Low Base Flow Research Articles

Climate-change impacts on hydrology and nutrients in a Danish lowland river basin

The Mike 11–TRANS modelling system was applied to the lowland Gjern river basin in Denmark to assess climate-change impacts on hydrology and nitrogen retention processes in watercourses, lakes and riparian wetlands. Nutrient losses from land to surface waters were assessed using statistical models incorporating the effect of changed hydrology. Climate-change was predicted by the ECHAM4/OPYC General Circulation Model (IPCC A2 scenario) dynamically downscaled by the Danish HIRHAM regional climate model (25 km grid) for two time slices: 1961–1990 (control) and 2071–2100 (scenario). HIRHAM predicts an increase in mean annual precipitation of 47 mm (5%) and an increase in mean annual air temperature of 3.2 °C (43%). The HIRHAM predictions were used as external forcings to the rainfall-runoff model NAM, which was set up and run for 6 subcatchments within and for the entire, Gjern river basin. Mean annual runoff from the river basin increases 27 mm (7.5%, p < 0.05) when comparing the scenario to the control. Larger changes, however, were found regarding the extremes; runoff during the wettest year in the 30-year period increased by 58 mm (12.3%). The seasonal pattern is expected to change with significantly higher runoff during winter. Summer runoff is expected to increase in predominantly groundwater fed streams and decrease in streams with a low base-flow index. The modelled change in the seasonal hydrological pattern is most pronounced in first- or second-order streams draining loamy catchments, which currently have a low base-flow during the summer period. Reductions of 40–70% in summer runoff are predicted for this stream type. A statistical nutrient loss model was developed for simulating the impact of changed hydrology on diffuse nutrient losses (i.e. losses from land to surface waters) and applied to the river basin. The simulated mean annual changes in TN loads in a loamy and a sandy subcatchment were, respectively, + 2.3 kg N ha − 1 (8.5%) and + 1.6 kg N ha − 1 (6.9%). The rainfall-runoff model and the nutrient loss model were chained with Mike 11–TRANS to simulate the combined effects of climate-change on hydrology, nutrient losses and nitrogen retention processes at the scale of the river basin. The mean annual TN export from the river basin increased from the control to the scenario period by 7.7%. Even though an increase in nitrogen retention in the river system of 4.2% was simulated in the scenario period, an increased in-stream TN export resulted because of the simulated increase in the diffuse TN transfer from the land to the surface-waters.

The influence of grazing on vegetation, soil properties and stream discharge in a small Dartmoor catchment, southwest England, UK

AbstractIntensive grazing has been responsible for environmental degradation of moorland landscapes over the last 50 years. Anecdotal evidence on Dartmoor, UK, for example, has suggested that it is particularly associated with a greater frequency of large floods, lower baseflow and a greater occurrence of erosion. A study was therefore conducted at Holne Moor, a small catchment on east Dartmoor, to investigate the impacts of grazing animals on hillslope hydrology and stream discharge. The vegetation in the study area ranged from heather (Calluna vulgaris), in areas with low grazing intensity, to short grass with occasional erosion scars visible associated with larger sheep numbers. The physical characteristics of the topsoil beneath heather species were very different compared with short grass. This was not due so much to the direct physical impact of grazing animals, but indirectly to the chemical and biological differences brought about by changes in vegetation response and soil structure. The experiment at Holne Moor indicated that soil water content on the hillslope was linked directly to the stream's rainfall–runoff response via the soil water pathways. In relatively dry conditions, the soil water content is mainly determined by local factors such as vegetation. In wet periods, non‐local influences such as slope and contributing area are more important. A soil water content threshold of around field capacity separated the two conditions. A comparison between soil physical properties and soil moisture under different vegetation types indicated that this ‘runoff’ threshold between dry and wet states is lower in heavily grazed areas. It is argued that more intense grazing causes conditions suited to the increased delivery of soil water to rapid flowpaths such as sheep tracks and other rapid routes which connect the source of this hillslope water to the contributing areas during large floods. Any alteration of vegetation and soil properties due to management causes a substantial increase in ‘active’ source areas and therefore discharge during large storm events even before any erosion occurs. Copyright © 2006 John Wiley &amp; Sons, Ltd.

