Search USGS⌕ Search

SEARCH · Search USGS

Results for “Soil Systems”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7Linked to original sources
USGS1952

Pimpled plains of Eastern Oklahoma

Patterns formed by the networks of furrows separating the natural mounds of pimpled plains in eastern Oklahoma are regarded as attributable to shrinkage-polygon systems of coarse texture comparable to those occurring (1) in mound-studded parts of the northwestern United States that are underlain by vertically jointed basalt, (2) in tracts of Alaskan tundra that are occupied by ice-wedge networks, and (3) in beds of playa lakes in the arid southwest that are cut by systems of giant desiccation fissures. While the inter-mound furrow networks characteristic of pimpled plains in the mid-continent region may owe their origin to former frigid climatic conditions, one of a number of alternative possibilities is that the peculiar configuration of these surfaces, as exemplified in eastern Oklahoma , has resulted from erosion of systems of giant soil polygons caused by desiccation. The time of origin of the pimpled plains of eastern Oklahoma is believed to have been not earlier than late Pleistocene.

Oklahoma↗
USGS2025

Geochemical processes related to mined, milled, or natural metal deposits in a rapidly changing global environment

The demand for metals and raw materials, such as nickel and copper, has been projected to expand in the coming decades, driven by the global energy transition, the need for green technologies, and expanding infrastructure. Consequently, the increasing extraction and production of mining waste can have adverse impacts on surrounding environments and human health. The aim of this thematic collection is to fill critical knowledge gaps in the present-day cycles of metal(loid)s from source to larger sinks, and the effect of environmental management, anthropogenic development, and climate change. Altogether, the studies have been conducted in different natural settings around the world and comprise investigations in laterites, a soil-medicinal plant system, watersheds, and banded iron formations, among others. The geochemical applications in tracing mineralization, its secondary products, and/or potential impact on the immediate environment are highly diverse with applied tools ranging from isotope tracers to major and trace element systematics. Particularly the use of rare earth elements, their patterns and anomalies are methods employed by several studies in this collection. We summarize the findings to offer a potential future direction for the use of geochemical tracing techniques in resource exploration in the context of climate change and environmental challenges.

Geochemistry: Exploration, Environment, Analysis↗
USGS2010

Assessing the vulnerability of public-supply wells to contamination: Glacial aquifer system in Woodbury, Connecticut

This fact sheet highlights findings from the vulnerability study of a public-supply well in Woodbury, Connecticut. The well typically produces water at the rate of 72 gallons per minute from the glacial aquifer system in the Pomperaug River Basin. Water samples were collected at the public-supply well and at monitoring wells installed in or near the simulated zone of contribution to the supply well. Samples of untreated water from the public-supply wellhead contained several types of undesirable constituents, including 11 volatile organic compounds (VOCs), nitrate, pesticides, uranium, and radon. Most of these constituents were detected at concentrations below drinking-water standards, where such standards exist. Only concentrations of the VOC trichlorethylene exceeded the Maximum Contaminant Level (MCL) of 5 micrograms per liter (ug/L) established by U.S. Environmental Protection Agency for drinking water. Radon concentrations exceeded a proposed-but not finalized-MCL of 300 picocuries per liter (pCi/L). Overall, the study findings point to four main factors that affect the movement and fate of contaminants and the vulnerability of the public-supply well in Woodbury: (1) groundwater age (how long ago water entered, or recharged, the aquifer); (2) the percentage of recharge received through urban areas; (3) the percentage of recharge received through dry wells and their proximity to the public-supply well; and (4) natural geochemical processes occurring within the aquifer system; that is, processes that affect the amounts and distribution of chemical substances in aquifer sediments and groundwater. A computer-model simulation of groundwater flow to the public-supply well was used to estimate the age of water particles entering the well along the length of the well screen. About 90 percent of the simulated flow to the well consists of water that entered the aquifer 9 or fewer years ago. Such young water is vulnerable to contaminants resulting from human activities, as indicated by the solvents, fuel components, road salt, and septic-system leachate that were detected in the glacial aquifer system during the current study. Age-dating combined with chemical modeling suggests that less than 2 percent of water produced by the public-supply well is water from the deep bedrock that is "old" (water that recharged, or entered, the aquifer before 1952). Such a small percentage of old groundwater entering the public-supply well offers little potential for dilution of young waters containing contaminants from human activities. Shallow groundwater that originated as recharge through urban areas generally had higher median concentrations and more detections of volatile organic compounds (VOCs) than did groundwater from the deep glacial deposits or fractured bedrock that originated mainly as recharge through agricultural and undeveloped land. Shallow groundwater was also found to be affected by road salt and septic-system leachate. A chemical mixing model indicates that up to 15 percent of nitrate in water from the supply well is likely from septic-system leachate. The Connecticut Department of Public Health has identified several potential sources of contamination in the commercial area of Woodbury (several light industrial or commercial properties where hazardous materials and petroleum products are used and stored). To reduce stormwater runoff in the commercial area, water from the parking lots and pavement is channeled into dry wells-drains that shunt water directly into the aquifer system, bypassing the soil and unsaturated zones. A computer-model simulation of groundwater flow indicates that approximately 16 percent of the water produced by the public-supply well is derived from runoff captured by these drains. Traveltime for water from the dry wells to the public-supply well ranges from about 1.5 to less than 4 years. Dry wells have the potential to enhance contaminant movement to the supply well, suggesting that stormwater-control methods cannot be considered separately from groundwater quality—they are linked. Water-quality protection in this setting depends on the entire community. If residents and businesses take steps to reduce input of manmade contaminants to groundwater, a positive effect on quality of the supply-well water might begin to be seen in less than 10 years, owing to the short residence time of water in the aquifer.

