Geology topics

Toby L. Welborn

Publications and source records attributed to Toby L. Welborn.

16 recordsLinked to original sources

USGS2022

Predictions and drivers of sub-reach-scale annual streamflow permanence for the upper Missouri River basin: 1989-2018

The presence of year-round surface water in streams (i.e., streamflow permanence) is an important factor for identifying aquatic habitat availability, determining the regulatory status of streams, managing land use change, allocating water resources, and designing scientific studies. However, accurate, high resolution, and dynamic prediction of streamflow permanence that accounts for year-to-year variability at a regional extent is a major gap in modeling capability. Herein, we expand and adapt the U.S. Geological Survey (USGS) PRObability of Streamflow PERmanence (PROSPER) model from its original implementation in the Pacific Northwest (PROSPER PNW ) to the upper Missouri River basin (PROSPER UM ), a geographical region that includes mountain and prairie ecosystems of the northern United States. PROSPER UM is an empirical model used to estimate the probability that a stream channel has year-round flow in response to climatic conditions (monthly and annual) and static physiographic predictor variables of the upstream basin. The structure and approach of PROSPER UM are generally consistent with the PROSPER PNW model but include improved spatial resolution (10 m) and a longer modeling period. Average model accuracy was 81 %. Drainage area, upstream proportion as wetlands, and upstream proportion as developed land cover were the most important predictor variables. The PROSPER UM model identifies decreases in streamflow permanence during climatically drier years, although there is variability in the magnitude across basins highlighting geographically varying sensitivity to drought. Variability in the response of perennial streams to drought conditions among basins in the study area was also observed.

Montana, North Dakota, South Dakota, Wyoming↗

USGS2022

Upper Rio Grande Basin water-resource status and trends: Focus area study review and synthesis

The Upper Rio Grande Basin (URGB) is a critical international water resource under pressure from a myriad of climatic, ecological, infrastructural, water-use, and legal constraints. The objective of this study is to provide a comprehensive assessment of the spatial distribution and temporal trends of selected water-budget components (snow processes, evapotranspiration (ET), streamflow processes, and groundwater storage) using integrated analyses, such as watershed modeling and water availability and use data in the URGB over the past three decades. A spatially distributed snow evolution modeling system simulated snowpack processes over 34 years (1984–2017). It highlighted snow water equivalent declines from -35 to -77 mm/decade with widespread variability across elevation zones and land cover types. Gridded actual ET data from the SSEBop model were developed and tested for the URGB and demonstrated that all land-cover types had significant decreasing trends (1986-2015) ranging from -14 to -80 mm/decade. Conductivity-mass-balance (CMB) hydrograph separation results found that baseflow forms a large component of total streamflow, ranging from 29 to 69% (49% average) of total streamflow at 17 URGB sites upstream of Albuquerque, NM. Three of 4 graphical hydrograph separation methods in the U.S. Geological Survey Groundwater Toolbox were found to be inappropriate for estimating baseflow in the URGB; the most promising method, baseflow index (BFI) Standard, was optimized using CMB data and tested at three URGB sites, with resulting overestimation of 0 to 47%. Simulated changes in groundwater storage were extracted from historical and recent groundwater-flow models of select alluvial basins (San Luis, Española, Middle Rio Grande, and Tularosa-Hueco). In general, decreases in groundwater storage were observed from 1903 to 2013 except for the San Luis alluvial basin (Colorado), where periods of recovery are observed. The PRMS hydrologic model was successfully calibrated for 9 near-native subbasins (Nash-Sutcliffe efficiency 0.47 to 0.85) and parameters translated to the remaining subbasins; compared to simulated near-native flows (with minimal influence of reservoirs or diversions), observed Rio Grande streamgage flows demonstrated reductions of 40% or more for New Mexico and Texas areas of the basin. Significant decreasing trends (1980-2015) in precipitation, snowmelt rate, streamflow, and baseflow were observed at many of the 12 streamgage basins studied, which suggests that the decreasing trends for actual ET may be related to overall decreasing water availability in the basin, with negative implications for agricultural production and groundwater abstraction. Water security concerns arise from our findings of higher fraction precipitation as rain, slower snowmelt rates leading to decreasing streamflow production, and an increasing fraction of baseflow, all of which will affect the timing and magnitude of water available for human needs in the basin.

