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Abstract:Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors in¯uencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26±75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20±47% and 6±28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15±70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3±9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76±82% of the annual TN transport. The remaining dissolved N species, ammonium (NH 4 -N) and organic N (DON) account for 0 . 3±1 . 2% and 13±16% of the annual TN transport, respectively. The TPN and TPP¯uxes closely re¯ect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient¯uxes are in¯uenced by point source inputs to the study basins, especially for P, although the TON¯ux is primarily in¯uenced by diuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn in¯uenced by land use and basin size. The magnitude of the NH 4 -N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitri®cation within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the dierent nutrient fractions exhibit signi®cant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes. #

The results of two comprehensive catchment-scale flow accretion surveys that were conducted in the lowland permeable catchments of the Rivers Frame and Piddle in Dorset, UK are presented. The surveys, undertaken as part of the NERC lowland catchment and research (LOCAR) program, were conducted under baseflow conditions during the summer of 2005.Results were adjusted for seasonal variation, the occurrence of intense precipitation events and anthropogenic inputs. A number of significant groundwater-surface water interactions reiated to various geological formations and boundaries were identified in this study. These included inputs of groundwater associated with the Greensand/Chatk (18-51 Ls ^ km"'') and Chaik/Palaeogene (ó8-i52Ls"''km""') boundaries and the Oakdaie Clay Member (52-79Ls"^km"''}, as well as substantial surface water losses associated with the Broadstone Sand Formation (158-171 Ls" ^ km " \ The study suggests that it may be possibie to predict groundwater-surface water interactions from basic information on solid geology, which would be of considerable benefit to the implementation of the Water Framework Directive In England and Wales.

This paper describes the development of a field-scale model that simulates the nitrogen (N) cycle in grazed grassland within a catchment-scale management model which can predict the loading and concentration of nitrate in rivers. The development is comprised of the addition of two sub-models of nitrate transport: one relating the amount of soil nitrate to its concentration in drainage water for different types of soil, and the second accounting for the proportion of permeable rock underlying the catchment. The sub-model that calculates the supply and transport of soil nitrate has been made sensitive to annual patterns of weather according to a classification based on the maximum soil water deficit. The model predictions were tested against best estimates of annual load and peak concentration of nitrate in rivers draining 11 small, predominantly grassland, catchments in the UK during the period 1974-1987.Un modèle de gestion pour la prévision du lessivage des nitrates dans des bassins herbeux du Royaume Uni: 1. Développement du modèle Résumé Cet article décrit un modèle simulant le cycle de l'azote à l'échelle d'une parcelle de prairie pâturée, inclus dans un modèle de gestion à l'échelle du bassin, susceptible de décrire la charge et la concentration des nitrates dans les rivières. Le modèle comprend deux sous-modèles de transport des nitrates: l'un reliant la concentration des nitrates dans les eaux drainées à travers différents types de sols à la quantité de nitrates stockés dans ces sols, le second prenant en compte la fraction de roches perméables du bassin. Le sous-modèle calculant l'approvisionnement et le transport des nitrates du sol répond aux variations climatiques annuelles selon un schéma fondé sur le maximum du déficit en eau du sol. Les prévisions du modèle ont été confrontées aux meilleures estimations de la charge annuelle et de la concentration de pointe de 11 rivières du Royaume Uni drainant de petits bassins versants à dominante herbeuse pour la période allant de 1974 à 1987.