Given a precision threshold to be imposed on the group velocity error and a finite difference scheme for the acoustic wave equation, it is possible to determine time-step and grid-spacing in an optimal manner, i.e., so as to minimize the computational cost. Using this optimal cost as a criterion, it becomes easy to compare schemes for efficiency in homogeneous media. Heterogeneous media with constant density can be accommodated to a certain extent by minimizing the cost over a range of Courant numbers. Such analysis shows that, amongst the second-order Taylor series schemes in time, higher-order schemes are generally more efficient than lower-order schemes. However, this result does not extend to very high order schemes. 相似文献
There is increased interest in the potential of tree planting to help mitigate flooding using nature-based solutions or natural flood management. However, many publications based upon catchment studies conclude that, as flood magnitude increases, benefit from forest cover declines and is insignificant for extreme flood events. These conclusions conflict with estimates of evaporation loss from forest plot observations of gross rainfall, through fall and stem flow. This study explores data from existing studies to assess the magnitudes of evaporation and attempts to identify the meteorological conditions under which they would be supported. This is achieved using rainfall event data collated from publications and data archives from studies undertaken in temperate environments around the world. The meteorological conditions required to drive the observed evaporation losses are explored theoretically using the Penman–Monteith equation. The results of this theoretical analysis are compared with the prevailing meteorological conditions during large and extreme rainfall events in mountainous regions of the United Kingdom to assess the likely significance of wet canopy evaporation loss. The collated dataset showed that event Ewc losses between approximately 2 and 38% of gross rainfall (1.5 to 39.4 mm day−1) have been observed during large rainfall events (up to 118 mm day−1) and that there are few data for extreme events (>150 mm day−1). Event data greater than 150 mm (reported separately) included similarly high percentage evaporation losses. Theoretical estimates of wet-canopy evaporation indicated that, to reproduce the losses towards the high end of these observations, relative humidity and the aerodynamic resistance for vapour transport needed to be lower than approximately 97.5% and 0.5 to 2 s m−1 respectively. Surface meteorological data during large and extreme rainfall events in the United Kingdom suggest that conditions favourable for high wet-canopy evaporation are not uncommon and indicate that significant evaporation losses during large and extreme events are possible but not for all events and not at all locations. Thus the disparity with the results from catchment studies remains. 相似文献
Cold-seep environments and their associated symbiont-bearing megafaunal communities create islands of primary production for macro- and meiofauna in the otherwise monotonous and nutrient-poor deep-sea environment. To examine the spatial variation and distribution patterns of metazoan meiobenthos in different seepage-related habitats, samples were collected in two regions off Norway: several pockmarks associated with the Storegga Slide including the Nyegga pockmark area (730 m; 64°N), and the active, methane-venting Håkon Mosby Mud Volcano (HMMV) west of the Barents Sea (1280 m; 72°N). Based on sediment geochemistry and associated epifauna, three different habitat types were distinguished across the two regions: (1) reduced sediment with suboxic conditions, sometimes covered by bacterial mats, (2) sediment colonised by chemosynthetic, siboglinid tubeworms, and (3) sediment outside the influence of seepage and without a large chemosynthetic fauna. Meiofaunal communities varied strongly in terms of generic diversity and dominance among the different habitat types. Control sites and Siboglinidae polychaete fields both supported high nematode genus richness similar to normal deep-sea sediments, whereas the reduced sediments yielded a genus-poor nematode community dominated by one or two successful species. Meiofaunal densities in the different habitats were negatively correlated with macrobenthic densities. An extremely dense (>11,000 ind. 10 cm–2), mono-specific nematode population appeared to be restricted to the bacterial mats at HMMV. It consisted of a new cryptic species of the Halomonhystera disjuncta complex, which has been described from intertidal habitats in the North Sea. The reduced seep sediments at Nyegga did not yield H. disjuncta but were dominated by Terschellingia longicaudata, another cosmopolitan nematode species known to be abundant in organic-rich, oxygen-poor, shallow-water environments. These observations point to a past or recent connection between margins and shallow-water habitats. 相似文献
The hydro-mechanical behaviour of a clay-based buffer material for nuclear waste disposal has been investigated in a laboratory program. In this program, the main focus was on the influence of confinement on water uptake and swelling pressure during suction decrease. The laboratory program and some of the results are presented by Dueck [Dueck, A., 2006. Laboratory results from hydro-mechanical tests on a water unsaturated bentonite. submitted for publication.].
The results from the laboratory tests were used to find a relationship between water content, void ratio, swelling pressure and suction. Two equations for swelling pressure represent the outline of the model.
In the first equation, the swelling pressure developed during water uptake is normalised by a pressure corresponding to the swelling pressure at saturation. This is done in order to be independent of void ratio. A relationship between the normalised swelling pressure and the degree of saturation is suggested.
The second equation describes a relationship between the swelling pressure, the water content and the actual suction (or relative humidity). The equation is based on a thermodynamic relationship and includes the retention curve (i.e. water content vs. suction under free swelling conditions).
The model can be used for a state where two of the four variables; water content, void ratio, swelling pressure and suction are known and can thus be useful to evaluate field measurements and model late stages of the wetting process. An example of an application is given. The equations are mainly based on results from tests with increasing degrees of saturation under constant void ratio but are also suggested for use with increasing void ratio. 相似文献