ALBERT

Beschreibung: Suspended sediment sources in the Upper Severn catchment are quantified using a composite fingerprinting technique combining statistically-verified signatures with a multivariate mixing model. Composite fingerprints are developed from a suite of diagnostic properties comprising trace metal (Fe, Mn, AI), heavy metal (Cu, Zn, Pb, Cr, Co, Ni), base cation (Na, Mg, Ca, K), organic (C, N), radiometric (137Cs, 210Pb), and other (total P) determinands. A numerical mixing model, to compare the fingerprints of contemporary catchment source materials with those of fluvial suspended sediment in transit and those of recent overbank floodplain deposits, provides a means of quantifying present and past sediment sources respectively. Sources are classified in terms of eroding surface soils under different land uses and channel banks. Eroding surface soils are the most important source of the contemporary suspended sediment loads sampled at the Institute of Hydrology flow gauging stations at Plynlimon and at Abermule. The erosion of forest soils, associated with the autumn and winter commercial activities of the Forestry Commission, is particularly evident. Reconstruction of sediment provenance over the recent past using a sediment core from the active river floodpiain at Abermule, in conjunction with a 137Cs chronology, demonstrates the significance of recent phases of afforestation and deforestation for accelerated catchment soil erosion.

Beschreibung: In a field experiment to investigate potential impacts of climatic change, a small area of flush wetland in Wales was subjected to three successive years of simulated summer drought/rewetting (autumn-spring) cycles (1992–94). Drought was simulated achieved by diverting stream water around the experimental wetland during the summer, so that the wetland received only precipitation inputs during that time. The effects on peat-water chemistry in the rhizosphere were monitored at regular intervals until spring 1996, and comparisons made with a control. Simulated summer drought decreased, significantly, the natural summer peaks in dissolved organic carbon (DOC) and iron, whilst subsequently increasing the natural autumn-winter peaks in sulphate concentrations and acidity in the peat water. The effects of simulated drought on SO4 concentrations in the peat water compared favourably with subsequent events monitored following a natural summer drought in 1995. Autumn-winter peaks in SO4 concentrations in the control wetland following the natural drought were of similar magnitude to those induced by the drought simulated in the experimental wetland in the previous three years.

Beschreibung: A summary of hydrochemical research in the Plynlimon catchments from 1983 to the present is related to identifying water flow pathways within catchments and the modelling of soil and stream water acidification. The study reveals a highly heterogeneous system that barely conforms with current understanding of hydrology and acidification theory. The role of hydrochemical mixing processes and groundwater flow routing is emphasised as is the need for maintaining long term monitoring studies and enhancing process based studies of water and chemical fluxes through catchments. The applicability of current environmental impact models for predictive and environmental management purposes is questioned and it is proposed that new hydrochemical modelling structures are needed to examine the highly heterogeneous systems being modelled.

Beschreibung: The concept of potential evaporation is defined on the basis of the following criteria: (i) it must establish an upper limit to the evaporation process in a given environment (the term "environment" including meteorological and surface conditions), and (ii) this upper limit must be readily calculated from measured input data. It is shown that this upper limit is perfectly defined and is given by the Penman equation, applied with the corresponding meteorological data (incoming radiation and air characteristics measured at a reference height) and the appropriate surface characteristics (albedo, roughness length, soil heat flux). Since each surface has its own potential evaporation, a function of its own surface characteristics, it is useful to define a reference potential evaporation as a short green grass completely shading the ground. Although the potential evaporation from a given surface is readily calculated from the Penman equation, its physical significance or interpretation is not so straightforward, because it represents only an idealized situation, not a real one. Potential evaporation is the evaporation from this surface, when saturated and extensive enough to obviate any effect of local advection, under the same meteorological conditions. Due to the feedback effects of evaporation on air characteristics, it does not represent the "real" evaporation (i.e. the evaporation which could be physically observed in the real world) from such an extensive saturated surface in these given meteorological conditions (if this saturated surface were substituted for an unsaturated one previously existing). From a rigorous standpoint, this calculated potential evaporation is not physically observable. Nevertheless, an approximate representation can be given by the evaporation from a limited saturated area, the dimension of which depends on the height of measurement of the air characteristics used as input in the Penman equation. If they are taken at a height of 2 m (the height of the meteorological observations), the dimension of the saturated surface in the direction of the wind ranges roughly from 50 to 200 m for a short green grass completely shading the ground.

Beschreibung: The increasing incidence of groundwater pollution has led to recognition of a need to develop objective techniques for designing reniediation schemes. This paper outlines one such possibility for determining how many abstraction/injection wells are required, where they should be located etc., having regard to minimising the overall cost. To that end, an artificial neural network is used in association with a 2-D or 3-D groundwater simulation model to determine the performance of different combinations of abstraction/injection wells. Thereafter, a genetic algorithm is used to identify which of these combinations offers the least-cost solution to achieve the prescribed residual levels of pollutant within whatever timescale is specified. The resultant hybrid algorithm has been shown to be effective for a simplified but nevertheless representative problem; based on the results presented, it is expected the methodology developed will be equally applicable to large-scale, real-world situations.

