ALBERT

Description: The influence of winter on methane (CH4) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow‐pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m−3 caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration‐depth profiles in pore water steepened and concentrations increased by as much as 400 μM at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat‐ice cover plays an important part in CH4 build‐up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH4 in pore water over two winters and one summer showed an average annual amplitude of 1.3 gCH4m−2 (25–75cm depth range), with a winter maximum of 4.7gCH4m−2. Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993–1994) of total annual emission than the winter with below‐average snowfall and warmer air temperature (2% in 1994–1995). Emissions averaged 56 and 26mg m−2 day−1 during the first and second winter (December, January and February), respectively. Copyright © 1995 John Wiley & Sons, Ltd

Description: The influence of trees on the ground thermal regime is important to the overall winter energy exchange in a snow‐covered, forested watershed. In this work, spatial zones around a single conifer tree were defined and examined for their controls on the snow cover, snow‐ground interface temperatures and frozen ground extent. A large white spruce (Picea glauca), approximately 18 m tall with a crown diameter of 7.5 m and located in northern Vermont, was the subject of this study. The tree was instrumented with thermistors to measure the snow‐ground interface temperature between the tree trunk and 6 m from the tree into undisturbed snow. Four distinct zones around the conifer are defined that affect the snow distribution characteristics: adjacent to the trunk; the tree well; the tree crown perimeter; and the unaffected area away from the tree. At the time of peak snow accumulation and during the ablation season, snow depth and density profiles were measured. The area beneath the canopy accumulated 34% of the snow accumulated in the undisturbed zone. By the end of the ablation season, the depth of snow under the canopy had decreased to 18% of the undisturbed snow depth. The tree and branch characteristics of spruce in this temperate climate resulted in a different snow depth profile compared with previous empirical relationships around a single conifer. A new relationship is presented for snow distribution around conifer trees that has the ability to better fit data from a variety of conifer types than previously published relationships. Less snow beneath the canopy led to colder snow‐ground interface temperatures than measured in undisturbed snow. The depth of frozen ground in the different zones was modelled using a simple analytical solution that showed deeper frost penetration in the tree well than beneath the undisturbed snow. Copyright © 1995 John Wiley & Sons, Ltd

Description: An approach to spatially distribute a snow process model by segmenting images of land cover, terrain and snow properties is reported. A small 1.7 ha study area with an existing database was selected for this preliminary evaluation. The methodology was carried out over a relatively flat valley bottom at Camp Grayling, Michigan. Meteorological measurements on two sides of the area showed only small differences, so uniform meteorological variables were assumed over the site. Initial snow cover conditions were reconstructed and were distributed over the area using snow maps and sparse snow pit measurements. One metre resolution terrain, soil, vegetation and snow type maps were individually processed into class maps. These layers were then combined to produce a segmented class map, where the attributes from the data layers were known for each class. A one‐dimensional model of snow processes was run for each class, then the results were mapped back into images. Shallow snow conditions provided high sensitivity of ablation patterns to meteorological conditions over a 72 h period. The model performance was assessed by comparing predicted and observed ablation patterns. The error in total snow‐covered area was less than 9%. However, the location errors were greater (predicted snow where no snow was observed and observed snow where no snow was predicted). Extensive error analysis was not justified because of the lack of multiple point measurements of snow properties. Copyright © 1995 John Wiley & Sons, Ltd

Description: It is argued that the aggregation approach towards macroscale hydrological modelling, in which it is assumed that a model applicable at small scales can be applied at larger scales using ‘effective’ parameter values, is an inadequate approach to the scale problem. It is also unlikely that any general scaling theory can be developed due to the dependence of hydrological systems on historical and geological perturbations. Thus a disaggregation approach to developing scale‐dependent models is advocated in which a representation of the distribution of hydrological responses is used to reflect hydrological heterogeneity. An appropriate form of distribution may vary with both scale and environment. Such an approach is dependent on the data available to define and calibrate the chosen subgrid parameterization. A parameterization based on a minimum patch representation is suggested and the problems of identification at the larger scale discussed. Copyright © 1995 John Wiley & Sons, Ltd

Description: Investigations of the impact of the ground surface on energy and mass fluxes in the atmosphere as well as on the dynamic response of the atmosphere at different scales are reviewed to emphasize the requirements for hydrological schemes for atmospheric models. Based on these investigations, a discussion on the scaling of the most important land processes and characteristics, from an atmospheric modelling perspective, is provided. It appears that because of the strong, non‐linear response of the atmosphere to the spatial variability of land‐surface characteristics, the scaling of hydrological processes and parameters is not linear. This implies that appropriate hydrological schemes for atmospheric models need to provide higher statistical moments and characteristic length scales of the spatial distribution of water availability for evapotranspiration. Copyright © 1995 John Wiley & Sons, Ltd

Description: A catchment‐wide soil moisture index based on spatially distributed point measurements of soil moisture is used to describe the temporal trend in regional soil moisture status in a 26 km2 catchment in south‐eastern Australia. The temporal variation in runoff, evaporation and soil moisture storage is simulated with a modification of the lumped SFB water balance model of Boughton (1984), which assumes a fixed bucket size, and with the variable infiltration capacity (VIC) model of Wood et al. (1992), which assumes a variable bucket representation. Comparison of simulated catchment soil moisture storage and the soil moisture index based on measurements indicates that both models can make useful predictions of soil moisture status at the catchment scale, with the VIC model performing slightly better than the SFB model. It is also shown that the quasi‐distributed VIC model can predict the relative wetness at individual locations, given their relative frequency of occurrence, thus allowing the disaggregation of catchment‐scale storage values to point‐scale soil moisture values. Copyright © 1995 John Wiley & Sons, Ltd

Description: A model is proposed for predicting the spatial variation in colluvial soil depth, the results of which are used in a separate model to examine the effects of root strength and vertically varying saturated conductivity on slope stability. The soil depth model solves for the mass balance between soil production from underlying bedrock and the divergence of diffusive soil transport. This model is applied using high‐resolution digital elevation data of a well‐studied site in northern California and the evolving soil depth is solved using a finite difference model under varying initial conditions. The field data support an exponential decline of soil production with increasing soil depth and a diffusivity of about 50 cm2/yr. The predicted pattern of thick and thin colluvium corresponds well with field observations. Soil thickness on ridges rapidly obtain an equilibrium depth, which suggests that detailed field observations relating soil depth to local topographic curvature could further test this model. Bedrock emerges where the curvature causes divergent transport to exceed the soil production rate, hence the spatial pattern of bedrock outcrops places constraints on the production law. The infinite slope stability model uses the predicted soil depth to estimate the effects of root cohesion and vertically varying saturated conductivity. Low cohesion soils overlying low conductivity bedrock are shown to be least stable. The model may be most useful in analyses of slope instability associated with vegetation changes from either land use or climate change, although practical applications may be limited by the need to assign values to several spatially varying parameters. Although both the soil depth and slope stability models offer local mechanistic predictions that can be applied to large areas, representation of the finest scale valleys in the digital terrain model significantly influences local model predictions. This argues for preserving fine‐scale topographic detail and using relatively fine grid sizes even in analyses of large catchments. Copyright © 1995 John Wiley & Sons, Ltd

