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At least 865 records · Page 48Linked to original sources
USGS1990

Hydrologic framework of Long Island, New York

Long Island, N.Y., is underlain by a mass of unconsolidated geologic deposits of clay, silt, sand, and gravel that overlie southward-sloping consolidated bedrock. These deposits are thinnest in northern Queens County (northwestern Long Island), where bedrock crops out, and increase to a maximum thickness of 2,000 ft in southeastern Long Island. This sequence of unconsolidated deposits consists of several distinct geologic units ranging in age from late Cretaceous through Pleistocene, with some recent deposits near shores and streams. These units are differentiated by age, depositional environment, and lithology in table 1. Investigations of ground-water availability and flow patterns may require information on the internal geometry of the hydrologic system that geologic correlations and interpretation alone cannot provide; hydrologic interpretations in which deposits are differentiated on the basis of water-transmitting properties are generally needed also. This set of maps and vertical sections depicts the hydrogeologic framework of the unconsolidated deposits that form Long Island's ground-water system. These deposits can be classified into eight major hydrogeologic units (table 1). The hydrogeologic interpretations presented herein are not everywhere consistent with strict geologic interpretation owing to facies changes and local variations in the water-transmitting properties within geologic units. These maps depict the upper-surface altitude of seven of the eight hydrogeologic units, which, in ascending order, are: consolidated bedrock, Lloyd aquifer, Raritan confining unit, Magothy aquifer, Monmouth greensand, Jameco aquifer, and Gardiners Clay. The upper glacial aquifer—the uppermost unit—is at land surface over most of Long Island and is, therefore, not included. The nine north-south hydrogeologic sections shown below depict the entire sequence of unconsolidated deposits and, together with the maps, provide a detailed three-dimensional interpretation of Long Island's hydrogeologic framework. The structure-contour map that shows the upper-surface altitude of the Cretaceous deposits is included to illustrate the erosional unconformity between the Cretaceous and overlying Pleistocene deposits. Pleistocene erosion played a major role in determining the shape and extent of the Lloyd aquifer, the Raritan confining unit, and the Magothy aquifer, and thus partly determined their hydrogeologic relation with subsequent (post-Cretaceous) deposits.

Hydrologic Atlas↗
USGS2012

Assessing future risks to agricultural productivity, water resources and food security: How can remote sensing help?

Although global food production has been rising, the world sti ll faces a major food security challenge. Over one billion people are currently undernourished (Wheeler and Kay, 2010). By the 2050s, the human population is projected to grow to 9.1 billion. Over three-quarters of these people will be living in developing countries, in regions that already lack the capacity to feed their populations . Under current agricultural practices, the increased demand for food would require in excess of one billion hectares of new cropland, nearly equivalent to the land area of the United States, and would lead to significant increases in greenhouse gases (Tillman et al. , 2011). Since climate is the primary determinant of agricultural productivity, changes to it will influence not only crop yields, but also hydrologic balances and supplies of inputs to managed farming systems, and may lead to a shift in the geographic location of some crops . Therefore, not only must crop productivity (yield per unit of land; kg/m 2 ) increase, but water productivity (yield per unit of water or "crop per drop"; kg/m 3 ) must increase as well in order to feed a burgeoning population against a backdrop of changing dietary consumption patterns, a changing climate and the growing scarcity of water and land (Beddington, 2010). The impact from these changes wi ll affect the viability of both dryland subsistence and irrigated commodity food production (Knox, et al. , 2010a). Since climate is a primary determinant of agricultural productivity, any changes will influence not only crop yields, but also the hydrologic balances, and supplies of inputs to managed farming systems as well as potentially shifting the geographic location for specific crops . Unless concerted and collective action is taken, society risks worldwide food shortages, scarcity of water resources and insufficient energy. This has the potential to unleash public unrest, cross-border conflicts and migration as people flee the worst-affected regions to seck refuge in "safe havens", a situation that Beddington described as the "perfect storm" (2010).

