ALBERT

Description: Measurements of evapotranspiration from contrasting stands of Phragmites australis [ET(Reed)] were obtained over several years at three sites within England between 1994 and 1998. A comparison with published ET(Reed) values showed the UK rates to be considerably lower than those measured in continental Europe. From the ET(Reed) data monthly crop coefficients [Kc(Reed)] have been developed, which when used with Reference Crop Evapotranspiration will enable reedbed water use to be determined. Due to the differences in study site characteristics, values for Kc varied widely, and it was not considered appropriate to combine results to produce a single set of monthly mean values. However, suggestions are made regarding the application of the Kc(Reed) values to wetland design. Copyright © 2001 John Wiley and Sons, Ltd.

Description: Consistent relationships between groundwater conditions and topographic properties in small drainage basins would assist the study and modelling of basin hydrology and hydrochemistry. Hydrochemical simulation using topographically based models would benefit in particular from the linkage of groundwater residence times to topographic indices, such as the In(a/ tan β) index of Beven and Kirkby. Hypothesized associations between this index and groundwater characteristics were tested during the spring 2000 snowmelt in two sub-basins in the Turkey Lakes Watershed near Sault Ste Marie, ON. One sub-basin is in mature hardwood forest, and the other was clearcut in the fall of 1997. Piezometric surface elevations were monitored in each sub-basin throughout the melt, and the topographic index value for each piezometer was determined. The δ18O signatures of input water and groundwater were also measured. Mean groundwater residence times were obtained using the exponential system response function, which generally produced good fits to the observed groundwater δ18O time series in both sub-basins. There were significant contrasts in groundwater conditions between the sub-basins. The forested sub-basin exhibited higher and more temporally variable piezometric surface elevations and a greater contrast between δ18O signatures and water residence times in shallow versus deeper groundwater relative to the harvested sub-basin. Patterns of groundwater residence times in the sub-basins were supported by pre-harvest groundwater chemistry; however, data from the two sub-basins largely failed to support the hypothesized relationships between groundwater conditions and topographic index values. Methodological limitations that may have precluded a more rigorous test of the hypotheses are reviewed, and the potential for using groundwater residence times to evaluate the impacts of forest harvesting on basin hydrochemistry is briefly discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Description: In this paper we consider the concept of modelling dynamical systems using numerical-experimental substructuring. This type of modelling is applicable to large or complex systems, where some part of the system is difficult to model numerically. The substructured model is formed via the adaptive minimal control synthesis (MCS) algorithm. The aim of this paper is to demonstrate that substructuring can be carried out in real time, using the MCS algorithm. Thus, we reformulate the MCS algorithm into a substructuring form. We introduce the concepts of a transfer system, and carry out numerical simulations of the substructuring process using a coupled three mass example. These simulations are compared with direct simulations of a three mass system. In addition we consider the stability of the substructuring algorithm, which we discuss in detail for a class of second-order transfer systems. A numerical-experimental system is considered, using a small-scale experimental system, for which the substructuring algorithm is implemented in real time. Finally we discuss these results, with particular reference to the future application of this method to modelling large-scale structures subject to earthquake excitation. Copyright © 2001 John Wiley & Sons, Ltd.

Description: Shaking-table data for a tuned liquid damper with a sloped bottom of 30° with the horizontal are investigated using a non-linear numerical model previously developed by Yu, Jin-kyu, Nonlinear characteristics of tuned liquid dampers. Ph.D. Thesis, Department of Civil Engineering, University of Washington, Seattle, WA, 98195 (1997). Stiffness and damping parameters for this model are obtained and compared with those previously derived for box-shaped tanks. The values for these parameters reflect the softening spring behaviour of the sloped-bottom system in contrast to the hardening system evident for the box-shaped TLD. Consequently, the sloped-bottom tank should be tuned slightly higher than the fundamental structural frequency in order to obtain the most effective damping. © 2001 John Wiley & Sons, Ltd.

Description: Nitrate (NO3) and other nutrients discharged by the Mississippi River are suspected of causing a zone of depleted dissolved oxygen (hypoxic zone) in the Gulf of Mexico each summer. The hypoxic zone may have an adverse affect on aquatic life and commercial fisheries. The amount of NO3 delivered by the Mississippi River to the Gulf of Mexico is well documented, but the relative contributions of different sources of NO3, and the magnitude of subsequent in-stream transformations of NO3, are not well understood. Forty-two water samples collected in 1997 and 1998 at eight stations located either on the Mississippi River or its major tributaries were analysed for NO3, total nitrogen (N), atrazine, chloride concentrations and NO3 stable isotopes (δ15N and δ18O). These data are used to assess the magnitude and nature of in-stream N transformation and to determine if the δ15N and δ18O of NO3 provide information about NO3 sources and transformation processes in a large river system (drainage area ∼ 2 900 000 km2) that would otherwise be unavailable using concentration and discharge data alone. Results from 42 samples indicate that the δ15N and δ18O ratios between sites on the Mississippi River and its tributaries are somewhat distinctive, and vary with season and discharge rate. Of particular interest are two nearly Lagrangian sample sets, in which samples from the Mississippi River at St Francisville, LA, are compared with samples collected from the Ohio River at Grand Chain, II, and the Mississippi River at Thebes, IL. In both Lagrangian sets, mass-balance calculations indicate only a small amount of in-stream N loss. The stable isotope data from the samples suggest that in-stream N assimilation and not denitrification accounts for most of the N loss in the lower Mississippi River during the spring and early summer months.

