Estimation of nutrient load from urban diffuse sources: experiments with runoff sampling at pilot catchments of Lake Balaton, Hungary.

About a quarter of the total nutrient loading of Lake Balaton (Hungary) originates from urban diffuse sources, mostly from direct shoreline watersheds. This load cannot be measured directly. Sampling of urban runoff can help improving load estimations. The dynamic processes characterizing the accumulation and washoff of contaminants suggest that randomly observed concentrations are likely under- or overestimated. The results of two recent pilot programs aimed towards achieving continuous measurement of nutrient load carried by urban runoff are introduced. Stations were implemented in two pilot catchments located on the shore of Lake Balaton. Storm event runoff was sampled automatically and manually. Discharge, precipitation and rainfall intensities were also recorded. Results proved that the more a specific pollutant is associated with solid particles, the more of its load comes from a few but large storm events, nevertheless the cumulative effect of small rainfall events is not negligible, either. Event mean concentrations of solid-related pollutants were found to be dependent on rainfall intensity. The derived empirical relationships for SS, TP and TN event mean concentrations were indeed found to be applicable for reducing the uncertainty of load estimations of these pollutants significantly, as compared to using long-time average (i.e. annual mean) concentration values.