Dynamics of connectivity and diffuse pollution transfers in agricultural catchments
Abstract
Hydrological connectivity is often assumed as a driver of nutrient and suspended sediment loss to watershed drainage networks which can have damaging impacts on natural biogeochemical processes. Targeted management of areas of higher connectivity and high pollutant loading is becoming widely accepted, however the assumption that connectivity increases sediment and nutrient transfer has rarely been tested at field and catchment scale. In order to test this hypothesis, the role of connectivity was investigated using empirical field experiments and catchment scale modelling. Overland flow connectivity and nutrient and sediment response were monitored from a hydrologically isolated 0.2ha grassland plot. Results demonstrated that the relationship between phosphorus (P) loads and connectivity was fraction specific with total soluble P (TSP) and soluble reactive P (SRP) highly significant (P=<0.05), while particulate P (PP) proved insignificant (P=>0.05). In-storm dynamics of TSP and SRP concentrations were variable, however could be predicted by the change in the rate of overland flow (dilution) and the size of the contributing area (P=<0.05). Building on these data at the catchment scale, sediment land use contributions were traced using geochemical fingerprinting within two small agricultural watersheds. It was assumed land use contributions derived from geochemical fingerprinting to be a result of land erodability (land use risk) and hydrological connectivity. Using a Monte Carlo approach the land use risk for each catchment was optimised by combining random risk weightings and connectivity indices using the Network Index version of TOPMODEL. Only model outputs which sufficiently replicated sediment land use contributions derived from geochemical fingerprinting were accepted. Results from watershed scale investigation showed that sediment land use contributions are strongly dependant on land use erodability (land use risk) and the spatial distribution of land use across both watersheds. At both field and catchment scales, connectivity is key determinant on the delivery of P and sediment to the watershed drainage network; however field investigations indicated the response of overland flow quality to the temporal development of connectivity was complex and storm dependent. This study highlights that while connectivity is a useful tool to predict pollutant loads, its ability to predict temporal nutrient loss may be limited.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPA53A2081T
- Keywords:
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- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 1804 HYDROLOGY / Catchment;
- 1815 HYDROLOGY / Erosion;
- 1871 HYDROLOGY / Surface water quality