Description: The surface hydrological output of a regional climate model is investigated with implications for process controls on the spatial‐temporal variability of the water cycle over the continental USA. Principal component analysis was performed on the seasonal and annual hydrological cycles to determine their dominant modes of spatial variability. At both seasonal and annual time‐scales, the first principal component is dominated by precipitation, which controls seasonal wetness and evaporation and accounts for only 52 to 58% of the variability in the continental‐scale hydrological cycle. The second principal component is related to both snowmelt runoff and the time variability of weather (via its influence on the residence time of soil moisture near the land surface) and explains another 22% to 34% of the variability in the hydrological cycle. Based on these findings, a classification of hydroclimatological similarity is proposed in which two areas are similar in their hydroclimatology if their first and second principal components are similar. The classification scheme differs from classical approaches because it is based on dominant modes of variability rather than specific indices such as vegetation or seasonal wetness. Copyright © 1995 John Wiley & Sons, Ltd

Description: Topographic indices may be used to attempt to approximate the likely distribution of variable source areas within a catchment. One such index has been applied widely using the distribution function catchment model, TOPMODEL, of Beven and Kirkby (1979). Validation of the spatial predictions of TOPMODEL may be affected by the algorithm used to calculate the model's topographic index. A number of digital terrain analysis (DTA) methods are therefore described for use in calculating the TOPMODEL topographic index, In(a/tanβ) (a = upslope contributing area per unit contour; tanβ = local slope angle). The spatial pattern and statistical distribution of the index is shown to be substantially different for different calculation procedures and differing pixel resolutions. It is shown that an interaction between hillslope contributing area accumulation and the analytical definition of the channel network has a major influence on calculated In(a/tanβ) index patterns. A number of DTA tests were performed to explore this interaction. The tests suggested that an ‘optimum’ channel initiation threshold (CIT) may be identified for positioning river headwaters in a raster digital terrain model (DTM). This threshold was found to be dependent on DTM grid resolution. Grid resolution is also suggested to have implications for the validation of spatial model predictions, implying that ‘optimum’ TOPMODEL parameter sets may be unique to the grid scale used in their derivation. Combining existing DTA procedures with an identified CIT, a procedure is described to vary the directional diffusion of contributing area accumulation with distance from the channel network. Copyright © 1995 John Wiley & Sons, Ltd

Description: The advent of digital elevation models (DEMs) has made it possible to objectively extract, calculate and store geomorphological parameters for hydrological modelling at several scales. For a grid‐based DEM, the threshold area used to extract the channel network is analogous to the scale of the map produced. In addition to the map scale, the effects of the vertical resolution of the DEM on some frequently used geomorphological parameters in hydrology are examined using high‐resolution DEMs of two natural and two artificial catchments. The vertical resolution was varied between 1 cm and 1 m, the most common vertical resolution of DEMs. At a fixed map scale, the mean absolute percentage error in the geomorphological parameters caused by a decrease in vertical resolution is within the range 0–5% for the medium‐sized catchments and 0–10% for the small catchments studied. Although it is true that a change in vertical resolution may cause a change in the individual pixel slope, area and topographic index (area/slope), particularly in low relief terrain, their cumulative distributions do not show any significant change with the vertical resolution. The shape of the normalized width function is not very sensitive to the vertical resolution and the map scale. For small catchments order change may occur at different map scales for the different vertical resolution DEMs of the same catchment, causing a significant change in order‐related parameters such as Horton ratios. It is suggested that the vertical resolution of the DEM of a catchment be considered satisfactory for most hydrological applications if the ratio of the average drop per pixel and vertical resolution is greater than unity. This ratio criterion could be used to define the minimum pixel area for reliable channel network definition for any given vertical resolution. The minimum pixel area places a lower bound on the horizontal resolution with which a channel network can be extracted from a DEM. These results could potentially be used to assess the adequacy for hydrological purposes of existing and proposed digital elevation databases. Copyright © 1995 John Wiley & Sons, Ltd

Description: The chemistry of bulk precipitation and stream water was monitored in an acidic afforested catchment at Llyn Brianne in upland Wales between 1985 and 1990. Throughfall, stemflow and soil water chemistry were also monitored between 1988 and 1989. Marine‐derived solutes dominated the ionic composition of precipitation and stream water, which had mean Cl concentrations of 113 μequiv. 1−1 and 245 μequiv. 1−1, respectively. The higher concentrations in stream water reflect occult and dry deposition on the forest canopy and the effect of interception and transpiration losses. Chloride variations in stream water (112‐454μequiv. 1−1) were damped compared with bulk precipitation (28‐762μequiv. 1−1) due to the mixing of event (‘new’) water with pre‐event (‘old’) water in the catchment soils. A storm episode monitored in the catchment in April 1989 was associated with high sea salt inputs and Cl concentrations in throughfall (1466μequiv. 1−1) and storm runoff were exceptionally high (392μequiv. 1−1). The Cl signal in stream water during the episode was consistent with an event (‘new’) water contribution to the storm response. However, a short‐term hydrochemical budget estimated that although Cl outputs from the catchment during the event (1.17 kg ha−1) were equivalent to 8% of inputs in throughfall and stemflow, the storm runoff was equivalent to 32% of effective precipitation. This indicates that pre‐event (‘old’) water was the dominant source (〉 75%) of storm runoff. Although sea salt inputs during the event had a marked impact on stream water chemistry, the anomalously high levels of acidity sometimes associated with sea salt events were not observed in this particular study. Copyright © 1995 John Wiley & Sons, Ltd

Description: Field infiltration tests using portable rainfall infiltrometers were conducted in the Wangjiagou experimental basin in the hilly loess region of north China. Based on data collected at 27 sites, a topographic zonation of infiltration characteristics was observed. The average steady infiltration rate and the average ponding time decreased from the hilltop to the hillslope and further decreased to the gully wall. Such a zonation is closely related to the variations of topography, soil and land use conditions in the study area. A general infiltration model is proposed. Collected field data are used to establish the applicability of the proposed model in the study area. Copyright © 1995 John Wiley & Sons, Ltd

Description: Groundwater circulation is known to be one of the agents responsible for the redistribution of geothermal energy by acting as a source or sink in the course of its movement through porous media. Heat transport in groundwater systems is considered to be a coupled process and the theory based on this was used to analyse temperature profiles of 30 thermally stable observation wells in a deep, semi‐confined aquifer system in the Tokyo Metropolitan area. Vertical water fluxes in the semi‐confined aquifers and the associated upward heat fluxes were estimated from a heat flux equation that describes convection and conduction processes of heat transport in one dimension. The vertical downward water fluxes in Shitamachi lowland, Musashino and Tachikawa terraces were 0.69.26.91 × 10−9, 1.46‐70.92 × 10−9 and 2.61.2204 × 10−9 m/s, respectively. A vertical upward water flux of 1.80‐33.60 × 10−9 m/s was estimated in Shitamachi lowland. The water flux generally decreased with increasing depth for observation wells which intercepted more than one semi‐confining layer. The estimated upward heat fluxes for Shitamachi lowland, Musashino and Tachikawa terraces were 0.32‐1.12, 0.49‐1.21 and 1.00‐11.62 W/m2, respectively. The heat flux was highest in Tachikawa terrace where a major fault, the Tachikawa fault, is located. Generally, the estimated heat flux was higher in the semi‐confining layers than in the aquifers. Areas with heat sources and sinks as well as groundwater flow patterns in the semi‐confined aquifers were revealed by heat flux and temperature distributions in the study area. Copyright © 1995 John Wiley & Sons, Ltd

