Small-scale Temporal Variation Research Articles

Estimation of pumpage and recharge in alluvial fan topography under multiple irrigation practices

► Estimate small-scale temporal variation in pumpage and recharge on a monthly scale. ► Identify flow amount in a groundwater system under multiple irrigation practices. ► Can estimate illegal pumpage for irrigation or non-irrigation use. ► Recharge sources can be individually identified as rainfall, river, boundary inflow, and irrigation. ► Semi-arid multiple irrigation region shows huge temporal variation in groundwater budget. In this study, we estimate small-scale temporal variations in groundwater pumpage and recharge in alluvial fan topography. We study these variations on a monthly scale using groundwater storage hydrograph and isotope analysis along with area rainfall hydrographs and data of historical irrigation pumpage variations. We consider multiple irrigation practices whose effect was measured only on an annual basis in mostly previous studies. In previous studies, pumpage for non-irrigation or irrigation purposes was regarded as known but illegal pumping was not accounted for. Also, recharge sources include rainfall, rivers, boundary inflow, and groundwater-irrigation, which have not been individually accounted for in previous studies. Pumpage can be estimated from the pumping rate and the number of pumping days. In alluvial fan topography, the actual pumping rate, including that because of illegal wells, can be deduced from the recession slope of the groundwater hydrograph. The annual pumping rate of non-irrigation pumping is assumed to be constant, and can be estimated as the slope of the groundwater hydrograph over consecutive non-rainy days during the non-irrigation periods of the dry season. For cases where there are multiple irrigation practices, the temporal distribution of the irrigation pumping rate can be estimated from the slope of the groundwater hydrograph and also from records provided by the Irrigation Association. In this study, we regard recharge as the difference between inflow and loss. Different types of inflow can be inferred from the storage hydrograph in rainy or non-rainy days, and the loss can then be obtained by using the groundwater balance equation. The isotope mass balance equation is then used to quantify various sources of recharge. In this study, we apply the proposed methodology to the Cho-Shui River alluvial fan, Taiwan. The results show that the proposed methodology can overcome previous restrictions of groundwater hydrograph and isotope analyses to effectively and rationally estimate monthly variations in pumpage and recharge, thus making it possible to research fine flow mechanisms within a multi-layer groundwater system.

Small-scale temporal variation in propagule supply of an intertidal red alga

A. Bellgrove and M.N. Aoki. 2006. Small-scale temporal variation in propagule supply of an intertidal red alga.Phycologia 45: 458–464. DOI: 10.2216/05-53.1Information on the variability in supply of algal propagules is scarce, hindered by the difficulty in identifying propagules, but this variability may affect the distribution and abundance of algal assemblages. This study examined the small-scale (½ hourly to hourly) temporal variation in propagule supply of Chondrus verrucosus (Gigartinaceae, Rhodophyta) over a dense, isolated bed in south-eastern Japan in summer and winter of 1999. Either 0.5 litre scoop samples or 5 litre pump samples were collected ½ hourly to hourly over 13, 22.5, and 30 h on three occasions in summer (June & July) and 32 h on one occasion in winter (December). Sampling was conducted around either the new moon (two occasions in summer) or full moon (one occasion in both summer and winter) and incorporated full tidal sequences including daytime (summer) and nighttime (winter) low-low (LL) tides. Chondrus verrucosus was the only red alga with spores within the size range of 15–20 μm that was fertile in the study area and surrounds at the time of sampling facilitating identification of spores. Spores in scoop samples were settled onto Petri dishes and identified on the basis of cell shape, colour and size. Pump samples were filtered onto transparent membrane filters and identified using epifluorescence microscopy: C. verrucosus spores fluoresced bright yellow and were easily distinguished from other micro-organisms of similar size, which fluoresced red or green. Results showed that while propagules could be found in the water column at most times, propagule supply of C. verrucosus was greatest during the 1–2 h period following LL tides. Variability in propagule supply was less than in previous studies examining surface or offshore waters. Spore release is thought to be stimulated by either desiccation or salinity changes associated with periods of emersion at low tide followed by re-immersion on incoming tides.

