Autumn Storms Research Articles

The sublimation of falling snow over the Mackenzie River Basin

The sublimation of falling snow may be an important component of the atmospheric water budget of the Mackenzie River Basin and many parts of the Arctic. To investigate this issue, a simple sublimation model is used along with surface precipitation observations and sonde data obtained during the autumn 1994 Beaufort and Arctic Storms Experiment (BASE). Model results are then compared with actual precipitation measurements at Inuvik and Tuktoyaktuk, sites in Northern Canada, to approximate mass loss due to sublimation. The sublimation results are found to vary in concert with cloud base height, precipitation intensity aloft and the nature of the precipitation. Atmospheric conditions are furthermore examined over a wide range of the Arctic, especially the Mackenzie River Basin, to assess to what degree the results can be generalized. The presence of a relatively dry near-surface layer, a favourable environment for sublimation, is a key feature of most sites during the early autumn storm period. Estimates of sublimational mass losses are found over Inuvik and Tuktoyaktuk using sonde derived cloud base heights and temperature and humidity profiles. Sublimation losses for such sites are found to be of the order of 40–60%, which shows that sublimation is indeed a significant process over the Mackenzie Basin and needs to be well handled in climate models. However, increasing the vertical resolution of the sublimation model to that of climate scales can dramatically affect predicted sublimation amounts; how to properly account for sublimation then remains a difficult task.

Flow variability at the continental shelf break of the Mackenzie Shelf in the Beaufort Sea

Observations from four, yearlong current meter deployments made in 1987/1988 near the shelf break in the Canadian Beaufort Sea are analyzed by statistical and spectral methods to characterize the structure and variability (vertical and horizontal) of the velocity field, and to compare measurements obtained during summer (ice‐free) and winter (ice‐covered) conditions. Three main frequency bands are defined: low frequencies (0.01–0.07 cpd), intermediate or synoptic‐scale frequencies (0.07–0.7 cpd), and high frequencies (>0.7 cpd; including tides and inertial currents). Within this division, about 65% of the kinetic energy is in the low‐frequency band, 15% in the intermediate band, and 20% is in the high‐frequency band; the latter is dominated by tidal motions. Low‐frequency variability is well correlated both vertically and horizontally along the shelf for separation distances exceeding 200 km. Variability in this band appears to be related to wind forcing and topographic wave propagation over large horizontal scales. Low‐frequency variability is highest during October and November in association with the occurrence of intense autumn storms, and lowest during periods of ice cover due to wind shielding and increased frictional damping by ice cover. Intermediate‐scale motions have significant correlation in the vertical, but poor correlation in the horizontal. Motions in this frequency band appear to be associated with mesoscale eddies advected by the mean flow. There is also evidence of local generation of intermediate‐frequency current fluctuations by storms in late autumn and, again, an apparent damping during winter periods of ice cover. Finally, high‐frequency motions show significant peaks in diurnal and semidiurnal tidal bands which are well correlated horizontally and vertically. However, other high‐frequency motions in this band show no significant correlation in either the vertical or horizontal direction. There is no clear evidence in the current meter records of coupling between high‐frequency motions and direct atmospheric forcing.

Reactions of phosphorus with sediments in fresh and marine waters

Abstract. The interactions of P with soils and sediments are examined in the context of transport processes from land, through rivers to estuaries and coastal waters. In soil erosion, selective size fractionation and preferential sorption to finer solids is crucial in the transport of P to water courses. Problems in quantifying the sorption affinity and equilibrium phosphate concentration (EPC) of mixtures of different soils and sediments are identified. Riverine transport of P by suspended solids is usually very important and examples of the changes in the amount and composition of particulate P (PP) concentration during storm events are discussed. Increased P content of solids during the first autumn storms, probably reflect the resuspension of accumulated stream bed‐deposits. The fate of P in estuaries and their importance as possible long‐term sinks of P are discussed. The relatively high concentrations of dissolved P associated with riverine inputs are to some extent buffered by the relatively high concentrations of suspended sediments resulting from tidal flows. Phosphorus may be released during transport to the sea due to decreases in the EPC, increases in salinity and release from bottom sediments as a result of low oxygen conditions.

