Results emerging from the long-term forestry versus upland grassland paired catchment study conducted by the Institute of Hydrology at Plynlimon, mid-Wales, and from studies of the processes controlling the hydrological responses of areas under forest and grassland, have been accepted by the water and forestry industries as a means of determining the probable effects of afforestation in other areas of Britain. When considering proposals in the late 1970s for a further major expansion of forestry, mainly in Highland Scotland, it became apparent that insufficient information was available to predict with confidence the effects in areas where forestry would replace medium height vegetation (heather sp., bracken) or in areas where a significant proportion of the precipitation falls as snow. Against this background a consortium of interested parties (see Hall 1987), agreed in 1981 to fund parallel systems and process studies of the effects in appropriate areas of Scotland. In this paper the initial stages of the systems study, on two catchments in the Balquhidder area of Central Region, are described. Some preliminary results from phase I, in which the catchment water balances under a mature forest and a mixed heather, bracken, grass cover were obtained, are presented. These water balances suggest that water use by the partly forested catchment is lower than that by the control and also lower than Penman ET. These findings are discussed in relation to the Plynlimon results and to information gained in the current process studies. Plans for phase II of the study, in which the mature forest will be clear-felled and part of the heather, bracken, grass control will be planted, are outlined.

Loch Fleet is an upland loch in SW Scotland, distant from significant local pollution, and unlikely to be affected by changes during the five-year project. The underlying geology is granitic. The 111 ha catchment is 90% moorland, dominated by heather and flying bent; 10% is planted with conifers.

Rainfall is approximately 2200 mm per year, predominantly from W and SW. More than half of rain events are pH>4·7; mean pH is 4·84 (1985). Marine salts are the major solutes. Deposition of acidity is about 0·1 g H+ m−2 per year, about twice that of eastern Scotland; deposition of non-seasalt sulphur is 1·6 g S m−2 per year. Soils are shallow and comprise organic peats, peaty gleys, and peaty rankers; base saturation is 3–19%. Soils (pH 2·5 to 4·6) are most acid in the forested area.

The loch is more acid (×5) than bulk deposition; additional acidity is mobilised from soils by the predominantly (>90%) superficial drainage. Moorland drainage dominates; that from the forest is more acid, but very variable in quantity and composition.

Catchment interventions in 1986 and 1987 are designed to alter the chemistry of drainage waters and the loch, so that the fishery can be restored. In addition to the work reported, extensive soils, sediment and biological investigations are underway.

A description is given of the hydrological regime and associated surface water quality recorded at a multi-disciplinary long-term investigation into the effects of acid precipitation and afforestation. The study catchment at Loch Dee is situated on the margins of the Loch Doon granite complex in SW Scotland. Annual rainfall is in excess of 2200 mm. The three main tributary burns exhibit a rapid response to rainfall with a high percentage of direct runoff. The loch with an average volume of 3650 M1 and a contributing catchment area of 15·6 km2 has an average turn-over of approximately forty days but this has varied from 15 to 100 days. Local physical variations are shown to influence both hydrological response and surface water quality. Generally stream water during spate conditions reflects rainfall chemistry. Occasionally, however, moderately acid rainfall containing large concentrations of sea salts has produced highly acidic runoff through a process of ion-exchange within the surface horizons of the soils. The quality of base flows shows a spatial variation which reflects various lithologies to the extent that pH values ranging from 4·9 to 6·9 have been observed.

Snowmelt modelling is of potential value in flood forecasting, reservoir management, and understanding stream acidification. It involves meteorological extrapolation, snowmelt calculation, meltwater routing, and snowpack depletion. A simple conceptual model using air temperature can reproduce the general pattern of daily streamflow in basins of >100 km2 but is prone to parameter instability. At a point scale and with the benefit of automatic weather station data the energy balance approach is superior to temperature index methods, but the roughness length parameter is again unstable in time and space. Even in a small (0·4 km2) basin this approach has to be coupled with an adequate flow routing model. Current research is comparing alternative models and data inputs in an intermediate-sized basin.