English

Biological monitoring was initiated on the Red River in 1997 to evaluate the effects on aquatic biota of open pit mine operations and mine rock piles over nearly a 40-year period. The general public perception was that the mine has had severe effects on the aquatic biota due to the absence of robust biological populations adjacent to the mine. However, initial biological monitoring data provided evidence that the observed negative impacts to fish and benthic invertebrates in the Red River near the mine were actually caused by naturally occurring thermal scars upstream of, and within, the mine area in the Red River drainage. These data indicated the open pit mine and mine rock piles did not measurably impact the suitability of the Red River to support aquatic organisms. Continued biological monitoring has reinforced these earlier conclusions, and has shown that the aquatic biota is most likely limited by episodic summer rain storms which simultaneously add large amounts of sediment and degrade water quality in the Red River downstream of the hydrothermal scars, although groundwater appears to be a contributing factor in low baseflow periods. Fish and macroinvertebrate parameters decreased immediately downstream of the town of Red River, demonstrating that the negative impacts to the aquatic biota begin well upstream of the Molycorp Mine property. Long term biological monitoring has demonstrated the complex nature of factors structuring the aquatic biota in the Red River, and demonstrated the value of well-designed monitoring programs. Additional

Seasonal evolution of runoff from Haut Glacier d'Arolla, Switzerland and implications for glacial geomorphic processes

Statistical classification of hydrograph form is used to elucidate the controls on diurnal runoff cycle evolution at Haut Glacier d'Arolla, Switzerland during the 1998 and 1999 melt seasons. Hydrographs are objectively grouped using statistical techniques into four principal types that are qualitatively interpreted as rising, falling, peaked-falling and peaked hydrographs. Peaked hydrographs are further grouped on the basis of the magnitude of their bulk flow, baseflow and diurnal flow components. Comparison with the evolution of meltwater sources and pathways demonstrates that runoff cycles evolve systematically during the melt season in response to removal of the seasonal snowpack from the ablation area. Peaked hydrographs predominate following the onset of snowpack removal and demonstrate an increasing and progressively earlier diurnal peak, but also an unusually low baseflow component that is probably due to surface melt mainly contributing direct to subglacial channels. Runoff cycle evolution has potentially significant geomorphic implications because peaked surface runoff cycles result in the formation of hydraulically efficient, channelised subglacial drainage and a significant increase in the gradient of the relationship between suspended sediment transport and discharge. Increasingly peaked diurnal cycles also result in increased basal sediment availability, most likely related to high diurnal water pressure variation within subglacial channels that may also have enhanced rates of basal sliding and hence subglacial erosion. Differences in runoff cycle form and evolution therefore have the potential to significantly influence glacial erosion rates and sediment yields.

N budgets and aquatic uptake in the Ipswich River basin, northeastern Massachusetts

We calculated N budgets and conducted nutrient uptake experiments to evaluate the fate of N in the aquatic environment of the Ipswich River basin, northeastern Massachusetts. A mass balance indicates that the basin retains about 50% of gross N inputs, mostly in terrestrial components of the landscape, and the loss and retention of total nitrogen (TN) in the aquatic environment was about 9% of stream loading. Uptake lengths of PO4 and NH4 were measurable in headwater streams, but NO3 uptake was below detection (minimum detection limit = 0.05 μM). Retention or loss of NO3 was observed in a main stem reach bordered by wetland habitat. Nitrate removal in urban headwater tributaries was because of water withdrawals and denitrification during hypoxic events and in ponded wetlands with long water residence times. A mass balance using an entire river network indicates that basin‐wide losses due to aquatic denitrification are considerably lower than estimates from several recent studies and range from 4 to 16% of TDN in stream loading. Withdrawals for domestic use restrict the runoff of headwater catchments from reaching the main stem during low base flow periods, thereby contributing to the spatial and temporal regulation of N export from headwater tributaries.