Connecticut↗
USGS1977

A FORTRAN program for calculating nonlinear seismic ground response

The program described here was designed for calculating the nonlinear seismic response of a system of horizontal soil layers underlain by a semi-infinite elastic medium representing bedrock. Excitation is a vertically incident shear wave in the underlying medium. The nonlinear hysteretic behavior of the soil is represented by a model consisting of simple linear springs and Coulomb friction elements arranged as shown. A boundary condition is used which takes account of finite rigidity in the elastic substratum. The computations are performed by an explicit finite-difference scheme that proceeds step by step in space and time. A brief program description is provided here with instructions for preparing the input and a source listing. A more detailed discussion of the method is presented elsewhere as is the description of a different program employing implicit integration.

Open-File Report↗
USGS2007

Thermal Methods for Investigating Ground-Water Recharge

Recharge of aquifers within arid and semiarid environments is defined as the downward flux of water across the regional water table. The introduction of recharging water at the land surface can occur at discreet locations, such as in stream channels, or be distributed over the landscape, such as across broad interarroyo areas within an alluvial ground-water basin. The occurrence of recharge at discreet locations is referred to as focused recharge, whereas the occurrence of recharge over broad regions is referred to as diffuse recharge. The primary interest of this appendix is focused recharge, but regardless of the type of recharge, estimation of downward fluxes is essential to its quantification. Like chemical tracers, heat can come from natural sources or be intentionally introduced to infer transport properties and aquifer recharge. The admission and redistribution of heat from natural processes such as insolation, infiltration, and geothermal activity can be used to quantify subsurface flow regimes. Heat is well suited as a ground-water tracer because it provides a naturally present dynamic signal and is relatively harmless over a useful range of induced perturbations. Thermal methods have proven valuable for recharge investigations for several reasons. First, theoretical descriptions of coupled water-and-heat transport are available for the hydrologic processes most often encountered in practice. These include land-surface mechanisms such as radiant heating from the sun, radiant cooling into space, and evapotranspiration, in addition to the advective and conductive mechanisms that usually dominate at depth. Second, temperature is theoretically well defined and readily measured. Third, thermal methods for depths ranging from the land surface to depths of hundreds of meters are based on similar physical principles. Fourth, numerical codes for simulating heat and water transport have become increasingly reliable and widely available. Direct measurement of water flux in the subsurface is difficult, prompting investigators to pursue indirect methods. Geophysical approaches that exploit the coupled relation between heat and water transport provide an attractive class of methods that have become widely used in investigations of recharge. This appendix reviews the application of heat to the problem of recharge estimation. Its objective is to provide a fairly complete account of the theoretical underpinnings together with a comprehensive review of thermal methods in practice. Investigators began using subsurface temperatures to delineate recharge areas and infer directions of ground-water flow around the turn of the 20th century. During the 1960s, analytical and numerical solutions for simplified heat- and fluid-flow problems became available. These early solutions, though one-dimensional and otherwise restricted, provided a strong impetus for applying thermal methods to problems of liquid and vapor movement in systems ranging from soils to geothermal reservoirs. Today?s combination of fast processors, massive data-storage units, and efficient matrix techniques provide numerical solutions to complex, three-dimensional transport problems. These approaches allow researchers to take advantage of the considerable information content routinely achievable in high-accuracy temperature work.