Upper Rio Grande Basin↗

USGS2021

Hydrogeologic framework and groundwater characterization in selected alluvial basins in the upper Rio Grande basin, Colorado, New Mexico, and Texas, United States, and Chihuahua, Mexico, 1980 to 2015

Increasing demand for the limited water resources of the United States continues to put pressure on resource management agencies to balance the competing needs of ecosystem health with municipal, agricultural, and other uses. To meet these needs, the U.S. Geological Survey conducted a multiyear study to evaluate water resources in the upper Rio Grande Basin in the southwestern United States. The upper Rio Grande Basin extends from south-central Colorado, through New Mexico, into west Texas near Fort Quitman, including parts of Chihuahua, Mexico. The upper Rio Grande Basin consists of a sequence of alluvial basins that formed in the Rio Grande rift approximately 30 million years ago. This report describes the hydrogeology of the upper Rio Grande Basin and how the groundwater resources in the basin have changed from 1980 to 2015. The hydrogeologic framework includes the horizontal delineation of the alluvial basins within the upper Rio Grande Basin from the headwaters in Colorado to Fort Quitman, Texas, including part of Mexico. Groundwater-level measurements from existing State and Federal data were used to construct groundwater-level altitude and groundwater-level change maps. Of the 2,699 wells with groundwater-level data used in this study, 1,055 wells had data for only a single 5-year period, 703 wells had data for 50 percent or more of the 35 years of the study, and only 57 wells have 5-year groundwater-level data for the entire study period. The median decline in water levels in the upper Rio Grande Basin was 0.13 foot (ft) per 5-year period, and declines were measured in 53 percent of the 703 wells that contained data for 50 percent or more of the study period. Rates of groundwater-level decline greater than 1 ft per 5-year period were measured in 17 percent of the wells, greater than 2 ft per 5-year period, in 3 percent of the wells, and greater than 3 ft per 5-year period, in 1 percent of the wells. Overall, groundwater levels rose in 6 percent of the 703 wells that contained data for 50 percent or more of the study period, and in 4 percent of the wells, groundwater levels rose by 1 ft or more per 5-year period. Groundwater-level changes in wells with consecutive 5-year measurement periods exhibited the most variability in the Española, Middle Rio Grande, and Mesilla/Conejos-Médanos alluvial basins. The largest declines in groundwater-level altitudes in individual wells were observed in the Española alluvial basin during 1995–2000, in the Palomas alluvial basin during 2010–2015, and in the Jornada del Muerto alluvial basin during 2005–10. The largest rises in groundwater-level altitudes in individual wells were observed in the Española alluvial basin during 2005–10, in the Middle Rio Grande alluvial basin during 1995–2000, and in the Mesilla/Conejos-Médanos alluvial basin during 1980–85. Changes in groundwater storage throughout the study period varied by alluvial basin, likely based largely on changes in groundwater withdrawals because of increased demands during drier periods and population growth. All alluvial basins except the Tularosa-Hueco alluvial basin were evaluated for changes in groundwater storage from 1980 to 2015. Extremely limited data availability in 2010–15 for the Tularosa-Hueco alluvial basin led to this 5-year period being dropped from the groundwater-level change map and storage analysis for this basin. In the San Luis Valley in southern Colorado, efforts to reverse groundwater depletion in the unconfined aquifer recovered approximately 250,000 acre-feet in storage between late 2013 and early 2018, following the implementation of a “pay-to-pump” groundwater program. However, severe drought that persists in the upper Rio Grande Basin, particularly in southern Colorado, has undone some of the conservation efforts. Within the Española alluvial basin, groundwater storage varied because municipal demand increased the demand on groundwater resources and conservation efforts were implemented. A groundwater-flow model evaluated for the Española alluvial basin indicated declines in groundwater storage from 1947 through 1982. Groundwater storage decreased in the Española alluvial basin in 1980–85, 1985–90, 1990–95, 1995–2000, and 2005–10 and increased in 2000–05 and 2010–15 leading to groundwater storage in 2015 about even with that in 1985. Based on gridded groundwater-level altitudes, groundwater storage decreased in the Middle Rio Grande Basin from 1980 to 2015, except for during the 1980–85, 2000–05, and 2010–15 periods with an overall cumulative storage decrease from 1980 to 2015. Groundwater-flow models evaluated for the Middle Rio Grande alluvial basin showed groundwater storage in the Middle Rio Grande alluvial basin has been reduced since the mid-1950s through the end of the study period except for a brief recovery (reduction in storage outflow) in the mid-1980s. Simulated groundwater storage has also decreased in parts of the Palomas and Mesilla/Conejos-Médanos alluvial basins, and the northern part of the Conejos-Médanos alluvial basin starting in 1995 (excluding 2005 and 2007) and in the Tularosa-Hueco alluvial basin from the early 1940s to the end of the study period. Groundwater storage increased in the Mesilla/Conejos-Médanos alluvial basin during 1980–85 and slightly during 1990–95 and then decreased in the other 5-year periods. Groundwater storage in the Tularosa-Hueco alluvial basin increased from 1985 to 1990, but otherwise decreased, leading to an overall net groundwater-level decline in this part of the basin from 1980 to 2010.