Effects of a beaver pond on runoff processes: comparison of two headwater catchments

Natural variations in concentrations of 18O, D, and H 4SiO 4 in two tributary catchments of Woods Lake in the west-central Adirondack Mountains of New York were measured during 1989–1991 to examine runoff processes and their implications for the neutralization of acidic precipitation by calcium carbonate treatment. The two catchments are similar except that one contained a 1.3 ha beaver pond. Evaporation from the beaver pond caused a seasonal decrease in the slope of the meteoric water line in stream water from the catchment with a beaver pond (WO2). No corresponding change in slope of the meteoric water line was evident in stream water from the other catchment (WO4), nor in ground water nor soil water from either catchment, indicating that evaporative fractionation was not significant. Application of a best-fit sine curve to δ 18O data indicated that base flow in both catchments had a residence time of about 100 days. Ground water from a well finished in thick till had the longest residence time (160 days); soil water from the O-horizon and B-horizon had residence times of 63 and 80 days, respectively. Water previously stored within each catchment (pre-event water) was the predominant component of streamflow during spring snowmelt and during spring and autumn rainfall events, but the proportion of streamflow that consisted of pre-event water differed significantly in the two catchments. The proportion of event water (rain and snowmelt) in WO2 was smaller than at WO4 early in the spring snowmelt of March 13–17, 1990, but the proportions of source water components for the two catchments were almost indistinguishable by the peak flow on the third day of the melt. The event water was further separated into surface-water and subsurface-water components by utilizing measured changes in H 4SiO 4 concentrations in stream water during the snowmelt. Results indicated that subsurface flow was the dominant pathway by which event water reached the stream except during the peak flow of a rain-on-snow event on the last day of the melt. Streamflow from a spring rain storm with dry antecendent conditions two months later (May 16–18, 1990), was less than 5% event water at peak flow in WO2 and 26% in WO4. This change from the runoff pattern in March is attributed to retention of event water in the beaver pond favored by relatively low pre-event storage and isothermal (nonstratified) conditions in the pond that allowed mixing. Streamflow during several autumn storms was about 15–25% event water at peak flow in WO4; the highest values for event water were associated with wet antecedent moisture conditions. These results indicate that a beaver pond can significantly affect the downstream delivery of event water through evaporation and mixing, but provides minimal retention during large runoff events such as snowmelt. Beaver ponds are expected to provide greater opportunity for neutralization of acidic waters during most of the year in catchments treated with calcium carbonate, but little neutralization effect during snowmelt.

Intertidal habitat loss and wildfowl numbers: applications of a spatial depletion model

1. A spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation at Lindisfarne National Nature reserve, north‐east England, was used to investigate the capacity of the site’s beds of Zostera and other intertidal vegetation to support brent geese Branta bernicla hrota and wigeon Anas penelope. 2. Recent total winter counts of brent geese and wigeon were both only 40% of the maximum that the food supply at the site could theoretically support. Other factors must have been restricting their numbers. Earlier arrival of brent geese at the site could increase the number of brent goose‐days which could be supported, but would have only a slight negative effect on the wigeon‐days. 3. The model was used to examine three conservation issues: encroachment of Spartina anglica, sea level rise and loss of food plants from the whole site (which could result from increased autumn storms or plant disease). Loss from the top of the shore through encroachment by Spartina anglica had the greatest effect on the site’s capacity to sustain geese and wigeon. Loss from the bottom of the shore, as would occur through sea level rise, had less impact. Increased loss of vegetation over the whole site would have an intermediate effect. 4. This work has important implications for the management of the site. Factors such as hunting, that may be restricting current numbers below those that could be supported by the food supply, require urgent investigation. Model predictions indicate that encroachment of Spartina is likely to depress local populations of brent geese and wigeon under current conditions only if it results in the complete loss of Zostera from the top 500 m of the shore.

Sources and flowpaths of dissolved organic carbon during storms in two forested watersheds of the Precambrian Shield

Dissolved organic carbon (DOC) concentrations and export were studied in two small catchments in central Ontario to examine DOC sources and to assess the hypothesis that organic matter adjacent to the stream is a significant contributor of DOC during storms. Different DOC dynamics and exports were observed according to the depth of the riparian water table. In Harp 4-21, riparian flowpaths were predominantly through A and upper B soil horizons and riparian soils contributed between 73 and 84% of the stream DOC export during an autumn storm. In Harp 3A, riparian flowpaths were predominantly through lower B horizons. Consequently, riparian soils were less important and hillslopes contributed more than 50% of the stream DOC export in subcatchments without wetlands during storms. Wetlands and adjacent soils contributed significantly to DOC export in Harp 3A; 8% of the total catchment area exported 32 to 46% of the storm runoff DOC. DOC export dynamics in wetlands and riparian soils were distinctly different. In wetlands, transport was affected by leaching and flushing of DOC at the wetland surface leading to lower DOC concentrations with successive storms. In riparian soils, groundwater flowpaths were more important and stronger positive relationships between discharge and DOC concentration were observed. Precipitation, throughfall and stemflow were minor sources of stream DOC during storms and contributed less than 20% of the total export.