Professional Paper↗
USGS2007

Application of the Basin Characterization Model to Estimate In-Place Recharge and Runoff Potential in the Basin and Range Carbonate-Rock Aquifer System, White Pine County, Nevada, and Adjacent Areas in Nevada and Utah

A regional-scale water-balance model was used to estimate recharge and runoff potential and support U.S. Geological Survey efforts to develop a better understanding of water availability for the Basin and Range carbonate-rock aquifer system (BARCAS) study in White Pine County, Nevada, and adjacent areas in Nevada and Utah. The water-balance model, or Basin Characterization Model (BCM), was used to estimate regional ground-water recharge for the 13 hydrographic areas in the study area. The BCM calculates recharge by using a distributed-parameter, water-balance method and monthly climatic boundary conditions. The BCM requires geographic information system coverages of soil, geology, and topographic information with monthly time-varying climatic conditions of air temperature and precipitation. Potential evapotranspiration, snow accumulation, and snowmelt are distributed spatially with process models. When combined with surface properties of soil-water storage and saturated hydraulic conductivity of bedrock and alluvium, the potential water available for in-place recharge and runoff is calculated using monthly time steps using a grid scale of 866 feet (270 meters). The BCM was used with monthly climatic inputs from 1970 to 2004, and results were averaged to provide an estimate of the average annual recharge for the BARCAS study area. The model estimates 526,000 acre-feet of potential in-place recharge and approximately 398,000 acre-feet of potential runoff. Assuming 15 percent of the runoff becomes recharge, the model estimates average annual ground-water recharge for the BARCAS area of about 586,000 acre-feet. When precipitation is extrapolated to the long-term climatic record (1895-2006), average annual recharge is estimated to be 530,000 acre-feet, or about 9 percent less than the recharge estimated for 1970-2004.

Scientific Investigations Report↗
USGS1999

Hydrogeologic assessment of the Sequim-Dungeness area, Clallam County, Washington

