Abstract
Abstract. This paper quantifies and maps the effects of coupled physical and biogeochemical variability on diffuse hydrological mass transport through and from catchments. It further develops a scenario analysis approach and investigates its applicability for handling uncertainties about both physical and biogeochemical variability and their different possible cross-correlation. 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. The geographic mapping can be used to identify potential hotspot areas with large mass loading to downstream surface and coastal waters, as well as their opposite, potential lowest-impact areas within the catchment. Results for alternative travel time distributions show that neglect or underestimation of the physical advection variability, and in particular of those transport pathways with much shorter than average advective solute travel times, can lead to substantial underestimation of pollutant and nutrient loads to downstream surface and coastal waters. This is particularly true for relatively high catchment-characteristic product of 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, with a significant fraction of relatively short travel times, combined with a relevant average biogeochemical mass attenuation rate, emerges consistently from the present 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.
Highlights
Model estimations of diffuse hydrological mass transport are critical for biogeochemical cycle understanding, and successful and efficient environmental management
Pollutants and excess nutrients that are transported from diffuse sources through the subsurface water system may yield considerable long-term loading to downstream surface and coastal waters (Carpenter et al, 1998; Baresel and Destouni, 2007; Darracq et al, 2008; Basu et al, 2010; Howden et al, 2011)
We have developed and shown the applicability of a scenario analysis approach to quantify and map the effects of uncertainty about physical and biogeochemical variability, cross-correlation and uncertainty on expected mass loading from diffuse sources
Summary
Model estimations of diffuse hydrological mass transport are critical for biogeochemical cycle understanding, and successful and efficient environmental management. In many hydrological catchments with human activities, there are, apart from direct nutrient and pollutant discharges into surface waters, typically diffuse sources at the land surface and below it, in soil, in mobile and immobile groundwater, and in sediments. Pollutants and excess nutrients that are transported from diffuse sources through the subsurface water system may yield considerable long-term loading to downstream surface and coastal waters (Carpenter et al, 1998; Baresel and Destouni, 2007; Darracq et al, 2008; Basu et al, 2010; Howden et al, 2011). Many successive publications have quantified different types of uncertainty and investigated their implications for the modelling and management of water pollution
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