Epsilon Open Archive

Abstract

Risk assessment and mitigation of pesticides losses to surface waters is a major challenge due to spatial and temporal variation in the factors that influence underlying transport processes. This thesis examines effects of spatial and temporal variation in soil properties on such losses and the pathways along which the transport occurs. Spatial variation in pesticide concentrations in streamflow were monitored in a small Swedish agricultural catchment with a large variation in soil types. Temporal variability in the structural and hydraulic properties that largely control the partitioning between surface and subsurface runoff was examined through field measurements and laboratory experiments.
Considerable changes in the volume, size distribution and connectivity of structural pores due to rainfall were observed in the harrowed layer after tillage, both in the field and the laboratory. In the field these changes were associated with decreases in near-saturated hydraulic conductivities of around one order of magnitude. Effects of wetting and drying on total porosity and the pore size distribution (PSD) varied between soils of different texture and organic carbon content. Post-tillage changes in soil structural and hydraulic properties could be accounted for in mechanistic models as changes in total porosity and the PSD and functions relating soil properties to the magnitude of these changes should be useful for parameterization. The ability to predict pesticide losses through surface runoff could thus be expected to increase.
At the catchment scale, consistently larger numbers of compounds at larger concentrations were found in a sub-catchment with a relatively large proportion of clay soils than in a sub-catchment with a smaller proportion of such soils. Only a few compounds at trace concentration were found in a third sub-catchment with coarser textured soils. Temporal stability of this spatial pattern suggests that the relative risk of pesticide losses to surface waters is related to soil properties under Swedish agro-environmental conditions. Soil texture maps could thus be used as a simple method for identification of high-risk areas. In the studied catchment, which is to a large extent subsurface drained and where surface connectivity between fields and the stream is limited, drainage was found to be a more likely dominant transport pathway than surface runoff.