Description: River discharge measurements downstream of the Russell Glacier, near Kangerlussuaq (Søndre Strømfjord), West Greenland revealed the occurrence of short‐term discharge fluctuations during the 1991 melt season. Frontal ice‐cliff collapse results in temporary river damming, producing initial decreases in discharge and subsequent sudden flood peaks on dam failure. Fluctuations are less than two hours in duration, with a maximum discharge fluctuation magnitude of 50m3s−1, double that of normal ablation‐controlled diurnal fluctuations. As such, these events are exceeded in magnitude only by periodic jökulhlaups resulting from the drainage of an ice‐dammed lake further up‐glacier. The concentration of discharge fluctuations at the beginning of the melt season, the large number of ice blocks within the flow and the confinement of flows between ice block levees all add to the effectiveness of these events in terms of channel erosion and sediment transport. Copyright © 1995 John Wiley & Sons, Ltd

Description: The roles and limitations of geographical information systems (GISs) in scaling hydrological models over heterogeneous land surfaces are outlined. Scaling is defined here as the extension of small‐scale process models, which may be directly parameterized and validated, to larger spatial extents. A process computation can be successfully scaled if this extension can be carried out with minimal bias. Much of our understanding of land surface hydrological processes as currently applied within distributed models has been derived in conjunction with ‘point’ or ‘plot’ experiments, in which spatial variations and patterns of the controlling soil, canopy and meteorological factors are not defined. In these cases, prescription of model input parameters can be accomplished by direct observation. As the spatial extent is expanded beyond these point experiments to catchment or larger watershed regions, the direct extension of the point models requires an estimation of the distribution of the model parameters and process computations over the heterogeneous land surface. If the distribution of the set of spatial variables required for a given hydrological model (e.g. surface slope, soil hydraulic conductivity) can be described by a joint density function, f(x), where x = x1, x2, x3,… are the model variables, then a GIS may be evaluated as a tool for estimating this function. In terms of the scaling procedure, the GIS is used to replace direct measurement or sampling of f(x) as the area of simulation is increased beyond the extent over which direct sampling of the distribution is feasible. The question to be asked is whether current GISs and current available spatial data sets are sufficient to adequately estimate these density functions. Copyright © 1995 John Wiley & Sons, Ltd

Description: Groundwater heads and chemical composition were measured at approximately two week intervals during the summer of 1993 along a 1 km transect across the Insh Marshes floodplain mire in Inverness‐shire, Scotland. Groundwater heads were generally higher near the valley side slope, with lower pH values and greater dissolved organic carbon, A1 and C1 concentrations. In the centre of the transect, upward groundwater heads were identified and pH, conductivity and concentrations of base cations were much greater. Near the River Spey, pH and base cation concentrations decreased and A1 and C1 concentrations increased. Deep groundwater followed a similar spatial trend but was generally more base‐rich than shallow groundwater. These variations reflect the influence of three major water sources with different chemical signatures. Runoff from the valley side slope increased dissolved organic carbon and A1 in the shallow groundwater, the upward flow of groundwater increased the pH and Ca concentration and inundation near the river decreased the base status and increased C1 and A1. Copyright © 1995 John Wiley & Sons, Ltd

Description: The space and time resolutions used for the input variables of a distributed hydrological model have a sufficient impact on the model results. This resolution depends on the required accuracy, experimental site and the processes and variables taken into account in the hydrological model. The influence of space and time resolution is studied here for the case of TOPMODEL, a model based on the variable contributing area concept, applied to an experimental 12 km2 catchment (Coët‐Dan, Brittany, France) during a two month winter period. A sensitivity analysis to space and time resolution is performed first for input variables derived from the digital elevation data, secondly for the optimized values of the TOPMODEL parameters and finally for modelling efficiency. This analysis clearly shows that a relevant domain of space and time resolutions where efficiency is fairly constant can be defined for the input topographic variables, as opposed to another domain of larger resolutions that induces a strong decrease of modelling efficiency. It also shows that the use of a single set of parameters, defined as mean values of parameters on this relevant domain of resolution, does not modify the accuracy of modelling. The sensitivity of the parameters to space and time resolution allows the physical significance of the parameter values to be discussed. Copyright © 1995 John Wiley & Sons, Ltd

Description: C. H. TAYLOR Methodological issues associated with isotopic hydrograph separations (IHSs) in built‐up environments are explored using results from the 1990 spring melt in a suburban basin in Peterborough, Ontario, Canada. The hetrogeneous nature of suburban environments complicates the selection of appropriate isotopic signatures for event and pre‐event waters. Near‐stream groundwater δ18O sampled from wells was poorly mixed, such that the pre‐event water signature was best characterized by δ18O in pre‐melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using δ18O in the pre‐melt snowpack, surface runoff samples or meltwater from lysimeters. However, the use of snowpack δ18O may be inappropriate in suburban basins where meltwater from thin snowcover may exhibit pronounced responses to δ18O in rainfall contributions. Intensive sampling of the spatial variability of runoff or meltwater δ18O may be required to characterize the average event water signature adequately. Rainfall δ18O provided an appropriate event water signal during a large rain on snow event, and differences between this IHS and one generated using an event water signature that included meltwater contributions from snow‐covered surfaces were within the uncertainty attributable to the analytical error in δ18O values. Event water supplied 55‐63% of the peak discharge and 48‐58% of total runoff from the basin during the melt, which is consistent with the fraction of the basin that has been developed. These results contrast with IHSs conducted in forested basins that suggest that stormflow is dominated by pre‐event water contributions. Copyright © 1995 John Wiley & Sons, Ltd

Description: The effects of timber‐cutting on sediment concentrations, soil loss and overland flow in an insigne pine (Pinus radiata) plantation were studied in a mountain watershed of the Cordillera de la Costa, central Chile. Soil formation rates for the lithological conditions of the watershed were estimated. Soil loss measurements on the plantation were taken in 100 m2 plots, equipped with Coshocton samplers, during the years 1991 and 1992. Treatments were: clear‐cutting no residues/burned, clear‐cutting with residues and undisturbed controls. First‐year soil losses were greater from the no residues/burned (2128 kg ha−1) than from the residues (1219 kg ha−1) or undisturbed (48 kg ha−1) plots. During the second post‐treatment year, soil loss was greater from the burned plots (1349 kg ha−1) than from the residues (243 kg ha−1) or the undisturbed (72 kg ha−1) plots. Sediment concentrations for the three treatments were 561, 340 and 59 mgl‐1 during the first year, and 400, 150 and 83 mgl−1 in the second year. Runoff from the no residues/burned plots was greater than from residues or undisturbed plots during the two post‐treatment years. Long‐term soil losses were projected to average 240 kg ha−1 yr−1 from areas without residues/burned and 120 kg ha−1 yr−1 in areas with residues treatment, over a 25 year rotation period, whereas control areas were projected to average 60 kg ha−1 yr−1. Copyright © 1995 John Wiley & Sons, Ltd

Description: This pilot study uses a chemical technique (KEtX) to establish vertical profiles of average monthly water temperature within river beds: the hyporheic zone at the interface between surface water and groundwater. Data are presented for two gravel‐bed sites and one sand‐bed site on the River Wissey, Norfolk, UK. From February to October 1992, average monthly temperatures were determined at 10cm intervals down 1 m profiles. A strong seasonal pattern was defined at all sites with hyporheic temperatures being relatively warm in winter and cool in summer. Isothermal periods occurred in March‐April and September‐October. Temperature ranges within the hyporheic were decreased (4.6–7.7°C) compared with those of surface waters (10 and 10.9°C). Temperature profiles were similar at all sites during winter, but the sand‐bed site had relatively low temperatures at a standard depth within the hyporheic during summer and the temperature gradient became isothermal later in the autumn at this site. It is suggested that the influence of flows and substratum characteristics on temperature patterns, especially in regulated rivers, may have significant ecological implications, for example for determining the timing of salmonid fry and invertebrate emergence. Copyright © 1995 John Wiley & Sons, Ltd

