CONCURRENT STABILIZATION OF SOME INTERIOR DUNE FIELDS IN MICHIGAN

Inland dunes occur over a large part of east-central lower Michigan, where they mantle glaciolacustrine and outwash surfaces that were exposed around 12,000 yrs. B.P. The dunes are parabolic, with northwest-oriented limbs, and occur in swampy landscapes, suggesting that paleoclimatic conditions at the time of their formation were much drier and possibly windier. In order to determine whether the dunes stabilized concurrently or randomly in time and space, surface soils were studied on 30 dunes in the area and quantitatively analyzed for relative differences. Soils data from the dunes indicate concurrent stabilization, following a period of regional mobilization of aeolian sand. Surface soils have formed by podzolization, in uniform parent materials, and are morphologically similar throughout the area. All the soils are weakly developed, with subtle variations on a A-E-Bs-BC-C horizonation sequence. Munsell colors of Bs horizons are remarkably uniform, with 27 of 30 sites exhibiting values of 4 and chromas of 6. Chemical data suggest that Fe and Al translocation has been uniform throughout the region. When compared with soils of known age in northwest lower Michigan, the data indicate that dunes in the region had stabilized at least by 4000 yrs. B.P., leaving an approximately 8000-yr. interval in which they could have formed. In contrast to the prevailing southwesterly winds of today, dune-forming winds were dominantly from the northwest. [Key words: aeolian sedimentation, dunes, soil, Great Lakes, Michigan.]     

TOPOCLIMATIC CONTROLS IN AN ALPINE FELLFIELD AND THEIR ECOLOGICAL SIGNIFICANCE

The interaction of topography and ambient atmospheric conditions and their combined effect on surface climate are studied in the alpine fellfield on Niwot Ridge, Colorado Front Range. Insolation, net shortwave and all-wave radiation, surface and air temperatures, ground heat flux, wind speed, and evapotranspiration are analyzed in order to characterize moisture and energy exchanges. The spatial pattern of daily energy and moisture fluxes is influenced more by cloud cover than topographic position. The daily radiation load is usually highest on east-facing slopes because clear mornings are commonly followed by cumulus development during midday and early afternoon. In the dominant westerly wind regime, wind speeds on east-facing slopes are lower than elsewhere. Strong insolation produces high rates of evapotranspiration from the thin, sandy/gravelly fellfield soil when moisture is abundant. After a few consecutive precipitation-free days, large radiation loads produce high surface temperatures, especially on east-facing slopes where wind speed and thus the sensible heat flux are small. During the 1985 summer drought period, the impact of hot, dry weather on plant water stress was most pronounced on east-facing slopes. The well-defined distribution of Dryas octopetala on Niwot Ridge, near the southern extent of its circumpolar range, is an example of the control of topoclimate over plant distribution owing to water stress during periods of drought The absence of Dryas from south-facing slopes does not correspond well with present topoclimatic patterns. The present distribution of Dryas may be a consequence of topoclimatic patterns throughout the middle of the last millennium when the summer climate in the Colorado Front Range was hotter and drier than today. Summer droughts then were probably more prolonged, and a combination of clearer days, lower wind speeds, or an increased frequency of northerly wind would have created more intense water stress on south-facing than east-facing slopes. [Key words: Topoclimate, alpine tundra, Dryas octopetala, energy balance.]     

THE EFFECT OF SLOPE-ASPECT ON TURBULENT TRANSFER IN AN ALPINE FELLFIELD: NIWOT RIDGE,FRONT RANGE,COLORADO

This study investigates the impact of local convergence and divergence of air on turbulent transfer at a mid-latitude alpine fellfield site. Pairs of simultaneous wind and temperature profile measurements on a knoll crest and primary slope (aspect =0, 90, 180, and 270°) are analyzed assuming the logarithmic law. Friction velocity (u*) and roughness length (z₀) vary systematically from windward to leeward slopes. General relationships of the form u* = f(u) are derived for conditions of westerly flow with wind speed (u) ranging between 2 and 24 m s^-1. Coefficients in the empirical equations for estimating u* at sites on the knoll from wind speed measurements on the crest vary in a consistent manner with respect to local convergence and divergence. Large differences in z₀ values between windward and leeward sites correspond with contrasts in surface temperature.     

