Scale Effect in Nutrient Transport along a Rural River System: THE River Eden, Cumbria, Northwest, England
Abstract:Only a limited amount of information derived from studies conducted at small catchment scales can be transferred to large scales because of the non-linear scale effects, thus necessitating studies (including nutrient concentrations and yields) across a range of scales. Here we present results from an investigation of spatial scale pattern and temporal variability of nutrient concentration in the River Eden in northwest England, a nested catchment stretching from Gais Gill (1.1 km2) to Great Corby (1373 km2). The monitoring involved seasonal campaigns and spot sampling of river water quality, using two United Kingdom national catchment study platforms. Nitrate concentration shows a clear increasing trend with the catchment area and there is highly significant difference (P<0.001) among the catchments. Compared with the headwater areas, phosphorus (P) and suspended sediment (SS) concentrations are significantly higher (P<0.05) downstream but do not show a very clear spatial pattern. An alternative explanation was therefore sought for their distribution along the river. Generally, intensity of agricultural activities appears to influence the concentrations of these water quality parameters. The field data show that the amount of nutrients and suspended sediment is higher in catchments with higher farming activities and this increase downstream. This underscores the importance of the distribution of agricultural land use as a driving force in nutrient transport in River Eden. Higher nitrate concentration is associated with the period of low flow (strongest negative relationship, R2 = 0.97, was recorded in autumn sampling campaign at a gauging station). In contrast, phosphorus and suspended sediment are positively associated with discharge (strongest relationship (R2= 0.97) for total P were recorded in spring campaign at a gauging station). Similarly the dryness or wetness of a season affects the nutrient concentrations.
Thus, it appears that hydrology and land use distribution, from the headwater to the lowland areas downstream, control the spatial behaviour of the nutrients and suspended sediment concentration in the River Eden. The findings are being generalized for wider applicability in the UK using the TOPCAT-NP model.