Description: Maps of the potential waterlogging of soils were generated using hypotheses about the effect of topography on the soil water regime inspired by Beven and Kirkby's concept of saturation overland flow. The procedure was validated by comparing the simulated maps with maps derived from a 1: 25 000 soil survey for two contrasting catchments. The value and limitations of the method are discussed in the light of this comparison. The approach proposed here is relevant to modelling the distribution of intensely waterlogged soils, provided the relationship between bedrock and the limit values is established. This approach can be used for several purposes: (1) to distinguish positional waterlogging from other types of waterlogging; (2) to control the quality and consistency of waterlogging maps; and (3) to create soil water regime maps for non‐surveyed catchments. Conversely, soil water regime maps can be compared with contributing areas simulated by hydrological distributed models for validation purposes. Copyright © 1995 John Wiley & Sons, Ltd

Description: During the summer of 1990, 12 gravity cores were collected in Lake Coeur d'Alene, Idaho at various depths and in a variety of depositional environments. All core subsamples were analysed to determine the bulk sediment chemistry; selected subsamples were analysed for trace element partitioning and 137Cs activity. The purpose of these analyses was to determine the trace element concentrations and distributions in the sediment column and to try to establish a trace element geochemical history of the lake in relation to mining and mining‐related discharge operations in the area. Substantial portions of the near‐surface sediments in Lake Coeur d'Alene are markedly enriched in Ag, As, Cd, Hg, Pb, Sb and Zn, and slightly enriched in Cu, Fe and Mn. Variations in the thickness of the trace element‐rich sediments, which range from more than 119 cm to as little as 17 cm, indicate that the source of much of this material is the Coeur d'Alene River. An estimated 75 million tonnes of trace element‐rich sediments have been deposited on or in the lake bed. Estimated trace element masses in excess of those considered representative of background conditions range from a high of 468 000 tonnes of Pb to a low of 260 tonnes of Hg. The similarity between the trace element‐rich surface and subsurface sediments with respect to their location, their bulk chemistry, their interelement relations and their trace element partitioning indicate that the sources and/or concentrating mechanisms causing the trace element enrichment in the lake sediments have probably been the same through‐out their depositional history. Based on a Mt St Helens'ash layer from the 1980 eruption, ages estimated from 137Cs activity and the presence of 80 discernible and presumably annual layers in a core collected near the Coeur d'Alene River delta indicate that deposition rates for the trace element‐rich sediments have ranges from 2.1 to 1.3 cm/year. These data also indicate that the deposition of trace element‐rich sediments began, at least in the Coeur d'Alene River delta, some time between 1895 and 1910, dates consistent with the onset of mining and ore processing activities that began in the area in the 1880s. Copyright © 1995 John Wiley & Sons, Ltd

Description: Selecting the correct resolution in distributed hydrological modelling at the watershed scale is essential in reducing scale‐related errors. The work presented herein uses information content (entropy) to identify the resolution which captures the essential variability, at the watershed scale, of the infiltration parameters in the Green and Ampt infiltration equation. A soil map of the Little Washita watershed in south‐west Oklahoma, USA was used to investigate the effects of grid cell resolution on the distributed modelling of infiltration. Soil‐derived parameters and infiltration exhibit decreased entropy as resolutions become coarser. This is reflected in a decrease in the maximum entropy value for the reclassified/derived parameters vis a vis the original data. Moreover, the entropy curve, when plotted against resolution, shows two distinct segments: a constant section where no entropy was lost with decreasing resolution and another part which is characterized by a sharp decrease in entropy after a critical resolution of 1209 m is reached. This methodology offers a technique for assessing the largest cell size that captures the spatial variability of infiltration parameters for a particular basin. A geographical information system (GIS) based rainfall‐runoff model is used to simulate storm hydrographs using infiltration parameter maps at different resolutions as inputs. Model results up to the critical resolution are reproducible and errors are small. However, at resolutions beyond the critical resolution the results are erratic with large errors. A major finding of this study is that a large resolution (1209 m for this basin) yields reproducible model results. When modelling a river basin using a distributed model, the resolution (grid cell size) can drastically affect the model results and calibration. The error structure attributable to grid cell resolution using entropy as a spatial variability measure is shown. Copyright © 1995 John Wiley & Sons, Ltd

Description: A qualitative discussion is presented on the concept of a representative elementary area (REA) proposed by Wood et al. (1988). Their work represents a systematic effort of looking at spatial variability as functions of scale, and over the past few years has stimulated new thoughts and research findings. However, it is argued here that the REA concept has limited utility in catchment hydrology for the following reasons. Firstly, the method of finding the REA using distributed modelling appears flawed. In estimating local runoff, point‐process models are applied to elements of 30 × 30 m2 or larger, but the effect of this particular upscaling on the calculated REA has not been addressed. This causes the REA to be relative to the resolution of the modelling. Secondly, an REA does not seem to exist in a natural environment because of its relativeness in the presence of multi‐scale heterogeneity and its dependence on individual storm events. Based on the above arguments and following a discussion of the hydrological implications of REA, it is pointed out that the REA does not provide a definite and robust measure of the inherent spatial variability in runoff and that its utility in formulating large‐scale hydrology is not as clear as it was first envisioned. Copyright © 1995 John Wiley & Sons, Ltd

Description: Errors in the kinematic wave and diffusion wave approximations for time‐independent (or steady‐state) cases of channel flow were derived for three types of boundary conditions: zero flow at the upstream end, and critical flow depth and zero depth gradient at the downstream end. The diffusion wave approximation was found to be in excellent agreement with the dynamic wave approximation, with errors in the range 1–2% for values of KF 02 (⩾ 7.5), where K is the kinematic wave number and F 0 is the Froude number. Even for small values of KF 02 (e.g. KF 2 0 = 0.75), the errors were typically less than 15%. The accuracy of the diffusion wave approximation was greatly influenced by the downstream boundary condition. The error of the kinematic wave approximation was found to be less than 13% in the region 0.1 ⩽ x ⩽ 0.95 for KF 02 = 7.5 and was greater than 30% for smaller values of KF 02 (⩽ 0.75). This error increased with strong downstream boundary control. Copyright © 1995 John Wiley & Sons, Ltd

Description: Experimental results are reported for dissolved solids concentration increase when clear meltwaters from the surface of an Alpine glacier were reacted with suspended sediment derived from the subsole. The experiments, at Findelengletscher, Switzerland, were conducted at temperatures of about 1°C. Rates of reaction, which decrease through time as solute content rises, were slower in meltwaters which had not been allowed to equilibrate with the atmosphere before sediment was added. Diurnal variations in transit times of meltwater from the surface to the glacier portal were derived from injections of rhodamine dye into large moulin‐conduit systems. Experimentally‐determined rates of increase of electrical conductivity were used together with observed transit times in a Lagrangian formulation. This model was used to assess the contributions of rate of reaction and flow‐through velocity to production of observed variations of solute concentration with portal meltwater discharge. The observed diurnal concentrations varied inversely but in phase with portal discharge. Diurnal variations in discharge through such moulin‐conduit systems lead to substantial changes in flow‐through velocity, altering the periods of time available for parcels of meltwater to react with basally‐entrained suspended sediment en route to the portal. Hence, there is considerable variability in the amount of solute acquired by meltwaters in transit. A diurnal pattern of solute concentration variation with discharge is generated by the model, although absolute levels of solute concentration are underestimated. Parcels of meltwaters flowing more slowly, having entered the glacier down small cracks, crevasses and tiny moulins, acquire larger concentrations of solute during longer transit times. These waters, which cumulatively account for a significant proportion of total discharge, mix with those traversing moulin‐conduit systems and presumably raise the overall solute content of the total discharge from Findelengletscher to the measured levels. Copyright © 1995 John Wiley & Sons, Ltd