Photogrammetric Engineering and Remote Sensing↗
USGS1999

Stable isotopes and mineral resource investigations in the United States

The elements oxygen, hydrogen, sulfur, and carbon are important constituents of hydrothermal ore-forming systems and the weathering processes of mineral deposits in the surficial environment. They also play key roles in volcanic activity, ecosystem dynamics, climate change, and hydrologic and atmospheric processes. Therefore, study of the stable isotopes of these elements can provide powerful insights into these processes. This is especially true for ongoing U.S. Geological Survey (USGS) projects in the Eastern United States that are concerned with the origins of base (copper, lead, and zinc) and precious (gold and silver) metal deposits in the Carolina slate belt and northern Maine and with the environmental effects of weathering of mineral deposits (fig. 1).

Information Handout↗
USGS2014

Identifying dominant controls on hydrologic parameter transfer from gauged to ungauged catchments: a comparative hydrology approach

Daily streamflow information is critical for solving various hydrologic problems, though observations of continuous streamflow for model calibration are available at only a small fraction of the world’s rivers. One approach to estimate daily streamflow at an ungauged location is to transfer rainfall–runoff model parameters calibrated at a gauged (donor) catchment to an ungauged (receiver) catchment of interest. Central to this approach is the selection of a hydrologically similar donor. No single metric or set of metrics of hydrologic similarity have been demonstrated to consistently select a suitable donor catchment. We design an experiment to diagnose the dominant controls on successful hydrologic model parameter transfer. We calibrate a lumped rainfall–runoff model to 83 stream gauges across the United States. All locations are USGS reference gauges with minimal human influence. Parameter sets from the calibrated models are then transferred to each of the other catchments and the performance of the transferred parameters is assessed. This transfer experiment is carried out both at the scale of the entire US and then for six geographic regions. We use classification and regression tree (CART) analysis to determine the relationship between catchment similarity and performance of transferred parameters. Similarity is defined using physical/climatic catchment characteristics, as well as streamflow response characteristics (signatures such as baseflow index and runoff ratio). Across the entire US, successful parameter transfer is governed by similarity in elevation and climate, and high similarity in streamflow signatures. Controls vary for different geographic regions though. Geology followed by drainage, topography and climate constitute the dominant similarity metrics in forested eastern mountains and plateaus, whereas agricultural land use relates most strongly with successful parameter transfer in the humid plains.

Journal of Hydrology↗
USGS2010

The relationship of catchment topography and soil hydraulic characteristics to lake alkalinity in the northeastern United States

We undertook the task of determining whether base flow alkalinity of surface waters in the northeastern United States is related to indices of soil contact time and flow path partitioning that are derived from topographic and soils information. The influence of topography and soils on catchment hydrology has been incorporated previously in the variable source area model TOPMODEL as the relative frequency distribution of ln ( a/Kb tan B ), where ln is the Naperian logarithm, “ a ” is the area drained per unit contour, K is the saturated hydraulic conductivity, b is the soil depth, and tan B is the slope. Using digital elevation and soil survey data, we calculated the ln ( a/Kb tan B ) distribution for 145 catchments. Indices of flow path partitioning and soil contact time were derived from the ln ( a/Kb tan B ) distributions and compared to measurements of alkalinity in lakes to which the catchments drain. We found that alkalinity was, in general, positively correlated with the index of soil contact time, whereas the correlation between alkalinity and the flow path partitioning index was weak at best. A portion of the correlation between the soil contact time index and alkalinity was attributable to covariation with soil base saturation and cation exchange capacity, while another portion was found to be independent of these factors. Although our results indicate that catchments with long soil contact time indices are most likely to produce high alkalinity base flow, a sensitivity analysis of TOPMODEL suggests that surface waters of these same watersheds may be susceptible to alkalinity depressions during storm events, due to the role of flow paths.