Beschreibung: Three upland Lake District Tarns, Scoat, Greendale and Burnmoor, have been evaluated using MAGIC (Model of Acidification of Groundwater In Catchments) to reconstruct past, present and future chemical behaviour. The modelled historical changes in acidity are compared with palaeoecological estimation of pH to demonstrate model validity. Chemistry as simulated for all anions and cations and two of the three lakes are shown to have undergone significant acidification. The effects of changing atmospheric pollution levels on lake chemistry is evaluated and 80-90% sulphur reduction levels are required to achieve zero alkalinity. The impacts of increased nitrogen deposition are assessed and are shown to further delay reversibility.

Beschreibung: Soil moisture gradients along hillslopes in humid watersheds, although indicated by vegetation gradients and by studies using models, have been difficult to confirm empirically. While soil properties and topographic features are the two general physio-graphic factors controlling soil moisture on hillslopes, studies have shown conflicting results regarding which factor is more important. The relative importance of topographic and soil property controls was examined in an upland forested watershed at the Coweeta Hydrologic Laboratory in the southern Appalachian mountains. Soil moisture was measured along a hillslope transect with a mesic-to-xeric forest vegetation gradient over a period spanning precipitation extremes. The hillslope was transect instrumented with a time domain reflectometry (TDR) network at two depths. Soil moisture was measured during a severe autumn drought and subsequent winter precipitation recharge. In the upper soil depth (0-30 cm), moisture gradients persisted throughout the measurement period, and topography exerted dominant control. For the entire root zone (0-90 cm), soil moisture gradients were found only during drought. Control on soil moisture was due to both topography and storage before drought. During and after recharge, variations in soil texture and horizon distribution exerted dominant control on soil moisture content in the root zone (0-90 cm). These results indicate that topographic factors assert more control over hillslope soil moisture during drier periods as drainage progresses, while variations in soil water storage properties are more important during wetter periods. Hillslope soil moisture gradients in southern Appalachian watersheds appear to be restricted to upper soil layers, with deeper hillslope soil moisture gradients occurring only with sufficient drought.

Beschreibung: Dambos, seasonally saturated wetlands, are widespread in headwater catchments in southern Africa and play an important role in the regional hydrological cycle. However, the processes influencing runoff from these catchments are poorly understood. This paper reports an isotopic investigation of runoff-generating mechanisms within a Zimbabwean catchment containing a dambo. Hydrograph separation using deuterium reveals that, once the dambo is saturated, up to 70% of total storm flow can be considered "new" water (i.e. derived directly from rainfall generating the runoff event). However, both the total proportion and the instantaneous maximum amount of "new" water in hydrographs are sensitive to rainfall characteristics and antecedent conditions. These results are (1) compatible with observations made in catchments in temperate climates when wetlands are present, and contrast with results obtained when wetlands are absent and (2) consistent with saturation overland flow, generated in saturated regions of the dambo, being the major storm runoff mechanism. To reconcile these observations with past perceptions that dambos attenuate flood flows, a dual role for dambos in storm flow production is postulated.

Beschreibung: The use of the nonparametric nearest-neighbour resampling technique is studied for generating time series of daily rainfall and temperature for seven stations in the German part of the Rhine basin. The emphasis is on the reproduction of extreme N-day precipitation amounts. The daily temperatures are used to determine snow accumulation and melt in winter. Two versions of the resampling method, conditional on the atmospheric circulation and unconditional, show comparable results. For precipitation, the autocorrelation properties are well reproduced, whereas for temperature the autocorrelation coefficients are systematically underpredicted. The distributions of the N-day annual maximum precipitation amounts are adequately preserved. Despite the systematic underprediction of the temperature autocorrelation, the distributions of N-day maximum snowmelt are well reproduced. A 1000-year simulation for the seven stations shows that unprecedented rainfall situations can be generated.

Beschreibung: By using topographic indices as derived from a Digital Terrain Models (DTM), it is possible to represent the heterogeneity within a landscape. This heterogeneity can reflect both long term evolutionary patterns seen in a landscape and the short term forcing of flow dynamics during storm events. By spatial analysis, the linkage between the geomorphological- hydrological-plant physiological phenomena can be examined. In this study, a direct link will be established between the topographically-driven hydrological phenomena and the eco-physiological response. The topographic distribution function of TOPMODEL is used to control the spatial and temporal flux of the channel flow and water table. The plant physiological model GAS-FLUX is used to give a spatially and temporally dissaggregated species-sensitive estimate of evapotranspiration flux. Evapotranspiration is sensitive to the vegetation phonology, to tundra community physiology and to the temperature regime. A simple linking of TOPMODEL and the GAS-FLUX model is applied to a summer snow-free period to the Imnavait catchment, Alaska (2.2 km2). A species-sensitive evapotranspiration model proved to give the highest quality results when validated against flow observations. Predicted dynamics of variable source area and the component hydrological processes are illustrated.