Monitoring spatio-temporal variation in molluscan grazing pressure in seasonal, tropical rock pools

In Hong Kong, low-shore rock pools support a patchy mosaic of different successional stages of crustose coralline algae. Echinoid and molluscan grazers are abundant in these low-shore rock pools and spatial and temporal variation in grazing pressure is thought to play an important role in structuring these assemblages. Direct records of graze marks using a modified wax-disc method were used to monitor spatial (different pools) and temporal (periods, 3 or 4 days each; spring vs. neap tides; summer vs. winter) variations in grazing pressure. Radula marks of Chlorostoma argyrostoma, Lunella coronata and Nerita albicilla were common in all pools. Grazing pressure was highly variable with tidal state and season but also within these two factors. Lunella and Nerita exerted highest grazing pressure at neap tides during summer, whilst Chlorostoma exerted highest grazing pressure in winter. Although overall patterns of temporal variation were similar and consistent in all pools, there was between pool variation in grazing pressure and individual species showed different patterns. Within the seasonal and tidal variations, large period-to-period differences were found for all species, illustrating small-scale temporal variation. Fine-scale spatial and temporal patterns of variation in grazing pressure of different species, therefore, contribute to the highly dynamic nature of these low-shore rock pools.

Multiscale Spatial and Small-Scale Temporal Variation in the Composition of Riverine Fish Communities

We studied the multiscale (sites, river reaches and rivers) and short-term temporal (monthly) variability in a freshwater fish assemblage. We found that small-scale spatial variation and short-term temporal variability significantly influenced fish community structure in the Macquarie and Namoi Rivers. However, larger scale spatial differences between rivers were the largest source of variation in the data. The interaction between temporal change and spatial variation in fish community structure, whilst statistically significant, was smaller than the variation between rivers. This suggests that although the fish communities within each river changed between sampling occasions, the underlying differences between rivers were maintained. In contrast, the strongest interaction between temporal and spatial effects occurred at the smallest spatial scale, at the level of individual sites. This means whilst the composition of the fish assemblage at a given site may fluctuate, the magnitude of these changes is unlikely to affect larger scale differences between reaches within rivers or between rivers. These results suggest that sampling at any time within a single season will be sufficient to show spatial differences that occur over large spatial scales, such as comparisons between rivers or between biogeographical regions.

Small-scale temporal variation and the effect of urbanisation on extracellular enzyme activity in streams

The activity of six extracellular enzymes involved in the degradation of dissolved organic carbon compounds was measured in two highly urbanised and two minimally impacted streams east of Melbourne, Australia, using 4-methylumbelliferyl-substrates. Small-scale temporal variation in enzyme activity was determined by repeatedly sampling the same point in the water column, while the effect of flow was determined by sampling in regions of higher and lower flow in both stream types. Replicate samples showed that enzyme activity was not significantly different over small (minutes) time scales. On five of six sampling occasions the enzyme activity was unaffected by flow. On one sampling occasion in a minimally disturbed stream, the difference between the high- and low-flow regions was statistically significant (ANOSIM, Global R= 0.78, P= 0.03). Enzyme activity profiles (activities of the suite of enzymes) of the streams in urbanised catchments were different to those in minimally disturbed catchments. The measurements made in four different streams showed high reproducibility over short time periods (minutes) which lends greater credibility to analogous spatial studies. Although these results determined that small-scale temporal variability was not significant, and that the effects of flow were generally minimal, it is recommended that spatial and temporal variability in the stream be at least considered before any studies measuring extracellular enzyme activity in stream waters are carried out. Such an approach will lead to conclusions from measurements that are not likely to be confounded by variables such as flow rate or time.

Inconsistency in short-term temporal variability of microgastropods within and between two different intertidal habitats

Small-scale temporal variation in abundances of fauna in marine soft sediments has long been recognised. Many studies on rocky intertidal shores have, however, focused on larger fauna in single habitats and have primarily examined relatively long time-scales. The implications of small-scale variability are frequently not adequately addressed in the studies of changes in fauna over longer time-scales. Without knowledge of the magnitude of variation at smaller scales, comparisons across longer time-scales may be confounded. In this study, the temporal variability of a number of co-existing species of microgastropods in patches of two different intertidal habitats (coralline turf and sediment) in Botany Bay, New South Wales, Australia, was measured using a nested, hierarchical sampling design incorporating temporal scales of weeks, 1 and 3 months. In addition to habitats, there were also spatial scales of metres between plots and 100s of metres between the locations. There was generally a lack of consistency in the trends of variance for the three temporal scales at the smallest spatial scale of plots. In addition, the different species, including those that were closely related, showed different patterns of variation, depending on the habitat and site. These data show the importance of incorporating adequate scales of sampling in different habitats when analysing the distribution and abundance of microbenthos in intertidal habitats.