Water Resources Research↗
USGS2017

Hydrologic modeling in a marsh-mangrove ecotone: Predicting wetland surface water and salinity response to restoration in the Ten Thousand Islands region of Florida, USA

At the fringe of Everglades National Park in southwest Florida, United States, the Ten Thousand Islands National Wildlife Refuge (TTINWR) habitat has been heavily affected by the disruption of natural freshwater flow across the Tamiami Trail (U.S. Highway 41). As the Comprehensive Everglades Restoration Plan (CERP) proposes to restore the natural sheet flow from the Picayune Strand Restoration Project area north of the highway, the impact of planned measures on the hydrology in the refuge needs to be taken into account. The objective of this study was to develop a simple, computationally efficient mass balance model to simulate the spatial and temporal patterns of water level and salinity within the area of interest. This model could be used to assess the effects of the proposed management decisions on the surface water hydrological characteristics of the refuge. Surface water variations are critical to the maintenance of wetland processes. The model domain is divided into 10 compartments on the basis of their shared topography, vegetation, and hydrologic characteristics. A diversion of + 10 % of the discharge recorded during the modeling period was simulated in the primary canal draining the Picayune Strand forest north of the Tamiami Trail (Faka Union Canal) and this discharge was distributed as overland flow through the refuge area. Water depths were affected only modestly. However, in the northern part of the refuge, the hydroperiod, i.e., the duration of seasonal flooding, was increased by 21 days (from 115 to 136 days) for the simulation during the 2008 wet season, with an average water level rise of 0.06 m. The average salinity over a two-year period in the model area just south of Tamiami Trail was reduced by approximately 8 practical salinity units (psu) (from 18 to 10 psu), whereas the peak dry season average was reduced from 35 to 29 psu (by 17%). These salinity reductions were even larger with greater flow diversions ( + 20 % ). Naturally, the reduction in salinity diminished toward the open water areas where the daily flood tides mix in saline bay water. Partially restoring hydrologic flows to TTINWR will affect hydroperiod and salinity regimes within downslope wetlands, and perhaps serve as a management tool to reduce the speed of future encroachment of mangroves into marsh as sea levels rise.

Journal of Hydrologic Engineering↗
USGS2022

Natural and anthropogenic influences on benthic cyanobacteria in streams of the northeastern United States

Benthic cyanobacteria are widespread in streams and rivers and have the potential to release toxins. In large numbers, these microorganisms and their toxins present a risk to human health. Cyanobacterial abundance in stream biofilms is typically related to single or a limited set of environmental factors, mainly light availability, water temperature, and nutrient concentrations. However, these factors may act synergistically with watershed characteristics and other stressors, such as anthropogenic pollutants, to affect cyanobacteria. We investigated the influence of multiple regional and local variables on the abundance of benthic cyanobacterial genera in streams using all subsets generalized additive modeling. We examined watershed factors (topography, geology, and climate) alongside in-stream factors (geomorphology, hydrology, pH, specific conductance, nutrients, organic contaminants, and dissolved metals) from 76 sites along an urban gradient in the northeast United States. Each genus responded to a distinct combination of environmental variables, demonstrating strong intergeneric variation in environmental selection of realized niches. Four of the 7 potentially toxigenic genera that we modeled were positively influenced by water temperature or nutrients. Nonetheless, watershed characteristics, streamflow, and/or other water quality pollutants were equally or more influential for the potentially toxigenic genera. Additionally, the relationships between cyanobacterial abundance and environmental factors varied in shape and direction across many genera. In particular, with increasing concentrations of herbicides, polychlorinated biphenyls, or metals, the abundance of roughly half of the affected genera decreased, while the others increased. These results likely demonstrate novel toxic effects of the pollutants on cyanobacterial genera in the environment, while indicating that unmeasured biotic interactions may lead to positive responses for other genera. Our results emphasize the need to consider variables beyond those that are most frequently measured or implicated (e.g., water temperature and nutrients) to more fully understand the environmental conditions that influence the distributions and abundance of potentially harmful cyanobacteria.