Description: The event-and physics-based KINEROS2 runoff/erosion model for predicting overland flow generation and sediment production was applied to unpaved mountain roads. Field rainfall simulations conducted in northern Thailand provided independent data for model calibration and validation. Validation shows that KINEROS2 can be parameterized to simulate total discharge, sediment transport and sediment concentration on small-scale road plots, for a range of slopes, during simulated rainfall events. The KINEROS2 model, however, did not accurately predict time-dependent changes in sediment output and concentration. In particular, early flush peaks and the temporal decay in sediment output were not predicted, owing to the inability of KINEROS2 to model removal of a surface sediment layer of finite depth. After 15-20 min, sediment transport declines as the supply of loose superficial material becomes depleted. Modelled erosion response was improved by allowing road erodibility to vary during an event. Changing the model values of erosion detachment parameters in response to changes in surface sediment availability improved model accuracy of predicted sediment transport by 30-40%. A predictive relationship between road erodibility 'states' and road surface sediment depth is presented. This relationship allows implementation of the dynamic erodibility (DE) method to events where pre-storm sediment depth can be estimated (e.g., from traffic usage variables). Copyright © 2001 John Wiley & Sons, Ltd.

Description: Direct, in situ measurements of glacier mass balance are expensive. Remote sensing would be an attractive alternative if remotely observable quantities could be interpreted in terms of mass gain or loss. A system developed recently for the analysis of Radarsat browse images is used here to explore temporal and environmental controls of melting on glaciers on Axel Heiberg Island, Nunavut, Canada. The browse images have an effective spatial resolution of 2 km, are georeferenced to single-pixel accuracy, and number about 200 over the two study years, 1998 and 1999. Surface melting in the accumulation zone is readily recognized by the contrast between bright frozen firn, which exhibits a microwave signal dominated by volume scattering from ice lenses and pipes, and dark wet firn, the signal from which is muted by either absorption or near-specular reflection at the surface. In the ablation zone, radar images, apart from showing the brief spring period of snowmelt, contain little information about the intensity or even the fact of melting. However, in the accumulation zone there is a definite dependence of brightness on temperature: at temperatures above freezing, the hotter it is the darker is the appearance of the glacier. This is most naturally (and encouragingly) interpreted as a relation between brightness and melting rate. Analysis of same-day image pairs shows that, as expected, the terrain slope and the aspect influence the radar brightness strongly and must therefore be allowed for in future modelling of the hydrology of ablation on glaciers. Copyright © 2001 John Wiley & Sons, Ltd.

Description: An algorithm for estimating areal snowmelt infiltration into frozen soils is developed. Frozen soils are grouped into classes according to surface entry condition as: (a) Restricted-water entry is impeded by surface conditions, (b) Limited-capillary flow predominates and water entry is influenced primarily by soil physical properties, and (c) Unlimited-gravity flow predominates and most of the meltwater infiltrates. For Limited soils cumulative infiltration over time is estimated by a parametric equation from surface saturation, initial soil moisture content (water + ice), initial soil temperature and infiltration opportunity time. Total infiltration into Unlimited and Limited soils is constrained by the available water storage capacity. This constraint is also used to determine when Limited soils have thawed. The minimum spatial scale of the infiltration model is established for Limited soils by the variabilities in surface saturation, snow water equivalent, soil infiltrability, soil moisture (water + ice) and depth of soil freezing. Since snowmelt infiltration is influenced by other processes and factors that affect snow ablation, it is assumed that the infiltrability spatial scale should be consistent with the scales used to describe these variables. For open, northern, cold regions the following order in spatial scales is hypothesized: frozen ground ≥ snowmelt ≥ snow water equivalent ≥ frozen soil infiltrability ≥ soil moisture (water + ice) and snow water. For mesoscale application of the infiltration model it is recommended that the infiltrability scale be taken equal to the scale used to describe the areal extent and distribution of the water equivalent of the snowcover that covers frozen ground. Scaling the infiltrability of frozen soils in this manner allows one to exploit established landscape-stratification methodology used to derive snow accumulation means and distribution. Scaling of soil infiltrability at small scales (microscale) is complicated and requires information on the association(s) between the spatial distributions of soil moisture (water + ice) and snow water. A flow chart of the algorithm is presented. Copyright © 2001 John Wiley & Sons, Ltd.