Colorado, New Mexico, Texas, Chihuahua↗

USGS2020

Delineation of flood-inundation areas in Grapevine Canyon near Scotty’s Castle, Death Valley National Park, California

On October 18, 2015, a large flood caused considerable damage in Grapevine Canyon near Death Valley Scotty Historic District, in Death Valley National Park, California. Significant channel changes had limited the applicability of previously created flood-inundation maps to current conditions. Predicted flood-inundation maps for Scotty’s Castle were updated using one-dimensional hydraulic models. A digital terrain model was created for the study area using a terrestrial laser scanner for use in the hydraulic models. Estimations of the 4, 2, 1, 0.5, and 0.2-percent annual exceedance probability flood streamflows (previously known as the 25, 50, 100, 250, and 500-year floods) were computed from regional flood regression equations. The estimated flood streamflows were used with the hydraulic models to compute water surface elevations that were mapped on the digital terrain model. The results indicate inundation of the visitor center and park offices occurs by the 4-percent annual exceedance probability flood. Bridge and embankment overtopping occurs by the 2-percent annual exceedance probability flood. Sections of Grapevine Canyon Road and the parking lot are inundated by the 4-percent annual exceedance probability flood and above streamflows. None of the computed streamflows reach Scotty’s Castle main building.

California↗

USGS2019

Flood-inundation mapping of a steep, gravel desert stream in Death Valley National Park, California

In desert landscapes, flooding can result in dramatic changes to streams. However, the frequency, magnitude, and geomorphic effects of floods in such environments are less understood compared to wetter environments (Tooth, 2000). In desert landscapes, steep slopes and sparse vegetation result in runoff and flashy flood peaks, often lasting for only a few hours. Many floods are the result of isolated, convective thunderstorms that cannot be predicted easily in advance and frequently occur at night. Therefore, direct observations or measurements of streamflow during floods often are limited, with data collection mostly occurring after the event. In ephemeral streams, limited vegetation within channels and on overbanks result in large stream velocity and higher probability for erosion during flooding. Moreover, flood occurrence in desert streams is often highly variable and some sites may go for years without streamflow, complicating flood frequency analyses. Finally, data sets in desert environments are often short and have few long-term, systematic collection sites.

California↗

USGS2018

Characterization of peak streamflows and flood inundation of selected areas in southeastern Texas and southwestern Louisiana from the August and September 2017 flood resulting from Hurricane Harvey

Hurricane Harvey made landfall near Rockport, Texas, on August 25, 2017, as a Category 4 hurricane with wind gusts exceeding 150 miles per hour. As Harvey moved inland, the forward motion of the storm slowed down and produced tremendous rainfall amounts over southeastern Texas, with 8-day rainfall amounts exceeding 60 inches in some locations, which is about 15 inches more than average annual amounts of rainfall for eastern Texas and the Texas coast. Historic flooding occurred in Texas as a result of the widespread, heavy rainfall; wind and flood damages were estimated to be $125 billion, and the storm resulted in at least 68 direct fatalities. In the immediate aftermath of the Harvey-related flood event, the U.S. Geological Survey (USGS) and the Federal Emergency Management Agency initiated a cooperative study to evaluate the magnitude of the flood, determine the probability of occurrence, and map the extent of the flood in Texas. Seventy-four USGS streamflow-gaging stations in Texas with at least 15 years of record and no large data gaps in the period of record had a 2017 annual peak streamflow related to Harvey ranking in the top five of all annual peaks for each given station. New peaks of record streamflow were recorded at 40 of the 74 USGS streamflow-gaging stations. The number of years of peak streamflow record for the 74 analyzed streamflow-gaging stations ranged from 18 to 105, with a mean number of 55 years. The annual exceedance probability estimates for the analyzed streamflow-gaging stations ranged from less than 0.2 to 14.0 percent. USGS field crews surveyed 2,123 high-water marks to obtain water-surface elevations, in feet above the North American Vertical Datum of 1988. In some locations, several water-surface elevations were averaged to obtain 1 water-surface elevation, resulting in 1,258 water-surface elevations. Some of these high-water marks were used, along with peak-stage data from USGS streamflow-gaging stations, to create 19 inundation maps to document the areal extent of the maximum depth of the flooding. Digital datasets of the inundation area, modeling boundary, water-depth rasters, and final map products are available from the USGS data release associated with this report ( https://doi.org/10.5066/F7VH5N3N ).