Controlling Storm-Water Runoff with Tradable Allowances for Impervious Surfaces

Storm-water flow from an impervious surface can lead to stream degradation, habitat alteration, low base flows, and increased toxic loadings from nonpoint sources, a problem that has resisted traditional command and control regulatory approaches. We explore the thesis that a well-designed, tradable runoff allowance system can create economic incentives for landowners to employ low-cost runoff management practices to reduce excess stormwater flow to more ecologically sound levels. Attributes such as percent impervious surface, soil type, and so on, determine a given land parcel’s runoff potential and management alternatives and, by extension, its allowance requirements. Using a small watershed in Cincinnati, Ohio, as a case study, we show that a tradable runoff allowance system carries promise as a low-cost method for attaining reductions in storm-water runoff.

Comparative study of bedload sediment yield processes in small mountainous catchments covered by secondary and disturbed forests, western Japan

To clarify the sediment yield processes following a disturbance by a forest fire in a mountainous catchment, and considering the hydrological and geomorphological processes in the headwater, we measured bedload sediment yield at rainfall events in disturbed and secondary forest catchments in the western part of Japan. The three catchments were under different hydrogeological conditions. The IK, TB and TY catchments were disturbed by forest fires in 2000, 1994, and 1978, respectively. In the IK catchment, although runoff response to rainfall was fastest with high peak flows, the catchment also had the highest base flow. Moreover, the annual sediment yield there was about ten times as high as in the other two catchments, and it was found that there was a steep linear curve in the relationship between precipitation and bedload sediment yield. This is thought to be caused by overland flow generation following water repellency on the slopes, and by the accumulated sediment that forms the thick soil layer on the valley bottom. On the other hand, in the TB catchment runoff experienced high peak flows at rainfall events and low base flows, and there was a gradual linear curve in the precipitation–sediment yield relationship. This might be the result of there being a thin soil layer on the hillslope and on the valley bottom because of successive erosion after the fire. In the TY catchment, runoff had a low peak flow at rainfall events and a high base flow; and the bedload sediment yield increased exponentially with increasing precipitation. Therefore, sediment yield in the TB catchment was more than that in the TY during storm events with precipitation of less than 100 mm, whereas it was the opposite during heavier rainfalls. It indicates that there is a thick soil layer on the slope and a thin soil layer on the valley bottom in the TY catchment following the recovering of vegetation, and that the sediment yield process predominates only during big rainfall events, only then does subsurface flow generate.

NWSRFS CALIBRATION PARAMETER SELECTION AND GEOLOGIC REASONING: PACIFIC NORTHWEST CASES1

ABSTRACT: The National Weather Service River Forecast System (NWSRFS) is the new hydrologic prediction model for the National Weather Service (NWS) and provides guidance to meteorologists who issue NWS Flood Warnings to the public. The primary submodel within NWSRFS is the Sacramento Soil Moisture Accounting (SAC‐SMA) model, which predicts surface runoff as a function of meteorological, geological, and soil data calibrated over a watershed. The research presented here focuses on a different approach to NWSRFS calibrations: greater utilization of geologic and soil data, in order to give the model better predictive capability. Geologic understanding can create better insights for the initial estimation and subsequent adjustment of SAC‐SMA parameters. Fifteen calibrated Pacific Northwest drainages reveal a variety of hydrogeologic responses. For example, results for the Mount Rainier drainages show the complex interaction between active glaciers, impermeable volcanic surfaces, and glacial sedimentary valleys. Unweathered volcanic terrains show flashy peak flows, fast flow recessions, and low baseflow. Sedimentary terrains display subdued peak flows, slow flow recessions, and higher baseflow. Operational implementation of these calibrations at the NWS's Northwest River Forecast Center has yielded more accurate predictive results since 1995. NWS hydrologic forecasters nationwide could benefit from using drainage basin geologic characteristics in understanding and improving model calibrations and real time forecasts.

A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (&lt;0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.

