Comparison of nitrate attenuation characterization methods at the Uranium mill tailing site in Monument Valley, Arizona

Several methods for characterizing the occurrence and rate of nitrate attenuation were tested at a field site near Monument Valley, Arizona. Spatial and temporal nitrate concentration data collected from a transect of monitoring wells located along the plume centerline were analyzed to evaluate the overall rates of natural attenuation. The occurrence and rate of denitrification was evaluated through microcosm experiments, nitrogen isotopic fractionation analysis, and solute-transport modeling. First-order denitrification-rate coefficients calculated with each method were comparable. In addition, the composite natural attenuation rate coefficient was similar to the denitrification-rate coefficients, which suggests that microbially induced decay primarily controls nitrate attenuation at the site. This research highlights the benefits associated with a multiple-method approach for the characterization of natural attenuation.     

Determination of the Tropospheric Hydroxyl Radical by Liquidphase Scrubbing and HPLC: Preliminary Results

A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×10⁵ radicalscm^-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×10⁶ radicals cm^-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×10⁶ radicals cm^-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit.     

Steady state hodographs in a baroclinic boundary layer

If the steady-state geostrophic wind vector varies exponentially with height in the planetary boundary layer, calculated hodographs of the 24-hour mean wind resemble the classic Ekman spiral distorted by thermal effects. For such an assumed distribution, Lettau's (1967) conclusion that the effects of thermal influence on the steady-state boundary-layer winds can be linearly superimposed on the effects of internal friction is justified.The minimum value of the cross-isobar angle of the surface wind for a given magnitude of the thermal wind vector occurs when that vector points about 345° to the right of the surface geostrophic wind vector and the maximum value occurs when it points at an azimuth of about 120° relative to the surface geostrophic wind vector. The range of values of the cross-isobar angle is almost directly proportional to the magnitude of the thermal wind vector.Hodographs resulting from the assumed variation of the geostrophic wind have approximately the same shape as 24-hour mean hodographs at two locations over the Great Plains.This work is part of a thesis submitted to the University of Wisconsin in partial fulfillment of the requirements for the Ph.D. Degree, written under the supervision of Professor H. Lettau, Department of Meteorology.     

Forecasting in the social and natural sciences: An overview and analysis of isomorphisms

This article identifies and analyzes several points of similarity in the structure and context of forecasting in the social and natural sciences. These include: the limits of identities or universal laws as a basis for forecasts; the corresponding need for simplifying parametric representations of one or more of the variables that enter into identities; various sources of uncertainty about parameterizations; intrinsic limitations on predictability or forecasting accuracy in large-scale systems; the need for sensitivity analyses of model responses to changes in exogenous variables and/or parametric structures; problems of model linkage; and the social (organizational and political) context of forecasts. Suggestions for future lines of inquiry are made in each case. Several of these are such that they can benefit from a sharing of experience and expertise across disciplinary lines.The research reported herein was supported in part by the IC² Institute, The University of Texas at Austin, and in part by National Science Foundation Grant Number SES-8411702. However, the opinions expressed in the paper are those of the authors and do not necessarily reflect those of the sponsoring organizations. We appreciate the advice and comments of Jesse H. Ausubel, Robert S. Chen, Judith Jacobsen, and Richard C. Rockwell on earlier versions of this paper.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Climate Change and Water Resources

Current perspectives on global climate change based on recent reports of the Intergovernmental Panel on Climate Change (IPCC) are presented. Impacts of a greenhouse warming that are likely to affect water planning and evaluation include changes in precipitation and runoff patterns, sea level rise, land use and population shifts following from these effects, and changes in water demands. Irrigation water demands are particularly sensitive to changes in precipitation, temperature, and carbon dioxide levels. Despite recent advances in climate change science, great uncertainty remains as to how and when climate will change and how these changes will affect the supply and demand for water at the river basin and watershed levels, which are of most interest to planners. To place the climate-induced uncertainties in perspective, the influence on the supply and demand for water of non-climate factors such as population, technology, economic conditions, social and political factors, and the values society places on alternative water uses are considered.     

Sediment deposition in the flood plain of Stemple Creek Watershed, northern California

Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the ¹³⁷Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year⁻¹ for the period from 1964 to 2002 with an average of 0.85±0.41 cm year⁻¹. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year⁻¹ and an average of 1.29±1.04 cm year⁻¹. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain.     

Electrical current along balloon rigging line inside thunderstorms

We have designed a new instrument to measure the current flowing along balloon rigging line during flights through thunderstorms. This instrument was tested in a high voltage facility and used to collect line current data during one balloon flight into a thunderstorm. Using these data, worst-case calculations are made; as such, we claim that they are the upper limits of any alteration (to the measured electric field or particle charge) that may occur, and the real number is likely much less. It is postulated the rigging-line current could have two separate effects on the measured electric field: (1) reduction of the field due to emission of corona ions, and (2) enhancement of the field due to the insertion of a long thin ‘conductor.' Even with current as high as 1 μA (the largest measured was around 50–100 nA), these two effects were found to be about −1% and +1%, respectively. Also, the calculated worst-case alteration to charged precipitation measurements is about 0.1 pC. Thus, with proper efforts to make the rigging line as poor a conductor as possible, it seems that we are justified in stating that these effects are negligible.     

Impacts of recent climate change on wheat production systems in Western Australia

The wheatbelt of Western Australia shows a distinct Mediterranean climate with most of the rainfall occurring in the winter months. The main factor limiting plant production in this region is rainfall. Due to clearing of native vegetation, dryland salinity is a major problem in south-west Australia. Since the mid 1970s the region has experienced a significant decrease in winter rainfall. Across nine sites, growing season rainfall (May to October) decreased by an average of 11% and the sum of rainfall in June and July (June + July) decreased by 20%. We used the ASPIM-Nwheat model in combination with historic climate data to study the impact of recent climate change on the hydrology and production of wheat based farming systems by comparing results for before and after 1975. Despite the large decline in rainfall, simulated yields based on the actual weather data did not fall. At the same time, simulated drainage decreased by up to 95% which will significantly reduce the spread of dryland salinity. These results were due to the rainfall changes mainly occurring in June and July, a period when rainfall often exceeds crop demand and large amounts of water are usually lost by deep drainage. The findings will have significant implications for estimates of future climate change impacts in this region with changes in rainfall causing non-proportional impacts on production and hydrological aspects, such as deep drainage and waterlogging, where proportionality is often presumed.