Diffuse hydrological mass transport through catchments: scenario analysis of physical and biogeochemical uncertainty effects

Abstract

Abstract. This paper develops and investigates the applicability of a scenario analysis approach to quantify and map the effects of physical and biogeochemical variability, cross-correlation and uncertainty on expected hydrological mass loading from diffuse sources. The approach enables identification of conservative assumptions, uncertainty ranges, as well as pollutant/nutrient release locations and situations for which further investigations are most needed in order to reduce the most important uncertainty effects. The present scenario results provide different statistical and geographic distributions of advective travel times for diffuse hydrological mass transport, and show that neglect or underestimation of the physical advection variability implies substantial risk to underestimate pollutant and nutrient loads to downstream surface and coastal waters. This is particularly true for relatively high catchment-characteristic product between average attenuation rate and average advective travel time, for which mass delivery would be near zero under assumed transport homogeneity but can be orders of magnitude higher for variable transport conditions. A scenario of high advection variability, combined with a relevant average biogeochemical mass attenuation rate, emerges consistently from the example catchment results as a generally reasonable, conservative assumption for estimating maximum diffuse mass loading when the prevailing physical and biogeochemical variability and cross-correlation are uncertain. The geographic mapping of advective travel times for this high-variability scenario identifies also directly the potential hotspot areas with large mass loading to downstream surface and coastal waters, as well as their opposite, the potential lowest-impact areas within the catchment.