Allometric Responses in Coupled Groundwater - Surface Water - Reservoir Systems

Thomas S. Buqo

ABSTRACT
In this research, the concept of allometry (the rates of change of two or more aspects of a system) is applied to two coupled groundwater-surface water-reservoir systems, the Regional Carbonate Aquifer – Muddy Springs – Muddy River - Lake Mead system in southern Nevada, and the Navajo Sandstone – Lake Powell system in southern Utah and northern Arizona. Springs discharging in the Muddy Springs area, in northwestern Clark County, Nevada, provide the source for the Muddy River and habitat for the endangered Moapa Dace. The springs discharge about 33,000 acre feet per year, and have been described as the terminal discharge point for the White River flow system. Discharge at the springs depends upon a number of independent variables including geology, climate and recharge, water withdrawals, water level variations, aquifer confinement, and base level elevation.
Since the mid 1980’s, data set have been collected on the climate, water levels, water withdrawals, and base level. The other independent variables are constant over the geologically short time frame. Observed changes in water levels, precipitation, and groundwater withdrawals can not fully account for observed variations in the discharge at springs in the area. Examination of lake stage conditions at Lake Mead suggests that a decline in stage that began in 2000 is a contributing factor in the decline of spring discharge rates. Simple hydraulic calculations and cross-sectional numerical models of the groundwater regime verify that this type of response is numerically possible.
Because data are lacking on water levels in the Muddy Springs area prior to 1980, water levels in the Navajo Sandstone near Lake Powell were evaluated to examine the effects of reservoir filling. Prior to the construction of the dam, the local hydrologic base level was represented by the top of base flow in the Colorado River. During the impoundment of the Colorado River by the Glen Canyon Dam, the local base level rapidly rose as the reservoir filled. The effect on water levels in the Navajo Sandstone was pronounced, with groundwater level rises of more than 300 feet documented as far as sixteen miles from Lake Powell.
These findings suggest that the Muddy Spring area is not the terminus of the White River flow system. Under pre-lake conditions, a significant quantity of groundwater discharged along the Muddy River and Virgin River bottoms. It is postulated that the change in head that resulted from lake filling, and the corresponding decrease in groundwater discharge along the river bottoms, caused an allometric response through the system that resulted in rises in both water levels in the carbonate aquifer and spring discharge rates in the Muddy Springs area. Analyses are continuing to determine the response of spring discharge to each of the independent variables that form the system to establish the actual impacts of groundwater withdrawals on this complex hydrogeologic regime. Final confirmation may not be possible until the current drought ends and the stage of Lake Mead rises to its 1998 to 1999 levels.

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Updated Estimates of the Distribution of Average Annual Precipitation in Carson Valley, 1971-2000, Douglas County, Nevada, and Alpine County, California

Douglas K. Maurer and Keith J. Halford

ABSTRACT
Rapid growth and development in Carson Valley is causing concern over the continued availability of water resources to sustain such growth into the future. To address concerns over continued growth, the U.S. Geological Survey, in cooperation with Douglas County, began a study to refine estimates of water-budget components in Carson Valley. Precipitation data collected at 14 sites in and near Carson Valley were used to determine observed and adjusted average annual precipitation at the sites for the period 1971–2000. The averages for this period were used to estimate the distribution of annual precipitation in Carson Valley using two methods. The first method applied relations between precipitation and altitude for the western and eastern sides of Carson Valley to digital elevation models of the area. The second method adjusted a precipitation distribution developed using a climatological model called Precipitation-elevation Regressions on Independent Slopes Model (PRISM) to the 1971–2000 average at the 14 sites.
The distribution derived from the precipitation–altitude relations estimates as much as 40 inches per year near the southern part of the crest of the Carson Range and as much as 18 inches per year near the crest of the Pine Nut Mountains. The adjusted PRISM distribution also shows about 40 inches per year near the southern part of the Carson Range and about 15 inches per year near the crest of the Pine Nut Mountains. The total volume of precipitation from the two methods used for Carson Valley are in fairly close agreement; about 270,000 acre-feet per year from the precipitation–altitude relation, and about 250,000 acre-feet per year from the adjusted PRISM distribution. The overall uncertainty for both estimates is about 15 percent, or from 38,000 to 41,000 acre- ft/yr. Total precipitation from the unadjusted PRISM distribution is about 330,000 acre- feet per year; an apparent overestimation.
The volumes of precipitation for areas receiving more than 15 inches per year estimated by the precipitation-altitude relations are more than twice those estimated by the adjusted PRISM method. Similarly, the volumes estimated using the unadjusted PRISM distribution are 2 to almost 5 times those of the other two methods. Applying the Maxey-Eakin method to the unadjusted PRSIM distribution would greatly overestimate ground-water recharge.
Comparison of the total estimated precipitation with previous studies is difficult because of additional acreage included in the calculation of previous estimates. The updated estimates are somewhat less per acre than previous estimates. In contrast to previous estimates, they are based on recent data collected in and near Carson Valley.