Land use, agronomic management and water quality in a small Northern Italian watershed

European union (EU) agricultural policy has induced significant changes in crop rotations, especially in marginal areas. The evaluation of the impact on water quality induced by this new agronomic framework is presented in this paper. The discharge, the sediment content and the concentrations of herbicides and nitrates in the Centonara creek, draining a hilly watershed near Bologna, were measured from October 1994 to September 1996. A geographic information system (GIS) and the crop simulation model CropSyst were used to characterize the relationships between cropping systems, land use, pedological and morphological properties of the watershed as well as nitrate losses. Hydrological results showed that the Centonara creek discharge was characterized by low base flows and by fast increments during flooding. Herbicide concentrations were above the EU 0.1 μg l −1 limit on several occasions, whereas nitrate concentrations were always below the 50 mg l −1 limit established by EU for drinking waters. It was estimated that more than 30% of the nitrogen input in the watershed is due to atmospheric depositions. The purpose of GIS was to subdivide the watershed in 86 “agronomically homogeneous areas”, which were then utilized as the basis for the application of CropSyst. Simulations obtained by the model showed that the greatest leaching losses of nitrates were higher than 10 kg ha −1 yr −1 exclusively in the agronomically homogeneous areas characterized by coarser textured soils. Overall nitrate and herbicide losses were low, mainly due to the differentiation of the cropping systems in the watershed. The combined use of GIS and CropSyst enabled the characterization of the environmental vulnerability in relation to the land use in the watershed by means of pedologic cartography, land use maps and meteorological data. In particular, erosion and herbicide losses were higher in sloping areas planted with spring–summer crops. The increase in row crops cultivations, determined by EU agricultural policy, represents the main impact on water quality of the investigated area.

Variability in Flood Disturbance and the Impact on Riparian Tree Recruitment in Two Contrasting River Systems

The vegetation within the riparian zone performs animportant ecological function for in-stream processes.In Australia, riparian zones are regarded as the mostdegraded natural resource zone due to disturbancessuch as river regulation and livestock grazing. Thisstudy looks at factors influencing vegetation dynamicsof riparian tree species on two contrasting riversystems in Western Australia. The Blackwood River insouth-western Australia is influenced by aMediterranean type climate with regular seasonalwinter flows. The Ord River in north-western Australiais characterized by low winter base flows andepisodic, extreme flows influenced by monsoon rains inthe summer. For both rivers, reproductive phenology ofstudied overstory species is timed to coincide withseasonal river hydrology and rainfall. An evendistribution of size classes of trees on the BlackwoodRiver indicated recruitment into the population iscontinual and related to the regular predictableseasonal river flows and rainfall. In contrast, on theOrd River tree size class distribution was clustered,indicating episodic recruitment. On both rivers treeestablishment is also influenced by elevation abovethe river, microtopography, moisture status and soiltype. In terms of vegetation dynamics riparianvegetation on the Ord River consists of long periodsof transition with short lived stable states incontrast to the Blackwood river where tree populationstructure is characterized by long periods of stablestates with short transitions.

Channel precipitation dynamics in a forested Pennsylvania headwater catchment (USA)

Seasonal and event variations in stream channel area and the contributions of channel precipitation to stream flow were studied on a 106-ha forested headwater catchment in central Pennsylvania. Variations in stream velocity, flowing stream surface width and widths of near-stream saturated areas were periodically monitored at 61 channel transects over a two-year period. The area of flowing stream surface and near-stream saturated zones combined, ranged from 0·07% of basin area during summer low flows to 0·60% of total basin area during peak storm flows. Near-stream saturated zones generally represented about half of the total channel area available to intercept throughfall and generate channel precipitation. Contributions of routed channel precipitation from the flowing stream surface and near-stream zones, calculated using the Penn State Runoff Model (PSRM, v. 95), represented from 1·1 to 6·4% of total stream flow and 2·5–29% of total storm flow (stream flow–antecedent baseflow) during the six events. Areas of near-stream saturated zones contributed 35–52% of the computed channel precipitation during the six events. Channel precipitation contributed a higher percentage of stream flow for events with low antecedent baseflow when storm flow generated by subsurface sources was relatively low. Expansion of channel area and consequent increases in volumes of channel precipitation with flow increases during events was non-linear, with greater rates of change occurring at lower than at higher discharge rates. Copyright © 1999 John Wiley & Sons, Ltd.