Arkansas, Louisiana, Texas↗

USGS2016

Groundwater-level change and evaluation of simulated water levels for irrigated areas in Lahontan Valley, Churchill County, west-central Nevada, 1992 to 2012

The acquisition and transfer of water rights to wetland areas of Lahontan Valley, Nevada, has caused concern over the potential effects on shallow aquifer water levels. In 1992, water levels in Lahontan Valley were measured to construct a water-table map of the shallow aquifer prior to the effects of water-right transfers mandated by the Fallon Paiute-Shoshone Tribal Settlement Act of 1990 (Public Law 101-618, 104 Stat. 3289). From 1992 to 2012, approximately 11,810 water-righted acres, or 34,356 acre-feet of water, were acquired and transferred to wetland areas of Lahontan Valley. This report documents changes in water levels measured during the period of water-right transfers and presents an evaluation of five groundwater-flow model scenarios that simulated water-level changes in Lahontan Valley in response to water-right transfers and a reduction in irrigation season length by 50 percent. Water levels measured in 98 wells from 2012 to 2013 were used to construct a water-table map. Water levels in 73 of the 98 wells were compared with water levels measured in 1992 and used to construct a water-level change map. Water-level changes in the 73 wells ranged from -16.2 to 4.1 feet over the 20-year period. Rises in water levels in Lahontan Valley may correspond to annual changes in available irrigation water, increased canal flows after the exceptionally dry and shortened irrigation season of 1992, and the increased conveyance of water rights transferred to Stillwater National Wildlife Refuge. Water-level declines generally occurred near the boundary of irrigated areas and may be associated with groundwater pumping, water-right transfers, and inactive surface-water storage reservoirs. The largest water-level declines were in the area near Carson Lake. Groundwater-level response to water-right transfers was evaluated by comparing simulated and observed water-level changes for periods representing water-right transfers and a shortened irrigation season in areas near Fallon and Stillwater, Nevada. In the Stillwater modeled area, water rights associated with nearly 50 percent of the irrigated land were transferred from 1992 to 1998, represented by the model scenario reduction in groundwater recharge by 50 percent. The scenario resulted in a simulated average decline of 0.6 foot; average observed water-level change for the modeled area was estimated to be 0.0 foot, or no change. In the Fallon modeled area, transfers of water rights associated with 180 acres of land occurred from 1994 to 2008. The transfer is most similar to the scenario for removal of 320 acres of irrigated land. The model scenario resulted in simulated water-level declines of 0.1; water levels measured from 1994 to 2012 indicate no significant trends in water levels, or approximately zero change in water levels, for the Fallon modeled area. The model scenarios included the simulation of a irrigation season shortened by 50 percent, which was determined to have occurred in the 1992 irrigation season in both modeled areas. The shortening of the irrigation season in the Fallon modeled area resulted in simulated water-level declines of 1.1 feet; observed declines were estimated to be 1.3 feet. The Stillwater model simulations resulted in a simulated decline of 1.4 feet, and observed water levels declined an estimated 2.3 feet for the area. The estimated difference between simulated and observed water levels are 0.2 and 0.9 foot for the Fallon and Stillwater modeled areas, respectively. Observed water-level changes were generally within one standard deviation of changes from model simulations, based on the selected periods of comparison. Simulated and observed water-level changes agree well, generally within 1 foot; however, the model scenarios were only approximately similar to the observed conditions, and periods of comparison were generally shorter for the observed periods and included additional cumulative effects of water-right transfers. Climate variability was not considered in the model scenarios.