Sediment transport in a high mountain catchment in the central Spanish Pyrenees

This paper reports on sediment transport rates from a 0.33 km 2 high mountain catchment in the central Spanish Pyrenees. Sediment transport from three distinct periods are compared: winter, when discharge is very low and regular; the spring snowmelt season, when discharge is high during several weeks, with daily oscillations; and the rest of the year when discharge is controlled by fluctuations in the precipitation regime. It is shown that sediment transport is dominated by the summer and autumn storm events, with the storms of October 1987 accounting for 61% of the bedload transport in seven years. In addition the sediment transport is considered in three components: solutes, suspended sediment load and bedload. In years with a normal pluviometric regime, the ratio between the three modes of sediment transport is between 82 and 94 percent solutes, 5 to 7.5 percent suspended sediments and 1 to 10 per cent bedload. This too varies greatly from year to year, with the October 1987 storms transporting 17 tons of bedload transport whilst the dissolved and suspended load only reached 2,000 and 750 kg respectively.

NORTH-SOUTH SHIFTS OF THE GULF STREAM: OCEAN-ATMOSPHERE INTERACTIONS IN THE NORTH ATLANTIC

Year-to-year changes in the latitude of the north wall of the Gulf Stream are very similar to those seen in the abundances of zooplankton observed by the Continuous Plankton Recorder Survey around the British Isles and also to those in the abundance of zooplankton in Lake Windermere. These connections must reflect changing weather patterns across the North Atlantic. The index of Gulf Stream position was constructed from the north wall data by principal components analysis. The first principal component, the index used, has eigenvector coefficients that all have the same sign, and is a measure of the latitude of the whole of the north wall. However, the component may represent the occurrences of meanders that are extensive in space and time rather than displacements of the Gulf Stream as a whole. This principal component has been used to calculate weighted averages of monthly mean sea-level pressure and of monthly mean numbers of cyclone tracks in order to show the changes in weather patterns associated with displacements of the north wall. Northward displacements of the north wall were accompanied by significantly reduced cyclone numbers in the northernmost regions of the Atlantic (annually and in the autumn) and, in spring, summer, and autumn, a region of reduced atmospheric pressure in the central Atlantic area 40°–60°N, 30°–50°W (locally significant). There was some tendency (not significant) for storm tracks to be deflected around the south side of this region. The pattern in winter is less clear and shows no statistical significance. Changes in the vicinity of the British Isles were generally too small to be statistically significant but were generally consistent with a lower frequency of storms in spring and autumn. As the biological changes appear to be caused by variations in the onset of thermal stratification during the spring they may be the result of relatively small changes in the atmospheric forcing. The atmospheric changes show no indications of the sources of the Gulf Stream displacements, the anomaly winds opposing the displacements. This may be because meanders of the Gulf Stream are not simply related to any single atmospheric variable. The clearest and most statistically significant meteorological signals were all well downstream from the north wall. Although the displacements of the north wall are caused by changing weather patterns over the North Atlantic, the Gulf Stream is also a region of strong heat transfers from the ocean to the atmosphere. Sawyer's criterion indicates that distortions of this heat source could cause noticeable disturbances to the atmospheric circulation over the North Atlantic. A numerical model based on the analytical model of Smagorinsky is used to investigate the perturbations of the zonal circulation that might be caused by displacements of this local heat source. The predictions are in agreement with the changes seen in the central Atlantic during summer, spring, and autumn (but not those during winter). In the region where the model predicts atmospheric pressure reductions should occur, there are no positive correlation coefficients between the position of the north wall and the surface atmospheric pressure but a significant excess of negative correlation coefficients compared with chance, and northward shifts of the Gulf Stream were accompanied by significant reductions in atmospheric pressure. It is therefore possible that displacements of the north wall could influence weather patterns further east. The model predicts that any changes over the European continental shelf will be weak. An accurate description of the dynamics of the Gulf Stream may be an important requirement of coupled ocean–atmosphere models.