Description: Methodologies for developing a macro‐scale model of subsurface stormflow generation on a steep forested catchment in Japan are addressed. Field studies on this catchment have indicated that subsurface flow, consisting mainly of ‘old’ water displaced by ‘new’ rain water, dominates the storm response. Detailed field measurements on the catchment allowed simple, catchment‐scale relationships to be developed between the volume of saturated groundwater, on the one hand, and discharge, and surface and subsurface saturated areas, on the other. Attempts are made to find linkages between these empirical relationships and physically‐based descriptions of hydrological processes operating at smaller scales. A distributed model based on the saturated‐unsaturated groundwater flow on steep catchments was developed and tested with field data collected on this catchment. Derivation of catchment‐scale relationships can be carried out by a straightforward integration of the distributed model output. An alternative disaggregation‐aggregation approach is presented, whereby the catchment is divided into a number of hillslope flow strips. By applying the distributed model on each flow strip, under steady‐state conditions, it is possible to infer the spatial variability of groundwater volume in the various flow strips. This variability is related to a measure of hillslope topography and geometry. Knowing the catchment topography and geometry for any catchment, it is then possible to derive useful catchment‐scale relationships which are applicable under quasi‐steady‐state conditions. Hydrological relationships derived in this manner are compared with corresponding empirical relationships. The methodologies presented are effective for understanding the linkages between catchment scale response and small‐scale flow processes. Copyright © 1995 John Wiley & Sons, Ltd

Description: In a headwater basin covered with boreal forest in northern Japan, the summer dry flow was monitored each summer from 1985 to 1993. Streamflow and specific electrical conductance fluctuated diurnally and these variations were attributed to daytime evapotranspiration. In 1989, the daytime reduction in streamflow and conductance were accompanied by a reduction in the HCO−3 concentration. The low flow hydrograph was separated into two components using HCO−3 and Cl− concentrations in August 1989, assuming low flow to be a mixture of delayed subsurface flow and of quick shallow flow. The slight diurnal variation in the ratio of shallow flow to subsurface flow caused the diurnal variation in conductance by changing the HCO−3 concentration. Copyright © 1995 John Wiley & Sons, Ltd

Description: Runoff from a small glacierized catchment in the Canadian high Arctic was monitored throughout one melt season. The stream discharge record is one aspect of a larger project involving glacier mass balance, superimposed ice formation and local climate on a glacier in the Sawtooth Range, Ellesmere Island, Northwest Territories, Canada. To better understand the main factors influencing the production of runoff on the glacier during the period of main summer melt, regression analyses were performed relating daily air temperature, shortwave incoming and net radiation, absorptivity and wind speed to daily glacier discharge. Air temperature at the glacier meteorological station on rain‐free days is the element with the greatest correlation with runoff (r2 = 0.57; n = 34). A multiple regression of discharge with air temperature, shortwave incoming radiation, net radiation hours and wind speed achieved the best fit (r2 = 0.84; n = 34). Rain events (〉 10mmd−1) can dominate daily discharge when they occur during the period of ice melt, creating more runoff per unit area than can be produced by melt alone, and significantly reduce the accuracy of runoff predictions. Copyright © 1995 John Wiley & Sons, Ltd

Description: Toxic cyanobacteria have become a common nuisance in freshwater lakes and reservoirs throughout the world, sometimes resulting in the closure of sites with high amenity value. Cyanobacteria are able to regulate their buoyancy state in response to changing photosynthetic rates. Additionally, the cyanobacteria are liable to become entrained within wind‐induced near‐surface turbulent currents, resulting in mixing and mass transport. These movement processes have been modelled. A mathematical function is presented which describes light‐ and nutrient‐limited cyanobacterial growth. The growth model is integrated with a previous movement model (SCUM: simulation of cyanobacterial underwater movement) as movement patterns and wind‐induced lake mixing strongly affect the intensity and duration of light received by the cyanobacteria and thereby determine the photosynthetic potential. Results of the model suggest that cyanobacteria are resistant to periods of lake mixing and continue to increase their biomass, but at a depressed rate. Growth is most rapid under calm conditions. The results agree well with field‐based findings, confirming the validity of the growth function. Copyright © 1995 John Wiley & Sons, Ltd

Description: Many existing general circulation models (GCMs) use so‐called ‘bucket algorithms’ to represent land‐surface hydrology. Wood et al. (1992) presented a generalization of the simple bucket representation, based on their variable infiltration capacity (VIC) model. The VIC model, in essence, assumes a statistical distribution of bucket sizes within the grid square. In this way, it provides a simple, computationally efficient and yet physically realistic model of land‐surface hydrology. A preliminary attempt is made here, using this simple model, to evaluate the effects of the spatial variability of rainfall intensities and the resulting soil moisture within a hypothetical GCM grid square. Rainfall is assumed to be patchy, with only a fraction of the grid square being wetted by rainfall at any given time. Within the wetted area, however, rainfall is allowed to vary randomly in space. Evaporation during interstorm periods is estimated from the rest of the grid square by a simple non‐linear function of the available soil moisture. The soil moisture is also assumed to be patchy and spatially variable due to the antecedent rainfall that caused it. A number of simplifying assumptions have been made in the model about the redistribution of soil moisture at the end of storm and interstorm periods, and the effects of a vegetation canopy have been ignored. The model is applied, under a variety of conditions, to estimate the biases in the modelled water balance fluxes if the assumed spatial heterogeneity is neglected. The model is also used to simulate the long‐term water balance dynamics under assumed hypothetical storm and inter‐storm climatic inputs, to see how the steady‐state hydrological regime is affected by spatial variability. These simulations are relevant to current efforts towards developing simple parameterizations of land‐surface hydrology that explicitly incorporate subgrid heterogeneity. Copyright © 1995 John Wiley & Sons, Ltd

Description: Extrapolation of measurements of water use by individual trees to that for a stand of trees is a critical step in linking plant physiology and hydrology. Limitations in sampling resources and variation in tree sizes within a stand necessitate the use of some scaling relationship. Further, to scale tree water use in space as well as time, the relationship must reflect the changing availabilities of energy and water supply. It is argued here that tree leaf area is the most appropriate covariate of water use to achieve this aim. However, empirical results show that the relationship is not always linear. A theory is developed, based on the concepts of hydrological equilibrium (sensu Nemani and Running, 1989) and ecological field theory (Walker et al., 1989) which accounts for (occasional) non‐linear behaviour of the flux/ leaf area relationship in evergreen trees. A key feature of this theory is the notion of a non‐linear, quasi‐equilibrium reflecting plant water stress. An equation is derived from these concepts and a standard, explicit treatment of tree water use (Landsberg and McMurtrie, 1984), which is used to characterise this relationship. This equation has the form Q = aIA + bΨsAf. The theory is tested against field data and published reports on Eucalyptus tree water use. Copyright © 1995 John Wiley & Sons, Ltd

Description: To parameterize the land surface in global climate models (GCMs) data must be provided at a variety of resolutions. In GCMs, high‐resolution data must be aggregated to the coarser resolution of the GCM. The method used for parameter aggregation can lead to the simulation of very different land surface climatologies. It is shown that the most important transition from a simulation of homogeneous tundra to one of homogeneous coniferous forest is the initial increase in forest. The differences between the simulation of tundra and the simulation of tundra with two‐ninths coniferous forest are considerable. This suggests that the land surface is sensitive to the aggregated parameters in non‐linear ways. It suggests that more care is needed in data aggregation, and that improved algorithms for data aggregation must be developed, because these data sets represent the foundations on which advanced land surface parameterizations are built. Finally, it shows that the influence of relatively small amounts of secondary vegetation should be represented in GCMs. Copyright © 1995 John Wiley & Sons, Ltd