The Sequim-Dungeness area covers 116 square miles (mi 2 ) on the northern part of the Olympic Peninsula in northwestern Washington. The central part of this area (74 mi 2 ) was designated as a primary study area. During the past two decades, the population has rapidly increased, land use has changed from mostly agricultural to residential, and salmon populations in the Dungeness River have appreciably declined. The increasing competition for water combined with a close relation between ground water, the Dungeness River, and an extensive irrigation system has created a need for a better understanding of ground water and the relation between ground water and surface water in the study area. The Sequim-Dungeness area is underlain by as much as 2,000 feet of unconsolidated Quaternary deposits that are mostly of glacial origin. Interpretation of 10 hydrogeologic cross sections and lithologic logs of about 600 wells led to the delineation of three aquifers, two confining beds, and a lower unit of undifferentiated deposits. A bedrock unit at the bottom is considered the base of the ground-water system. Ground water in the study area is recharged from infiltration and percolation of precipitaton, percolation of unconsumed irrigation water, leakage from irrigation ditches, subsurface inflow through the southern study-area boundary, and leakage from streams. Average annual recharge for the study period (December 1995 to September 1997) was estimated to be 17.7 inches (in.) ( 151 cubic feet per second (ft 3 /s)). The distribution of recharge was 8.6 in. (74 ft3fs) from precipitation, 2.7 in. (23 ft 3 /s) from subsurface inflow, 3.1 in. (26 ft 3 /s) from irrigation, and 3.3 in. (28 ft 3 /s) from leakage from the Dungeness River. The 8.6 in. of recharge from precipitation is much higher than would be expected in an average year because average annual precipitation during the study period was about 28 in., which is 1.35 times higher than long-term average annual precipitation. The long-term average annual recharge from precipitation was estimated to be 5.4 in. (48 ft 3 /s). Ground water discharges as subsurface flow to saltwater bodies, flow to streams, flow to springs, and as withdrawals from wells. Subsurface flow to saltwater bodies and flow to springs were not estimated in this study. Estimated average annual discharge was 3.2 inches (in.) (27 ft 3 /s)) to the Dungeness River and 4.6 in. (39 ft 3 /s) to other streams in the study area. Gross withdrawals from wells in 1996 were estimated to be 1.0 in. (8.4 ft 3 /s). There was a small but statistically significant increase in nitrate concentrations in ground water from 1980 to 1996. Median concentrations in the primary study area were 0.37 milligrams per liter (mg/L) in 1980 and 0.46 mg/L in 1996. The areal pattern of elevated nitrate concentrations has not changed appreciably during the past 15 years. Elevated concentrations were found in a large area east of the Dungeness River and at scattered locations west of the Dungeness River. About 543,200 pounds of nitrogen are estimated to enter the ground-water system in the primary study area each year. Four sources account for about 85 percent of the nitrogen; residential fertilizers, septic systems, mineralization of soil organic matter, and agricultural fertilizers .. It appears that the four major sources are approximately equivalent in amounts of nitrogen. Concentrations of nitrate in the shallow aquifer were significantly higher under residential areas than under natural grasslands or forests. Median nitrate concentrations were 1.3 mg/L under residential areas, 0.55 mg/L under agricultural areas, and 0.12 mg/L under natural grasslands or forests.

Washington↗
USGS2008

Climatically driven loss of calcium in steppe soil as a sink for atmospheric carbon

During the last several thousand years the semi-arid, cold climate of the Russian steppe formed highly fertile soils rich in organic carbon and calcium (classified as Chernozems in the Russian system). Analysis of archived soil samples collected in Kemannaya Steppe Preserve in 1920, 1947, 1970, and fresh samples collected in 1998 indicated that the native steppe Chernozems, however, lost 17-28 kg m-2 of calcium in the form of carbonates in 1970-1998. Here we demonstrate that the loss of calcium was caused by fundamental shift in the steppe hydrologic balance. Previously unleached soils where precipitation was less than potential evapotranspiration are now being leached due to increased precipitation and, possibly, due to decreased actual evapotranspiration. Because this region receives low levels of acidic deposition, the dissolution of carbonates involves the consumption of atmospheric CO2. Our estimates indicate that this climatically driven terrestrial sink of atmospheric CO2 is ???2.1-7.4 g C m-2 a-1. In addition to the net sink of atmospheric carbon, leaching of pedogenic carbonates significantly amplified seasonal amplitude of CO2 exchange between atmosphere and steppe soil. Copyright 2008 by the American Geophysical Union.

Global Biogeochemical Cycles↗
USGS2019

Ecological effects of establishing a 40-year oasis protection system in a Northwestern China Desert

Aims: Desertification around oasis areas is a serious problem in semi-arid and arid regions, which is expected to continue into the future due to a rapidly increasing human population. Oasis protection systems are created to reverse desertification by recovering degraded soil and vegetation properties and improving ecosystem services. Most research has focused on the short-term effects of a single restoration practice using an individual vegetation or soil metric, while more complete assessments along a gradient of an entire oasis protection system have seldom been studied. In this study, soil and vegetation properties were measured along a 40-year oasis-protection system to assess the effectiveness of increasing land protection belts along a gradient from prohibiting grazing, to fencing shrubland, to establishing shrub- and tree-plantations in northwestern China. Methods: Three sites in each belt of the oasis-protection system were selected to measure soil texture, soil organic carbon, total nitrogen, total phosphorus, pH, electrical conductivity, and eight ions under the plant canopy and in the inter-canopy area, as well as to investigate plant community composition, diversity and productivity. Wind velocity and sand transportation rates were measured in each of the five belts during storm events. Results: Compared with shifting dunes in unprotected desert settings, the wind velocity and sand transportation rate decreased by 75 and 98%, respectively, when spring storms passed through the most protected plantation belts in the oasis system. Soil organic carbon, total nitrogen, total phosphorus significantly increased, along with the silt and clay contents, across the protection gradient, and reached their highest levels at the shrub- and tree-plantation belts. Similarly, the density, cover, and biomass of herbaceous plants also increased along the gradient. Despite these positive effects, there was a significant increase in soil salinity, sodicity, and desiccation at the shrub- and tree-plantation belts, which may negatively affect the future sustainability of the oasis-protection system under a predicted future drier and warming climate. Conclusions Although shrub and tree plantations improve soil fertility and favor the development of the herbaceous plant community, their environmental consequences (i.e., soil salinization and desiccation) need to be evaluated in the context of ecosystem restoration over the long-term, especially in arid regions.