Development of Small Mytllus Beds and its Effects on Resident Intertidal Macro fauna

Abstract. The development of temporary Mytilus edulis beds and their associated macro fauna was studied on an intertidal sand flat in the German Wadden sea during 1986 by comparing mussel beds with bare sandy areas of the same tidal elevation. Macro fauna samples We taken with a 100 cm2 corer (penetration depth 27 cm, mesh size 0.5 mm, 5 parallel samples).Numbers of taxa and diversity were higher in the mussel beds compared to the sandy areas. The abundance of macro fauna organisms decreased with the age of the mussel bed, whereas the relative abundance of species was not strongly affected. Hydrobia ulvae, oligochaetes, and Heteromastus filiformis dominated the macro fauna inside as well as outside the mussel beds. Among the lower ranks of dominance changes in species composition were observed due to increasing densities of opportunistic polychaetes.About six weeks after the establishment of the mussel beds a higher abundance of Capitella spp., Polydora ligni, Tharyx marioni, and nemerteans occurred within the mussel beds while other species such as Hydrobia ulvae and Pygospio elegans showed a significantly lower abundance compared to the adjacent flat. Macoma balthica as well as Mya arenaria were not affected by the alteration of a bare sand flat to a mussel bed. O‐group Carcinus maenas reached their highest abundance in the mussel beds (over 1000 indiv. m‐2). In contrast juvenile Crangon crangon preferred the bare sand flat. After destruction of the mussel beds by an early autumn storm, species abundance and composition was similar to the situation before the mussel beds had become established.

Seasonal variation in the intertidal seagrass Zostera noltii Hornem.: coupling demographic and physiological patterns

The considerable seasonal variation in biomass (2–130 g ash-free dry weight (AFDW) m −2) and cover of intertidal Zostera noltii Hornem. in the Zandkreek estuary (SW Netherlands) was mainly caused by changes in shoot density (1000–23000 m −2) and not in shoot size (shoot weight 1–3.6 mg AFDW, shoot leaf area 0.3–1.5 cm 2). The spring increase in shoot density was realised through continuous monopodial branching of the rhizome, which commenced when light available during low tide increased above 15 E m −2 day −1, in mid April. Branching stopped by the end of July, about 6 weeks before the onset of the annual biomass decline, due to a combination of: (a) self-shading during low tide; (b) high respiratory demand by the expanded rhizome network; (c) the seasonal decline in light availability during late summer. Nutrients were probably not limiting since concentrations in shoots remained high: 3.6% N and 0.6% P of dry weight are seasonal means. During the period of maximal biomass the primary rhizome axes decayed, leaving single shoots on short pieces of rhizome, the former secondary axes. The rapid decline in biomass from mid September onwards could be attributed to grazing by herbivorous migratory waterfowl. It was estimated that brent geese, Branta bernicla (L.), and wigeon, Anas penelope L. (about 200 and 300 birds, respectively, on the bed of 30 ha) together removed 45 g AFDW m −2 month −1, whilst autumn storms were insignificant. As established experimentally, winter survival was by single shoots enclosing an active meristem, and not by rhizome fragments without leaves. Sucrose was the main storage carbohydrate. The rhizome was the main storage organ, with maximal carbohydrate content observed in mid July (190 mg g −1 dry weight, sucrose + starch in glucose weight equivalents), and a gradual decline during autumn and winter. We estimated that the storage carbohydrates could cover 28% of the respiratory needs during winter, which would necessitate a substantial photosynthesis to meet the remaining 72%. From iteratively fitted photosynthesis-light curves we conclude that this intertidal Z. noltii population is high-light adapted compared with permanently submerged seagrasses and freshwater angiosperms: estimates for the light compensation point (LCP) and half-saturation constant ( K m) were comparatively high (July LCP and K m: 98 μE m −2 s −1 and 236 μE m −2 s −1, respectively), maximum photosynthetic rate was high ( P max: 236 μg O 2 g −1 AFDW min −1) and the initial slope of the curve was low (α: 0.63 μg O 2 g −1 AFDW min −1/μE m −2 s −1). Estimated daily oxygen balances confirmed that positive net photosynthesis was largely limited to low tide daylight in this turbid estuary (mean high tide light attenuation coefficient: 2.1 m −1).