The Scottish Hydrometric Network consists of a number of river gauging stations which have been located at sites considered suitable to provide long term flow records. Economic recession has placed some stress on the gauging programme, and has given rise to extensive closures of gauging stations in England and, to a minor extent so far, in Scotland. The way in which the network became established provides a mixture of strengths and weaknesses which could have unpredictable consequences in an adverse economic climate. Changing technology provides some opportunity to reduce the cost of data acquisition and improve the deployment of manpower, while maintaining data standards. In these changing circumstances, particularly with extensive use of computer systems, it is important that standards are established for data returned to the Water Archive and that the network is not allowed to degenerate by default.

Establishing the magnitude and frequency of floods within upland catchments on the basis of short-term gauged runoff records is crucially dependent upon the extent to which the record is truly representative. In the case of the River Dee, upstream of Crathie in Aberdeenshire, gauged discharge records are limited in length. Although the middle Dee has been gauged since 1929, the gauge within the upper catchment has only ten years of record. Thus, reliable estimates of the return intervals of extreme floods for this part of the Dee can only be obtained by using a variety of historical sources to extend the flood series.

Long-term rainfall records, where available, provide a valuable independent check on the reconstructed flood series. Such rainfall records, when analysed in terms of the magnitude, frequency and duration of major events, should, in general terms, correspond with the flood series. In this paper, the recurrence interval of extreme rainfalls of varying magnitude and duration in upper Deeside is estimated by extreme value analysis of the annual maximum series. The frequency of rainfall events above varying thresholds is also assessed. The existence of climatic fluctuations giving highly variable recurrence intervals for rainfall events of the same magnitude is demonstrated. Finally, the seasonality of frequent flood-producing storms is analysed. Patterns observed within the rainfall record are compared with those previously established within the historic flood series to substantiate and augment the flood record.

Mires are terrestrial ecosystems which conserve organic production in the form of peat because their soils are waterlogged. Scotland's damp climate makes it rich in mires, with fens, raised mires and blanket mires occurring widely. Intact examples are mostly treeless due to wind exposure. They have two functional layers of soil: a shallow acrotelm overlying the catotelm or peat deposit proper. The acrotelm is the main region of diagenesis and contains the water table, which lies close to the surface for most of the time. Evapotranspiration depends on water table depth and surface vegetation. In the catotelm, water transmission is described by Darcy's law with sufficient exactness to allow prediction of the water table profile. An analysis of seepage using Dupuit–Forchheimer theory predicts a hemi-elliptical profile whose curvature is governed by the water balance in dry years and which, in its turn, closely controls the overall shape of the mire. Water flow is more rapid in the acrotelm, where it creates a regular series of different soil physical regimes which are linked with distinct combinations of surface topography and vegetation arranged sequentially from the centre of a mire to its edge. Such regular arrangements are most clearly seen in the Flowe Country of E Sutherland and Caithness. Acrotelm structure also permits variation in flow rate with depth, so that the amplitude of water table oscillations is narrowly confined and the mire thus protected against both desiccation and sheet flow.

This paper reports on the progress to date of a long-term investigation of the effects of drainage and afforestation on the hydrology of a peat bog near Edinburgh based on the use of runoff plots and other related equipment. The hydrology of the area immediately after ploughing is described and recently collected data are presented to show how this has changed over a six-year period. The main changes appear to be a substantial reduction in water yield and a move towards a less flashy runoff regime. Data are presented to show that these are most probably due to an increase in the amount of rainfall intercepted by the vegetation now covering the area and to a deterioration in the hydraulic efficiency of the drainage network.

The index flood/regional growth curve method is the most commonly used procedure for estimating a design flood at an ungauged site in the United Kingdom when only the instantaneous peak discharge is required. This paper summarises recent work in Scotland in which the authors have refined the equations for estimating the index flood from the physical characteristics of the drainage basin and have proposed new methods for classifying basins to increase the hydrological homogeneity of the regions on which the growth curves are based. A new algorithm for estimating regional growth curves is reported which allows for correlation between flood magnitude at different sites. Simulation experiments are described which highlight the consequences of the data failing to meet the assumptions of the models used.