Small-scale temporal variations in biogeochemical features in the Strait of Gibraltar, Mediterranean side—the role of NACW and the interface oscillation

On the Mediterranean side of the Strait of Gibraltar, the distribution of physical, chemical and biological variables (temperature, salinity, nutrients, chlorophyll a, lipids, particles size and plankton abundance) was examined. Sampling was carried out between the surface and 150 m at a fixed station over a 24-h time series. The patterns observed were related to the overlaying of different processes. The Atlantic–Mediterranean interface acts as a strong pycnocline and its vertical oscillation accounts for the gross distribution of nutrients, particles, and living biomass. Injection of North Atlantic Central Water (NACW) into the upper layer occurs at the sill each semidiurnal tidal cycle (every 12 h). As a consequence, in the upper Atlantic layer the NACW was observed every 12 h in the trough of the interface oscillation, whereas Surface Atlantic Water (SAW) dominated in the crest at the fixed station. The initially nutrient-rich NACW was associated with eutrophic signatures such as high chlorophyll, large cells and low turbidity; The nutrient depleted SAW was associated with oligotrophic signatures such as low chlorophyll, small cells and high turbidity. The distribution of lipid biotracers at the depth of the chlorophyll maxima (10–40 m) depicted a similar trend with abundant chloroplast lipids and a low lipolysis index in NACW-enriched waters, and a high lipolysis index and abundant zooplankton tracers in SAW especially at night. During the eastward advection of Atlantic water, the nutrient content of NACW is likely to be assimilated by phytoplankton. A scenario is proposed for explaining changes in phytoplankton maxima composition during the time series observations, taking into account the timing of the NACW injection at the sill, the diurnal cycle and zooplankton grazing. Although more studies over a longer temporal scale are necessary to validate this scenario, our observations show the scale of daily variations in the physical/biological coupling in the Strait and the implications for nutrient and matter exchanges between the Atlantic and Mediterranean.

Trace metal and total suspended solids concentrations in freshwater: the importance of small-scale temporal variation.

The temporal variation in the concentrations of particulate trace metals (Cu, Pb and Zn) and total suspended solids (TSS) was examined in three rivers that drain into the Port Jackson estuary, Australia, using a nested, hierarchical sampling design. Sampling was conducted between March and June 1999, under low flow conditions. The sampling design incorporated four temporal scales (hours, days, weeks and months). It was considered that hours, days, weeks and months were representative of such time scales and could be analyzed as random, nested sources of variation in an analysis of variance (ANOVA). Significant variation was found at temporal scales ranging from hours, within the same day, to months. The amount and scales of variation differed between particulate trace metals and TSS concentrations and between rivers. In many cases, differences between small-scale were as important as differences between months. The results suggest that higher anthropogenic influences cause higher variability at small temporal scale. Results indicate the need for nested sampling designs to be incorporated into studies of temporal variation in order to unconfound small-scale temporal variation. The conclusions of this study are likely to be applicable to other water quality variables and pollutants.

Boundary layer formation in the magnetotail: Geotail observations and comparisons with a global MHD simulation

We present Geotail plasma and field observations from the middle magnetotail near XGSE=−46 RE for the time period 1400 to 1800 UT on December 14, 1994. During that period, the Wind satellite monitored the solar wind plasma and interplanetary magnetic field (IMF) upstream of the bow shock. The IMF was northward and the plasma parameters near average. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the plasma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. Comparison of the simulation results with the observational data show very good overall agreement of the magnitudes of the plasma and field parameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal variations are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which formed by magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotail is located very close to the TFBL and may have entered the TFBL proper. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF conditions.

Small-scale temporal variability in assemblages of larval fishes: implications for sampling

Short-term changes in the assemblages of larval fishes in waters over the inner continental shelf off Sydney (Australia) were examined.This was done to highlight the potential sampling problem of confounding short-term variability with longer-term and spatial variability in plankton communities. Assemblages at the surface and at 30 m depth were sampled twice a day over consecutive days at two locations 5 km apart. The assemblages and the densities of individual taxa varied greatly between sampling times and days. The small-scale temporal changes in densities of larvae were not the same at both locations or between depths. Large and consistent depth-related differences in assemblages and abundances of most taxa were evident The consequences of small-scale temporal variations are discussed relative to the design of sampling programs for ecological and environmental purposes. Studies that include the double sampling of some stations are recommended. Greater emphasis needs to be placed on fine-scale oceanography and larval behaviour to determine causative mechanisms of shortterm changes in assemblages of larval fishes.

Temporal variation in soft-sediment benthos

The existence of small-scale temporal variation in the distribution of the fauna of marine soft sediments has long been recognised. The implications of this variation have rarely been adequately addressed in studies of changes in the fauna over longer time-scales. Without knowledge of the presence or absence of variation at smaller scales, comparisons across longer time-scales must be confounded. Temporal variation in the distribution of the macrofauna of sediments in Botany Bay, New South Wales, Australia, is described using a nested, hierarchical sampling design. Significant variation was found at temporal scales from days to months. Implications of short-term variation for environmental sampling and monitoring, and the means of overcoming associated problems, are discussed. The conclusions of the present study are applicable to studies of other variables, such as pollutants.