Connecticut, Massachusetts, New Hampshire. New Yor↗
USGS2022

An extrapolation method for estimating loads from unmonitored areas using watershed model load ratios

It is important to routinely estimate loads from an entire watershed to describe current conditions and evaluate how watershed-wide management efforts have affected the nutrient and sediment export that affect downstream water quality. However, monitoring in most areas, including the Great Lakes watershed, consists of sampling at a limited number of sites that are only periodically used to estimate total watershed loading. Here, we describe a technique to extrapolate loads measured at a limited number of reference sites to the total load from a large watershed using load ratios between monitored sites and unmonitored areas obtained from a watershed model (i.e., model load ratio, MLR, approach). In this study, modeled nonpoint-source load ratios between monitored tributaries (reference sites) and nearby unmonitored areas and point-source delivery factors for all areas were obtained from a Spatially Referenced Regression On Watershed attributes (SPARROW) model and used to extrapolate the measured loads from an ongoing monitoring program (Great Lakes Restoration Initiative Tributary monitoring program) to the entire Great Lakes watershed. The MLR approach incorporates spatial variability in nonpoint- and point-source delivery, watershed characteristics, and hydrology that are often not considered when estimating loads from unmonitored areas, such as using the unit area load (UAL) extrapolation approach. The MLR approach provided smaller watershed loads than the UAL approach because yields from monitored sites, in general, were larger than from unmonitored areas. When both approaches were used to estimate loads at adjacent monitored sites, the MLR approach provided more accurate estimates than the UAL approach.

Great Lakes Basin↗
USGS2020

COVID-19 pandemic impacts on global inland fisheries

The COVID-19 pandemic has led to environmental recovery in some ecosystems from a global “anthropause,” yet such evidence for natural resources with extraction or production value (e.g., fisheries) is limited. This brief report provides a data-driven global snapshot of expert-perceived impacts of COVID-19 on inland fisheries. We distributed an online survey assessing perceptions of inland fishery pressures in June and July 2020 to basin-level inland fishery experts (i.e., identified by the Food and Agriculture Organization of the United Nations across the global North and South); 437 respondents from 79 countries addressed 93 unique hydrological basins, accounting for 82.1% of global inland fish catch. Based on the responses analyzed against extrinsic fish catch and human development index data, pandemic impacts on inland fisheries 1) add gradation to the largely positive environmental narrative of the global pandemic and 2) identify that basins of higher provisioning value are perceived to experience greater fishery pressures but may have limited compensatory capacity to mitigate COVID-19 impacts along with negative pressures already present.

PNAS↗
USGS2022

Mapped predictions of manganese and arsenic in an alluvial aquifer using boosted regression trees

Manganese (Mn) concentrations and the probability of arsenic (As) exceeding the drinking-water standard of 10 μg/L were predicted in the Mississippi River Valley alluvial aquifer (MRVA) using boosted regression trees (BRT). BRT, a type of ensemble-tree machine-learning model, were created using predictor variables that affect Mn and As distribution in groundwater. These variables included iron (Fe) concentrations and specific conductance predicted from previously developed BRT models, groundwater flux and age estimates from MODFLOW, and hydrologic characteristics. The models also included results from the first airborne geophysical survey conducted in the United States to target an entire aquifer system. Predictions of high Mn and As occurred where Fe was high. Predicted high Mn concentrations were correlated with fraction of young groundwater (less than 65 years) computed from MODFLOW results. High probabilities of As exceedance were predicted where groundwater was relatively old and airborne electromagnetic resistivity was high, typically proximal to streams. Two-variable partial-dependence plots and sensitivity analysis were used to provide insight into the factors controlling Mn and As distribution in groundwater. The maps of predicted Mn concentrations and As exceedance probabilities can be used to identify areas where these constituents may be high, and that could be targeted for further study. This paper shows that incorporation of a selected set of process-informed data, such as MODFLOW results and airborne geophysics, into a machine-learning model improves model interpretability. Incorporation of process-rich information into machine-learning models will likely be useful for addressing a wide range of problems of interest to groundwater hydrologists.