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Estimation of Impervious Cover in the Lake Tahoe Basin Using Remote Sensing and Geographic Information Systems Data Integration

Timothy B. Minor and Mary E. Cablk

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The objective of this study was to directly measure hard impervious cover (roads, buildings, parking lots) using high-resolution IKONOS satellite (Space Imaging Inc., Thornton, Colorado) imagery in the Lake Tahoe basin. The research presented attempted to assess the ability of multispectral, high-resolution imagery to derive accurate impervious cover estimates critical for evaluating the impacts to water quality, wildlife, and vegetation. A combination of image processing and GIS data integration techniques were employed to delineate hard impervious cover for the 832 km2 land area of the Lake Tahoe basin. The methods applied produced very accurate delineations of a variety of impervious surfaces in a region dominated by dense conifer canopy. Sub-canopy and sub-shadow surfaces were detectable. Where the canopy cover was found too dense to discern the underlying impervious features, both existing and developed spatial data were added to the image processing results using GIS data integration techniques. The resultant data set produced an overall accuracy of 92%, based on 500 ground truth sample points. For this application, the higher spatial resolution of the image data proved a better operator than spectral resolution. The addition of vector road data greatly improved the accuracy of the hard impervious cover data set. Although the total amount of hard impervious cover found in the basin is less than 4%, of this total amount 76% is found within 3 km of the lakeshore. Results from this analysis will be used to better understand the impacts of impervious surfaces on the ecology of the Lake Tahoe basin.

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Climate and Barometric Pressure Influences on Pederson Spring Discharge and the Carbonate Aquifer near the Muddy Springs, Southern Nevada

Dwight L. Smith, Jeffrey A. Johnson, David J. Donovan, Gavin M. Kistinger and Andrew Burns

ABSTRACT
The Muddy Springs, including Pederson Spring, derive flow from a regional carbonate aquifer in central-southern Nevada. Annual potentiometric water level fluctuations near Muddy Springs range from 0.6 to 1.2 feet, which are attributed predominantly to barometric pressure responses. Computed barometric efficiencies are 0.42 to 0.67 at well MX-4 situated 9 miles west of Muddy Springs, 0.60 at well UMVM-1 situated 5 miles west, 0.50 at well EH-5B located near the southwestern edge of the springs, and decreasing to 0.25 at well EH-4 located 2 miles east of EH-5B and ¼-mile south of Pederson Spring. Pederson Spring barometric efficiency is calculated at 0.065 cfs per foot of barometric pressure change. Since 1998, declining water levels in nearby observation wells and spring discharges are observed, being generally coincident with both a pronounced dry trend in central-southern Nevada and increased production from a nearby municipal well completed in the carbonate aquifer. Declining trends appear to have commenced in 1998, one year prior to the 5-year dry climate trend which began in 1999. These declining trends appear to be more pronounced than preceding climate influences since the mid-1980s, supporting the hypothesis of pumping influences. These observations are less evident in Pederson Spring discharge, as the declining discharge began in 1999, supporting the hypothesis of climate dominated influences on spring discharge, and suggesting a hydraulic discontinuity between the pumping well and spring. Several other lines of evidence suggest that hydraulic discontinuities exist between the up-gradient carbonate wells and Pederson Spring, including: 1.) fault structures cross cutting the region of the springs, 2.) differences in barometric efficiencies up-gradient and down-gradient of fault structures, and 3.) deviations in degrees of interpreted drawdown effects at well EH-5b, and between well EH-4 and Pederson Spring.

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Evaluation of Climate Factors to Forecast Streamflow of the Upper Truckee River

Glenn A. Tootle and Thomas C. Piechota

ABSTRACT
Rapid development in eastern California and the Truckee Meadows (Reno) region of Nevada has resulted in an increased need for water. The primary water supply for the region is the Truckee River, which originates in the Sierra Mountains of California and Nevada and flows east until its terminus into Pyramid Lake. The Upper Truckee River Basin contributes the majority of runoff to the Truckee River. This is primarily due to higher precipitation (snowfall) in the Sierra Mountains and the resulting snowmelt during the spring-summer season. Currently, a spring-summer (April, May, June and July) forecast is provided by the Natural ResourcesConservation Service for the Upper Truckee River station at Farad, California, considered the terminus (lower or downstream boundary) of the Upper Truckee River Basin. Water planners rely on this forecast, as well as other forecast models, to assist with water supply decisions. Cyclonic (frontal) storms that originate in the Pacific Ocean and move eastward account for the majority of wintertime precipitation (snowfall) in the western United States and the resulting spring-summer runoff. Seasonal averages of (a) persistence (previous season’s streamflow); (b) Pacific Ocean climate indices for climate patterns such as the El Niño-Southern Oscillation, the Pacific Decadal Oscillation and the Pacific – North American index; and, (c) Pacific Ocean sea surface temperatures will be examined for long lead-times (3 to 9 months in advance) and correlated with the spring-summer streamflow for the Truckee River at Farad, California. Based on the results of this analysis, the more highly correlated “predictors” will be used in a statistically based streamflow forecast methodology previously applied to watersheds in Australia and the United States.

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