Modeling Surface Runoff

The objective of this study was to compare approaches to modeling surface runoff due to summer and autumn storms on a cultivated field. The data consisted of measurements performed every 15 minutes during rainfall-surface runoff events in 1993. A transfer function model was formulated using measured rainfall or rainfall excess as an input and surface runoff as an output. The physical models were based on the kinematic wave approximation of the Saint Venant equations. Surface runoff was assumed to flow first as an overland flow on a level field and second in rills. The results showed that the transfer function model using rainfall excess as an input, and the implicitly solved rill flow model performed the best with respect to the fitness coefficients, which denoted the efficiency of the model. The testing of the models using fixed parameter combinations indicated that an event based parameter estimation was not applicable in verifying the models to changing conditions.

Seasonal dynamics and spatial distribution of chydorid cladocerans in relation to chironomid larvae in the sandy littoral zone of an oligo-mesotrophic lake

The seasonal variation of the principal macro- and meiobenthic taxa in the sandy littoral zone of the oligomesotrophic Dutch Lake Maarsseveen I was studied during two years. Population peaks of the different taxa were clearly separated in time. In early spring there were chironomid density peaks of Stictochironomus sticticus, in one year followed by a Polypedilum maximum. From June–July chydorid cladocerans dominated, with a peak of Monospilus dispar followed by a peak of Rhynchotalona falcata. In autumn the chironomid Cladotanytarsus mancus became dominant, remaining so throughout the winter. To study the spatial heterogeneity of the major macro- and meiofaunal taxa, samples were taken in a grid of 2.5 × 10 m. Distributions of all but one taxon were significantly different from random, with Morisita indices varying from 1.23 (chironomids) to 2.10 (the chydorid Monospilus dispar). Wind-induced disturbance presumably had strong and species-specific effects on littoral macro- and meiobenthic taxa. Chydorid heterogeneity increased immediately following the first autumn storm of the season; this may be explained by the tendency of these organisms to remain attached to detritus particles. Temporal and spatial correlation coefficients between macro- and meiobenthic taxa were generally low, suggesting that interactions between these groups are weak, with distribution patterns that are independent of each other. Among the meiofaunal taxa, positive spatial and temporal correlation coefficients were found; apparently, the similarity in both seasonal dynamics and spatial distribution is larger among the meiofaunal taxa than between macro- and meiofaunal taxa. Also among most of the chironomid species significant positive temporal correlations were found, except for Stictochironomus sticticus and Polypedilum sp.; these species have similar life-cycles, but were during one year temporally separated by several alternating settlement peaks. In the next year, Polypedilum disappeared completely following an extremely strong Stictochironomus settlement peak.

Submersed macrophytes of two habitats in Lake Constance (Untersee) and their relations to chemical composition of surface and sediment interstitial water

Summary Habitat conditions of two macrophyte stands in Lake Constance with different species composition were investigated to find out determinating factors. The results of surface water analyses hardly showed any differences. Marked differences were found in chemical composition of sediment interstitial water. This was explained by different wind exposition of the two habitats. One is exposed to the main wind direction and can be characterized as erosion shore, the other one is mostly protected against wind and can be characterized as sedimentation shore. In first case, locally produced organic matter is transported away from littoral zone by autumn storms onto the shore or to deeper sites of the lake, where decomposition of organic matter takes place. In the other case, decomposition of locally produced organic matter takes place directly at production site. This results in high concentrations of ammonium and sulfid in sediment interstitial water showing strictly anaerobic conditions, whereas at the wind exposed site only little concentrations of ammonium and no sulfid was detectable. Apart from different resistence against wave action of the macrophyte species, different sediment interstitial water quality is considered to be the reason for the differences in species composition of the two macrophyte habitats.

Streamflow generation on a small agricultural catchment during autumn recharge: II. Stormflow periods