Seasonal snowpack influence on the hydrology of a sub-arctic catchment in Scotland

Snow cover, depth and water equivalent have been measured in each snow season between 1989/90 and 1993/94 in the Alit a' Mharcaidh catchment in the western Cairngorms. Spatial and temporal patterns of snow accumulation vary: the peak measured water equivalent held in the catchment snowpack ranged from around 50 mm in mild winters to 200 mm in more severe ones. Snow accumulation was concentrated on north and east facing slopes in the parts of the catchment above 700 m. Point measurements of water equivalent in the areas of greatest accumulation exceeded 2000 mm as a result high densities produced by prolonged melt metamorphosis. Patterns of accumulation and melt strongly influenced the hydrology and hydrochemistry of the stream which has a distinct alpine like regime. Low base flows of high pH (>6.5) characterise cold periods of snow accumulation. Melt events are variable; slight increases in temperatures commonly melt all snow lying at low altitudes. More substantial melts involving the extensive snowpack at higher altitudes occur in the early spring as temperatures increase more significantly. Melt events strongly influence the hydrochemistry of the stream; preferential elution of Cl and SO4 were distinct characteristics of each melt season. Moreover, the most acidic (pH < 5.5) streamflows, and the highest levels of inorganic aluminium, were usually observed during snowmelt.

Evaluating the Reliability of the Stream Tracer Approach to Characterize Stream‐Subsurface Water Exchange

Stream water was locally recharged into shallow groundwater flow paths that returned to the stream (hyporheic exchange) in St. Kevin Gulch, a Rocky Mountain stream in Colorado contaminated by acid mine drainage. Two approaches were used to characterize hyporheic exchange: sub‐reach‐scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes (hydrometric approach) and reachscale modeling of in‐stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones (stream tracer approach). Subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange. The reach‐averaged hyporheic exchange flux (1.5 mL s−1 m−1), determined by hydrometric methods, was largest when stream base flow was low (10 L s−1); hyporheic exchange persisted when base flow was 10‐fold higher, decreasing by approximately 30%. Reliability of the stream tracer approach to detect hyporheic exchange was assessed using first‐order uncertainty analysis that considered model parameter sensitivity. The stream tracer approach did not reliably characterize hyporheic exchange at high base flow: the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone. At low base flow the stream tracer approach reliably characterized exchange between the stream and gravel streambed (timescale of hours) but was relatively insensitive to slower exchange with deeper alluvium (timescale of tens of hours) that was detected by subsurface measurements. The stream tracer approach was therefore not equally sensitive to all timescales of hyporheic exchange. We conclude that while the stream tracer approach is an efficient means to characterize surface‐subsurface exchange, future studies will need to more routinely consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange. Stream tracer models with multiple rate constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted.

The effectiveness of management options for dryland salinity control at Wanilla, South Australia

A calibrated groundwater flow model was used to assess several options for management of dryland salinity at Wanilla on Lower Eyre Peninsula of South Australia. The management options were chosen as potentially able to rectify the hydrological imbalance produced by the clearing of native vegetation which will reverse the salinisation process. The model predicted that current agricultural practices will result in groundwater levels continuing to rise in saline areas over the next 20 years (although tending towards equilibrium) increasing soil salinisation. As groundwater levels rise seepage of groundwater to the local drainage (natural) line will also increase, flowing down-gradient towards the catchment outlet and a seasonally flooded lower lying basin. A 50–90% reduction in catchment recharge, achieved with improved management of existing crops and establishment of perennial pastures, is predicted to reduce groundwater levels across the catchment over a 10–20 year period. This will lead to recovery of saline areas and lower baseflow. However, the model predicts that management of recharge needs to be implemented at least at the catchment scale to be effective. Treatments at the sub-catchment scale (e.g. property) have limited impact. Short term benefits (less than 1 year) can be gained from pumping groundwater from a high transmissivity zone close to discharge area. Groundwater levels are lowered over a small area leading to less evaporative discharge and reduced salt accumulation in the root zone. This option needs to work in conjunction with longer term management such as recharge control.