The propagation of a flood wave through a river system is influenced by a complex process of energy dissipation. This is especially the case when overbank flows are a feature of the flood movement, producing more defined lateral movement of flow and high rates of fluid shear at the main channel/floodplain interface. The present paper concentrates on the physical and numerical modelling of river flows during overbank flow. A general description of the flow behaviour is outlined, including an investigation into the nature of the shear interaction between channel and floodplain. The paper advances a method for determining energy losses for the case of meandering channels with overbank flow and a method of sub-dividing the floodway for stage-discharge prediction. A state-of-the-art study of numerical modelling of flood propagation is outlined, including selected case histories on how the overbank flow situation has been handled to date, as well as comment on the current means of representing energy dissipation relying on calibration of the numerical model against observed field data.

The results of a series of experiments to study the physical processes governing the evaporation from upland vegetation in Scotland, i.e. coniferous forest, heather and grass, are described. Particular attention is given to the interception process occurring in heather, one of the dominant indigenous species in Scottish upland catchments. Attention is also given to the interception of snow precipitation which in the Scottish uplands is a significant proportion of the total precipitation. A comparison of the parameters describing the efficiency of the transport process of water vapour for coniferous forest and heather indicates that the process is more efficient than predicted by classical diffusion theory: additional transport mechanisms are considered. A simple model, based upon the results of the process studies, was applied to data from the Monachyle catchment (Balquhidder) and the model predictions compared with observations. This study, in conjunction with recent results from the Balquhidder catchment experiment, illustrates the necessity of further investigations to give a fuller understanding of the evaporation from high altitude grassland.

Intensive coniferous afforestation of many water supply catchments is taking place in Strathclyde Region. The current concern over the impact of this significant change in land use on the quantity and quality of water supplies is discussed. Some effects, such as high sediment run-off associated with ploughing and road development, may be of dramatic short-term effect, while others, such as reduction in quantitative yield, the development of eutrophication or acidification with aluminium leaching from soils, may take longer to materialise. Selective routine water quality monitoring work commenced in 1984 in a number of water supply catchments aimed at identifying forestry-related water quality trends. The initial results of this work are presented. Against the background of catchment land use changes, new European Community water supply standards were imposed in 1985 committing the Region to a substantial capital expenditure programme over a ten-year period in order to upgrade water treatment works. Any significant long-term changes in water quality associated with forestry activity will make this programme more costly to achieve. It is considered that the water supplies most susceptible to forestry related quantity or quality problems are the small supplies in areas of Argyll, Ayrshire and the Islands. It is suggested that the way forward is for improved advance consultation and planning on a site specific basis to protect public water supply interests.

The leaching of non-adsorbed tracer ions was studied at two field sites and in undisturbed columns. Results of field work are compared with leaching predicted using a simple chromatographic plate model. This comparison shows that in most cases less leaching than predicted occurs. This suggests that nitrate ions may be protected from leaching in field soils because of the presence of immobile water, and column experiments support this hypothesis. The extent of protection is larger than observed in other work for soils of similar texture.

Concern over the effects of eutrophication of Loch Leven led to an assessment of the phosphorus (P) loading and the contributions from runoff (from a catchment area of 145 km2), rain on the loch, roosting wildfowl, treated sewage effluent and P-rich industrial waste. Integrated programmes of flow gauging and nutrient analyses on twelve stream systems were carried out at eight-day intervals from January 1985 to February 1986. The paper concentrates on runoff and is restricted largely to total phosphorus (TP) loadings estimated from instantaneous flow and concentration data. The potential for deriving continuous loadings is discussed; examination of instantaneous and continuous discharge figures for a major inflow suggests that the present sampling regime has characterised its hydrological behaviour well. Nearly 90% of the water enters the loch in four streams of 0·1–1·0 m3 s−1 mean flow. The total input of water was high in 1985–134·4 × 106 m3 which is equivalent to a flushing rate of 2·6 (loch volumes); the effect on the loading results of the unusually wet study period is discussed.

Inflow concentrations of TP ranged from <10μg 1−1 to c. 4 mg 1−1; they generally increased with increasing flow but co-efficients of determination (r2 values) were only ≦0·57. Rating curves based on log load/log flow gave, for most waters, r2 values of 0·85, suggesting that load-flow, rather than concentration-flow models, will prove more favourable for predicting loads on non-sampling dates.