Description: Surface observations and NOAA advanced very high resolution radiometer (AVHRR) satellite data are combined to provide area‐averaged values of albedo, canopy resistance, leaf area index and fractional vegetation cover. Albedo, fractional vegetation cover and leaf area index are derived from the reflectance of the visible and infra‐red NOAA AVHRR channels. Canopy resistance is estimated by closing the surface energy balance equation using the surface infra‐red temperature and the normalized difference vegetation index. These land surface parameters are evaluated against independent measurements and used as input into a numerical model to simulate the energy exchange between the surface and the overlying atmosphere. Simulation results are validated against detailed aircraft observations undertaken in south‐western Australia over both natural and agricultural vegetation. Copyright © 1995 John Wiley & Sons, Ltd

Description: The concepts of simple scaling and multiscaling provide a new theoretical framework for the study of spatial or regional flood frequency relations and their underlying physical generating mechanisms. In particular, the scaling exponents in the power law relationship between flood quantiles and drainage areas contain a ‘basic signature of invariance’ regarding the spatial variability of floods, and therefore suggest different hypotheses regarding their physical generating mechanisms. If regional floods obey simple scaling, then the slopes do not vary with return periods. On the other hand, if regional floods obey multiscaling, then the slopes vary with return periods in a systematic manner. This premise is expanded here by investigating the empirical variations in the scaling exponents in three states of the USA: New York, New Mexico and Utah. Distinct variations are observed in the exponents among several regions within each state. These variations provide clear empirical evidence for the presence of both simple scaling and multiscaling in regional floods. They suggest that snowmelt‐generated floods exhibit simple scaling, whereas rainfall‐generated floods exhibit multiscaling. Results from a simple rainfall‐runoff experiment, along with the current research on the spatial scaling structure of mesoscale rainfall, are used to give additional support to these physical hypotheses underlying two different scaling structures observed in floods. In addition, the rainfall‐runoff experiment suggests that the behaviour of the flood exponents in small basins is determined by basin response rather than precipitation input. This finding supports the existence of a critical drainage area, as has been reported for the Appalachia flood data in the USA, such that the spatial variability in floods in basins larger than the critical size is determined by the precipitation input, and in basins smaller than the critical size is determined by the basin response. Copyright © 1995 John Wiley & Sons, Ltd

Description: Runoff generation in natural catchments due to storm rainfall is highly complex and spatially and temporally heterogeneous. In recent work on seven small experimental catchments Larsen et al. (1994) showed that underlying the heterogeneity of runoff generation within the catchments, there is a degree of regularity between the catchments that could be quantified in terms of two dimensionless similarity parameters K0* and f*. These two parameters, constants for a catchment, were able to characterize the relative dominance of the saturation excess (Dunne‐type) and infiltration excess (Horton‐type) mechanisms of runoff generation. Given that K*0 and f* can characterize the type of runoff generation on any catchment, it may follow that they can be used to define a catchment‐scale runoff generation model. This idea is pursued in this paper. For the same catchments as studied by Larsen et al. (1994), a lumped, physically based model is developed that describes both the extent of saturated areas and the average infiltration capacity of the unsaturated areas during a storm. This is achieved by utilizing the distributed model used by Larsen et al. (1994) to aggregate the point‐scale runoff generation responses, up to the catchment scale, from which the functional form and the parameters of the catchment‐scale runoff generation model are inferred. The parameters of this lumped model are defined entirely in terms of the underlying distribution of topography, three similarity parameters K*0, f* and B*, the normalized average water‐table depth, z*, and the normalized cumulative volume of infiltration, G*. Copyright © 1995 John Wiley & Sons, Ltd

Description: The Tretyakov non‐recording precipitation gauge has been used historically as the official precipitation measurement instrument in the Russian (formerly the USSR) climatic and hydrological station network and in a number of other European countries. From 1986 to 1993, the accuracy and performance of this gauge were evaluated during the WMO Solid Precipitation Measurement Intercomparison at 11 stations in Canada, the USA, Russia, Germany, Finland, Romania and Croatia. The double fence intercomparison reference (DFIR) was the reference standard used at all the Intercomparison stations in the Intercomparison. The Intercomparison data collected at the different sites are compatible with respect to the catch ratio (measured/DFIR) for the same gauge, when compared using mean wind speed at the height of the gauge orifice during the observation period. The Intercomparison data for the Tretyakov gauge were compiled from measurements made at these WMO intercomparison sites. These data represent a variety of climates, terrains and exposures. The effects of environmental factors, such as wind speed, wind direction, type of precipitation and temperature, on gauge catch ratios were investigated. Wind speed was found to be the most important factor determining the gauge catch and air temperature had a secondary effect when precipitation was classified into snow, mixed and rain. The results of the analysis of gauge catch ratio versus wind speed and temperature on a daily time step are presented for various types of precipitation. Independent checks of the correction equations against the DFIR have been conducted at those Intercomparison stations and a good agreement (difference less than 10%) has been obtained. The use of such adjustment procedures should significantly improve the accuracy and homogeneity of gauge‐measured precipitation data over large regions of the former USSR and central Europe. Copyright © 1995 John Wiley & Sons, Ltd

Description: A study was undertaken during the winter of 1990–1991 in a small (3.7 ha) Canadian Shield catchment to examine the hydrological and hydrochemical response during rain‐on‐snow events. The results are presented of two large (37.9 and 34.6 mm) rain‐on‐snow events occurring in early and late March 1991. Peak and total runoff and the groundwater response from the two events are significantly different. Hydrological data indicate that these differences can be attributed to a combination of meteorological (temperature) and physical conditions (antecedent snowpack ripeness, soil moisture and groundwater levels). An immature snowpack (low temperature and density) combined with low antecedent soil moisture conditions significantly reduced the magnitude of the net hydrological input and runoff from the catchment during the early March event, whereas a more mature snowpack and high antecedent soil moisture conditions led to a large runoff event during late March. During both rain‐on‐snow events a significant portion of the pre‐event snowpack chemical load was lost. Based on the maximum snowpack chemical load measured before the events, the two large rain‐on‐snow events and a brief mid‐March warm period during which there were two much smaller rain‐on‐snow events removed 78% of the hydrogen ion and 63% of the sulphate and nitrate load from the snowpack, while only reducing snowpack water equivalence by 7%. A two‐component (rain and snowmelt) isotopic (δ18O SMOW %0) separation of snowmelt lysimeter water during the two events indicated that snowmelt was an important (50 and 65%, respectively) water source available for infiltration and runoff at the snow‐soil interface. Considering the high hydrogen ion loadings to the catchment during these two events (3.3 and 3.0 mequiv.m−2, respectively) streamflow pH was not significantly reduced due to an increase in the discharge of well‐buffered groundwater. A two‐component isotopic hydrograph separation of peak stream discharge during the 2–3 March event indicated that 75% of the total flow was groundwater. In mid‐latitude acid‐sensitive catchments, winter rain‐on‐snow events are an important hydrological occurrence due to their ability to elute much of the chemical load (H+, SO4, NO3) from the snowpack before the onset of spring melt when the maximum annual hydrological input typically occurs. Copyright © 1995 John Wiley & Sons, Ltd