Nevada↗

USGS2014

Soil, plant, and terrain effects on natural perchlorate distribution in a desert landscape

Perchlorate (ClO 4 − ) is a contaminant that occurs naturally throughout the world, but little is known about its distribution and interactions in terrestrial ecosystems. The objectives of this Amargosa Desert, Nevada study were to determine (i) the local-scale distribution of shallow-soil (0–30 cm) ClO 4 − with respect to shrub proximity (far and near) in three geomorphic settings (shoulder slope, footslope, and valley floor); (ii) the importance of soil, plant, and terrain variables on the hillslope-distribution of shallow-soil and creosote bush [ Larrea tridentata (Sessé & Moc. ex DC.) Coville] ClO 4 − ; and (iii) atmospheric (wet plus dry, including dust) deposition of ClO 4 − in relation to soil and plant reservoirs and cycling. Soil ClO 4 − ranged from 0.3 to 5.0 μg kg −1 . Within settings, valley floor ClO 4 − was 17× less near shrubs due in part to enhanced leaching, whereas shoulder and footslope values were ∼2× greater near shrubs. Hillslope regression models (soil, R 2 = 0.42; leaf, R 2 = 0.74) identified topographic and soil effects on ClO 4 − deposition, transport, and cycling. Selective plant uptake, bioaccumulation, and soil enrichment were evidenced by leaf ClO 4 − concentrations and Cl − /ClO 4 − molar ratios that were ∼8000× greater and 40× less, respectively, than soil values. Atmospheric deposition ClO 4 − flux was 343 mg ha −1 yr −1 , ∼10× that for published southwestern wet-deposition fluxes. Creosote bush canopy ClO 4 − (1310 mg ha−1) was identified as a previously unrecognized but important and active reservoir. Nitrate δ 18 O analyses of atmospheric deposition and soil supported the leaf-cycled–ClO 4 − input hypothesis. This study provides basic data on ClO 4 − distribution and cycling that are pertinent to the assessment of environmental impacts in desert ecosystems and broadly transferable to anthropogenically contaminated systems.

Nevada↗

USGS2014

ARkStorm@Tahoe: Stakeholder perspectives on vulnerabilities and preparedness for an extreme storm event in the greater Lake Tahoe, Reno, and Carson City region

Atmospheric rivers (ARs) are strongly linked to extreme winter precipitation events in the Western U.S., accounting for 80 percent of extreme floods in the Sierra Nevada and surrounding lowlands. In 2010, the U.S. Geological Survey developed the ARkStorm extreme storm scenario for California to quantify risks from extreme winter storms and to allow stakeholders to better explore and mitigate potential impacts. To explore impacts on natural resources and communities in montane and adjacent environments, we downscaled the scenario to the greater Lake Tahoe, Reno and Carson City region of northern Nevada and California. This ArkStorm@Tahoe scenario was presented at six stakeholder meetings, each with a different geographic and subject matter focus. Discussions were facilitated by the ARkStorm@Tahoe team to identify social and ecological vulnerabilities to extreme winter storms, science and information needs, and proactive measures that might minimize impacts from this type of event. Information collected in these meetings was used to develop a tabletop emergency response exercise and set of recommendations for increasing resilience to extreme winter storm events in both Tahoe and the downstream communities of Northern Nevada. Over 300 individuals participated in ARkStorm@Tahoe stakeholder meetings and the emergency response exercise, including representatives from emergency response, natural resource and ecosystem management, health and human services, public utilities, and businesses. Interruption of transportation, communications, and lack of power and backup fuel supplies were identified as the most likely and primary points of failure across multiple sectors and geographies, as these interruptions have cascading effects on natural and human systems by impeding emergency response efforts. Other key issues that arose in discussions included contamination risks to water supplies and aquatic ecosystems, especially in the Tahoe Basin and Pyramid Lake, interagency coordination, credentialing, flood management, and coordination of health and human services during such an event. Mitigation options were identified for each of the key issues. Several science needs were identified, particularly the need for improved flood inundation maps. Finally, key lessons learned were identified and may help to increase preparedness, response and recovery from extreme storms in the future.