Effects of hydropower peaking on epilithon and invertebrate community trophic structure

AbstractHydropower generation induces rapid and frequent fluctuations of hydrodynamic parameters in rivers downstream of hydroelectric impoundments. On the FontauliéGre river (ArdéGche basin, France), water releases come from the Loire basin and the flow varies from 1.3 to 20 m3/s for one reach and from 0.1 to 20 m3/s for another reach. A reference site upstream was chosen to compare the communities with two downstream impacted sites. These two sites enabled two different base flow situations to be studied simultaneously and also the effect of base flow enhancement at one site. Benthic and epilithic samples were collected at the three sites on three dates. Epilithon development was linked to hydraulic regimes, with a major increase in algal biomass, dominated by filamentous algae, linked to the minimum base flow level and to the duration of periods without peaks. The macroinvertebrate communities of impacted reaches were less diversified and more specialized, with predominance of one or two taxa, whereas communities in upstream reaches were more diverse. The different morphological units specificity was attenuated in hydropeaking situations. In the case of a very low base flow, the lentic units with very low velocities (less than 4 cm s−1) had different fauna. The downstream algal development explained part of the change in benthic community structure in trophic groups (i.e. dominance of scrapers) in the site with the higher base flow. When the base flow was too low, a trophic discordance was observed and excess algal biomass represented a limiting factor. Thus, this study indicated that base flow level and duration had important effects on epilithic development and on invertebrate trophic structure in terms of mean velocities and of number of days without peaks.

Chemical characteristics and temporal trends in eight streams of the Catskill Mountains, New York

Discharge to concentration relationships for eight streams studied by the U.S. Geological Survey (USGS) as part of the U.S. Environmental Protection Agency's (U.S. EPA) Long-Term Monitoring Project (1983–89) indicate acidification of some streams by H2SO4 and HNO3 in atmospheric deposition and by organic acids in soils. Concentrations of major ions in precipitation were similar to those reported at other sites in the northeastern United States. Average concentrations of SO4 2− and NO3 − were similar among streams, but base cation concentrations differed widely, and these differences paralleled the differences in acid neutralizing capacity (ANC). Baseflow ANC is not a reliable predictor of stream acidity at high flow; some streams with high baseflow ANC (>150 Μeq L−1) declined to near zero ANC at high flow, and one stream with low baseflow ANC (<50 Μeq L−1) did not approach zero ANC as flow increased. Episodic decreases in ANC and pH during peak flows were associated with increased concentrations of NO3 − and dissolved organic carbon (DOC). Aluminum concentrations exceeding 300 Μg L−1 were observed during peak flows in headwater streams of the Neversink River and Rondout Creek. Seasonal Kendall Tau tests for temporal trends indicate that SO4 2− concentrations in streamwater generally decreased and NO3 − concentrations increased during the period 1983–1989. Combined acid anion concentrations (SO4 2− + NO3 −) were generally unchanged throughout the period of record, indicating both that the status of these streams with respect to acidic deposition is unchanged, and that NO3 − is gradually replacing SO4 2− as the dominant acid anion in the Catskill streams.

The rivers of North East Scotland: physicochemical characteristics

Multivariate analysis of spatial variation in 15 physicochemical characteristics grouped the seven major north-east rivers into two sets — highland and lowland rivers. The highland rivers formed a continuum that could be further divided into rivers with low concentrations of dissolved materials including the Spey and Dee (full highland rivers), and a more variable group with higher concentrations of dissolved materials including the Don, Deveron and Lossie (intermediate highland rivers) emphasising the diversity of river types classically described as highland. The Lossie was differentiated from the other intermediate highland rivers by lower oxygen concentration and pH. The major differences in physicochemistry between rivers were consistent with differences in catchment geology, soil type, climate, and land use. The large amounts of dissolved material and high nutrient status of the lowland Ythan and Ugie rivers reflected the high proportion of their catchments under agriculture, and the acidity of the Lossie reflected the high proportion of its catchment under forest. The small amounts of dissolved material in the Spey and Dee were attributable to the high proportion of slow weathering acidic rocks and acid soils in their catchments, and to the high volume flows of these rivers relative to the others. The amounts of dissolved material were greater in summer during low base flows and were accomparied by an increase in phosphate concentration in the lowland rivers. Oxygen saturation increased in summer in the Ythan and the Don, presumably as a result of greater photosynthetic activity. Marked declines in silica were thought to result from significant diatom growth in the highland and intermediate highland rivers, particularly the Don, during summer.