Catena↗
USGS1978

Predictive analyses of ground-water discharges in the Willow Creek Watershed, northeast Nebraska

Ground-water discharge to Willow Creek, which drains a 204 mi 2 watershed in northeast Nebraska was predicted for six combinations of conditions of climate and ground-water development. A digital model of the ground water/surface-water system was stressed with recharge and withdrawal functions determined from a linear reservoir model of the soil zone. The geohydrologic system includes a regional aquifer which containssand and gravel ranging in age from Pliocene through Pleistocene. The regional aquifer is unconfined in the western part of the watershed and confined in the eastern part. The confining layer, or blue clay, in the east is of Pleistocene age and comprises principally saturated eolian silts with very fine sand interbeds overlying a basal lacustrine clay. Where unconfined conditions exist in the regional aquifer, perennial flow of Willow Creek is sustained principally by ground-water discharge. Where confined conditions exist, the low hydraulic conductivity of the blue clay effectively isolates the regional aquifer from Willow Creek. The stream-aquifer models were tested by comparing simulated against measured water levels and streamflow during 1975 and 1976. Agreement between simulated and observed values was obtained by modifying initial estimates of aquifer hydraulic conductivity and specific storage. Future ground-water discharge to Willow Creek (base flow) was simulated by super-imposing different patterns of ground-water withdrawals upon variations from average recharge for a monthly climatic sequence identical with the period 1931 to 1954. Frequency analyses of the simulated monthly base flows showed that they would be less than 12 feet 3 per second at least 50 percent of the time under all but completely undeveloped conditions. Simulated water-level declines in the regional aquifer at the end of the pumping season after a drought period similar to that of the 1930's were less than 5 feet where unconfined conditions exist and were more than 80 feet where confined conditions exist.

Nebraska↗
USGS2019

Geographic attribution of soils using probabilistic modeling of GIS data for forensic search efforts

Examinations of soil traces associated with forensic evidence can be used to narrow potential source area(s) by characterizing features of the trace soil assemblage, some of which are limited to specific regions. Soil characteristics may be used to infer the likelihoods of the soil trace being derived from distinct areas within digital maps, including both maps of discrete classes such as formations on geologic maps and land cover, and continuous geospatial data, such as distance from a point source. Seldom do digital maps precisely represent the observable characteristics in a soil trace. Nevertheless, logical assigned likelihoods based on the correspondence between the mapped characteristics and the observed soil particulate assemblage permit creation of a model of the more probable sources of the soil trace. This approach is applied to a 2003 case in which forensic soil samples derived from digging tools were characterized for investigative leads and to narrow the search area of a clandestine grave. This grave site was located in 2005. The suspect traveled approximately 5,000 km before arrest, so narrowing the prioritized search area for law enforcement would be beneficial. Soil examination and case circumstances were used to assign relative likelihoods within digital maps (GIS or Geographic Information Systems data) of geology, soil mineralogy, plant distributions, power plant locations, and proximity to the known travel path. The product of these individual probability maps generates joint probability models to narrow the recommended search area. The digital model output can be easily overlaid on infrastructure maps to aid law enforcement searches.