Report↗

USGS2013

The distribution and modeling of nitrate transport in the Carson Valley alluvial aquifer, Douglas County, Nevada

Residents of Carson Valley in Douglas County, Nevada, rely on groundwater from an alluvial aquifer for domestic use and agricultural irrigation. Since the 1970s, there has been a rapid increase in population in several parts of the valley that rely on domestic wells for drinking water and septic systems for treatment of household waste. As a result, the density of septic systems in the developed areas is greater than one septic system per 3 acres, and the majority of the domestic wells are shallow (screened within 250 feet of the land surface). The distribution of nitrate as nitrogen (referred to herein as nitrate-N) concentrations in groundwater was determined by collecting more than 200 samples from 8 land-use categories: single family residential, multifamily residential, rural (including land use for agriculture), vacant land, commercial, industrial, utilities, and unclassified. Nitrate-N concentrations ranged from below detection (less than 0.05 milligrams per liter) to 18 milligrams per liter. The results of nitrate-N concentrations that were sampled from three wells equalled or exceeded the maximum contaminant level of 10 milligrams per liter set by the U.S. Environmental Protection Agency. Nitrate-N concentrations in sampled wells showed a positive correlation between elevated nitrate-N concentrations and the percentage of single-family land use and septic-system density. Wells sampled in other land-use categories did not have any correlation to nitrate-N concentrations. In areas with greater than 50-percent single-family land use, nitrate-N concentrations were two times greater than in areas with less than 50 percent single-family land use. Nitrate-N concentrations in groundwater near septic systems that had been used more than 20 years were more than two times greater than in areas where septic systems had been used less than 20 years. Lower nitrate-N concentrations in the areas where septic systems were less than 20 years old probably result from temporary storage of nitrogen leaching from septic systems into the unsaturated zone. In areas where septic systems are abundant, nitrate-N concentrations were predicted to 2059 by using numerical models within the Ruhenstroth and Johnson Lane subdivisions in the Carson Valley. Model results indicated that nitrate-N concentrations will continue to increase and could exceed the maximum contaminant level over extended areas inside and outside the subdivisions. Two modeling scenarios were used to simulate future transport as a result of removal of septic systems (source of nitrate-N contamination) and the termination of domestic pumping of groundwater. The models showed the largest decrease in nitrate-N concentrations when septic systems were removed and wells continued to pump. Nitrate-N concentrations probably will continue to increase in areas that are dependent on septic systems for waste disposal either under current land-use conditions in the valley or with continued growth and change in land use in the valley.

Nevada↗

USGS2011

Phreatophytic land-cover map of the northern and central Great Basin Ecoregion: California, Idaho, Nevada, Utah, Oregon, and Wyoming

Increasing water use and changing climate in the Great Basin of the western United States are likely affecting the distribution of phreatophytic vegetation in the region. Phreatophytic plant communities that depend on groundwater are susceptible to natural and anthropogenic changes to hydrologic flow systems. The purpose of this report is to document the methods used to create the accompanying map that delineates areas of the Great Basin that have the greatest potential to support phreatophytic vegetation. Several data sets were used to develop the data displayed on the map, including Shrub Map (a land-cover data set derived from the Regional Gap Analysis Program) and Gap Analysis Program (GAP) data sets for California and Wyoming. In addition, the analysis used the surface landforms from the U.S. Geological Survey (USGS) Global Ecosystems Mapping Project data to delineate regions of the study area based on topographic relief that are most favorable to support phreatophytic vegetation. Using spatial analysis techniques in a GIS, phreatophytic vegetation classes identified within Shrub Map and GAP were selected and compared to the spatial distribution of selected landforms in the study area to delineate areas of phreatophyte vegetation. Results were compared to more detailed studies conducted in selected areas. A general qualitative description of the data and the limitations of the base data determined that these results provide a regional overview but are not intended for localized studies or as a substitute for detailed field analysis. The map is intended as a decision-support aide for land managers to better understand, anticipate, and respond to ecosystem changes in the Great Basin.

Scientific Investigations Map↗

USGS2009

Hydrogeology of the Lake Tahoe Basin, California and Nevada

Ground water in the Lake Tahoe basin is the primary source of domestic and municipal water supply and an important source of inflow to Lake Tahoe. Over the past 30-40 years, Federal, State, and local agencies, and research institutions have collected hydrologic data to quantify the ground-water resources in the Lake Tahoe basin. These data are dispersed among the various agencies and institutions that collected the data and generally are not available in a format suitable for basin-wide assessments. To successfully and efficiently manage the ground-water resources throughout the Lake Tahoe basin, the U.S. Geological Survey (USGS) in cooperation with the U.S. Forest Service (USFS) compiled and evaluated the pertinent geologic, geophysical, and hydrologic data, and built a geodatabase incorporating the consolidated and standardized data for the Lake Tahoe basin that is relevant for examining the extent and characteristics of the hydrogeologic units that comprise the aquifers.

