TIME AGGREGATION OF NITROGEN LEACHING IN RELATION TO CRITICAL THRESHOLD VALUES

Leaching of nitrate contributes to the deterioration of groundwater an d can consequently have a negative influence on the quality of our dri nking water. Critical threshold values for nitrogen leaching are estab lished to preserve groundwater quality. A critical threshold value for nitrate leaching of 50 mg l(-1) (11.3 mg N l(-1)), similar to the dri nking water standard, serves as a threshold value for European countri es. However, the temporal aggregation scale on which this threshold va lue should be considered is unknown. A well tested simulation model wa s used to evaluate the exceedance of the threshold value at different time aggregation levels, ranging from one day till 30 yr. For three di fferent soil structure types within one soil type and a selected ferti lisation regime, the aggregated nitrogen leaching over 30 yr was 11.4, 19.2 and 10.6 mg l(-1) Considering an aggregation level of one day, t he critical threshold value of 11.3 mg N l(-1) was exceeded 2973, 5801 and 2556 times, respectively, for the three structure types during 30 yr. By considering other time aggregation levels, a clear relation re sulted between time aggregation level and the number of time elements during which the critical level was exceeded. Results strongly indicat e that a critical threshold value for leaching should include an assoc iated time-aggregation level. (C) 1998 Elsevier Science B.V.