Application of a digital filter for modelling river suspended sediment concentrations

The relationship between suspended sediment concentration and discharge in a river system is the result of a complex interaction of factors within the basin and of specific storm events. Nevertheless, estimation of suspended sediment concentrations during storm events is essential for prediction of loadings from storm-controlled river basins to the receiving waters. The Venoge River, a tributary of Lake Geneva, Switzerland, with a basin area of 237 km 2, has been monitored during eight storm events between October 1986 and November 1987. A digital filter is used to simulate the suspended sediment response during individual events with the principal factors being antecedent flow, maximum discharge and time during which maximum flow was attained. The measured and calculated sediment concentration curves were assessed using a x 2-test. It was concluded that the two curves were similar in shape at 95% confidence interval. However, the model is not always capable of simulating the phase lead effect of the sediment distribution curve relative to the hydrograph. This problem is most evident during high-intensity storms at periods of low base flow; and consequently, loadings calculated for individual storms of this type could be overestimated. Fortunately, the model provides good predictions during less-intense, longer-duration events and therefore loadings calculated by summing events for an annual period will tend not to be overestimated. Further work is necessary to evaluate the model for additional events, in particular spring snow melt, and to determine its applicability to other river systems.

タンク・モデル 河川の流量を雨量から算出する一つのモデルについて

The tank model is very simple as shown in Fig. 1. We can consider that it corresponds to the zonal structure of groundwater as shown in Fig. 2. In spite of its simple outlook, the behaviour of the tank model is not so simple. Corresponding to various types of input rainfall, it shows various types of response as shown in Fig. 5 by its nonlinear structure caused by the positions of side outlets which are set somewhat higher than the level of the bottom.The tank model shown in Fig. 1 is used to calculate the daily discharge from the daily precipitation for Japanese river basins. For the flood analysis, data of short time unit are necessary and an appropriate time unit is suggested to be given asT.U.=0.05√A, where T.U. is the time unit (hour) and A is the catchment area (km2). Table 1 shows some examples of appropriate time unit for various catchment areas. For the flood analysis the tank model with two tanks shown in Fig. 6 is applicable.In Japan, the tank model without soil moisture structure can give fairly good results because it is always very humid in Japan. However, for most river basins, the tank model with soil moisture structure shown in Fig. 7 must be applied. The assumed soil moisture structure is composed of two parts, the primary and the secondary soil moisture storages. When the primary soil moisture storage is not saturated, the water is absorbed from the lower tank and there is water transfer between the primary and the secondary soil moisture storages. These two kinds of water transfer are given as shown in Fig. 7c.In regions with long dry season, there is no tree on mountain area or trees have no leave in dry season and vegitation covering can be found on plains or along rivers. In such regions, mountain areas become dry during the dry season, because water moves to lower part of the basin by gravitation. To simulate such a basin, the basin is divided into zones each of which is simulated by the tank model. The tank model of 4X4 type shown in Fig. 8 is derived under such a consideration. During the dry season, zones become dry from mountain side and no evapotranspiration occurs in dry zones. In this model real evapotranspiration from the basin decreases with time corresponding to the dry condition of zones, i.e. areal real evapotranspiration of the basin decreases automatically.The tank model is considered as a black box model without physical meaning by most hydrologists. However, we can ask ourselves, if it is a mere black box, how can such a simple tank model successfully simulate river discharge from high flood to low base flow? There must be some physical meaning in the tank model. Very recently, we were able to find the phenomenon to prove the existence of two kinds of water storage corresponding to the top and the second tank of the tank model by analysing the record of crustal tilt meters affected by rainfall (Fig. 9). The crust is some sort of spring balance which weighs' and so measures the groundwater storage (Fig. 10).