STREAM NITRATE‐N LOADS IN RELATION TO VARIATIONS IN ANNUAL AND SEASONAL RUNOFF REGIMES1

Abstract

ABSTRACT: The calculation of stream nutrient loads from a sampling period of one year or, at most, a few years may provide an inaccurate estimate of average seasonal or annual loads due to considerable year‐to‐year variations in hydrological regime. The number of years of record required to give a reliable estimate of long‐term average NO3‐N loads was analyzed for E. Duffin Creek and the Nottawasaga River in Ontario, Canada. Nitrate load rating relationships were used in combination with a continuous stream discharge record for 22 years (E. Duffin Creek) and 34 years (Nottawasaga River) to simulate long‐term seasonal and annual variation in NO3.N loads. The errors involved in calculating average loads were examined by comparing the loads derived from sampling periods of one or more consecutive years duration with the estimated long‐term average load for the two rivers. Annual NO3‐N loads for a single year deviated from the long‐term average load by ± 20 to 53 percent in 8 out of 22 years in E. Duffin Creek and in 13 of 34 years in the Nottawasaga River. Six consecutive years of record would be required for both rivers to ensure that an error of > ± 20 percent would occur in only 5 percent of these observation periods. February‐April NO3‐N loads for a single year could deviate by up to +90 percent or ‐61 percent from the long‐term average spring period load for the two rivers. A sampling period of at least 6–7 years would be needed to estimate average NO3‐N loads for the spring runoff season with an error <± 20 percent.