Description: Since 1986 the multiple benefits of moso‐bamboo forest, a special forest type found mainly in south China, have been investigated in a small 11.7 ha watershed in Fenyi County, Jiangxi Province. The mean annual precipitation in the study area is 1593.3 mm. For the 0–60 cm soil layer the average soil bulk density is 1.00 g/cm3, and the mean values for other soil properties are: total porosity 71.74%; non‐capillary porosity 5.81%; and water retention capacity 430 mm. The maximum effective water retention capacity of 313 mm is 28% higher than that for Chinese fir (Cunninghamia lanceolata) plantations and natural broadleaved forest in the neighbouring area. The parameters f0, fc and k, in Horton's infiltration equation, measured using the double‐ring method under drought conditions, are 29.10 mm/min, 8.28 mm/min and 0.2391, respectively. These infiltration properties are more favourable than those under nearby Chinese fir plantations. Compared with a Chinese fir plantation, the canopy interception ratio of moso‐bamboo is lower, but the stemflow ratio is higher. The annual canopy interception ratio is 11.1%. Because of snowfall, the interception ratios in January, February and March are higher, with values of 12.1–17.2%, whereas during the period of leaf fall in April, May and June the interception ratios are lower with values of 9.2–9.5%. During the other months they are relatively constant. The annual stemflow ratio is 4.4%. Again, because of snowfall, the stemflow ratios in January, February and March are lower with values of 2.8–2.9%, whereas during the remaining months they are fairly constant. Runoff analysis shows that the annual runoff ratio in this research watershed is 54.8%, but the ratio for quick runoff, composed of direct runoff and surface runoff, is only 0.8%. The upper interflow ratio is 15% and the ratio for the slow runoff composed of deeper interflow and underflow is 39%. The moso‐bamboo forest is very effective in reducing peak runoff and increasing low flows. The annual nutrient element inputs (kg/ha) to the moso‐bamboo forest ecosystem associated with throughfall and stemflow are N 17.7, P 0.38, K 56.5, Ca 31.,4, Mg 4.8 and SiO2 26.2, respectively. All the measured element inputs, with the exception of P, are higher than those associated with precipitation in the open, where typical values are N 10.1, P 0.89, K 18.8, Ca 25.8, Mg 3.1 and SiO2 10.1. The annual outputs in streamflow are N 3.0, P 0.28, K 16.6, Ca 38.9, Mg 8.3 and SiO2 125.7, indicating that for N, P and K the moso‐bamboo forest ecosystem is an accumulating system, whereas for Ca, Mg and SiO2 the reverse applies. All the pH values associated with precipitation in the open, throughfall, stemflow, surface runoff from runoff plots and streamflow in the research watershed vary between 6.45 and 7.60 and are close to neutral. Copyright © 1995 John Wiley & Sons, Ltd

Description: Atmospheric vapour flux convergence is introduced for the estimation of the water balance in a river basin. The global distribution of vapour flux convergence, ‐ ΔH · Q is estimated using the European Centre for Medium‐Range Weather Forecasts global analysis data for the period 1980‐1988. From the atmospheric water balance, the annual mean ‐ ΔH · Q can be interpreted as the precipitation minus evaporation. The estimated ‐ ΔH · Q is compared with the observed discharge data in the Chao Phraya river basin, Thailand. The mean annual values are not identical, but their seasonal change corresponds very well. The four year mean ‐ ΔH · Q is also compared with the climatological runoff of nearly 70 large rivers. The multi‐annual mean runoff is calculated from the Global Runoff Data Centre data set and used for the comparison. There is generally a good correspondence between the atmospheric water balance estimates and the runoff observations on the ground, especially in the mid‐ and high latitudes of the northern hemisphere. However, there are significant differences in many instances. The results emphasize the importance of accurate routine observations in both the atmosphere and river runoff. The global water balance of the zonal mean is compared with prior estimates, and the estimated value from this study is found to be smaller than previous estimates. The annual water balance in each ocean and each continent are also compared with previous estimates. Generally, the global runoff estimation using the conventional hydrological water balance is larger than the result by the atmospheric water balance method. Annual freshwater transport is estimated by atmospheric water balance combined with geographical information. The results show that the same order of freshwater is supplied to the ocean from both the atmosphere and the surrounding continents through rivers. The rivers also carry approximately 10% of the global annual freshwater transport in meridional directions as zonal means. Copyright © 1995 John Wiley & Sons, Ltd

Description: Addressing scaling issues in hydrological modelling involves, among other things, the study of problems related to hydrological similarity between catchments of different scales. Recent research about catchment similarity relationships is based on distributed conceptual models of surface runoff production. In this type of hydrological modelling both infiltration excess and saturation excess runoff production mechanisms are considered. In many humid lowland areas overland flow is a rare phenomenon because of the specific conditions that prevail: moderate rainfall, high infiltration capacity and low relief. The complete drainage system in these regions consists of surface and subsurface components which have organized themselves in a given geological, geomorphological and climatic situation. A surface drainage network has developed through sapping erosion at the zone of groundwater exfiltration. The resulting hierarchical stream network is in equilibrium with large time‐scale conditions and adjusts itself dynamically to the inter‐year and seasonal meteorological fluctuations. Greater understanding of the interrelationships that underlie the storm response of catchments in humid lowland regions can be expected by focusing on stream network morphology as a function of topography, geology and climate. This paper applies the physically based mathematical model of stream network morphology, developed by De Vries (1977), to the Zwalmbeek catchment, Belgium. Based on this model and for different climatic conditions (expressed in terms of rainfall characteristics) the first‐order stream spacing versus average water‐table depth relationship is calculated. From field observations, digital elevation model derived channel network drainage densities and flood event analysis it is concluded that the 1% exceedance probability rainfall can be suggested as representative for the shaping climatic conditions in the catchment under study. The corresponding curves relating channel network characteristics, such as stream spacing, drainage density and channel geometry, to average water‐table depth are basin descriptors and could be used for comparative studies (e.g. regional flood frequency analysis). The model further allows for the prediction of the expansion and shrinkage of the first‐order channel network as a function of catchment wetness expressed in terms of the effective water‐table depth. Copyright © 1995 John Wiley & Sons, Ltd

Description: This study was conducted to estimate macropore space, macropore flow and matrix flow in an experimental forest plot in the Ouachita Mountains of Arkansas. Lateral soil water fluxes and soil capillary potentials were observed in the isolated plot during applied rainfall experiments. Rainfalls were applied 17 times during the period 17 July to 10 October 1991. The subsurface hydrograph separation technique was used to estimate macropore space, macropore flux and matrix flux. The boundary between macropore and matrix flow was statistically determined by covariance analysis. The maximum estimated lateral macropore space was approximately 0.006 (cm3 cm−3). The maximum estimated lateral macropore and matrix flow were 0.042 and 0.00066 cm s−1, respectively. This report also emphasizes the need for further research on the hydrograph separation procedure for estimating macropores and macropore flow. Copyright © 1995 John Wiley & Sons, Ltd