Geochemistry, Geophysics, Geosystems↗
USGS2020

Uptake and toxicity of clothianidin to monarch butterflies from milkweed consumption

Recent concern for the adverse effects from neonicotinoid insecticides has centered on risk for insect pollinators in general and bees specifically. However, natural resource managers are also concerned about the risk of neonicotinoids to conservation efforts for the monarch butterfly (Danaus plexippus) and need additional data to help estimate risk for wild monarch butterflies exposed to those insecticides. In the present study, monarch butterfly larvae were exposed in the laboratory to clothianidin via contaminated milkweed plants from hatch until pupation, and the effects upon larval survival, larval growth, pupation success, and adult size were measured. Soils dosed with a granular insecticide product led to mean clothianidin concentrations of 10.8–2,193 ng/g in milkweed leaves and 5.8–58.0 ng/g in larvae. Treatment of soils also led to clothianidin concentrations of 2.6–5.1 ng/g in adult butterflies indicating potential for transfer of systemic insecticides from the soil through plants and larvae to adult butterflies. Estimated LC50s for total mortality (combined mortality of larvae and pupae) and EC50 for larval growth were variable but higher than the majority of concentrations reported in the literature for clothianidin contamination of leaves.

PeerJ↗
USGS2014

Ecological site-based assessments of wind and water erosion: informing accelerated soil erosion management in rangelands

Accelerated soil erosion occurs when anthropogenic processes modify soil, vegetation or climatic conditions causing erosion rates at a location to exceed their natural variability. Identifying where and when accelerated erosion occurs is a critical first step toward its effective management. Here we explore how erosion assessments structured in the context of ecological sites (a land classification based on soils, landscape setting and ecological potential) and their vegetation states (plant assemblages that may change due to management) can inform systems for reducing accelerated soil erosion in rangelands. We evaluated aeolian horizontal sediment flux and fluvial sediment erosion rates for five ecological sites in southern New Mexico, USA, using monitoring data and rangeland-specific wind and water erosion models. Across the ecological sites, plots in shrub-encroached and shrub-dominated vegetation states were consistently susceptible to aeolian sediment flux and fluvial sediment erosion. Both processes were found to be highly variable for grassland and grass-succulent states across the ecological sites at the plot scale (0.25 Ha). We identify vegetation thresholds that define cover levels below which rapid (exponential) increases in aeolian sediment flux and fluvial sediment erosion occur across the ecological sites and vegetation states. Aeolian sediment flux and fluvial erosion in the study area can be effectively controlled when bare ground cover is <20% of a site or the cover of canopy interspaces >100 cm in length is less than ~35%. Land use and management activities that alter cover levels such that they cross thresholds, and/or drive vegetation state changes, may increase the susceptibility of areas to erosion. Land use impacts that are constrained within the range of natural variability should not result in accelerated soil erosion. Evaluating land condition against the erosion thresholds identified here will enable identification of areas susceptible to accelerated soil erosion and the development of practical management solutions.

New Mexico↗
USGS2007

Simulation and mapping of soil-water conditions in the Great Plains

Soil-water conditions provide valuable insight into the hydrologic system in an area. A soil-water balance quantitatively summarizes soil-water conditions and is based on climatic, soil, and vegetation characteristics that vary spatially and temporally. Soil-water balances in the Great Plains of the central United States were simulated for 1951-1980. Results of the simulations were mean annual estimates of infiltration, runoff, actual evapotranspiration, potential recharge, and consumptive water and irrigation requirements at 152 climatic data stations. A method was developed using a geographic information system to integrate and map the simulation results on the basis of spatially variable climatic, soil, and vegetation characteristics. As an example, simulated mean annual potential recharge was mapped. Mean annual potential-recharge rates ranged from less than 0.5 inch in much of the north-central and southwestern Great Plains to more than 10 inches in parts of eastern Texas and southwestern Arkansas.