Arkansas, Louisiana, Mississippi, Tennessee↗
USGS2005

The hydrologic reponses to development in regional sedimentary aquifers

The hydrologic response to development of three of the most heavily pumped sedimentary aquifer systems in the United States is similar in some aspects and different in others. In the semiarid West, an unconfined sand aquifer and a confined sand and clay aquifer system have been subjected to withdrawals that are far greater than predevelopment recharge rates. As a result, the aquifers have large losses of ground water from storage. In the humid East, pumpage from a carbonate aquifer system has resulted in induced recharge and diversion of natural discharge with insignificant loss from storage. However, the following responses to development are common in all three aquifer systems: (1) ground-water circulation has increased, (2) rates of recharge have increased—mostly due to recirculation of pumped ground water, or infiltration of imported surface water used for irrigation in the semiarid West, (3) locations of recharge areas have changed, and (4) natural discharge has decreased. Regional water-level declines associated with ground- water development are inevitably accompanied by some combination of elastic compaction of aquifer material, inelastic compaction of fine-grained sediments and land subsidence, dewatering of aquifer material near pumping centers, and induced formation of sinkholes. The degree to which these changes occur is dependent on: (1) rates of pumping in relation to available recharge, and (2) lithology, specifically the proportion of sand, gravel, silt, clay, and carbonate rock that comprise the aquifer system.

Groundwater↗
USGS2014

Physiographic and land cover attributes of the Puget Lowland and the active streamflow gaging network, Puget Sound Basin

Geospatial information for the active streamflow gaging network in the Puget Sound Basin was compiled to support regional monitoring of stormwater effects to small streams. The compilation includes drainage area boundaries and physiographic and land use attributes that affect hydrologic processes. Three types of boundaries were used to tabulate attributes: Puget Sound Watershed Characterization analysis units (AU); the drainage area of active streamflow gages; and the catchments of Regional Stream Monitoring Program (RSMP) sites. The active streamflow gaging network generally includes sites that represent the ranges of attributes for lowland AUs, although there are few sites with low elevations (less than 60 meters), low precipitation (less than 1 meter year), or high stream density (greater than 5 kilometers per square kilometers). The active streamflow gaging network can serve to provide streamflow information in some AUs and RSMP sites, particularly where the streamflow gage measures streamflow generated from a part of the AU or that drains to the RSMP site, and that part of the AU or RSMP site is a significant fraction of the drainage area of the streamgage. The maximum fraction of each AU or RSMP catchment upstream of a streamflow gage and the maximum fraction of any one gaged basin in an AU or RSMP along with corresponding codes are provided in the attribute tables.

Washington↗
USGS1995

Potentiometric surface of the upper Floridan Aquifer, west-central Florida, May 1995

The Floridan aquifer system consists of the Upper and Lower Floridan aquifers separated by a middle confining unit. The middle confining unit and the Lower Floridan aquifer generally contain highly mineralized water. In most reports on the hydrology of southwest Florida, the term "Floridan aquifer" has been applied to the water-bearing rocks, herin referred to as the Upper Floridan aquifer. The Upper Floridan aquifer is a productive aquifer and supplies more than 10 times the amount of water pumped from either the surfical aquifer system of the intermediate aquifer system in most of the study area (Duerr and other, 1988). This map report depicts the potentiometric surface of the Upper Floridan aquifer in May 1995. The potentipmetric surface represents to the level to which water will rise in tightly cased wells that tap a confined aquifer system. The surface is mapped by determining the altitude of water levels in a network of wells and is represented on maps by contours that connect point of equal altitude. This map represents water-level conditions near the end of the dry season when ground-water withdrawals for agricultural use are usually high. The cumulative rainfall for the study area was 2.98 inches above average for the period from June 1994 through May 1995 (Southwest Florida Water Management District, 1995)