A SYNOPTIC CLIMATOLOGICAL ANALYSIS OF SUMMERTIME PRECIPITATION INTENSITY IN THE EASTERN UNITED STATES

The spatial patterns of precipitation frequency and intensity over the eastern United States for summer from 1961 to 1990 are analyzed using a recently developed continental-scale air mass-based synoptic classification. This procedure, the spatial synoptic classification (SSC), is based on “seed” day identification of synoptic events and discriminant analysis to group together days that are within the same air-mass type. Results show differences in the types of precipitation associated with different air masses. Two air masses in particular-Moist Tropical and Moist Temperate-appear to be highly correlated with a majority of the precipitation, particularly in the southeast. The synoptic characteristics, daily intensity of rainfall, and radiosonde soundings during prevalence of these two air masses suggest that convective rainfall is common during Moist Tropical, and stratiform-type rainfall occurs during Moist Temperate. A simple stratification scheme based upon a synoptic-based air-mass delineation may be useful for studies that need to divide days into stratiform or convective regimes.     

Practice‐oriented estimation of the seismic demand hazard using ground motions at few intensity levels

This paper examines the calculation of the seismic demand hazard in a practice‐oriented manner via the use of seismic response analyses at few intensity levels. The seismic demand hazard is a more robust measure for quantifying seismic performance, when seismic hazard is represented in a probabilistic format, than intensity‐based assessments, which remain prevalent in seismic design codes. It is illustrated that, for a relatively complex bridge–foundation–soil system case study, the seismic demand hazard can be estimated with sufficient accuracy using as little as three intensity measure levels that have exceedance probabilities of 50%, 10% and 2% in 50 years which are already of interest in multi‐objective performance‐based design. Compared with the conventional use of the mean demand from an intensity‐based assessment(s), it is illustrated that, for the same number of seismic response analyses, a practice‐oriented ‘approximate’ seismic demand hazard is a more accurate and precise estimate of the ‘exact’ seismic demand hazard. Direct estimation of the seismic demand hazard also provides information of seismic performance at multiple exceedance rates. Thus, it is advocated that if seismic hazard is considered in a probabilistic format, then seismic performance assessment, and acceptance criteria, should be in terms of the seismic demand hazard and not intensity‐based assessments. Copyright © 2013 John Wiley & Sons, Ltd.     

Increasing temperature and flow management alter mercury dynamics in East Fork Poplar Creek

East Fork Poplar Creek (EFPC) is a mercury (Hg) contaminated creek in east Tennessee, USA. Stream restoration activities included the initiation of a flow management programme in 1996 in which water from a nearby lake was pumped to the head of the creek. We conducted regular water sampling for 2 years along the length of EFPC during active flow management and for 5 years after flow management stopped. Total Hg and total monomethylmercury (MMHg) concentration and flux decreased in the uppermost reaches of EFPC that were closest to the point of water addition. Most water quality parameters, including DOC concentration, remained unchanged after flow management termination. Nevertheless, SUVA₂₅₄, a measure of dissolved organic matter (DOM) composition, increased and coincided with increased dissolved Hg (Hg_D) concentration and flux and decreased Hg solid-water partitioning coefficients throughout EFPC. Higher SUVA₂₅₄ and Hg_D concentration have potential implications for bioavailability and MMHg production. Total and dissolved MMHg concentrations increased in lower reaches of EFPC after the end of flow management and these increases were most pronounced during spring and early summer when biota are more susceptible to exposure and uptake. A general warming trend in the creek after active flow management ended likely acted in concert with higher Hg_D concentration to promote higher MMHg concentration. Total and dissolved MMHg concentrations were positively correlated with water temperature above a threshold value of 10°C. Concentration changes for Hg and MMHg could not be accounted for by changes in creek discharge that accompanied the cessation of flow management. In addition to the changing DOM composition in-stream, other watershed-scale factors likely contributed to the observed patterns, as these changes occurred over months rather than instantaneously after flow management stopped. Nevertheless, similar changes in MMHg have not been observed in a tributary to EFPC.     

Longer term effects of biological control on tamarisk evapotranspiration and carbon dioxide exchange

Biological control of Tamarix spp. (tamarisk) with Diorhabda spp. (tamarisk beetle) was initiated in several states in the Western United States in 2001. We analysed 12 years of evapotranspiration (ET), net ecosystem production (NEP), and beetle abundance data from a tamarisk-invaded site in Western Nevada along the Truckee River. Diorhabda carinulata (northern tamarisk beetle) appeared at the site in 2007. Large beetle outbreaks and associated defoliation of the tamarisk occurred in 2008 and 2009, then the beetle population was highly variable from year to year. Since 2016, the beetle population declined. Growing season ET noticeably declined from direct beetle herbivory in 2008, 2009, and 2010, but the decline in ET was seasonally transient as trees regrew leaves. In 2012 and 2013, total growing season ET was low, likely due to the combined effects of drought and beetle herbivory pressure. Total seasonal ET losses and NEP were primarily driven by annual precipitation with higher values in wetter years and reduced values when precipitation fell below 100 mm. In the last 2 years of the study, 2017–2018, there were few to no beetles observed at the site, and we measured increased tamarisk leaf area index, ET, and NEP. Since 2010 at the study site, no further releases of the beetles have occurred due to wildlife concerns, and subsequent declines in beetle populations where such that the “outbreak” conditions apparently required to impair tamarisk physiological function and significantly reduce ET have not occurred. ET and photosynthesis were highly correlated (r² ≥ .91) to the Landsat-satellite normalized difference vegetation index (NDVI). Using a relationship between growing season ET and NDVI, we estimated ET for five additional tamarisk sites along several southwestern U.S. rivers. In the 2005 to 2018 analysis period, NDVI-estimated ET declined at all sites after beetle arrival with three sites showing a recovery in pre-beetle ET rates in subsequent years. At the other three sites, ET rates have not recovered to pre-beetle levels.     