The total TP loading in runoff in 1985 was 9·4 tonnes, 38% of which was in soluble reactive form (SRP). Losses of P varied between sub-catchments, with 0·41–1·67 kg TP ha−1 yr−1 and typically <0·25 kg SRP ha−1 yr−1—representing only 5% of the annual agricultural applications of P. Initial calculations of inputs from sewage treatment works and industry suggest that these point-sources together contribute at least as much TP as in runoff, and twice the SRP in runoff. The significance of these findings for P control is discussed.

The sediments on the floors of nine reservoirs in the Midland Valley of Scotland have been examined to establish rates of sedimentation and catchment sediment yields. Information from Cullaloe, Drumain, Harperleas and Lambieletham Reservoirs (Fife), is compared with data from Glenfarg and Glenquey Reservoirs (Tayside) and earlier studies on Hopes, N Esk (Lothian) and Kelly (Strathclyde) Reservoirs. The earth-fill dams were constructed between 1850 and 1935 below catchments ranging from 1·53 to 23·5 km2 in area. They are situated on sedimentary, volcanic or intrusive igneous rocks mainly of Devonian or Carboniferous age. In most areas bedrock is obscured by mantles of drift. Six of the catchments are dominated by moorland terrain with local afforestation. Mixed arable farming and woodland characterise the remainder. The higher basins receive on average over 1400 mm of precipitation per annum, while in the lower basins some 800 mm falls.

Measurements of thickness variations of the deposits have enabled the construction of isopachyte maps from which the volumes of entrapped materials have been derived. The computed catchment sediment yields vary from 2·l t km−2 yr−1 (Lambieletham) to 52·0 t km−2 yr−1 (Glenfarg). Sediments incorporate both mineral and organic matter, of which combustible material commonly forms 15–20% by weight. Siliceous tests of diatoms are also an important component of the deposits. The sediment yields computed are about an order of magnitude greater than those reported from rivers draining similar sizes of catchment within the Forth basin. There is a direct relationship between mean annual inflow to the reservoirs and sediment yield.

Outputs of suspended sediment and bedload from the 7·7 km2 moorland Monachyle basin and the 6·8 km2, 40%-forested Kirkton basin near Balquhidder, and inputs from tributary streams and mainstream bank erosion, are compared. Sediment yield is about three times higher in the forested basin and varies more sensitively with streamflow, suggesting greater availability of erodible sediment. The output is predominantly of suspended sediment and is derived mainly from tributary streams. Initial observations following partial moorland ploughing and forest clearfelling in 1986 indicate that erosion of timber loading areas and logging roads is the main sediment source.

As part of the joint British-Scandinavian Surface Waters Acidification Programme, the Institute of Hydrology is establishing catchment studies in Scotland. Data from these catchment studies are being used to develop a range of models for investigating short-term and long-term changes in catchment acidity. Information on the modelling techniques available at the Institute of Hydrology is presented together with applications of the models to catchments in Scotland.

Episodes of high streamwater acidity have been cited in the literature as causal in fish-kills; such episodes are often associated with acid precipitation. An investigation utilising continuous monitoring of both precipitation and streamwater for a remote upland site in SW Scotland is reported and the occurrence of two episodes of streamwater acidification is described. The apparent mechanism for these episodes is one of ion exchange in the catchment soils following precipitation inputs laden with sea-salts. The investigation indicates the potential importance of sea-salts in streamwater acidification within the British context. The data provide insights into the mechanisms of acidification from precipitation input to ftreamwater output.

The nature of soil-water interactions is discussed in the context of both soil formation and drainage water chemistry. The interacting factors regulating drainage water chemistry are then briefly considered, including soil type and parent material, soil atmosphere CO2 concentration, hydrological pathways, soluble salts, topography, climate and biotic effects. Based upon these factors, some sites in NE Scotland thought to be highly susceptible to water acidification are examined, and the low pH of their waters is confirmed. The acidity trends of two superficially similar streams apparently less at risk are compared, and the differences explained in terms of sub-surface soil characteristics. The possible impacts of both afforestation and of acidifying pollutants on water pH are considered briefly where appropriate.