To increase understanding of hydrologic flow paths, two- and three-component tracer models were used to determine stormflow components on a 19.8 ha agricultural catchment in central Pennsylvania. Three events were studied during the 1989 autumn recharge period with the first and largest storm having a 5–10 year return period, 80 mm rainfall. Continuous precipitation and streamflow records were augmented by observations and sampling of spring and seepage discharge along with shallow wells and soil water. During the largest storm event, a two-tracer, three-component model separation using 18O and Si indicated that 42% of total flow was derived from shallow subsurface stormflow, 11% from surface event water, and 47% from deep subsurface groundwater flow. Similar analysis using Si and NO 3 for a 34 mm rainfall event showed 17% of total flow to be from shallow subsurface flow, 10% from surface event water and 73% from deep subsurface groundwater flow. Based upon analysis of tracer concentration variations in soil water, shallow well water and springflow during these events, shallow subsurface stormflow was found to be a transient mixture of infiltrated event water (28% average maximum contribution) and stored pre-event water (72% average minimum contribution). Thus, two major pathways for transferring event water to streamflow existed in these two events: (1) overland flow and channel precipitation (10–11% of total flow); (2) shallow subsurface stormflow (5–12% of total flow). In the third event analyzed, 9 mm of rainfall was insufficient to generate shallow subsurface stormflow, and two-component models using 18O, Si and NO 3 indicated 10%, 4% and 3% event water contributions, respectively. Major early autumn storms, occurring before subsurface water becomes well mixed in this region, offer an opportunity to differentiate shallow and deep subsurface pathways for streamflow generation using two-tracer, three-component models.

Spatial and temporal variation of acoustic backscatter in the STRESS experiment

Acoustic backscatter measurements were made of the seabed with a bottom mounted, circularly scanning sonar. The placement was at 91 m depth, mid-shelf of Northern California (38° 34′N), site C3 of the experiment STRESS I (1988–1989). Our expectation was that sonar images (70 m radius, 12,000 m 2) would provide a means of observing, over a large field of view, changes in the bottom due to storm-induced sediment transport and due to bioturbation. This expectation was supported in part by towed sonar measurements at 35 kHz over a sandy area in the North Sea, where dramatic spatial variation in the level of the backseattered signal was observed during an Autumn storm on scales of a few km with no concomitant change in sediment grain size [ Jackson et al. (1986) The Journal of the Acoustical Society of America, 80, 1188–1199]. It appeared possible that storm-driven sediment transport might have been responsible for this patchiness, by altering bottom roughness and by redeposition of suspended material. At the California site, a conventional sonar processing of our data from the STRESS experiment reveals no such dramatic change in backscattered signal level due to storms. The sonar images contain random structures whose time evolution is subtle and difficult to interpret. A much clearer picture of temporal and spatial variations emerges from a processing scheme involving cross-correlation of time-separated acoustic views of the bottom. In effect, the sequence of correlation data images produces a movie in which patches of activity are seen to develop as functions of time. It appears that most of this activity is biological rather than hydrodynamic. A tentative explanation is two-fold. The bottom shear stress might have been considerably greater at the North Sea site (with depth only one-half of the California site). The seafloor at the California site was silty-clayey, and backscatter from such floor is less sensitive to the water-floor interface shape and roughness than it would be to the same parameters of a sandy bottom.

Varve Deposition and the Sediment Yield Record at Three Small Lakes of the Southern Canadian Cordillera

Lacustrine sediments deposited in three small glacier-fed lakes of the southern Canadian Cordillera are derived primarily from subglacial erosion and delivered via short proglacial streams or by direct melting and calving of cirque glaciers. Sediment transport and deposition during early summer is controlled by runoff-generated bottom currents and in the late summer through winter by settling from suspension. This forms distinct rhythmic laminations of silt and clay in distal lake areas. Cesium-137 content in all three lakes indicates that these are varve sediments. Time series of varve thickness covering the interval 1863 to present show distinct declines in sediment yield from 310 to less than 150 t km[sup [minus]2] a[sup [minus]1]. The decline is related to sediment exhaustion following glacier retreat from Little Ice Age maxima and the opening of intervening sediment storage sites. Annual varve thickness is significantly related to fluctuations in summer or late summer temperature highlighting the importance of ice ablation, melt-water runoff, and subglacial sediment sources in controlling deposition rates. Singular climate events, such as autumn storms provide distinctive sedimentary signatures in the varve record. Reconstructed sediment yield for the Little Ice Age is as much as 100% greater than the average Holocene rate. 39 refs.,more » 8 figs., 2 tabs.« less

Effects on bedload transport of experimental removal of woody debris from a forest gravel‐bed stream

AbstractExperimental removal of woody debris from a small, gravel‐bed stream in a forested area resulted in a four‐fold increase in bedload transport at bankfull discharge. This was caused by increased transportability of sediment previously stored upslope of debris buttresses or in low‐energy hydraulic environments related to debris. Bank erosion delivered additional sediment to the channel, and transport energy was increased by an inferred increase in the component of total boundary shear stress affecting grains on the bed. Increased transport following debris removal in May 1987 continued throughout the entire autumn storm season through late November 1987, indicating persistent adjustment of the stream bed and banks despite marked response to earlier flows as large as bankfull. Stream bed adjustments included development of a semi‐regular sequence of alternate bars and pools, many of which were spaced independently of former pool locations.