Description: A modified concept of hydrological response units (HRUs) for regional modelling of river basins using the PRMS/MMS model is presented. The HRUs are delineated by geographical information system (GIS) analysis from physiographic basin properties such as topography, soils, geology, rainfall and land use using a thorough hydrological systems analysis. The HRUs, once classified by GIS analysis, preserve the three‐dimensional heterogeneity of the drainage basin. The River Bröl basin (A = 216 km2), Rheinisches Schiefergebirge, Germany was selected to apply the concept. In total, 23 HRUs were delineated and tested with the PRMS/MMS model using a 20‐year hydrometeorological daily database. The hydrological systems analysis revealed that interflow is the dominant flow process through the basin's slopes and the major contribution to groundwater recharge and river runoff. This was accounted for by parameterizing the HRUs in the model control file to drain their surplus water not used for satisfying the demand of evapotranspiration to a common conceptual subsurface storage. This storage was simulated by interflow drainage to the groundwater aquifer in the valley floor, which in turn drained to the channel network. The PRMS/MMS model simulated the observed daily discharge very well and the fit was described by a daily correlation coefficient of r = 0.91. The NASIM and HSPF models using different means to represent the basin's physiographic heterogeneity were applied to the Bröl basin as well, but did not achieve this correlation. The HRU concept was found to be a reliable method for regional hydrological basin modelling and allows spatial up‐ and downscaling. Future research on this concept will focus on incorporating the variable precipitation distribution into the classification of HRUs and on the hydrodynamic routing of the modelled discharge. Additionally, satellite imagery must be used for classifying land use in macroscale drainage basins. Copyright © 1995 John Wiley & Sons, Ltd

Description: Since the paper of Wood et al. (1988), the idea of a representative elementary area (REA) has captured the imagination of catchment modellers. It promises a spatial scale over which the process representations can remain simple and at which distributed catchment behaviour can be represented without the apparently undefinable complexity of local heterogeneity. This paper further investigates the REA concept and reassesses its utility for distributed parameter rainfall‐runoff modelling. The analysis follows Wood et al. (1988) in using the same topography and the same method of generating parameter values. However, a dynamic model of catchment response is used, allowing the effects of flow routing to be investigated. Also, a ‘nested catchments approach’ is adopted which better enables the detection of a minimum in variability between large‐ and small‐scale processes. This is a prerequisite of the existence of an REA. Results indicate that, for an impervious catchment and spatially invariant precipitation, the size of the REA depends on storm duration. A ‘characteristic velocity’ is defined as the ratio of a characteristic length scale (the size of the REA) to a characteristic time‐scale (storm duration). This ‘characteristic velocity’ appears to remain relatively constant for different storm durations. Spatially variable precipitation is shown to dominate when compared with the effects of infiltration and flow routing. In this instance, the size of the REA is strongly controlled by the correlation length of precipitation. For large correlation lengths of precipitation, a separation of scales in runoff is evident due to small‐scale soil and topographic variability and large‐scale precipitation patterns. In general, both the existence and the size of an REA will be specific to a particular catchment and a particular application. However, it is suggested that a separation of scales (and therefore the existence of an REA), while being an advantage, is not a prerequisite for obtaining simple representations of local heterogeneity. Copyright © 1995 John Wiley & Sons, Ltd

Description: Changing the scale of observation or averaging has a significant, but poorly understood, impact on the apparent variability of hydrological quantities. The representative elementary area (REA) concept is used as a motivation for measuring inter‐storm streamflow and calculating wetness index distributions for the subcatchments of two small study areas in New Zealand. Small subcatchments are combined to provide larger scale samples, and then the variance of specific discharge between similar sized subcatchments is calculated. For small subcatchments (area less than ∼1 km2) this variance is found to decrease with area more quickly than might be expected if the catchments were random samples. Such behaviour is tentatively interpreted as evidence supporting the concept of ‘organization’. At larger scales, variance between catchments decreases in a way that is consistent with sampling from a stationary random field. The results from the streamflow data are reinforced by an analysis of topographic data for the two study areas, although some questions remain open. Both the flow and topographic data support the idea that it is possible to find an averaging scale where the variability between catchments is sufficiently small for a ‘distribution function’ approach to be used in distributed rainfall‐runoff modelling. Consistent estimates of the scale at which the study areas become stationary (0.5 km2 for Little Akaloa, 2 km2 for Lewis) are obtained using both flow and topographic data. The data support a pragmatic REA concept which allows meaningful averages to be formed: this may be a useful base for further conceptual developments, but it is not appropriate for a classical continuum approach. Further conceptual development combined with field measurement and computer simulation are still required for the REA to have operational impacts. In particular, it is not clear which models are appropriate for use at the REA scale. Copyright © 1995 John Wiley & Sons, Ltd

Description: The effects of small‐scale heterogeneity in land surface characteristics on the large‐scale fluxes of water and energy in the land‐atmosphere system has become a central focus of many of the climatology research experiments. The acquisition of high resolution land surface data through remote sensing and intensive land‐climatology field experiments (such as HAPEX and FIFE) has provided data to investigate the interactions between microscale land‐atmosphere interactions and macroscale models. One essential research question is how to account for the small‐scale heterogeneities and whether ‘effective’ parameters can be used in the macroscale models. To address this question of scaling, it is important to carry out modelling studies by analysing the spatial behaviour of process‐based, distributed land‐atmospheric models and available data from land surface climate experiments such as those designed under ISLSCP (e.g. FIFE and BOREAS) and HAPEX (e.g. HAPEX‐MOBILY, HAPEX‐SAHEL) and GEWEX (e.g. GCIP) and from smaller scale remote sensing experiments. Using data from FIFE'87 and WASHITA'92, a soil moisture remote sensing experiment, analyses are presented on how the land surface hydrology during rain events and between rain event varies; specifically, runoff during rain events, evaporation between rain events and soil moisture. The analysis with FIFE'87 data suggests that the scale at which a macroscale model becomes valid, the representative elementary scale (REA), is of the order of 1.5‐3 correlation lengths, which for the land processes investigated appear to be about 750‐1250 m. For the Washita catchment data, analysis of soil‐based infiltration data supports an REA of this spatial scale, but model derived and remotely sensed soil moisture data appear to suggest a larger scale. Statistical self‐similarity is investigated to further understand how soil moisture scales over the Washita catchment and to provide a basis for macroscale models. Copyright © 1995 John Wiley & Sons, Ltd

Description: Following the clearing of native perennial vegetation for agriculture based on winter growing annual species, the surface characteristics of south‐western Australia have been significantly altered. Analysis of the annual variation of these characteristics based on satellite data and a one‐dimensional boundary layer model suggests that convective mixing over the cleared land is no longer able to reach the lifting condensation level for a significant period of the year. This implies a decrease in convective cloud formation and a reduction in the convective enhancement of rainfall. Copyright © 1995 John Wiley & Sons, Ltd

Description: Modelling experiments have been undertaken to address the effects of land surface heterogeneity on the energy and water fluxes at catchment scales. The simulation results indicate that in the presence of strong contrasts (i.e. patchiness) in the land surface characteristics (for example, soil moisture, leaf area index or vegetation type) significant inter‐patch advection can result. For example, small areas with high levels of soil moisture surrounded by drier areas have a disproportionately high latent heat flux. The sensible heat flux showed a complementary suppression. The response to changes in leaf area index, however, was found to be much more complex. At constant soil moisture levels, it was found that under some conditions the latent heat flux of patches of high leaf area index increased as its proportion of the surface increased as a result of the increase in net radiation and roughness. There was also an increase in sensible heat flux. This effect was also found on surfaces with low moisture levels and strong contrasts in surface vegetation. Although these results do depend on the initial boundary layer and terrestrial conditions, a consequence of this is that significant biases can be generated in modelling catchment output if the heterogeneity effects are not fully accounted for. The model simulations demonstrate that the fluxes from a ‘homogenized’ surface with catchment‐average land surface properties (e.g. soil moisture) can be significantly higher than that for a heterogeneous surface with explicitly modelled inter‐patch interactions. These results have particular implications for nested catchment models where the responses of individual subcatchments are as important as that of the total catchment. They are also significant in efforts towards developing lumped land surface parameterizations for use in atmospheric models such as general circulation models. Copyright © 1995 John Wiley & Sons, Ltd