Water Resources Bulletin↗
USGS2021

Groundwater residence time estimates obscured by anthropogenic carbonate

Groundwater is an important source of drinking and irrigation water. Dating groundwater informs its vulnerability to contamination and aids in calibrating flow models. Here, we report measurements of multiple age tracers ( 14 C, 3 H, 39 Ar, and 85 Kr) and parameters relevant to dissolved inorganic carbon (DIC) from 17 wells in California’s San Joaquin Valley (SJV), an agricultural region that is heavily reliant on groundwater. We find evidence for a major mid-20th century shift in groundwater DIC input from mostly closed- to mostly open-system carbonate dissolution, which we suggest is driven by input of anthropogenic carbonate soil amendments. Crucially, enhanced open-system dissolution, in which DIC equilibrates with soil CO 2 , fundamentally affects the initial 14 C activity of recently recharged groundwater. Conventional 14 C dating of deeper SJV groundwater, assuming an open system, substantially overestimates residence time and thereby underestimates susceptibility to modern contamination. Because carbonate soil amendments are ubiquitous, other groundwater-reliant agricultural regions may be similarly affected.

California↗
USGS2009

The influence of use-related, environmental, and managerial factors on soil loss from recreational trails

Recreational uses of unsurfaced trails inevitably result in their degradation, with the type and extent of resource impact influenced by factors such as soil texture, topography, climate, trail design and maintenance, and type and amount of use. Of particular concern, the loss of soil through erosion is generally considered a significant and irreversible form of trail impact. This research investigated the influence of several use-related, environmental, and managerial factors on soil loss on recreational trails and roads at Big South Fork National River and Recreation Area, a unit of the U.S. National Park Service. Regression modeling revealed that trail position, trail slope alignment angle, grade, water drainage, and type of use are significant determinants of soil loss. The introduction of individual and groups of variables into a series of regression models provides improved understanding and insights regarding the relative influence of these variables, informing the selection of more effective trail management actions. Study results suggest that trail erosion can be minimized by avoiding “fall-line” alignments, steep grades, and valley-bottom alignments near streams, installing and maintaining adequate densities of tread drainage features, applying gravel to harden treads, and reducing horse and all-terrain vehicle use or restricting them to more resistant routes. This research also sought to develop a more efficient Variable Cross-Sectional Area method for assessing soil loss on trails. This method permitted incorporation of CSA measures in a representative sampling scheme applied to a large (24%) sample of the park's 526 km trail system. The variety of soil loss measures derived from the Variable CSA method, including extrapolated trail-wide soil loss estimates, permit an objective quantification of soil erosion on recreational trails and roads. Such data support relational analyses to increase understanding of trail degradation, and long-term monitoring of the natural and recreational integrity of the trail system infrastructure.

Journal of Environmental Management↗
USGS1999

Soil water retention and maximum capillary drive from saturation to oven dryness

This paper provides an alternative method to describe the water retention curve over a range of water contents from saturation to oven dryness. It makes two modifications to the standard Brooks and Corey [1964] (B-C) description, one at each end of the suction range. One expression proposed by Rossi and Nimmo [1994] is used in the high-suction range to a zero residual water content. (This Rossi-Nimmo modification to the Brooks-Corey model provides a more realistic description of the retention curve at low water contents.) Near zero suction the second modification eliminates the region where there is a change in suction with no change in water content. Tests on seven soil data sets, using three distinct analytical expressions for the high-, medium-, and low-suction ranges, show that the experimental water retention curves are well fitted by this composite procedure. The high-suction range of saturation contributes little to the maximum capillary drive, defined with a good approximation for a soil water and air system as H cM = ∫ 0 ∞ K rw dh c , where k rw is relative permeability (or conductivity) to water and h c is capillary suction, a positive quantity in unsaturated soils. As a result, the modification suggested to describe the high-suction range does not significantly affect the equivalence between Brooks-Corey (B-C) and van Genuchten [1980] parameters presented earlier. However, the shape of the retention curve near “natural saturation” has a significant impact on the value of the capillary drive. The estimate using the Brooks-Corey power law, extended to zero suction, will exceed that obtained with the new procedure by 25 to 30%. It is not possible to tell which procedure is appropriate. Tests on another data set, for which relative conductivity data are available, support the view of the authors that measurements of a retention curve coupled with a speculative curve of relative permeability as from a capillary model are not sufficient to accurately determine the (maximum) capillary drive. The capillary drive is a dynamic scalar, whereas the retention curve is of a static character. Only measurements of infiltration rates with time can determine the capillary drive with precision for a given soil.

Water Resources Research↗