Florida↗
USGS2011

Groundwater availability of the Denver Basin aquifer system, Colorado

The Denver Basin aquifer system is a critical water resource for growing municipal, industrial, and domestic uses along the semiarid Front Range urban corridor of Colorado. The confined bedrock aquifer system is located along the eastern edge of the Rocky Mountain Front Range where the mountains meet the Great Plains physiographic province. Continued population growth and the resulting need for additional water supplies in the Denver Basin and throughout the western United States emphasize the need to continually monitor and reassess the availability of groundwater resources. In 2004, the U.S. Geological Survey initiated large-scale regional studies to provide updated groundwater-availability assessments of important principal aquifers across the United States, including the Denver Basin. This study of the Denver Basin aquifer system evaluates the hydrologic effects of continued pumping and documents an updated groundwater flow model useful for appraisal of hydrologic conditions.

Colorado↗
USGS1971

Inventory of Glaciers in the North Cascades, Washington

Perennial bodies of ice in the North Cascades having areas of at least 0.1 km2 (square kilometer) are tabulated and classified. The inventory, a contribution to the International Hydrological Decade, includes 756 glaciers, covering 267 km2, about half of the glacier area in the United States south of Alaska. Listings include each glacier's location, drainage basin, area, length, orientation, altitude, and classification as to form, source, surface, nature of terminus, and activity. These glaciers contribute annually about 800 million cubic meters of water to streamflow in the State of Washington.

Professional Paper↗
USGS2008

Elevation-derived watershed basins and characteristics for major rivers of the conterminous United States

The U.S. Geological Survey Earth Resources Observation and Science Center Topographic Science Project has developed elevation-derived watershed basins and characteristics for major rivers of the conterminous United States. Watershed basins are delineated upstream from the mouth of major rivers by using the hydrologic connectivity of the Elevation Derivatives for National Applications (EDNA) seamless database. Watershed characteristics are quantified by integrating ancillary geospatial datasets, including land cover, population, slope, and topography, with elevation-derived watershed boundaries. The results are published in an online EDNA Watershed Atlas at http://edna.usgs.gov/watersheds. The atlas serves as a framework for evaluating and analyzing the physical, biological, and anthropogenic status of watersheds.

Scientific Investigations Report↗
USGS2014

Small reservoir distribution, rate of construction, and uses in the upper and middle Chattahoochee basins of the Georgia Piedmont, USA, 1950-2010

Construction of small reservoirs affects ecosystem processes in numerous ways including fragmenting stream habitat, altering hydrology, and modifying water chemistry. While the upper and middle Chattahoochee River basins within the Southeastern United States Piedmont contain few natural lakes, they have a high density of small reservoirs (more than 7500 small reservoirs in the nearly 12,000 km2 basin). Policymakers and water managers in the region have little information about small reservoir distribution, uses, or the cumulative inundation of land cover caused by small reservoir construction. Examination of aerial photography reveals the spatiotemporal patterns and extent of small reservoir construction from 1950 to 2010. Over that 60 year timeframe, the area inundated by water increased nearly six fold (from 19 reservoirs covering 0.16% of the study area in 1950 to 329 reservoirs covering 0.95% of the study area in 2010). While agricultural practices were associated with reservoir creation from 1950 to 1970, the highest rates of reservoir construction occurred during subsequent suburban development between 1980 and 1990. Land cover adjacent to individual reservoirs transitioned over time through agricultural abandonment, land reforestation, and conversion to development during suburban expansion. The prolific rate of ongoing small reservoir creation, particularly in newly urbanizing regions and developing counties, necessitates additional attention from watershed managers and continued scientific research into cumulative environmental impacts at the watershed scale.

