In intermittent streams, hydrological variation is probably the single-most important factor affecting fish assemblage structure. While the response of aquatic assemblages to seasonal or annual variation in hydrology is well-known, less attention has been devoted to how assemblages respond to natural, intraseasonal drought. To explore this question, I conducted summer surveys of fish and aquatic insect assemblages occupying pool refugia in first to third order, intermittent streams in the Saline river drainage in the Ouachita highlands (central Arkansas, U.S.A.)(2001?2003). The goals of this project were: 1) to relate assemblage variability of fishes and aquatic insects to environmental gradients during summer drying of streams; 2) to characterize the variability of fish assemblages occupying pool refugia, which differed in quality along a spatial gradient; and, 3) to determine the population genetic structure of five fish species across the intermittent landscape. Hydrological variables explained significant variation in assemblage variability for fishes. In contrast, variability in aquatic insect assemblages was related to water quality variables. These patterns are similar to those observed at larger scales of space and time. As pools dried, neither fish species richness nor the slope of the species-area relationship changed. However, the structure of many assemblages was variable over time. Pools with a relatively stable hydrology were sources of reproduction and high population growth, low extinction and high immigration. Sites that exhibited a more variable hydrology (drying completely or nearly-so) were sinks characterized by population declines. The majority of sites had minimal population growth, and intermediate immigration and extinction rates, and were dubbed metapopulations. Immigration and extinction dynamics had important effects on population genetics for common fish species. Two common species had relatively high immigration rates and showed no population differentiation. Populations of three species showed differentiation that was not related to geographic distance among sites. Instead, local extinction of rare haplotypes and evidence of recent bottlenecks suggested that ecological attributes associated with summer drought affected population differentiation. It is clear that retaining the natural hydrology of stream systems contributes to the maintenance of biodiversity, and the conservation of complex demographic processes and genetic patterns.

Headwater streams are extremely vulnerable to the consequences of land-use change as they are tightly coupled with the surrounding landscape. Understanding the natural processes that influence the structure and function of these ecosystems will improve our understanding of how land-use change affects them. Benthic substratum habitat was investigated in a sub-tropical headwater stream by quantifying temporal change to sediment texture of surface sediments (less than 10cm), over four years. Hydrological characteristics were also surveyed in detail, as hydrological regime is a primary determinant of sediment transportation. Additionally, measures of hydro-geological features - hydraulic conductivity and groundwater depth were made in order to explore features of sediment habitat that extend beyond the sediment-water interface. Whilst the typical discharge pattern was one of intermittent base flows and infrequent, yet extreme flood events associated with monsoonal rain patterns, the study period also encompassed a drought and a one in hundred year flood. Rainfall and discharge did not necessarily reflect the actual conditions in the stream. Surface waters were persistent long after discharge ceased. On several occasions the stream bed was completely dry. Shallow groundwater was present at variable depths throughout the study period, being absent only at the height of the drought. The sediments were mainly gravels, sand and clay. Changes in sediment composition were observed for fine particulates (size categories less than 2mm). The grain size change in the finer sediment fractions was marked over time, although bedload movement was limited to a single high discharge event. In response to a low discharge regimen (drought), sediments characteristically showed non-normal distributions and were dominated by finer materials. High-energy discharge regimes (flood) were characterised by coarsening of sands and a diminished clay fraction. Particulate organic matter from sediments showed trends of build-up and decline with the high and low discharge regimes, respectively. Benthic habitats were described according to prevailing hydro-geological parameters. Faunas from sediment substratum samples were associated with identified habitat categories. The fauna reflected the habitat variability in terms of hydrological disturbance of the substratum structure and intermittency of discharge. An applied multivariate procedure was used to correlate temporally changing environmental parameters and faunal abundance data. Faunas were correlated with a group of variables dominated by either discharge variables or sediment textural parameters. Sediment characteristics that affect substratum quality and substratum preference at the micro-scale were investigated via hypotheses testing. A model of carbon loss was used to determine how long particulate organic matter could potentially sustain microbial activity under experimental conditions. An estimate of up to 200 days was determined from this laboratory experiment. Secondly, enriched carbon isotopes were used in a field-based experiment to establish a link between sediments and macrofauna. Enrichment via organic sediments was found for various detritivorous and carnivorous taxa. In the 'third' experiment, artificial treatments were applied to elucidate substratum preference. Fauna was offered the choice of variable quantities of clay and/or quality of organic matter. There were no significant preferences found for the different substratum treatments, although further investigation is needed and a different outcome from this method may be achieved under more benign field conditions than those encountered during this experiment. Finally, the study was set within a context of the primary features of scale. Climate and hydrological features, including linkages with the alluvial aquifer and terrestrial ecosystem, and their potential to change within 'ecological time' are perceived as critical to understanding the role of benthic sediment substratum.

Dynamic expansions and contractions of stream networks can play an important role for hydrologic processes as they can connect different parts of the landscape to the stream channels. However, we know little about the temporal and spatial variations of stream networks during different flow and wetness conditions. This study focuses on the contraction and expansion of stream networks during different flow conditions in the boreal Krycklan catchment, located in Northern Sweden. The stream network and initiation points were extracted from a gridded digital elevation model (DEM) of 5-meter resolution, and then compared with the stream network initiation points (heads) observed during the spring flood (freshet) period in 2012. From the results of the study, it was clearly seen that the observed stream heads and the stream heads appearing in the stream network map extracted from DEM did not agree very well. 49% of the total observed stream heads (49) fell onto the low order stream branches and headwater streams derived from the DEM. Only few of them exactly matched the modeled stream heads. Moreover, the modeled stream network was much denser than the observed stream network, and so the simple raster based dynamic model developed could not well represent the dynamic stream network extension in the real system. Most headwater streams in the study catchment were man-made ditches, which were dug to drain water wetlands and to increase forest productivity. The majority of observed stream heads were formed by seepage from the saturated surrounding soils, while only a few of them were formed by saturation overland flow.  On the other hand, the dynamic stream network derived from the DEM suggested that the number of streams of lower order and their lengths was sensitive to change in streamflow, especially during the high flow episode.

From the Proceedings of the 1971 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 22-23, 1971, Tempe, Arizona / In order to construct a simulation model for ephemeral streamflow and to examine in depth the problem of the worth of data for that model, measurements of the ephemeral streamflow of Rillitto creek, Tucson, were analyzed for the period 1933-1965. The simulation model was based on several hypotheses: (1) flow durations and their succeeding dry periods (time when no flow is present) are independent; (2) the distribution of the lengths of the dry periods and flows is stationary over a certain period of the year (summer); (3) stationary probability distributions for flow durations and for dry period lengths can be derived. A related problem was how to derive a simulation model for the total amount of flow (in acre-ft) within 1 flow period. Three variables were considered: flow duration (minutes), peak intensity of flow (cu ft/sec) and antecedent dry period-minutes (ADP). Because the assumption of variance constancy does not hold, a multiplicative regression model was used. Using an analysis of variance, which is described in detail, the worth of the 3 kinds of data were examined in relation to total flow. It was concluded that there are at least 5 times during the year when the flow intervals differ significantly, and the ADP is not important in determining flow volume because of the poison flow arrival rate in summer. Events occur at random and are not clustered as in summer, indicating that channel moisture does not differ much between flow events.