A borehole investigation of zircon radiation damage annealing

This contribution reports Raman radiation damage measurements of zircons from the Kontinentale Tiefbohrung, on the western border of the Bohemian Massif. The mean wavenumbers (ω₃) and widths (Γ₃) of the ν₃(SiO₄) Raman band are constant down to 3 km, decrease (ω₃) resp. increase (Γ₃) between 3 and 5 km, and are again constant between 5 and 7 km. Uniform high Γ₃ values associated with ω₃ values close to those of undamaged zircon between 5 and 7 km are interpreted as due to residual damage predating the exhumation of the Bohemian Massif. A superimposed post‐exhumation signal indicates full damage retention down to 3 km depth, partial annealing between 3 and 5 km, and zero retention at greater depth. An attempt to calculate radiation damage ages gives results of a meaningful order of magnitude but also exposes difficulties associated with dating basement samples with complex damage accumulation and annealing histories.     

Assessing effects of model complexity and structure on predictions of hydrological responses using serial and parallel model design

By utilizing functional relationships based on observations at plot or field scales, water quality models first compute surface runoff and then use it as the primary governing variable to estimate sediment and nutrient transport. When these models are applied at watershed scales, this serial model structure, coupling a surface runoff sub-model with a water quality sub-model, may be inappropriate because dominant hydrological processes differ among scales. A parallel modeling approach is proposed to evaluate how best to combine dominant hydrological processes for predicting water quality at watershed scales. In the parallel scheme, dominant variables of water quality models are identified based entirely on their statistical significance using time series analysis. Four surface runoff models of different model complexity were assessed using both the serial and parallel approaches to quantify the uncertainty on forcing variables used to predict water quality. The eight alternative model structures were tested against a 25-year high-resolution data set of streamflow, suspended sediment discharge, and phosphorous discharge at weekly time steps. Models using the parallel approach consistently performed better than serial-based models, by having less error in predictions of watershed scale streamflow, sediment and phosphorus, which suggests model structures of water quantity and quality models at watershed scales should be reformulated by incorporating the dominant variables. The implication is that hydrological models should be constructed in a way that avoids stacking one sub-model with one set of scale assumptions onto the front end of another sub-model with a different set of scale assumptions.     

Influences of forest canopy on snowpack accumulation and isotope ratios

The stable water isotopes, ²H and ¹⁸O, can be useful environmental tracers for quantifying snow contributions to streams and aquifers, but characterizing the isotopic signatures of bulk snowpacks is challenging because they can be highly variable across the catchment landscape. In this study, we investigate one major source of isotopic heterogeneity in snowpacks: the influence of canopy cover. We measured amounts and isotopic compositions of bulk snowpack, throughfall, and open precipitation during seven campaigns in mid-winter 2018 along forest-grassland transects at three different elevations (1196, 1297, and 1434 m above sea level) in a pre-Alpine catchment in Switzerland. Snowpack storages under forest canopies were 67 to 93% less than in adjacent open grasslands. On average, the water isotope ratios were higher in the snowpacks under forest canopy than in open grasslands (by 13.4 ‰ in δ²H and 2.3 ‰ in δ¹⁸O). This isotopic difference mirrored the higher isotope values in throughfall compared with open snowfall (by 13.5 ‰ in δ²H and 2.2 ‰ in δ¹⁸O). Although this may suggest that most of the isotopic differences in snowpacks under forests versus in open grasslands were attributable to canopy interception effects, the temporal evolution of snowpack isotope ratios indicated preferential effluxes of lighter isotopes as energy inputs increased and the snowpack ripened and melted. Understanding these effects of forest canopy on bulk snowpack snow water equivalent and isotopic composition are useful when using isotopes to infer snowmelt processes in landscapes with varying forest cover.