 relationship between accretionary and substrate processes. Consequently, the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.

Marine Geology↗