Georgia↗
USGS2011

Factors that influence the hydrologic recovery of wetlands in the Northern Tampa Bay area, Florida

Reductions in groundwater withdrawals from Northern Tampa Bay well fields were initiated in mid-2002 to improve the hydrologic condition of wetlands in these areas by allowing surface and groundwater levels to recover to previously higher levels. Following these reductions, water levels at some long-term wetland monitoring sites have recovered, while others have not recovered as expected. To understand why water levels for some wetlands have not increased, nine wetlands with varying impacts from well field pumping were examined based on four factors known to influence the hydrologic condition of wetlands in west-central Florida. These factors are the level of the potentiometric surface of the Upper Floridan aquifer underlying the wetland, recent karst activity near and beneath the wetland, permeability of the underlying sediments, and the topographic position of the wetland in the landscape. The combination of two factors, the presence of recent karst activity below or near the wetlands and the depth to the potentiometric surface of the Upper Floridan aquifer below the wetlands, had the most influence on the hydrologic recovery of the study wetlands. The study wetlands are located in an area where numerous localized surface or buried depressions (karst features or sinkholes) are common throughout the mantled karst landscape, which increases the hydrologic connection between the wetlands and the underlying aquifers. Breaches or breaks in the underlying sediments or in the intermediate confining unit due to recent karst subsidence activity act as pathways for downward leakage. For the study wetlands, the leakage potential increased when the vertical separation between the potentiometric surface of the Upper Floridan aquifer and the wetland-bottom elevation (a surrogate for the wetland water level) increased. The increase in the potentiometric surface of the Upper Floridan aquifer below the wetland was the primary factor influencing the hydrologic recovery of the study wetlands, even in areas affected by karst subsidence. For one of the study wetlands influenced by karst subsidence (S-44 Cypress at Starkey well field), the potentiometric surface of the Upper Floridan aquifer increased to the level of the wetland-bottom elevation following the reductions in groundwater withdrawals. Despite the karst subsidence in the wetland, having the level of the potentiometric surface just below the wetland bottom limited the downward leakage potential and resulted in an increase in the flooded area and duration of the wetland hydroperiod. In contrast, two study wetlands affected by karst subsidence (W-12 Cypress and W-16 Marsh at Cypress Creek) remained mostly dry during the period of groundwater withdrawal reductions, even though the median elevation of the potentiometric surface of the Upper Floridan aquifer rose about 5 feet in this area of the well field. These wetlands are located in an area of the well field where large groundwater withdrawals are concentrated, and during the last 20 years (1989-2009) the wetlands were inundated only during periods of extreme rainfall. During these brief inundation periods, the wetland water levels receded after 1 to 2 months, much more rapidly than wetlands located in areas without karst subsidence or concentrated pumping, indicating the increased leakage between the wetlands and underlying aquifers. Because of this interconnection, water levels in these wetlands and others impacted by karst subsidence in this region will not recover if the potentiometric surface of the Upper Floridan aquifer remains at its current (2009) elevation (median distance of about 10 feet below the wetland-bottom elevation). Low permeability sediments and the absence of karst features underlying the wetlands had a positive influence on the wetland recovery following the reductions in groundwater withdrawals. In these settings, intact low permeability subsurface layers help maintain water within and beneath the wetland, and limit the downward leakage potential to the Upper Floridan aquifer. For wetlands in these settings, the increase in potentiometric surface of the Upper Floridan aquifer below the study wetland-bottom elevations resulted in an increase in the flooded area and the duration of the wetland hydroperiod. Although of less importance than the other three factors, a low-lying topographical position benefited the hydrologic condition of several of the study wetlands (S-68 Cypress and W-12 Cypress) both before and after the reductions in groundwater withdrawals. Compared to wetlands in a higher topographical position, those in a lower position had longer hydroperiods because of their greater ability to receive more runoff from higher elevation wetlands and to establish surface-water connections to other isolated wetlands and surface-water bodies through low-lying surface-water channels during wet conditions. In addition, wetlands in low-lying areas benefited from groundwater inflow when groundwater levels were higher than wetland water levels.

Scientific Investigations Report↗