California, Nevada↗

USGS2008

Hydrologic and Water-Quality Responses in Shallow Ground Water Receiving Stormwater Runoff and Potential Transport of Contaminants to Lake Tahoe, California and Nevada, 2005-07

Clarity of Lake Tahoe, California and Nevada has been decreasing due to inflows of sediment and nutrients associated with stormwater runoff. Detention basins are considered effective best management practices for mitigation of suspended sediment and nutrients associated with runoff, but effects of infiltrated stormwater on shallow ground water are not known. This report documents 2005-07 hydrogeologic conditions in a shallow aquifer and associated interactions between a stormwater-control system with nearby Lake Tahoe. Selected chemical qualities of stormwater, bottom sediment from a stormwater detention basin, ground water, and nearshore lake and interstitial water are characterized and coupled with results of a three-dimensional, finite-difference, mathematical model to evaluate responses of ground-water flow to stormwater-runoff accumulation in the stormwater-control system. The results of the ground-water flow model indicate mean ground-water discharge of 256 acre feet per year, contributing 27 pounds of phosphorus and 765 pounds of nitrogen to Lake Tahoe within the modeled area. Only 0.24 percent of this volume and nutrient load is attributed to stormwater infiltration from the detention basin. Settling of suspended nutrients and sediment, biological assimilation of dissolved nutrients, and sorption and detention of chemicals of potential concern in bottom sediment are the primary stormwater treatments achieved by the detention basins. Mean concentrations of unfiltered nitrogen and phosphorus in inflow stormwater samples compared to outflow samples show that 55 percent of nitrogen and 47 percent of phosphorus are trapped by the detention basin. Organic carbon, cadmium, copper, lead, mercury, nickel, phosphorus, and zinc in the uppermost 0.2 foot of bottom sediment from the detention basin were all at least twice as concentrated compared to sediment collected from 1.5 feet deeper. Similarly, concentrations of 28 polycyclic aromatic hydrocarbon compounds were all less than laboratory reporting limits in the deeper sediment sample, but 15 compounds were detected in the uppermost 0.2 foot of sediment. Published concentrations determined to affect benthic aquatic life also were exceeded for copper, zinc, benz[a]anthracene, phenanthrene, and pyrene in the shallow sediment sample. Isotopic composition of water (oxygen 18/16 and hydrogen 2/1 ratios) for samples of shallow ground water, lakewater, and interstitial water from Lake Tahoe indicate the lake was well mixed with a slight ground-water signature in samples collected near the lakebed. One interstitial sample from 0.8 foot beneath the lakebed was nearly all ground water and concentrations of nitrogen and phosphorus were comparable to concentrations in shallow ground-water samples. However, ammonium represented 65 percent of filtered nitrogen in this interstitial sample, but only 10 percent of the average nitrogen in ground-water samples. Nitrate was less than reporting limits in interstitial water, compared with mean nitrate concentration of 750 micrograms per liter in ground-water samples, indicating either active dissimilative nitrate reduction to ammonium by micro-organisms or hydrolysis of organic nitrogen to ammonium with concomitant nitrate reduction. The other interstitial sample falls along a mixing line between ground water and lake water and most of the nitrogen was organic nitrogen.

Scientific Investigations Report↗

USGS2007

Irrigated Acreage Within the Basin and Range Carbonate-Rock Aquifer System, White Pine County, Nevada, and Adjacent Areas in Nevada and Utah

Accurate delineations of irrigated acreage are needed for the development of water-use estimates and in determining water-budget calculations for the Basin and Range carbonate-rock aquifer system (BARCAS) study. Irrigated acreage is estimated routinely for only a few basins in the study area. Satellite imagery from the Landsat Thematic Mapper and Enhanced Thematic Mapper platforms were used to delineate irrigated acreage on a field-by-field basis for the entire study area. Six hundred and forty-three fields were delineated. The water source, irrigation system, crop type, and field activity for 2005 were identified and verified through field reconnaissance. These data were integrated in a geodatabase and analyzed to develop estimates of irrigated acreage for the 2000, 2002, and 2005 growing seasons by hydrographic area and subbasin. Estimated average annual potential evapotranspiration and average annual precipitation also were estimated for each field.The geodatabase was analyzed to determine the spatial distribution of field locations, the total amount of irrigated acreage by potential irrigation water source, by irrigation system, and by crop type. Irrigated acreage in 2005 totaled nearly 32,000 acres ranging from less than 200 acres in Butte, Cave, Jakes, Long, and Tippett Valleys to 9,300 acres in Snake Valley. Irrigated acreage increased about 20 percent between 2000 and 2005 and increased the most in Snake and White River Valleys. Ground-water supplies as much as 80 percent of irrigation water during dry years. Almost 90 percent of the irrigated acreage was planted with alfalfa.

