Non-native shrub species in the genus Tamarix (saltcedar, tamarisk) have colonized hundreds of thousands of hectares of floodplains, reservoir margins, and other wetlands in western North America. Many resource managers seek to reduce saltcedar abundance and control its spread to increase the flow of water in streams that might otherwise be lost to evapotranspiration, to restore native riparian (streamside) vegetation, and to improve wildlife habitat. However, increased water yield might not always occur and has been substantially lower than expected in water salvage experiments, the potential for successful revegetation is variable, and not all wildlife taxa clearly prefer native plant habitats over saltcedar. As a result, there is considerable debate surrounding saltcedar control efforts. We review the literature on saltcedar control, water use, wildlife use, and riparian restoration to provide resource managers, researchers, and policy-makers with a balanced summary of the state of the science. To best ensure that the desired outcomes of removal programs are met, scientists and resource managers should use existing information and methodologies to carefully select and prioritize sites for removal, apply the most appropriate and cost-effective control methods, and then rigorously monitor control efficacy, revegetation success, water yield changes, and wildlife use.

[1] Hydraulic head was overpressured at middepth in a 4.2-m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free-phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2-m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous-phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m À2 d À1 . These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas-flux chambers or tower-mounted sensors.

Water levels in wells screened only below the water table in unconfined aquifers fluctuate in response to atmospheric pressure changes. These fluctuations occur because the materials composing the unsaturated zone resist air movement and have capacity to store air with a change in pressure. Consequently, the translation of any pressure change at land surface is slowed as it moves through the unsaturated zone to the water table, but it reaches the water surface in the well instantaneously. Thus a pressure imbalance is created that results in a water level fluctuation. Barometric effects on water levels in unconfined aquifers can be computed by solution of the differential equation governing the flow of gas in the unsaturated zone subject to the appropriate boundary conditions. Solutions to this equation for two sets of boundary conditions were applied to compute water level response in a well tapping the Ogallala Formation near Lubbock, Texas from simultaneous microbarograph records. One set of computations, based on the step function unit response solution and convolution, resulted in a very good match between computed and measured water levels. A second set of computations, based on analysis of the amplitude ratios of simultaneous cyclic microbarograph and water level fluctuations, gave inconsistent results in terms of the unsaturated zone pneumatic properties but provided useful insights on the nature of unconfined‐aquifer water level fluctuations.

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Two stable, entirely man-made fluorocarbon gases, trichlorofluoromethane (F-11) and dichlorodifluoromethane (F-12), have been released into the atmosphere during the past 40 years as a consequence of their widespread commercial use. Production data for these compounds are well documented, and atmospheric buildup can be reconstructed on the basis of published release estimates. On-site measurements of F-I 1 and F-12 concentrations in soil air within unconsolidated sedimentary deposits were made using gas chromatography at four sites in the Southern High Plains of Texas. Measurable concentrations of both gases were found at depths as great as 43.9 m. An analytical model and a finitedifference model (both based on molecular-diffusion theory) were used to assess the on-site results. For the analytical model, effects of porosity, tortuosity, gas-liquid partitioning, and sorption were tested as part of a lumped diffusion coe•cient parameter. In the finite-difference model, effects of porosity, tortuosity, and gas-liquid partitioning were treated separately for individual layers, and sorption was treated as a single uniform parameter. Tortuosity values determined using the in situ measurement of F-11 and F-12 concentrations and the subsequent numerical modeling of results agree closely with values calculated using published theoretical and empirical relationships. These results thus confirm that the procedures for estimating tortuosity developed from theoretical considerations or by laboratory techniques are useful for predicting the transport of gases through even very thick and heterogeneous unsaturated zones. Such confirmation has not previously been available. the semiarid Southern High Plains of Texas (Figure 1) were investigated, and detectable concentrations of both gases were found at depths as great as 43.9 m. Soil gas samples were obtained from nests of air piezometers constructed by the U.S. Geological Survey for air permeability studies, as described by Weeks [ 1978]. All measurements of gas composition were made on site using a gas chromatograph.This study was designed to measure F-11 and F-12 concentrations in the interstitial gases of unsaturated soils, to derive effective diffusion coefficients for these gases, and to thereby determine the total retarding effect of tortuosity,

The Lisse effect is a rarely noted phenomenon occurring when infiltration caused by intense rain seals the surface soil layer to airflow, trapping air in the unsaturated zone. Compression of air by the advancing front results in a pressure increase that produces a water-level rise in an observation well screened below the water table that is several times as large as the distance penetrated by the wetting front. The effect is triggered by intense rains and results in a very rapid water-level rise, followed by a recession lasting a few days. The Lisse effect was first noted and explained by Thal Larsen in 1932 from water-level observations obtained in a shallow well in the village of Lisse, Holland. The original explanation does not account for the increased air pressure pushing up on the bottom of the wetting front. Analysis of the effect of this upward pressure indicates that a negative pressure head at the base of the wetting front, psi(f), analogous to that postulated by Green and Ampt (1911) to explain initially rapid infiltration rates into unsaturated soils, is involved in producing the Lisse effect. Analysis of recorded observations of the Lisse effect by Larsen and others indicates that the water-level rise, which typically ranges from 0.10 to 0.55 m, should be only slightly larger than psi(f) and that the depth of penetration of the wetting front is no more than several millimeters.

The vertical permeability to air of layered materials in the unsaturated zone may be determined from air pressure data obtained at depth during a period when air pressure is changing at land surface. Such data may be obtained by monitoring barometric pressure with a microbarograph or surveying altimeter and simultaneously measuring down-hole pneumatic head differences in specially constructed piezometers. These data, coupled with air-filled porosity data from other sources, may be compared with the results of electric-analog or numerical solution of the one-dimensional diffusion equation to make a trial-and-error determination of the air permeability for each layer. The permeabilities to air may in turn be converted to equivalent hydraulic conductivity values if the materials are well drained, are permeable enough that the Klinkenberg effect is small, and are structurally unaffected by wetting. Permeabilities to air determined by this method are representative of relatively large volumes of materials, and the hazard of disturbing the materials during measurement is small. Moreover, the method measures vertical permeability, rather than horizontal permeability or some composite of horizontal and vertical permeability. Hence, the method offers potential advantages over present methods to evaluate sites for artificial recharge by spreading; to evaluate groundwater pollution hazards from feedlots, sanitary landfills, and land irrigated with sewage effluent; and to evaluate sites for temporary storage of gas in the unsaturated zone.

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