Spatially correlated depth changes in the nearshore zone during autumn storms

Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October‐November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30‐min intervals with a range resolution of ±1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross‐correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10–15 cm) and heights (3–5 cm) are comparable to measurements made by divers. Ripple migration rates were 1–6 cm h−1 directed offshore in the direction of the mean shore normal near‐bottom current and increased with increasing mean current speed. Evidence of megaripple‐like features (2–5 cm height; 1.5–3 m length) is found, also migrating in the direction of the mean shore normal near‐bottom drift, at speeds of about 20 cm h−1. Bedload transport rates in the onshore‐offshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large‐scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabe's (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest‐energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h−1. This erosion event was associated with a general shoreward migration of the bar.

Wildfire Impacts on Soil-Erosion and Hydrology in Wet Mediterranean Forest, Portugal

The Agueda Basin, north-central Portugal is comparatively wet (rainfall, 1600-1800 mm/yr) with frequent, relatively large storms in autumn and winter yet the summer drought is sufficiently long and consistent for frequent forest wildfires. This paper discusses wildfire impacts in such a wet Mediterranean environment on soil hydrophobicity, infiltration capacity, overland flow coefficients, soil loss, rainsplash detachment and small-scale ground level changes for Eucalyptus globulus and Pinus pinaster forest: (1) 0-2 years after fire ('new' burn); (2) 3-4 years after fire ('old' burn); and (3) 'mature' (or long unburnt) sites. For 'new' burn sites, rainsplash detachment rates are an order of magnitude and soil losses two orders of magnitude higher than for 'old' burn sites and both are two orders of magnitude higher than for 'mature' sites. Soils are hydrophobic in all three categories of sites, but infiltration capacities are lower at 'new' burn and 'old' burn than at 'mature' sites. Overland flow coefficients on long unburnt sites were low while on burnt sites they were high and tended to be higher for summer and autumn than for winter and spring, implying enhanced hydrophobicity under summer drought conditions, causing decreased infiltration capacity and increased overland flow. The distinctiveness of fire effects on soil erosion and hydrology in this wet Mediterranean environment and implications for post-fire management are discussed.

The export of particulate matter, particulate phosphorus and dissolved phosphorus from two agricultural river basins: Implications on estimating the non-point phosphorus load

The transport of particulate matter (PM), particulate phosphorus (PP) and dissolved phosphorus (DP) in two agricultural river basins was intensively measured during 1 year (October 1986–September 1987). Seasonal and stage-dependent rating relationships between the concentration of particulate inorganic (PIM) and organic matter (POM) and discharge were calculated for the Lyngbygaards river whereas only single rating relationships were established for the Aarhus river, in which a high proportion of the catchment area drains to lakes (48%). The export coefficients for PIM were rather similar for the two catchments (228 and 271 kg ha −1 yr −1) when only including the part of the river basins actively involved in sediment delivery and in the range previously reported for lowland rivers in Canada and U.K. Significant relationships between PIM and POM and the associated phosphorus fractions were calculated. During low-flow periods the P-content of PIM were high (1.5 and 1.2%) decreasing to 0.5% during high flow. On the contrary, the P-content of POM was constantly around 1% except for a period in late summer in the Lyngbygaards river where the P-content decreased to 0.2–0.3% during the first autumn storms following weed cutting. A very high P-enrichment in PM compared with average soil P-content was documented (7–14) indicating the selective delivery and routing of PM in lowland rivers. In both rivers the concentration of dissolved inorganic P (DIP) were inversely related to discharge demonstrating the dilution of P from point sources at increasing flow. However, in the Lyngbygaards river significantly higher concentration levels of DIP were recognized at periods of storm run-off possibly indicating the importance of DIP delivery with surface run-off and P-releases during resuspension of PM from the river bottom. The non-point P-load was nearly equally high (1.15 and 0.90 kg ha −1 yr −1) in the two agricultural basins when omitting the part of the Aarhus river basin draining to the deep Stilling/Solbjerg lake which retains appreciable amounts of P. Equally high non-point P-loads are reported for agricultural basins in Finland and Norway whereas appreciable lower non-point P-loads are normally reported for agricultural areas in Sweden and Denmark.