Data Series↗

USGS2007

Mapping Evapotranspiration Units in the Basin and Range Carbonate-Rock Aquifer System, White Pine County, Nevada, and Adjacent Areas in Nevada and Utah

Accurate estimates of ground-water discharge are crucial in the development of a water budget for the Basin and Range carbonate-rock aquifer system study area. One common method used throughout the southwestern United States is to estimate ground-water discharge from evapotranspiration (ET). ET is a process by which water from the Earth's surface is transferred to the atmosphere. The volume of water lost to the atmosphere by ET can be computed as the product of the ET rate and the acreage of vegetation, open water, and moist soil through which ET occurs. The procedure used in the study groups areas of similar vegetation, water, and soil conditions into different ET units, assigns an average annual ET rate to each unit, and computes annual ET from each ET unit within the outer extent of potential areas of ground-water discharge. Data sets and the procedures used to delineate the ET-unit map used to estimate ground-water discharge from the study area and a qualitative assessment of the accuracy of the map are described in this report.

Scientific Investigations Report↗

USGS2006

Water-Table Levels and Gradients, Nevada, 1947-2004

In 1999, the U.S. Environmental Protection Agency began a program to protect the quality of ground water in areas other than ground-water protection areas. These other sensitive ground water areas (OSGWA) are areas that are not currently, but could eventually be, used as a source of drinking water. The OSGWA program specifically addresses existing wells that are used for underground injection of motor-vehicle waste. To help determine whether a well is in an OSGWA, the Nevada Division of Environmental Protection needs statewide information on depth to water and the water table, which partly control the susceptibility of ground water to contamination and contaminant transport. This report describes a study that used available maps and data to create statewide maps of water-table and depth-to-water contours and surfaces, assessed temporal changes in water-table levels, and characterized water-table gradients in selected areas of Nevada. A literature search of published water-table and depth-to-water contours produced maps of varying detail and scope in 104 reports published from 1948 to 2004. Where multiple maps covered the same area, criteria were used to select the most recent, detailed maps that covered the largest area and had plotted control points. These selection criteria resulted in water-table and depth-to-water contours that are based on data collected from 1947 to 2004 being selected from 39 reports. If not already available digitally, contours and control points were digitized from selected maps, entered into a geographic information system, and combined to make a statewide map of water-table contours. Water-table surfaces were made by using inverse distance weighting to estimate the water table between contours and then gridding the estimates. Depth-to-water surfaces were made by subtracting the water-table altitude from the land-surface altitude. Water-table and depth-to-water surfaces were made for only 21 percent of Nevada because of a lack of information for 49 of 232 basins and for most consolidated-rock hydrogeologic units. Depth to water is commonly less than 50 feet beneath valley floors, 50 to 500 feet beneath alluvial fans, and more than 500 feet in some areas such as north-central and southern Nevada. In areas without water-table information, greasewood and mapped ground-water discharge areas are good indicators of depth to water less than 100 feet. The average difference between measured depth to water and depth to water estimated from surfaces was 90 feet. More recent and detailed information may be needed than that presented in this report to evaluate a specific site. Temporal changes in water-table levels were evaluated for 1,981 wells with 10 or more years between the first depth-to-water measurement and last measurement made since 1990. The greatest increases in depth to water occurred where the first measurement was less than 200 feet, where the time between first and last measurements was 40 years or less, and for wells between 100 and 600 feet deep. These characteristics describe production wells where ground water is fairly shallow in recently developing areas such as the Las Vegas and Reno metropolitan areas. In basins with little pumping, 90 percent of the changes during the past 100 years are within ?20 feet, which is about the natural variation in the water table due to changes in the climate and recharge. Gradients in unconsolidated sediments of the Great Basin are generally steep near mountain fronts, shallow beneath valley floors, and depend on variables such as the horizontal hydraulic conductivity of adjacent consolidated rocks and recharge. Gradients beneath alluvial fans and valley floors at 58 sites were correlated with selected variables to identify those variables that are statistically related. Water-table measurements at three sites were used to characterize the water table between the valley floor and consolidated rock. Water-table gradients beneath alluvial fan

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